Lugtu & Sons

How to Calculate Satellite Bandwidth

2011-02-10T08:44:00.000-08:00

Satellites have a limited amount of bandwidth and they must use this space efficiently. The amount of bandwidth available on a satellite is easily calculated by adding together the bandwidth of each amplifier (transponder). Satellites typically have several carriers. Calculating the bandwidth for each individual carrier on the satellite is much more complicated and varies greatly from carrier to carrier. This is one of several calculations needed to effectively plan a satellite's carriers

1. Calculate satellite carrier bandwidth using the basic formula for carrier bandwidth: SR = DR / (m x CRv x CRrs), where SR = symbol rate, DR = data rate, m = modulation factor, CRv = Viterbi FEC rate and CRrs = Reed Solomon FEC rate.

2. Replace "DR" in the equation with the carrier's data rate in Mbp/s (megabits per second). The data rate should be available on the carrier parameter sheet and represents the amount of data being passed on the carrier. The data rate is also known as the information rate.

3. Substitute "m" in the equation with the carrier's modulation factor. The modulation factor is the transmit scheme used to lessen the bandwidth while still passing the necessary data. There are several different modulation schemes, including BPSK, QPSK, 8PSK, 8QAM and 16QAM. For the equation listed above, the following modulation factors can be used: BPSK = 1, QPSK = 2, 8PSK = 3, 8QAM = 3 and 16QAM = 4. The modulation scheme being used should be available on the carrier parameter sheet.

4. Swap the carrier's Viterbi FEC for the "CRv" in the equation. The Viterbi Forward Error Correction (FEC) rate is adjustable and adds bits to the data stream to allow for error correction. Typical Viterbi FEC settings are 1/2, 2/3, 3/4 or 7/8. The first number is the number of information bits, while the second number is the number of information bits plus added error correction bits. This setting also should be listed on the carrier parameter sheet.

5. Interchange 188/204 for "CRrs" in the equation if the carrier is using Reed Solomon FEC. It has the same purpose as Viterbi FEC. However, it cannot be changed, it is simply on or off. This information is also listed on the carrier parameter sheet.

How do Transistor work

2010-05-06T09:51:00.000-07:00

The design of a transistor allows it to function as an amplifier or a switch. This is accomplished by using a small amount of electricity to control a gate on a much larger supply of electricity, much like turning a valve to control a supply of water.

Transistors are composed of three parts – a base, a collector, and an emitter. The base is the gate controller device for the larger electrical supply. The collector is the larger electrical supply, and the emitter is the outlet for that supply. By sending varying levels of current from the base, the amount of current flowing through the gate from the collector may be regulated. In this way, a very small amount of current may be used to control a large amount of current, as in an amplifier. The same process is used to create the binary code for the digital processors but in this case a voltage threshold of five volts is needed to open the collector gate. In this way, the transistor is being used as a switch with a binary function: five volts – ON, less than five volts – OFF.

Semi-conductive materials are what make the transistor possible. Most people are familiar with electrically conductive and non-conductive materials. Metals are typically thought of as being conductive. Materials such as wood, plastics, glass and ceramics are non-conductive, or insulators. In the late 1940’s a team of scientists working at Bell Labs in New Jersey, discovered how to take certain types of crystals and use them as electronic control devices by exploiting their semi-conductive properties.Most non-metallic crystalline structures would typically be considered insulators. But by forcing crystals of germanium or silicon to grow with impurities such as boron or phosphorus, the crystals gain entirely different electrical conductive properties. By sandwiching this material between two conductive plates (the emitter and the collector), a transistor is made. By applying current to the semi-conductive material (base), electrons gather until an effectual conduit is formed allowing electricity to pass The scientists that were responsible for the invention of the transistor were John Bardeen, Walter Brattain, and William Shockley. Their Patent was called: “Three Electrode Circuit Element Utilizing Semi-conductive Materials.”

Understanding Zener Diode In FET

2010-04-08T10:02:00.000-07:00

Understanding Damper Diode

2010-04-08T09:55:00.000-07:00

Whenever I draw the diagram of the high voltage circuit of a Monitor, many times my students asked what was the function of the damper diode connected across collector and emitter pin of horizontal output transistor (HOT). For your information the damper diode can be found built into the HOT or in the circuit. Because of the high inductance and interwinding capacitance of the yoke and flyback transformer, the circuit must be damped out during retrace to prevent ringing. In other word, the damper is used to prevent oscillation in electronic circuit.

Note: Ringing means unwanted oscillation of a signal.
Source: http://www.electronicrepairguide.com/

Understanding Relay

2010-04-08T09:50:00.000-07:00

An electronic circuit consist of few or lots of electronic components for it to function properly. The electronic circuits such as the power supply, vertical, horizontal, high voltage, scanner, audio, color, memory, inverter, converter, feedback and etc are the circuits that forms the purpose of electronic equipment. In other words, different electronic equipment have different task because it has different types of electronic circuits in it. This month article is about to reveal the purpose of some electronic components in those electronic circuits. Let’s begin and analyse those components.

Relay
If you have repair electronic equipment before, I believe you have definitely seen a relay in electronic circuits. Sometimes in a circuit can have more than few relays. A relay is basically a switch operated by magnetic force. This magnetic force is generated by flow of current through a coil in the relay. The function of a relay is to open or close a circuit, when current through the coil is started or stopped. Now, the question is why there is a diode located parallel to the coil in the relay as seen from the schematic below?
Current flowing through a relay coil creates a magnetic field which collapses suddenly when the current is switched off. The sudden collapse of the magnetic field induces a brief high voltage ‘spikes’ across the relay coil which is very likely to damage ICs and Transistors. The diode which is also called as protection diode allows the induced voltage to drive a brief current through the coil (and diode) so the magnetic field dies away quickly rather than instantly. This prevents the induced voltage becoming high enough to cause damage to ICs and transistors.

Source: http://www.electronicrepairguide.com/

What is an audio signal?

2009-08-02T09:37:00.000-07:00

Actual sounds are converted to electrical signals for convenience of handling, recording and conveying from one place to another. This is the job of the microphone. There are two basic types of microphone, those which measure the variations in air pressure due to sound, and those which measure the air velocity due to sound, although there are numerous practical types which are a combination of both.

The sound pressure or velocity varies with time and so does the output voltage of the microphone, in proportion. The output voltage of the microphone is thus an analog of the sound pressure or velocity.As sound causes no overall air movement, the average velocity of all sounds is zero, which corresponds to silence.

As a result the bi-directional air movement gives rise to bipolar signals from the microphone, where silence is in the centre of the voltage range, and instantaneously negative or positive voltages are possible. Clearly the average voltage of all audio signals is also zero, and so when level is measured, it is necessary to take the modulus of the voltage, which is the job of the rectifier in the level meter. When this is done, the greater the amplitude of the audio signal, the greater the modulus becomes, and so a higher level is displayed.
Whilst the nature of an audio signal is very simple, there are many applications of audio, each requiring different bandwidth and dynamic range.

What is a video signal?
The goal of television is to allow a moving picture to be seen at a remote place. The picture is a two-dimensional image, which changes as a function of time. This is a three-dimensional information source where the dimensions are distance across the screen, distance down the screen and time. Whilst telescopes convey these three dimensions directly, this cannot be done with electrical signals or radio transmissions, which are restricted to a single parameter varying with time.
The solution in film and television is to convert the three-dimensional moving image into a series of still pictures, taken at the frame rate, and then, in television only, the two-dimensional images are scanned as a series of lines to produce a single voltage varying with time which can be digitized, recorded or transmitted. Europe, the Middle East and the former Soviet Union use the scanning standard of 625/50, whereas the USA and Japan use 525/59.94.

Introduction to MPEG-4

2009-08-01T08:05:00.000-07:00

Introduction to MPEG-4
MPEG-4 introduces a number of new coding tools as shown in Figure 1.13. In MPEG-1 and MPEG-2 the motion compensation is based on regular fixed-size areas of image known as macroblocks. Whilst this works well at the designed bit rates, there will always be some inefficiency due to real moving objects failing to align with macroblock boundaries. This will increase the residual bit rate. In MPEG-4, moving objects can be coded as arbitrary shapes.
Figure 1.14 shows that a background can be coded quite independently from objects in front of it. Object motion can then be described with vectors and much-reduced residual data. According to the Profile, objects may be twodimensional, three dimensional and opaque or translucent. The decoder must contain effectively a layering vision mixer which is capable of prioritizing image data as a function of how close it is to the viewer. The picture coding of MPEG-4 is known as texture coding and is more advanced than the MPEG-2 equivalent, using more lossless predictive coding for pixel values, coefficients and vectors.
In contrast, MPEG-4 may move the rendering process to the decoder, reducing the bit rate needed with the penalty
of increased decoder complexity.
In addition to motion compensation, MPEG-4 can describe how an object changes its perspective as it moves using a technique called mesh coding. By warping another image, the prediction of the present image is improved.
MPEG-4 also introduces coding for still images using DCT or wavelets.
Although MPEG-2 supported some scaleability, MPEG-4 also takes this further. In addition to spatial and noise scaleability, MPEG-4 also allows temporal scaleability where a base level bitstream having a certain frame rate may be augmented by an additional enhancement bitstream to produce a decoder output at a higher frame rate.
This is important as it allows a way forward from the marginal frame rates of today’s film and television formats whilst remaining backwards compatible with traditional equipment. The comprehensive scaleability of MPEG-4 is equally important in networks where it allows the user the best picture possible for the available bit rate.
MPEG-4 also introduces standards for face and body animation. Specialized vectors allow a still picture of a face and optionally a body to be animated to allow expressions and gestures to accompany speech at very low bit rates.
In some senses MPEG-4 has gone upstream of the video signal which forms the input to MPEG-1 and MPEG-2 coders to analyse ways in which the video signal was rendered. Figure 1.14(a) shows that in a system using MPEG-1 and MPEG-2, all rendering and production steps take place before the encoder. Figure 1.14(b) shows that in MPEG-4, some of these steps can take place in the decoder. The advantage is that fewer data need to be transmitted. Some of these data will be rendering instructions which can be very efficient and result in a high compression factor. As a significant part of the rendering takes place in the decoder, computer graphics generators can be designed directly to output an MPEG-4 bitstream. In interactive systems such as simulators and video games, inputs from the user can move objects around the screen. The disadvantage is increased decoder complexity, but as the economics of digital processing continues to advance this is hardly a serious concern.
As might be expected, the huge range of coding tools in MPEG-4 is excessive for many applications. As with MPEG-2 this has been dealt with using Profiles and Levels. Figure 1.15 shows the range of Visual

Introduction to MPEG-1

2009-07-30T19:53:00.000-07:00

Introduction to MPEG-1
As mentioned above, the intention of MPEG-1 is to deliver video and audio at the same bit rate as a conventional audio CD. As the bit rate was a given, this was achieved by subsampling to half the definition of conventional television. In order to have a constant input bit rate irrespective of the frame rate, 25 Hz systems have a picture size of 352 × 288 pixels whereas 30 Hz systems have a picture size of 352 × 240 pixels. This is known as common intermediate format (CIF). If the input is conventional interlaced video, CIF can be obtained by discarding alternate fields and downsampling the remaining active lines by a factor of two. As interlaced systems have very poor vertical resolution, down- sampling to CIF actually does little damage to still images, although the very low picture rates damage motion portrayal.
Although MPEG-1 appeared rather rough on screen, this was due to the very low bit rate. It is more important to appreciate that MPEG-1 introduced the great majority of the coding tools which would continue to be used in MPEG-2 and MPEG-4. These included an elementary stream syntax, bidirectional motion-compensated coding,
buffering and rate control. Many of the spatial coding principles of MPEG-1 were taken from JPEG. MPEG-1 also specified audio compression of up to two channels.

MPEG-2: Profiles and Levels
MPEG-2 builds upon MPEG-1 by adding interlace capability as well as a greatly expanded range of picture sizes and bit rates. The use of scaleable systems is also addressed, along with definitions of how multiple MPEG bitstreams can be multiplexed. As MPEG-2 is an extension of MPEG-1, it is easy for MPEG-2 decoders to handle MPEG-1 data. In a sense an MPEG-1 bitstream is an MPEG-2 bitstream which has a restricted vocabulary and so can be readily understood by an MPEG-2 decoder.
MPEG-2 has too many applications to solve with a single standard and so it is subdivided into Profiles and Levels.
Put simply a Profile describes a degree of complexity whereas a Level describes the picture size or resolution which goes with that Profile. Not all Levels are supported at all Profiles. Figure 1.11 shows the available combinations. In principle there are twenty-four of these, but not all have been defined. An MPEG-2 decoder having a given Profile and Level must also be able to decode lower Profiles and Levels.
The simple Profile does not support bidirectional coding and so only I and P pictures will be output. This reducesthe coding and decoding delay and allows simpler hardware. The simple Profile has only been defined at Main Level (SP ML).
The Main Profile is designed for a large proportion of uses. The Low Level uses a low resolution input having only 352 pixels per line. The majority of broadcast applications will require the MP ML (Main Profile at Main Level) subset of MPEG which supports SDTV (standard definition television). The High-1440 Level is a high-definition scheme which doubles the definition compared to Main Level. The High Level not only doubles the resolution but maintains that resolution with 16:9 format by increasing the number of horizontal samples from 1440 to 1920.
In compression systems using spatial transforms and requantizing it is possible to produce scaleable signals. A scaleable process is one in which the input results in a main signal and a ‘helper’ signal. The main signal can be decoded alone to give a picture of a certain quality, but if the information from the helper signal is added some aspect of the quality can be improved.
Figure 1.12(a) shows that in a conventional MPEG coder, by heavily requantizing coefficients a picture with moderate signal-to-noise ratio results. If, however, that picture is locally decoded and subtracted pixel by pixel from the original, a ‘quantizing noise’ picture would result. This can be compressed and transmitted as the helper signal.

A simple decoder only decodes the main ‘noisy’ bitstream, but a more complex decoder can decode both bitstreams and combine them to produce a low- noise picture. This is the principle of SNR scaleability.

As an alternative, Figure 1.12(b) shows that by coding only the lower spatial frequencies in a HDTV picture a base bitstream can be made which an SDTV receiver can decode. If the lower definition picture is locally decoded and subtracted from the original picture, a ‘definition- enhancing’ picture would result. This can be coded into a helper signal. A suitable decoder could combine the main and helper signals to re- create the HDTV picture. This is the principle of spatial scaleability.
The High Profile supports both SNR and spatial scaleability as well as allowing the option of 4:2:2 sampling.
The 4:2:2 Profile has been developed for improved compatibility with existing digital television production equipment. This allows 4:2:2 working without requiring the additional complexity of using the High Profile. For example a HP ML decoder must support SNR scaleability which is not a requirement for production.
MPEG-2 increased the number of audio channels possible to five whilst remaining compatible with MPEG-1 audio.
MPEG-2 subsequently introduced a more efficient audio coding scheme known as MPEG-2 AAC (advanced audio coding) which is not backwards compatible with the earlier audio coding schemes.

Introduction to motion compensation

2009-07-29T09:28:00.000-07:00

In real television program material objects move around before a fixed camera or the camera itself moves. Motion compensation is a process which effectively measures motion of objects from one picture to the next so that it can allow for that motion when looking for redundancy between pictures. Figure 1.9 shows that moving pictures can be expressed in a three-dimensional space which results from the screen area moving along the time axis. In the case of still objects, the only motion is along the time axis. However, when an object moves, it does so along the optic flow axis which is not parallel to the time axis. The optic flow axis is the locus of a point on a moving object as it takes on various screen positions.
Figure 1.9: Objects travel in a three dimensional space along the optic flow axis which is only parallel to the timeaxis if there is no movement.

It will be clear that the data values representing a moving object change with respect to the time axis. However, looking along the optic flow axis the appearance of an object only changes if it deforms, moves into shadow or rotates. For simple translational motions the data representing an object are highly redundant with respect to the optic flow axis. Thus if the optic flow axis can be located, coding gain can be obtained in the presence of motion.
A motion-compensated coder works as follows. A reference picture is sent, but is also locally stored so that it can be compared with another picture to find motion vectors for various areas of the picture. The reference picture is then shifted according to these vectors to cancel inter- picture motion. The resultant predicted picture is compared with the actual picture to produce a prediction error also called a residual. The prediction error is transmitted with the motion vectors. At the receiver the reference picture is also held in a memory. It is shifted according to the transmitted motion vectors to re-create the predicted picture and then the prediction error is added to it to re-create the original.

In prior compression schemes the predicted picture followed the reference picture. In MPEG this is not the case. Information may be brought back from a later picture or forward from an earlier picture as appropriate.

1.7.4 Film-originated video compression Film can be used as the source of video signals if a telecine machine is used. The most common frame rate for film is 24 Hz, whereas the field rates of television are 50 Hz and 60 Hz. This incompatibility is patched over in two different ways. In 50 Hz telecine, the film is simply played slightly too fast so that the frame rate becomes 25 Hz.
Then each frame is converted into two television fields giving the correct 50 Hz field rate. In 0 Hz telecine, the film travels at the correct speed, but alternate frames are used to produce two fields then three fields. The technique is known as 3:2 pulldown. In this way two frames produce five fields and so the correct 60 Hz field rate results. The
motion portrayal of telecine is not very good as moving objects judder, especially in 60 Hz systems. Figure 1.10shows how the optic flow is portrayed in film-originated video.
When film-originated video is input to a compression system, the disturbed optic flow will play havoc with the motion-compensation system. In a 50 Hz system there appears to be no motion between the two fields which have originated from the same film frame, whereas between the next two fields large motions will exist. In 60 Hzsystems, the motion will be zero for three fields out of five.
With such inputs, it is more efficient to adopt a different processing mode which is based upon the characteristics of the original film. Instead of attempting to manipulate fields of video, the system de-interlaces pairs of fields in order to reconstruct the original film frames. This can be done by a fairly simple motion detector. When substantial motion is measured between successive fields in the output of a telecine, this is taken to mean that the fields have come from different film frames. When negligible motion is detected between fields, this is taken to indicate that the fields have come from the same film frame.
In 50 Hz video it is quite simple to find the sequence and produce deinterlaced frames at 25 Hz. In 60 Hz 3:2pulldown video the problem is slightly more complex because it is necessary to locate the frames in which three fields are output so that the third field can be discarded, leaving, once more, de-interlaced frames at 25 Hz. Whilst it is relatively straightforward to lock-on to the 3:2 sequence with direct telecine output signals, if the telecine material has been edited on videotape the 3:2 sequence may contain discontinuities. In this case it is necessary to provide a number of field stores in the de-interlace unit so that a series of fields can be examined to locate the edits. Once telecine video has been de-interlaced back to frames, intra- and inter-coded compression can be employed using frame-based motion compensation.
MPEG transmissions include flags which tell the decoder the origin of the material. Material originating at 24 Hz but converted to interlaced video does not have the motion attributes of interlace because the lines in two fields have come from the same point on the time axis. Two fields can be combined to create a progressively scanned frame.
In the case of 3:2 pulldown material, the third field need not be sent at all as the decoder can easily repeat a field from memory. As a result the same compressed film material can be output at 50 or 60 Hz as required.
Recently conventional telecine machines have been superseded by the datacine which scans each film frame into a pixel array which can be made directly available to the MPEG encoder without passing through an intermediate digital video standard. Datacines are used extensively for mastering DVDs from film stock.

MPEG - Intra-coded compression

2009-07-28T09:43:00.000-07:00

Intra-coding works in three dimensions on the horizontal and vertical spatial axes and on the sample values. Analysis of typical television pictures reveals that whilst there is a high spatial frequency content due to detailed areas of the picture, there is a relatively small amount of energy at such frequencies. Often pictures contain sizeable areas in which the same or similar pixel values exist. This gives rise to low spatial frequencies. The average brightness of the picture results in a substantial zero frequency component. Simply omitting the highfrequencycomponents is unacceptable as this causes an obvious softening of the picture.
A coding gain can be obtained by taking advantage of the fact that the amplitude of the spatial components falls with frequency. It is also possible to take advantage of the eye’s reduced sensitivity to noise in high spatial frequencies. If the spatial frequency spectrum is divided into frequency bands the high-frequency bands can be described by fewer bits not only because their amplitudes are smaller but also because more noise can be tolerated. The wavelet transform (MPEG-4 only) and the discrete cosine transform used in JPEG and MPEG-1, MPEG-2 and MPEG-4 allow two-dimensional pictures to be described in the frequency domain.
Inter-coding takes further advantage of the similarities between successive pictures in real material. Instead of sending information for each picture separately, inter-coders will send the difference between the previous picture and the current picture in a form of differential coding.
Figure 1.8 shows the principle. A picture store is required at the coder to allow comparison to be made between successive pictures and a similar store is required at the decoder to make the previous picture available.
Figure 1.8: An inter-coded system (a) uses a delay to calculate the pixel differences between successive pictures.
To prevent error propagation, intra-coded pictures (b) may be used periodically.
The difference data may be treated as a picture itself and subjected to some form of transform-based spatial compression.
The simple system of Figure 1.8(a) is of limited use as in the case of a transmission error, every subsequent picture would be affected. Channel switching in a television set would also be impossible. In practical systems a modification is required. One approach is the so-called ‘leaky predictor’ in which the next picture is predicted from a limited number of previous pictures rather than from an indefinite number. As a result errors cannot propagate indefinitely. The approach used in MPEG is that periodically some absolute picture data are transmitted in place of difference data.
Figure 1.8(b) shows that absolute picture data, known as I or intra pictures are interleaved with pictures which are created using difference data, known as P or predicted pictures. The I pictures require a large amount of data, whereas the P pictures require fewer data. As a result the instantaneous data rate varies dramatically and buffering has to be used to allow a constant transmission rate. The leaky predictor needs less buffering as the compression factor does not change so much from picture to picture.
The I picture and all of the P pictures prior to the next I picture are called a group of pictures (GOP). For a high compression factor, a large number of P pictures should be present between I pictures, making a long GOP.
However, a long GOP delays recovery from a transmission error.
The compressed bitstream can only be edited at I pictures as shown.
In the case of moving objects, although their appearance may not change greatly from picture to picture, the data representing them on a fixed sampling grid will change and so large differences will be generated between successive pictures. It is a great advantage if the effect of motion can be removed from difference data so that they only reflect the changes in appearance of a moving object since a much greater coding gain can then be obtained.
This is the objective of motion compensation.

MPEG - Video compression

2009-07-26T10:30:00.000-07:00

Video compression
Video signals exist in four dimensions: these are the attributes of the pixel, the horizontal and vertical spatial axes and the time axis. Compression can be applied in any or all of those four dimensions. MPEG assumes an eight-bit colour difference signal as the input, requiring rounding if the source is ten-bit. The sampling rate of the colour signals is less than that of the luminance. This is done by downsampling the colour samples horizontally and generally vertically as well. Essentially an MPEG system has three parallel simultaneous channels, one for luminance and two colour difference, which after coding are multiplexed into a single bitstream.

Figure 1.7(a) shows that when individual pictures are compressed without reference to any other pictures, the time axis does not enter the process which is therefore described as intra-coded (intra = within) compression. The term spatial coding will also be found. It is an advantage of intra-coded video that there is no restriction to the editing which can be carried out on the picture sequence. As a result compressed VTRs such as Digital Betacam, DVC and D-9 use spatial coding. Cut editing may take place on the compressed data directly if necessary. As spatial coding treats each picture independently, it can employ certain techniques developed for the compression of still pictures. The ISO JPEG (Joint Photographic Experts Group) compression standards are in this category. Where a succession of JPEG coded images are used for television, the term ‘Motion JPEG’ will be found.
Greater compression factors can be obtained by taking account of the redundancy from one picture to the next. This involves the time axis, as Figure 1.7(b) shows, and the process is known as inter-coded (inter = between) or temporal compression.
Temporal coding allows a higher compression factor, but has the disadvantage that an individual picture may exist only in terms of the differences from a previous picture. Clearly editing must be undertaken with caution and arbitrary cuts simply cannot be performed on the MPEG bitstream. If a previous picture is removed by an edit, the difference data will then be insufficient to re-create the current picture.

Intra-coded compression
Intra-coding works in three dimensions on the horizontal and vertical spatial axes and on the sample values. Analysis of typical television pictures reveals that whilst there is a high spatial frequency content due to detailed areas of the picture, there is a relatively small amount of energy at such frequencies. Often pictures contain sizeable areas in which the same or similar pixel values exist. This gives rise to low spatial frequencies. The average brightness of the picture results in a substantial zero frequency component. Simply omitting the highfrequency components is unacceptable as this causes an obvious softening of the picture.

A coding gain can be obtained by taking advantage of the fact that the amplitude of the spatial components falls with frequency. It is also possible to take advantage of the eye’s reduced sensitivity to noise in high spatial frequencies. If the spatial frequency spectrum is divided into frequency bands the high-frequency bands can be described by fewer bits not only because their amplitudes are smaller but also because more noise can be tolerated. The wavelet transform (MPEG-4 only) and the discrete cosine transform used in JPEG and MPEG-1, MPEG-2 and MPEG-4 allow two dimensional pictures to be described in the frequency domain.

MPEG - Compression principles

2009-07-26T08:08:00.000-07:00

Compression principles
In a PCM digital system the bit rate is the product of the sampling rate and the number of bits in each sample and this is generally constant.
Nevertheless the information rate of a real signal varies. In all real signals, part of the signal is obvious from what has gone before or what may come later and a suitable receiver can predict that part so that only the true information actually has to be sent. If the characteristics of a predicting receiver are known, the transmitter can omit parts of the message in the knowledge that the receiver has the ability to re-create it. Thus all encoders must contain a model of the decoder.
One definition of information is that it is the unpredictable or surprising element of data. Newspapers are a good example of information because they only mention items which are surprising. Newspapers never carry items about individuals who have not been involved in an accident as this is the normal case. Consequently the phrase ‘no news is good news’ is remarkably true because if an information channel exists but nothing has been sent then it is most likely that nothing remarkable has happened.
The unpredictability of the punch line is a useful measure of how funny a joke is. Often the build-up paints a certain picture in the listener’s imagination, which the punch line destroys utterly. One of the author’s favourites is the one about the newly married couple who didn’t know the difference between putty and petroleum jelly – their windows fell out.
The difference between the information rate and the overall bit rate is known as the redundancy. Compression systems are designed to eliminate as much of that redundancy as practicable or perhaps affordable. One way in which this can be done is to exploit statistical predictability in signals. The information content or entropy of a sample is a function of how different it is from the predicted value. Most signals have some degree of predictability.
A sine wave is highly predictable, because all cycles look the same. According to Shannon’s theory, any signal which is totally predictable carries no information. In the case of the sine wave this is clear because it represents a single frequency and so has no bandwidth.
At the opposite extreme a signal such as noise is completely unpredictable and as a result all codecs find noise difficult. The most efficient way of coding noise is PCM. A codec which is designed using the statistics of real material should not be tested with random noise because it is not a representative test. Second, a codec which performs well with clean source material may perform badly with source material containing superimposed noise.
Most practical compression units require some form of pre-processing before the compression stage proper and appropriate noise reduction should be incorporated into the pre-processing if noisy signals are anticipated. It will also be necessary to restrict the degree of compression applied to noisy signals.
All real signals fall part-way between the extremes of total predictability and total unpredictability or noisiness. If the bandwidth (set by the sampling rate) and the dynamic range (set by the wordlength) of the transmission system are used to delineate an area, this sets a limit on the information capacity of the system. Figure 1.5(a) shows that most real signals only occupy part of that area. The signal may not contain all frequencies, or it may not have full dynamics at certain frequencies.

Figure 1.5: (a) A perfect coder removes only the redundancy from the input signal and results in subjectively lossless coding. If the remaining entropy is beyond the capacity of the channel some of it must be lost and the codec will then be lossy. An imperfect coder will also be lossy as it fails to keep all entropy. (b) As the compression factor rises, the complexity must also rise to maintain quality. (c) High compression factors also tend to increase latency or delay through the system.
Entropy can be thought of as a measure of the actual area occupied by the signal. This is the area that must be transmitted if there are to be no subjective differences or artifacts in the received signal. The remaining area is called the redundancy because it adds nothing to the information conveyed. Thus an ideal coder could be imagined which miraculously sorts out the entropy from the redundancy and only sends the former. An ideal decoder would then re-create the original impression of the information quite perfectly. As the ideal is approached, the coder complexity and the latency or delay both rise. Figure 1.5(b) shows how complexity increases with compression factor. The additional complexity of MPEG-4 over MPEG-2 is obvious from this. Figure 1.5(c) shows how increasing the codec latency can improve the compression factor.
Obviously we would have to provide a channel which could accept whatever entropy the coder extracts in order to have transparent quality. As a result moderate coding gains which only remove redundancy need not cause artifacts and result in systems which are described as subjectively lossless. If the channel capacity is not sufficient for that, then the coder will have to discard some of the entropy and with it useful information. Larger coding gains which remove some of the entropy must result in artifacts. It will also be seen from Figure 1.5 that an imperfect
coder will fail to separate the redundancy and may discard entropy instead, resulting in artifacts at a sub-optimal compression factor.
A single variable-rate transmission or recording channel is traditionally unpopular with channel providers, although newer systems such as ATM support variable rate. Digital transmitters used in DVB have a fixed bit rate. The variable rate requirement can be overcome by combining several compressed channels into one constant rate transmission in a way which flexibly allocates data rate between the channels. Provided the material is unrelated, the probability of all channels reaching peak entropy at once is very small and so those channels which are at one
instant passing easy material will make available transmission capacity for those channels which are handling difficult material. This is the principle of statistical multiplexing.
Where the same type of source material is used consistently, e.g. English text, then it is possible to perform a statistical analysis on the frequency with which particular letters are used. Variable-length coding is used in which frequently used letters are allocated short codes and letters which occur infrequently are allocated long codes. This results in a lossless code. The well-known Morse code used for telegraphy is an example of this approach. The letter e is the most frequent in English and is sent with a single dot. An infrequent letter such as z is allocated a
long complex pattern. It should be clear that codes of this kind which rely on a prior knowledge of the statistics of the signal are only effective with signals actually having those statistics. If Morse code is used with another language, the transmission becomes significantly less efficient because the statistics are quite different; the letter z, for example, is quite common in Czech.
The Huffman code is also one which is designed for use with a data source having known statistics. The probability of the different code values to be transmitted is studied, and the most frequent codes are arranged to be transmitted with short wordlength symbols. As the probability of a code value falls, it will be allocated longer wordlength.
The Huffman code is used in conjunction with a number of compression techniques and is shown in Figure 1.6.

MPEG - Lossless and perceptive coding

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Lossless and perceptive coding
Although there are many different coding techniques, all of them fall into one or other of these categories. In lossless coding, the data from the expander are identical bit-for-bit with the original source data. The so-called ‘stacker’ programs which increase the apparent capacity of disk drives in personal computers use lossless codecs.
Clearly with computer programs the corruption of a single bit can be catastrophic. Lossless coding is generally restricted to compression factors of around 2:1.
It is important to appreciate that a lossless coder cannot guarantee a particular compression factor and the communications link or recorder used with it must be able to function with the variable output data rate. Source data which result in poor compression factors on a given codec are described as difficult. It should be pointed out that the difficulty is often a function of the codec. In other words data which one codec finds difficult may not be found difficult by another. Lossless codecs can be included in bit-error-rate testing schemes. It is also possible to
cascade or concatenate lossless codecs without any special precautions.
Higher compression factors are only possible with lossy coding in which data from the expander are not identical bit-for-bit with the source data and as a result comparing the input with the output is bound to reveal differences.
Lossy codecs are not suitable for computer data, but are used in MPEG as they allow greater compression factors than lossless codecs. Successful lossy codecs are those in which the errors are arranged so that a human viewer or listener finds them subjectively difficult to detect. Thus lossy codecs must be based on an understanding of psycho-acoustic and psycho-visual perception and are often called perceptive codes.
In perceptive coding, the greater the compression factor required, the more accurately must the human senses be modelled. Perceptive coders can be forced to operate at a fixed compression factor. This is convenient for practical transmission applications where a fixed data rate is easier to handle than a variable rate. The result of a fixed compression factor is that the subjective quality can vary with the ‘difficulty’ of the input material. Perceptive codecs should not be concatenated indiscriminately especially if they use different algorithms. As the reconstructed signal from a perceptive codec is not bit-for-bit accurate, clearly such a codec cannot be included in any bit error rate testing system as the coding differences would be indistinguishable from real errors.
Although the adoption of digital techniques is recent, compression itself is as old as television. Figure 1.4 shows some of the compression techniques used in traditional television systems.
Most video signals employ a non-linear relationship between brightness and the signal voltage which is known as gamma. Gamma is a perceptive coding technique which depends on the human sensitivity to video noise being a function of the brightness. The use of gamma allows the same subjective noise level with an eight-bit system as
would be achieved with a fourteen-bit linear system.
One of the oldest techniques is interlace, which has been used in analog television from the very beginning as a primitive way of reducing bandwidth. As will be seen in Chapter 5, interlace is not without its problems, particularly in motion rendering. MPEG-2 supports interlace simply because legacy interlaced signals exist and there is a requirement to compress them. This should not be taken to mean that it is a good idea.

Figure 1.4:
Compression is as old as television.
(a) Interlace is a primitive way of halving the bandwidth.
(b) Colour difference working invisibly reduces colour resolution.
(c) Composite video transmits colour in the same bandwidth as monochrome.
The generation of colour difference signals from RGB in video represents an application of perceptive coding. The human visual system (HVS) sees no change in quality although the bandwidth of the colour difference signals is reduced. This is because human perception of detail in colour changes is much less than in brightness changes.
This approach is sensibly retained in MPEG.
Composite video systems such as PAL, NTSC and SECAM are all analog compression schemes which embed a subcarrier in the luminance signal so that colour pictures are available in the same bandwidth as monochrome. In comparison with a linear-light progressive scan RGB picture, gamma-coded interlaced composite video has a compression factor of about 10:1.
In a sense MPEG-2 can be considered to be a modern digital equivalent of analog composite video as it has most of the same attributes. For example, the eight-field sequence of the PAL subcarrier which makes editing difficult has its equivalent in the GOP (group of pictures) of MPEG.

MPEG - applications of compression

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The applications of audio and video compression are limitless and the ISO has done well to provide standards which are appropriate to the wide range of possible compression products.
MPEG coding embraces video pictures from the tiny screen of a videophone to the high definition images needed for electronic cinema. Audio coding stretches from speechgrade mono to multichannel surround sound.
Figure 1.3 shows the use of a codec with a recorder. The playing time of the medium is extended in proportion to the compression factor. In the case of tapes, the access time is improved because the length of tape needed for a given recording is reduced and so it can be rewound more quickly. In the case of DVD (digital video disk aka digital versatile disk) the challenge was to store an entire movie on one 12 cm disk. The storage density available with today’s optical disk technology is such that consumer recording of conventional uncompressed video would be out
of the question.

In communications, the cost of data links is often roughly proportional to the data rate and so there is simple economic pressure to use a high compression factor. However, it should be borne in mind that implementing the codec also has a cost which rises with compression factor and so a degree of compromise will be inevitable.

Figure 1.3: Compression can be used around a recording medium. The storage capacity may be increased or the access time reduced according to the application.
In the case of video-on-demand, technology exists to convey full bandwidth video to the home, but to do so for a single individual at the moment would be prohibitively expensive. Without compression, HDTV (high-definition television) requires too much bandwidth. With compression, HDTV can be transmitted to the home in a similar bandwidth to an existing analog SDTV channel. Compression does not make video-on-demand or HDTV possible; it makes them economically viable.
In workstations designed for the editing of audio and/or video, the source material is stored on hard disks for rapid access. Whilst top-grade systems may function without compression, many systems use compression to offset the high cost of disk storage. In some systems a compressed version of the top-grade material may also be stored for browsing purposes.
When a workstation is used for off-line editing, a high compression factor can be used and artifacts will be visible in the picture. This is of no consequence as the picture is only seen by the editor who uses it to make an EDL (edit decision list) which is no more than a list of actions and the timecodes at which they occur. The original uncompressed material is then conformed to the EDL to obtain a high-quality edited work. When on- line editing is being performed, the output of the workstation is the finished product and clearly a lower compression factor will have to be used. Perhaps it is in broadcasting where the use of compression will have its greatest impact. There is only one electromagnetic spectrum and pressure from other services such as cellular telephones makes efficient use of bandwidth mandatory. Analog television broadcasting is an old technology and makes very inefficient use of bandwidth. Its replacement by a compressed digital transmission is inevitable for the practical reason that the bandwidth is needed elsewhere.
Fortunately in broadcasting there is a mass market for decoders and these can be implemented as low-cost integrated circuits. Fewer encoders are needed and so it is less important if these are expensive. Whilst the cost of digital storage goes down year on year, the cost of the electromagnetic spectrum goes up. Consequently in the future the pressure to use compression in recording will ease whereas the pressure to use it in radio communications will increase.

MPEG - Why compression is necessary

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Compression, bit rate reduction, data reduction and source coding are all terms which mean basically the same thing in this context. In essence the same (or nearly the same) information is carried using a smaller quantity or rate of data. It should be pointed out that in audio compression traditionally means a process in which the dynamic range of the sound is reduced. In the context of MPEG the same word means that the bit rate is reduced, ideally leaving the dynamics of the signal unchanged. Provided the context is clear, the two meanings can co-exist without a great deal of confusion.
There are several reasons why compression techniques are popular:
(a) Compression extends the playing time of a given storage device.
(b) Compression allows miniaturization. With fewer data to store, the same playing time is obtained with smaller hardware. This is useful in ENG (electronic news gathering) and consumer devices.
(c) Tolerances can be relaxed. With fewer data to record, storage density can be reduced making equipment which is more resistant to adverse environments and which requires less maintenance.
(d) In transmission systems, compression allows a reduction in bandwidth which will generally result in a reduction in cost. This may make possible a service which would be impracticable without it.
(e) If a given bandwidth is available to an uncompressed signal, compression allows faster than real-time transmission in the same bandwidth.
(f) If a given bandwidth is available, compression allows a better-quality signal in the same bandwidth

MPEG-1, 2 and 4 contrasted
The first compression standard for audio and video was MPEG-1. Although many applications have been found, MPEG-1 was basically designed to allow moving pictures and sound to be encoded into the bit rate of an audio Compact Disc. The resultant Video-CD was quite successful but has now been superseded by DVD. In order to meet the low bit requirement, MPEG-1 downsampled the images heavily as well as using picture rates of only 24–30 Hz and the resulting quality was moderate.
The subsequent MPEG-2 standard was considerably broader in scope and of wider appeal. For example, MPEG-2 supports interlace and HD whereas MPEG-1 did not. MPEG-2 has become very important because it has been chosen as the compression scheme for both DVB (digital video broadcasting) and DVD (digital video disk).
Developments in standardizing scaleable and multi-resolution compression which would have become MPEG-3 were ready by the time MPEG-2 was ready to be standardized and so this work was incorporated into MPEG-2, and as a result there is no MPEG-3 standard.
MPEG-4 uses further coding tools with additional complexity to achieve higher compression factors than MPEG-2.
In addition to more efficient coding of video, MPEG-4 moves closer to computer graphics applications. In the more complex Profiles, the MPEG-4 decoder effectively becomes a rendering processor and the compressed bitstream describes three dimensional shapes and surface texture. It is to be expected that MPEG-4 will become as important to Internet and wireless delivery as MPEG-2 has become in DVD and DVB.

[1]ISO/IEC JTC1/SC29/WG11 MPEG, International standard ISO 11172, Coding of moving pictures and associated audio for digital storage media up to 1.5 Mbits/s (1992)
[2]LeGall, D., MPEG: a video compression standard for multimedia applications. Communications of the ACM, 34, No.4, 46–58 (1991)
[3]MPEG-2 Video Standard: ISO/IEC 13818–2: Information technology – generic coding of moving pictures and associated audio information: Video (1996) (aka ITU-T Rec. H-262 (1996))
[4]MPEG-4 Standard: ISO/IEC 14496–2: Information technology – coding of audio-visual objects: Amd.1 (2000)

MPEG - Introduction to compression

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What is MPEG?
MPEG is actually an acronym for the Moving Pictures Experts Group which was formed by the ISO (International Standards Organization) to set standards for audio and video compression and transmission.
Compression is summarized in Figure 1.1. It will be seen in (a) that the data rate is reduced at source by the compressor. The compressed data are then passed through a communication channel and returned to the original rate by the expander. The ratio between the source data rate and the channel data rate is called the compression

factor. The term coding gain is also used. Sometimes a compressor and expander in series are referred to as a compander. The compressor may equally well be referred to as a coder and the expander a decoder in which case the tandem pair may be called a codec.

Figure 1.1: In (a) a compression system consists of compressor or coder, a transmission channel and a matching expander or decoder. The combination of coder and decoder is known as a codec. (b) MPEG is asymmetrical since the encoder is much more complex than the decoder. Where the encoder is more complex than the decoder, the system is said to be asymmetrical. Figure 1.1(b) shows that MPEG works in this way. The encoder needs to be algorithmic or adaptive whereas the decoder is ‘dumb’ and carries out fixed actions. This is advantageous in applications such as broadcasting where the number of expensive complex encoders is small but the number of simple inexpensive decoders is large. In point-to-point applications the advantage of asymmetrical coding is not so great.
The approach of the ISO to standardization in MPEG is novel because it is not the encoder which is standardized.
Figure 1.2(a) shows that instead the way in which a decoder shall interpret the bitstream is defined. A decoder which can successfully interpret the bitstream is said to be compliant. Figure 1.2(b) shows that the advantage of standardizing the decoder is that over time encoding algorithms can improve yet compliant decoders will continue to function with them.
It should be noted that a compliant decoder must correctly be able to interpret every allowable bitstream, whereas an encoder which produces a restricted subset of the possible codes can still be compliant.
Figure 1.2: (a) MPEG defines the protocol of the bitstream between encoder and decoder. The decoder is defined by implication, the encoder is left very much to the designer. (b) This approach allows future encoders of better performance to remain compatible with existing decoders. (c) This approach also allows an encoder to produce a standard bitstream while its technical operation remains a commercial secret.
The MPEG standards give very little information regarding the structure and operation of the encoder. Provided the bitstream is compliant, any coder construction will meet the standard, although some designs will give better picture quality than others. Encoder construction is not revealed in the bitstream and manufacturers can supply encoders using algorithms which are proprietary and their details do not need to be published. A useful result is that there can be competition between different encoder designs which means that better designs can evolve. The user will have greater choice because different levels of cost and complexity can exist in a range of coders yet a compliant decoder will operate with them all.
MPEG is, however, much more than a compression scheme as it also standardizes the protocol and syntax under which it is possible to combine or multiplex audio data with video data to produce a digital equivalent of a television program. Many such programs can be combined in a single multiplex and MPEG defines the way in which such multiplexes can be created and transported. The definitions include the metadata which decoders require to demultiplex correctly and which users will need to locate programs of interest.
As with all video systems there is a requirement for synchronizing or genlocking and this is particularly complex when a multiplex is assembled from many signals which are not necessarily synchronized to one another.

Digital Headend System Integration 4

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SYSTEM MANAGEMENT AND MONITORING
Management systems available on the market today, provide valuable information on the status of every individual building block individually, and some even add the complexity to view the system installed.
Experience has learned that creating visibility of the building blocks alone is not enough, monitoring and analyzing the broadcasted streams is what provides the real details on the services supplied to the end-users.

As said before, customer satisfaction is key when deploying Digital Television, this is where operators have to for control: ‘You inform your customer!’, instead of ‘your customer informing you!’ IMPEQ Technologies has developed and built a Broadcast Management Suite, that includes SNMP management and the Service Analyser and Monitoring system called SAM, to secure full control over the quality of the supplied end-user service in the operators network.

MEET SAM
The IMPEQ Service Analyzer and M o n i t o r i n g (SAM) system
was first l a u n c h e d during IBC 2006, being part of the I M P E Q
B r o a d c a s t Management
Suite. SAM provides the q u a l i t y information needed to secure quality of service for program delivery to the end-users. SAM monitors and analyses the broadcasted streams in the various stages of audio/video processing in your digital headend, it measures all services on quality and the presence of mandatory information (ETR290, bit rate, PSI/SI and more).

YOUR VIEW ON SERVICE QUALITY SAM provides a user friendly interface that displays the information you require in a single or multiple screen environment.

THE DIGITAL ROLL-OUT
When starting preparations for the rollout, the balance between investment and revenue has to be taken into account. Finding the right balance between the technical and marketing plans is crucial, to secure the launch of Digital Television services.

PLAN IN PHASES
Starting with the basics has proven to be the way forward when installing the digital headend.
By doing so, operators gain experience and get comfortable with the new installation and the basic services offered. Our experience shows that t h e deployment of set-top boxes to friendly users in this phase, provides the operator with the time needed to build experience and to expand customer care in the path to full digital services. Digital Television in many cases provides unique situations when dealing with the end customers (one-to-one), this in comparison to broadcasting before (one-to-many).
The moment more complexity and intelligence is added to the digital headend, more services, like Pay TV, may be offered to the customers. In the near future interactivity like Video on Demand (VOD), Gaming and Voting can be offered. Also here we strongly recommend the use of friendly customers as a test-base for new services before moving to full interactivity.
The main purpose to plan in phases is to align the technical planning and the marketing roadmap, by doing so operators can work towards one common goal, the deployment of Digital Television services.

LESSONS LEARNED
Dealing with an international customer base, IMPEQ Technologies has over 15 years of experience in Digital Television. In many projects we have seen, that the following points need careful attention.

GUIDELINES
• Balance between investment and revenue
• Use of open or proprietary standards
• System Flexibility and Scalability
• Plan carefully and start with the basics
• Gradually add intelligence to the headend
• System and Service availability
• Monitor the broadcasted services
• Select the set-top-boxes carefully
• Pre test and validate the set-top-boxes prior to roll-out
• Customer care is of high importance

Customer satisfaction requires constant attention. In the world of Digital Television, the operator becomes an experience provider, giving the end-user the ability to enjoy interactive and individual and personalized services.

Digital Headend System Integration 3

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ACCESS

Access is important not only in the first phase of the roll-out, but also in the future, when adding new services to the network. Capacity is one of the main concerns to many operators who move to interactive services like Video on Demand (VOD).

Distribution to the Hybrid Fibre Coax (HFC) network, using Quadrature Amplitude Modulation (QAM) combining in the digital headend, provides direct access. IP transport from the headend to the regional HFC networks is often used to boost capacity and to facilitate remote regions when using a central headend.

When using IP transport, protection of the IP traffic is needed. In most cases 1+1 redundancy is used in order to secure high availability.The HFC network is being facilitated by means of Edge QAM equipment. This provides the operator with a high level of flexibility to roll-out the necessary capacity.

SETTOPBOX

A successful launch of Digital Television Services fully depends on the end-user experience and satisfaction. Set-top-box suppliers are actively working to improve and expand their portfolio, also new suppliers are entering the
Digital Television arena almost every day.
The cost aspect plays a major role in the acceptance of a profitable
business case, where the features available and the ease of use are of
major importance towards customer satisfaction. When selecting the set topbox, manageability is one aspect often forgotten together with the
distribution model (retail or direct supply).
It is of high importance to thoroughly test and validate set-top-boxes prior to the deployment of any kind. In terms of manageability, attention needs to be given to: remote monitoring, Over the Air (OTA) software and firmware updates, Conditional Access (CA) and middleware. Also the number of different set-top-box types should be kept to a minimum to reduce the operational costs.
IMPEQ Technologies has established an independent Test & Measurement facility, providing quality confirmation and improvement recommendations. At IMPEQ Test & Measurement we supply the necessary assurance, so that there will be no surprises while deploying set-top-boxes in the field. More services can be offered, so feel free to contact IMPEQ to see how and where we can assist our customers.

Digital Headend System Integration 2

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COMPRESSION
The compression stage in a digital headend is under continuous discussion and change. Quality of the Service and availability is of key importance in reference to customer satisfaction and the success of digital television. But what do we need;

MPEG2 or MPEG4? Standard Definition (SD) of High Definition (HD) services? Or a combination of these?
Questions an operator may need to answer already today or in a later phase, depending on the market demands. One certainty exists; flexibility and scalability is of high importance.

There are different compression methods to select from, being: Encoding, Transrating and Transcoding. Comparing these methods can be done from a pure technical level, but the cost aspect should be taken into consideration at the same time.

Compression
We strongly suggest the combination of Encoding and Transrating when building a digital headend. For the Premium services Encoding is advised, to secure full control over the video quality of the delivered service. Second and Third class services may use Transrating, as the video quality may generally be lower.
This combination of different compression methods provides the right balance between the technical requirements and the cost aspect.
To secure the delivery of end-user services, redundancy may be added at any time during operation. We advise the use of redundancy for all Premium channels to secure availability. The following redundancy options are available: N:1 or N:M.

MULTIPLEXING
The multiplexing stage is of key importance, here your stream adjustments are made. The more digital services that are available for end-users, the more complex the multiplexing stage becomes. Stream adjustments are of high importance when going digital. Not only maximizing the bandwidth use of the transport stream is important, also adding service information like an Electronic Program Guide (EPG), Scrambling of services and settopbox download/firmware features, to mention a few.

Digital Headend System Integration

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CONFIGURING THE DIGITAL HEADEND
When one starts specifying the digital headend, usually an overload of features and requirements are created to establish a full blown Digital Headend. A closer look at the end-users is necessary to establish what is needed ‘today’ and in ‘future’ stages. In the end it all comes down to finding the right balance, translating the set ambitions into a profitable business case. Identifying technical requirements and compliance to local and national regulations is only a part of the total exercise, finding the right match between the rollout of the digital headend and the launch of end-user services is key, to secure investment and revenue income.

TAKING THE RIGHT DECISIONS
When building a Digital Headend, operators are forced to take important decisions that form the foundation of the Digital Television Services ‘today’ versus ‘tomorrow’. One of these decisions is whether to build a traditional ASI based headend or implement a headend with an IP-core.

Building ASI based headends provides a wide selection of products to choose from, build on proven standards. One of the downsides of an ASI

based headend is the use of big matrixes, that limit the flexibility in configuration of services. Headends with an IP-core architecture provide this flexibility. All feeds are available on the IP-core and can be freely used as seen fit.
Redundancy is easily added and managed. Its scalability is unique in comparison to the more traditional ASI based headends. Movement in the market indicates that suppliers are moving their development towards IP, they are adopting the GbE interfaces, providing a wide selection of building blocks to choose from. This makes the IP-core headend a future proof solution.
Looking at the fast growing need for new end-user services, an IP based headend provides the flexibility needed for current and future requirements, providing the operator with all means to adjust quickly to the changing market.

BUILDING BLOCKS
When designing digital headends, five main areas need careful attention; Content Aggregation, Compression, Multiplexing, Access and System Management. In the following sections, some considerations and guidelines are given.

CONTENT AGGREGATION
Content is presented in different formats; the source can be feeds from Studio’s, Satellite, Terrestrial and direct IP through Gigabit Ethernet
(GbE) interfaces. Aggregation of these content types is mainly done by means of professional Integrated Receiver’s/ Decoder’s (IRD’s) with a dedicated IP output.
The use of IRD’s differ, depending on the use of scrambling or free to air services.
Redundancy plays an important role in the design. Flexibility in this matter is needed, to be able to add redundancy at any time necessary. For redundancy of IRD’s, the following methods may be used, being N:1, N:M or 1:1 depending on the end-user service and availability requirements.

RELATIVE MERITS OF TV SYSTEMS

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RELATIVE MERITS OF TV SYSTEMS
The differences between each of the main TV systems are not quite as clear cut as one might at first imagine. While NTSC has a reputation for poor colour accuracy, this is only really true of broadcast television and as a video format it has some distinct advantages over the other systems.
All these systems are a compromise and many efforts have been made over the years to address the shortcomings in each of the systems.

NTSC/525 Advantages
Higher Frame Rate - Use of 30 frames per second (really 29.97) reduces visible flicker. Atomic Colour Edits - With NTSC it is possible to edit at any 4 field boundary point without disturbing the colour signal. Less inherent picture noise - Almost all pieces of video equipment achieve better signal to noise characteristics in their NTSC/525 form than in their PAL/625.

NTSC/525 Disadvantages
Lower Number of Scan Lines - Reduced clarity on large screen TVs, line structure more visible. Smaller Luminance Signal Bandwidth - Due to the placing of the colour sub-carrier at 3.58MHz, picture defects such as moire, cross-colour, and dot interference become more pronounced. This is because of the greater likelihood of interaction with the monochrome picture signal at the lower sub-carrier frequency. Susceptibility to Hue Fluctuation - Variations in the colour subcarrier phase cause shifts in the displayed colour, requiring that the TV receivers be equipped with a Hue adjustment to compensate. Lower Gamma Ratio - The gamma value for NTSC/525 is set at 2.2 as opposed to focus the slightly higher 2.8 defined for PAL/625. This means that PAL/625 can produce pictures of greater contrast.

Undesirable Automatic Features - Many NTSC TV receivers feature an Auto-Tint circuit to make hue fluctuations less visible to uncritical viewers. This circuit changes all colours approximating to flesh tone into a "standard" flesh tone, thus hiding the effects of hue fluctuation. This does mean however that a certain range of colour shades cannot be displayed correctly by these sets. Up-market models often have this (mis)feature switchable, cheaper sets do not.

PAL/625 Advantages
Greater Number of Scan Lines - more picture detail. Wider Luminance Signal Bandwidth The placing of the colour Sub-Carrier at 4.43MHz allows a larger bandwidth of monochrome information to be reproduced than with NTSC/525. Stable Hues - Due to reversal of sub-carrier phase on alternate lines, any phase error will be corrected by an equal and opposite error on the next line, correcting the original error. In early PAL implementations it was left to the low resolution of the human eye's colour abilities to provide the averaging effect; it is now done with a delay line. Higher Gamma Ratio - The gamma value for PAL/625 is set at 2.8 as opposed to the lower 2.2 figure of NTSC/525. This permits a higher level of contrast than on NTSC/525 signals. This is particularly noticeable when using multi-standard equipment as the contrast and brightness settings need to be changed to give a similar look to signals of the two formats.

PAL/625 Disadvantages
More Flicker - Due to the lower frame rate, flicker is more noticeable on PAL/625 transmissions; particularly so for people used to viewing NTSC/525 signals. Lower Signal to Noise Ratio - The higher bandwidth requirements cause PAL/625 equipment to have slightly worse signal to noise performance than it's equivalent NTSC/525 version.

Loss of Colour Editing Accuracy - Due to the alternation of the phase of the colour signal, the phase and the colour signal only reach a common point once every 8 fields/4 frames. This means that edits can only be performed to an accuracy of +/- 4 frames (8 fields).

Variable Colour Saturation - Since PAL achieves accurate colour through canceling out phase differences between the two signals, the act of canceling out errors can reduce the colour saturation while holding the hue stable. Fortunately, the human eye is far less sensitive to saturation variations than to hue variations, so this is very much the lesser of two evils.

FREQUENCY STANDARDS

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In addition to the incompatibilities of 50 and 60Hz systems, and the different Colour systems, there is a further barrier to compatibility. Fortunately, video recordings themselves are not affected by this, only the TV signal reception equipment. For various reasons of number of stations and terrain, TV pictures can be transmitted in any of three main frequency ranges, VHF, UHF and Microwave (Satellite Direct Broadcasting). Equipment designed to receive signals in only one of these bands cannot receive transmissions in any of the other bands.
Further, there are differences between the encoding of the sound between countries using the same frequency bands. Within 50Hz PAL UHF transmissions, audio signals can be at 5.5Mhz offset (system G), or at 6MHz offset (system I). Similar differences exist between the Middle Eastern versions of SECAM (MESECAM) and the Eastern Bloc (OIRT) version.

RELATIVE MERITS OF TV SYSTEMS
The differences between each of the main TV systems are not quite as clear cut as one might at first imagine. While NTSC has a reputation for poor colour accuracy, this is only really true of broadcast television and as a video format it has some distinct advantages over the other systems.
All these systems are a compromise and many efforts have been made over the years to address the shortcomings in each of the systems. NTSC/525 Advantages Higher Frame Rate - Use of 30 frames per second (really 29.97) reduces visible flicker. Atomic Colour Edits - With NTSC it is possible to edit at any 4 field boundary point without disturbing the colour signal. Less inherent picture noise - Almost all pieces of video equipment achieve better signal to noise characteristics in their NTSC/525 form than in their PAL/625.

NTSC/525 Disadvantages
Lower Number of Scan Lines - Reduced clarity on large screen TVs, line structure more visible. Smaller Luminance Signal Bandwidth - Due to the placing of the colour sub-carrier at 3.58MHz, picture defects such as moire, cross-colour, and dot interference become more pronounced. This is because of the greater likelihood of interaction with the monochrome picture signal at the lower
sub-carrier frequency. Susceptibility to Hue Fluctuation - Variations in the colour subcarrier phase cause shifts in the displayed colour, requiring that the TV receivers be equipped with a Hue adjustment to compensate. Lower Gamma Ratio - The gamma value for NTSC/525 is set at 2.2 as opposed to focus2
the slightly higher 2.8 defined for PAL/625. This means that PAL/625 can produce pictures of greater contrast.

Undesirable Automatic Features - Many NTSC TV receivers feature an Auto-Tint circuit to make hue fluctuations less visible to uncritical viewers. This circuit changes all colours approximating to flesh tone into a "standard" flesh tone, thus hiding the effects of hue fluctuation. This does mean however that a certain range of colour shades cannot be displayed correctly by these sets.
Up-market models often have this (mis)feature switchable, cheaper sets do not.

PAL/625 Advantages
Greater Number of Scan Lines - more picture detail. Wider Luminance Signal Bandwidth The placing of the colour Sub-Carrier at 4.43MHz allows a larger bandwidth of monochrome information to be reproduced than with NTSC/525. Stable Hues - Due to reversal of sub-carrier phase on alternate lines, any phase error will be corrected by an equal and opposite error on the next line, correcting the original error. In early PAL implementations it was left to the low resolution of the human eye's colour abilities to provide the averaging effect; it is now done with a delay line. Higher Gamma Ratio - The gamma value for PAL/625 is set at 2.8 as opposed to the lower 2.2 figure of NTSC/525. This permits a higher level of contrast than on NTSC/525 signals. This is particularly noticeable when using multi-standard equipment as the contrast and brightness settings need to be changed to give a similar look to signals of the two formats.

PAL/625 Disadvantages
More Flicker - Due to the lower frame rate, flicker is more noticeable on PAL/625 transmissions; particularly so for people used to viewing NTSC/525 signals.
Lower Signal to Noise Ratio - The higher bandwidth requirements cause PAL/625 equipment to have slightly worse signal to noise performance than it's equivalent NTSC/525 version.

Loss of Colour Editing Accuracy - Due to the alternation of the phase of the colour signal, the phase and the colour signal only reach a common point once every 8 fields/4 frames. This means that edits can only be performed to an accuracy of +/- 4 frames (8 fields).

Variable Colour Saturation - Since PAL achieves accurate colour through canceling out phase differences between the two signals, the act of canceling out errors can reduce the colour saturation while holding the hue stable. Fortunately, the human eye is far less sensitive to saturation variations than to hue variations, so this is very much the lesser of two evils.

In addition to the incompatibilities of 50 and 60Hz systems, and the different Colour systems, there is a further barrier to compatibility. Fortunately, video recordings themselves are not affected by this, only the TV signal reception equipment. For various reasons of number of stations and terrain,
TV pictures can be transmitted in any of three main frequency ranges, VHF, UHF and Microwave (Satellite Direct Broadcasting). Equipment designed to receive signals in only one of these bands cannot receive transmissions in any of the other bands.
Further, there are differences between the encoding of the sound between countries using the same frequency bands. Within 50Hz PAL UHF transmissions, audio signals can be at 5.5Mhz offset (system G), or at 6MHz offset (system I). Similar differences exist between the Middle Eastern versions of SECAM (MESECAM) and the Eastern Bloc (OIRT) version.

Television System

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NTSC
Beyond the initial divide between 50 and 60Hz based systems, further sub-divisions have appeared within both camps since the inception of Colour broadcasting. The majority of 60Hz based countries use a technique known as NTSC originally developed in the United States by a focus committee called the National Television Standards Committee. NTSC (often scurrilously referred to as Never Twice the Same Colour) works perfectly in a video or closed circuit environment but can exhibit problems of varying hue when used in a broadcast environment.

PAL
This hue change problem is caused by shifts in the colour sub-carrier phase of the signal. A modified version of NTSC soon appeared which differed mainly in that the sub-carrier phase was reversed on each second line; this is known as PAL, standing for Phase Alternate Lines (it has a wide range of facetious acronyms including Pictures At Last, Pay for Added Luxury (re: cost of delay line), and People Are Lavender). PAL has been adopted by a few 60Hz countries, most notably Brazil.

SECAM
Amongst the countries based on 50Hz systems, PAL has been the most widely adopted. PAL is not the only colour system in widespread use with 50Hz; the French designed a system of their own - primarily for political reasons to protect their domestic manufacturing companies - which is known as SECAM, standing for Sequential Couleur Avec Memoire. The most common facetious acronym is System Essentially Contrary to American Method, SECAM was widely adopted in Eastern Block countries to encourage incompatibility with Western transmissions - again a political motive.

SECAM ON PAL
Some Satellite TV transmissions (usually Russian) that are available over India, are in SECAM Since the field (25 frames /sec) and scan rates are identical, a SECAM signal will replay in B&W on a PAL TV and vice versa. However, transmission frequencies and encoding differences make equipment incompatible from a broadcast viewpoint. For the same reason, system converters between PAL and SECAM, while often difficult to find, are reasonably cheap. In Europe, a few Direct Satellite Broadcasting services use a system called D-MAC. It's use is not wide-spread at
present and it is transcoded to PAL or SECAM to permit video recording of it's signals. It includes features for 16:9 (widescreen) aspect ratio transmissions and an eventual migration path to Europe's proposed HDTV standard. There are other MAC-based standards in use around the world including B-MAC in Australia and B-MAC60 on some private networks in the USA. There is also a second European variant called D2-MAC which supports additional audio channels making transmitted signals incompatible, but not baseband signals.

WORLD TV STANDARDS

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A number of TV standards now co-exist worldwide. This article takes a look at how and why they originated, as well as a comparison. It also offers detail listings of standards prevalent in various countries.
Television broadcast commenced approximately 50 years ago. The knowledge gained over the years has helped evolve better standards. As a result of this, the US which saw the birth of wide spread commercial television broadcasts, evolved the first system which predictably, is also the most primitive. Subsequent television systems have learnt from earlier mistakes.

Before we consider different television systems, we need to take a look at the basics of television transmissions. A television transmission consist of a series of rapidly changing pictures which convey to the viewer, an illusion of continuous motion. The pictures need to flash at a rate of more than 16 pictures per second, to fool the eye into seeing continuous motion. Each of these rapidly changing pictures is termed as a "frame". Typically a television transmission consists of either 25 or 30 frames per second. This is Each picture consists of several closely spaced lines. The lines are scanned (written) from left to right and from the top of the screen to the bottom of the screen. Typically a TV picture consists of 525 or 625 lines. In view of the large number of lines, if all lines were written one after the other on the screen, the picture would begin to fade at the top of the screen by the time the last few lines at the bottom of the screen are written. To avoid this, the first frame carries only the odd numbered lines e.g. line numbers 1, 3, 5 etc.
The next frame carries only even numbered lines e.g. line numbers 2, 4, 6 etc. In this manner, successive frames carry the odd and even numbered lines. This provides a uniform intensity to the picture, and is called "interlacing".

TIMING :
TV receivers require a source to time the rapid succession of frames on the screen. Designers decided to use the Mains power supply frequency as this source for two good reasons. The first was that with the older type of power supply, (non SMPS) you would get rolling hum bars on the TV picture if the mains supply and power source were not at exactly the same frequency. The second was that the TV studio lights or for that matter all florescent, non incandescent lights flicker at the mains frequency. Since this flicker is much higher than 16 times per second the eye does not detect it. However this flicker could evolve into an extremely pronounced low frequency flicker on TV screens due to a "beat" frequency generated between the light flicker and the mains frequency.This would have made programmes unviewable particularly in the early days of development of TV receivers.
There are two Mains power frequencies widely used around the World, 50Hz and 60Hz. This immediately divided the worlds TV systems into two distinct camps, the 25 frames per second camp (50Hz) and the 30 frames per second camp (60Hz). Later the 60Hz camp made a small adjustment and changed the field rate to 59.94Hz when they added colour to the signals. The issue of field frequency remained sufficiently deep rooted in both TV standards that the vested interest remained long after the original technical justification had gone.
The biggest compatibility problems between TV standards remain related to the field rate; these are also the hardest problems to solve.

Frequency Spectrum

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Trunk Amplifier Equalization

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GAIN & SLOPE
The signal partially amplified by the pre amplifier section now passes to the main gain and slope controls. As the name suggests, the gain control allows user setting of the output signal level. The slope control permits setting of the overall slope required by the system. While some products maintain fixed pads for specific, plug-in gain and slope control values, others offer a variable potentiometer for more versatile, on site control. The advantages and disadvantages of fixed pads versus potentiometers is elaborated a little later.

POST AMPLIFIER
After the signal has been corrected for its level and slope, it is fed through the post amplifier. The post amplifier is designed to provide maximum output with the least possible distortion. Since the post amplifier already receives an input signal of fairly high signal levels, it is not necessary to optimise the post amplifier for low noise performance. Ideally, the post amplifier should be a "Power Doubler" or "Power Quadrupler". The power doubler configuration offers double the output level (3 dB higher), with equivalent distortion compared to an ordinary push-pull Hybrid IC. The power quadrupler offers 4 times the output level (6 dB higher) than an ordinary push-pull Hybrid IC. The output of the post amplifier is fed to the output diplex filter of the amplifier.

AGC / ALC
Most trunk amplifiers offer the option of providing an automatic gain control or an automatic level control facility. The AGC / ALC module is usually offered as an optional extra. A detailed explanation on the function of the AGC / ALC circuit has been provided in earlier articles and therefore not included in this write up.
It would however suffice to inform readers that the AGC / ALC module automatically compensates for 6 to 12 dB of variation in the input signal. These variations could occur not only due to changes in cable temperature but also some times due to permanent changes such as the insertion of a one way tap off at some point in the trunk line, before the amplifier. An AGC specification of +/- 4 dB would imply a total AGC range of 8 dB.

FIXED PADS & VARIABLE ATTENUATORS
Most of the budget priced line extenders offer a variable control for adjusting the gain and slope of an amplifier. The continuous variable controls offer tremendous flexibility and ease of set up. Their operation is however, relatively unstable. The setting value tends to drift over a period of time. The settings can also be inadvertently disturbed by a mechanical movement of the spindle. Fixed pads on the other hand, offer extremely high stability as well as reliability. The fixed pads are offered in a range of values and are easily plugged in or removed at site. Gold plated contacts are used to ensure that the contacts do not tarnish or deteriorate due to the adverse outdoor ambient conditions.
The disadvantage for fixed pads ofcourse is that a large inventory needs to be maintained to cater to the specific requirement of each amplifier in the network. A good system design would typically dictate that each trunk amplifier should receive an input signal of 70 dBU to 75 dBU and provide an output signal of 95 dBU to 100 dBU. The specific figures would depend on the number of amplifiers in cascade, the distance between each amplifier, the gain of each amplifier and of course whether the output stage uses power doubling technology. However an output signal level of 95
dBU to 100 dBU will generally be ideal. Once such a logical and consistent system design has been implemented, most amplifiers in the system will require a similar set of fixed value equaliser and attenuation pads. A bulk of the inventory needs to accommodate values of only about 3 dB above and below the typical value. It must be noted that fixed pads are available not only for gain control but also for equalisation. Often even the signal path between the pre and post amplifiers incorporate both, fixed and variable control. The fixed pads provide a bulk of the compensation while the variable controls are used to simply "trim" or optimise to the exact required characteristic.

THE REVERSE PATH AMP
While the above discussions have been restricted to the forward path amplification section, the reverse path amplifier can also benefit from a similar circuit topology. The reverse path typically caters to signals from 5 MHz to 50 MHz. Manufacturers often provide a variety of diplexer options to cater to different forward and reverse path frequency splits. While a range of 5 to 50 MHz may
not seem large, it must be noted that these frequencies represent a 1:10 ratio i.e. a 1 decade frequency range. Slope compensation over this one decade is often desirable particularly if the entire reverse path is to be utilised. However, most Indian networks use less than adequately shielded cable. (e.g. 60% braid RG11). Also connectoring practice is shabby. As a result, noise ingress is very substantial over the 5 MHz to 20 MHz frequency range. Due to this, most CATV networks do not transmit signals in the 5 to 20 MHz range. Hence utilised reverse path is just 20 to 50 MHz. In this case, reverse path slope compensation is often not necessary.

THE TRUNK STATION
A trunk amplifier unit often houses several amplifiers and modules. As an example, the unit may house 1 Forward Path Amplifier Module + 1 Bridger Amplifier Module + 1 Reverse Path Amplifier Module and ofcourse a built in power supply. Other modules such as AGC, status monitoring, etc may also be incorporated. A unit consisting of 2 or more amplifier modules in a single housing is often referred to as a "Trunk Station".

Trunk Amplifier

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A SIMPLE AMPLIFIER
The block diagram of a simple amplifier is shown in Figure 1. The active electronics i.e. the section which actually amplifies the signal is clubbed together in the block diagram, as the amplifier module.

INPUT CONTROL
Since some means of varying the output signal level is required, a gain control is a must. The gain control is shown as a variable resistance connected across the input signal.
For lower output levels, the gain control is lower i.e. a smaller signal is fed into the amplifier module. Unfortunately, the setting of the external gain control does not reduce the noise generated inside the amplifier module. Hence small input signals will be adversely affected by the internal noise from the amplifier module. As a result, the Carrier to Noise ratio (C/N) deteriorates rapidly for low gain control settings, if the topology of Figure 1 is used. All practical CATV Amplifiers need some extent of slope equalization. This is shown as a box in Figure 1 and will add a further attenuation at the input, for lower frequencies. As a result, the slope control at the input further degrades the C/N performance at lower frequencies.

OUTPUT CONTROL
block diagram Where the output signals are varied by the use of an attenuator across the output of the CATV Amplifier. This layout has the
disadvantage that the amplifier module is constantly providing the highest output. Only apart of this output signalis utilised, as required. Readers who have read our earlier articles will recall that distortion increases very rapidly with output levels. A 5 dB increase in output level will increase the output distortion by 10 dB !
The slope control will have to be added across the output, as shown in Figure 2. This will present an increased attenuation or burden on the amplifier at lower frequencies, which will result in increased distortion at lower frequencies. If the ideal amplifier characteristics of zero distortion are to be achieved, the output signal level must be maintained to the lowest level actually required. Clearly, a line extender amplifier if based on Figure 1, will produce an increased level ofnoise (poor noise figure). Alternately, if Figure 2 is implemented, the distortion will suffer.

MULTI STAGE AMPLIFIERS
A much better performance level would there fore be achieved if the amplifier gain was split into 2 or more stages and the signal level attenuated partially over the first and second stages so that the input stage does not see too low a signal level. At the same time the output stage is not forced to operate at excessive output levels. Figure 3 shows a block diagram of a practical trunk amplifier (Forward Path Only). The total amplifier gain of approximately 32 dB is divided over 2 sections.

PRE AMPLIFIER
As the name suggests, the Pre Amplifier is the first amplifying stage. The input signal after passing through the forward / reverse path diplexer passes through an external plug-in attenuator. In some products, this plug-in input attenuator can either be a fixed attenuator or a Thermal Equaliser. The pre amplifier receives relatively low level signals and is therefore designed to operate with low noise, at relatively modest output levels. Since output levels are not large, the pre amplifier section does not generate significant distortion.

THERMAL EQUALISER
The thermal equaliser is basically an attenuator whose attenuation changes with the temperature. Since cable attenuation increases with temperature, the thermal attenuator is designed so that that its impedance decreases with temperature. As a result, the thermal attenuator compensates for cable attenuation with temperature and presents an approximately constant input signal, despite changes in ambient temperatures. If high gain is required, without Thermal Compensation, this stage can be simply bypassed, with a shorting link.

FIXED EQUALISER
The optional attenuator / thermal equaliser is followed by a fixed equaliser. This partially compensates for the cable slope. Hence the signal to the pre amplifier is equalised in two stages viz. an optional, external attenuator or thermal equaliser and subsequently a fixed equaliser. It is important to note that only partial cable equalisation is performed prior to the pre amplifier. This ensures that low frequency signals are not overly attenuated when a large equalisation slope is
required. As a result, the pre amplifier maintains a superior low frequency C/N ratio.

Understanding Multi Stage & Trunk Amplifiers

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INTRODUCTION
A variety of CATV Amplifiers are now available. Each type of CATV Amplifier offers a compromise between its technical specifications and price. An ideal CATV Amplifier should amplify the cable TV signal without adding any distortion or noise to the signal. While this definition may seem simple, it is almost impossible to implement.

TYPES OF AMPLIFIERS
The most basic type of commercial CATV Amplifier is the Transistorised Amplifier. These Amplifiers do not use a Hybrid IC. They use multiple amplifying stages implemented with discreet transistors. These low cost amplifiers are strictly speaking appropriate for Master Antenna TV applications rather than CATV applications. However, They are often used in small cable TV networks,
which deliver upto 24 channels for frequencies upto 300 MHz. Some manufacturers even offer transistorised amplifiers for operation upto 550 MHz. These low cost amplifiers are unsuitable for output levels above 90 dBU, with 67 channels driven simultaneously.

The line extender amplifier is a Hybrid IC based, single stage amplifier that offers reasonable performance at an attractive price. Ideally, it should only be used on the branch and last mile paths. It does not offer the lowest level of distortion or noise and hence its use should be avoided on the trunk line. Bridger Amplifiers are usually Hybrid IC based amplifiers that offer high output levels to a branch, for distribution. Some times the Bridge Amplifier; as a module; is housed within the trunk amplifier enclosure.

Trunk amplifiers typically offer the highest level of performance and also cost the most. A trunk amplifier approaches the ideal amplifier i.e. it offers amplification with the least distortion and noise. Let us take a closer look at how this is achieved.

Controlling Distortion in a CATV Network 6

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CALCULATION OF MODULATOR NOISE AT THE HEADEND
Let us suppose that a typical modulator has an in band Carrier to Noise ratio of -65 dB and an out of band Carrier to Noise ratio of -65 dB. Let’s assume that there are 64 such modulators at the Headend. The noise of all these 64 modulators will add up, lets calculate. The exact mathematical formula for calculation of noise is: C/Nn = C/N1 - 10 Log n where: C/N1 is the Carrier To Noise ratio of the 1st Modulator n: The Total Number of Modulators at the Headend C/Nn : Resulting C/N of the total n modulators. However let us ignore elaborate mathematics. Fairly accurate estimates can be arrived by simple "Rule of Thumb". "Doubling the number of Amplifiers or Modulators increases the total noise by 3 dB i.e. the Carrier to Noise ratio reduces by 3 dB". The out of band noise of 1 modulator is assumed to be -65 dB.
Therefore 2 modulators would reduce the C/N to -62 dB.
4 modulators C/N -59 dB.
8 modulators C/N -56 dB.
16 modulators C/N -53 dB.
32 modulators C/N -50 dB.
64 modulators C/N -47 dB.
It quickly becomes apparent that the out of band noise is now a bigger concern than the noise generated by each modulator within its own channel bandwidth!
In practice, the situation sometimes is a little better because manufacturers often provide output filters on their modulators which reduce all signals including noise at frequencies far away from the operating frequency. High quality international modulators such as BARCO have an out of band C/N of approximately -90dB and an In band C/N of -70 dB. A Headend of 64 Modulators would therefore reduce this by 18 dB i.e. the C/N of just the modulators put together would have deteriorated to -52 dB. This of course excludes the noise contributed by the Dish + LNB and the Satellite Receiver!

THE AMPLIFIER
The input level signal to an Amplifier is of crucial importance to the noise performance. Low level signals deteriorate rapidly with the injection of noise. On the other hand noise contributed to high level signals plays a less significant role. Given this basic understanding, a good starting point is to always ensure that input level signals are maintained fairly high for each Amplifier. An input signal level of at least 65 dBU, preferably 70 dBU should be maintained at the input of each Amplifier, throughout the entire Cable TV network. Let’s take a closer look. To start with, we need to estimate the noise performance of a single Amplifier. Once this noise is estimated, it would be a relatively simple matter to estimate the overall noise performance of the cascade of similar Amplifiers on the network. Keep in mind that we need to estimate the noise performance of the cascade of Amplifiers i.e. the chain of Amplifiers where the output of one feeds the other. Let us assume that a Cable TV network has 3 trunk lines. The first trunk has a chain (cascade) of 8 Amplifiers, the second trunk with 12 Amplifiers and the third trunk with 9 Amplifiers. The network therefore has a total of 29 Amplifiers but the worse noise will be at the end of trunk 2 which has 12 Amplifiers in cascade. Hence the total noise of 12 Amplifiers needs to be estimated, not of 29 Amplifiers.

NOISE OF THE FIRST AMPLIFIER
Most Hybrid Amplifiers utilize the Motorola 5342 (450 MHz) or 6342 (550 MHz) or their equivalent Philips ICs. These Hybrid ICs have similar noise figures of 5.5 dB. Let’s take this to be 6 dB. If we use this as a starting point, we would arrive at reasonably good estimates for cascades of Hybrid Amplifiers.
The noise performance of one Hybrid Amplifier is given by the equation:
C/N1 : C/N Ratio of the 1st Amplifier
C/N1 = ( i/p in dBU - 1 ) - Noise Figure of Amp
= ( i/p in dBU - 1 ) - 6
Clearly, the noise performance is better if the level of the input signal is higher. Let us assume that the input signal with the Amplifier is 70 dBU. C/N1 = (70-1) - 6
This yields a noise performance of the first Amplifier of 63 dB. However, if a network installs amplifiers so that each amplifier receives an input signal of 55 dB, the Noise of the first amplifier itself will be 48 dB. This clearly shows how the noise performance can be adversely affected, if levels are not correctly maintainer throughout the network. It is not necessary to install expensive amplifiers to keep noise to a low level. It simply requires proper signal levels.

NOISE OF THE CASCADE
The exact formula to calculate the Noise of a cascade of amplifiers is: C / Nn = C / N1 - 10 log ( n ) n = No. of Amplifiers in Cascade Further, the BIS specifies that C/Nn should not be worse than 43 dB. Of course this formula is daunting for anyone short of an engineer with a Scientific Calculator! Let’s take a simpler approach.
Once the noise of the first Amplifier is determined it is simple to estimate the total noise of the Amplifier cascade, without even a calculator! Similar to the case of Modulators, the noise Performance is reduced by 3 dB for a doubling of the number of Amplifiers. Hence:
2 Amplifiers 60 dB
4 Amplifiers 57 dB
8 Amplifiers 54 dB
16 Amplifiers 51 dB
32 Amplifiers 49 dB
In most actual systems, the cascade will not exceed 12 to 16 Amplifiers. Hence, for a 16 Hybrid Amplifier cascade, with an input signal level of 70 dBU for each Amplifier, the output signal will have a carrier to noise ratio of 51 dB. The BIS specifies that the carrier to noise ratio to the last customer should not be worse than 43 dB. Of course this 43 dB is a noise contribution of the Amplifier cascade, the Modulators and the Dish Antenna, LNB + Satellite Receiver, etc. However it is clear that the Amplifiers in such a cable network will not contribute significantly in terms of noise.

CONCLUSION
It is often believed that good noise performance can only be achieved by expensive trunk Amplifiers. The facts are quite different. In fact as shown in Table 2, the noise performance of the same amplifier reduces drastically, and can be used for a cascade of just amplifiers, if fed an input signal of 55dB per amplifier. The same product, if fed an input of 70dBU can be used for a cascade of amplifiers.
The key to good noise and distortion performance is the proper adjustment of signal level. Good noise performance dictates that the input levels be maintained fairly high. An input signal level of at least 65 dBU preferably 70 dBU for Hybrid IC based Amplifiers is ideal. The next article in this series will examine distortion in Amplifiers and once again look at the optimal settings for the lowest practical distortion.

Controlling Distortion in a CATV Network 5

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THE DISH ANTENNA

The Dish Antenna is required to collect a few Pico Watts of signal ( 1 Pico Watt is 1 Millionth of a Millionth of a Watt ! ). Clearly at such low levels of signal, great precaution must be taken to minimize noise. The best measure is to use as large a dish as possible to maximize the signal collected by the dish. The dish collects the signal and focuses it at the LNB (Low Noise Block Converter). This is shown in Figure 2.

The LNB's task is to receive this very tiny C Band signal and amplify it more than 100,000 times, then down convert its frequency so that it can be carried with a relatively low loss on an ordinary coax cable from the LNB to the satellite receiver. The amount of noise contributed to the LNB is usually expressed in degrees Kelvin (degree K). Without going into the physics aspect, it would suffice to know that the lower the noise figure in degree K, the better the LNB, because the LNB will contribute less noise of its own to the signal.
Typical, commercially available LNBs have a noise figure of 15 degree K to 30 degree K. All other things being equal, the 15 degree K LNB will amplify signals with less noise compared to a 30 degree K LNB. It may be relevant to point out that better performance would be obtained usually by installing a larger dish, than an LNB with a lower noise figure. Atleast a 12 foot dish antenna should always be used on cable networks even if you are receiving signals from a powerful satellite. The difference in picture quality may not be visible at the control room but would clearly show up at your most distant subscriber, after a long distribution network. Small dish sizes are best suited only for personal dish reception systems.

THE SATELLITE RECEIVER
Despite the large amplification provided by the LNB, signals arriving at a satellite receiver are of the level of a few micro volts only. The satellite receiver therefore needs to detect and further amplify these signals to provide a good picture. The lowest level of signal that a satellite receiver can linearly / proportionately detect is referred to as the receivers "threshold". A Low Threshold (LT) Satellite Receiver will linearly detect even low level satellite signals and provide good picture quality. If a large dish is used to receive a fairly strong satellite signal, an LT satellite receiver is not necessary.

SAT RECEIVER BANDWIDTH
Satellite receivers also try to improve picture quality of weak signals by reducing the bandwidth. As one can imagine, broad band noise would be less of a problem if the bandwidth was reduced to 18 MHz compared to a bandwidth of 27 MHz. Broad band noise is uniformly distributed at all frequencies. Hence reducing the range of frequencies (bandwidth) will reduce the noise picked up. Remember that the wider you open your window, the more dust flies in! Of course reducing the bandwidth also reduces picture sharpness. Hence the user should reduce the reception bandwidth of the satellite receiver, only if he is receiving a noisy picture in spite of using a large dish antenna. In a CATV network, the signal from the satellite receiver is fed to a modulator. So let’s look at the next link in the chain.

THE MODULATOR
The CATV Modulator receives an audio and video signal, and modulates it to an RF output frequency. This electronic signal processing generates its own noise. The amount of noise generated by a modulator is indicated by its Carrier-to-Noise ratio. A typical modulator output is shown in Figure 1.

IN BAND NOISE
The noise that a modulator generates within its operating channel is referred to as In Band Noise. Hence the noise generated by a Channel 2 modulator over the frequency span of 48 MHz to 55 MHz would be the modulator's In Band Noise.

OUT OF BAND NOISE
The Modulator also generates broad band noise, often at all frequencies in the CATV Spectrum. A channel 2 modulator would therefore generate some noise at all frequencies probably from 5 MHz to 1 GHz and above. While the out of band noise does not affect the picture quality of that specific modulator it will certainly affect the picture quality of other modulators operating at other frequencies. Most important, the out of band noise of all the modulators at the Headend adds up and causes and overall deterioration in the picture. Let us take an actual example.

Controlling Distortion in a CATV Network 4

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While the concept of noise in day to day life is well understood, giving it a specific, technical definition is a difficult task. Noise can probably be defined as random signals that are not part of the intended signal. Let us consider the case of a simple, small cable network transmitting signals on channels E-2 to E-12. Any randomly fluctuating signals on this network that are not part of the specific channel audio and video carriers are noise.
Noise is inevitable and occurs in every system. The key here is to keep noise below a level where it would cause deterioration in the received picture or sound at your most distant subscriber. Noise will occur within each channel. It will also occur across the entire frequency spectrum. The noise occurring across the entire spectrum is referred to as "Broadband Noise".
Figure

Figure 1 shows a Spectrum display of a particular channel of the audio and video carriers as well as the noise within that channel. As you will see from this picture the noise components (spikes) are approximately 50 dB less than the video carrier of this channel. This simply implies that the Carrier to Noise (C/N) ratio for this channel is approximately 50 dB. Noise occurs at every stage of electronic signal processing or amplification. Noise is also added during transmission of a signal even along a passive component such as a cable. This article does not aim to review the physics aspect of noise such as Shot Noise, Thermal Noise, Johnson noise, etc. But will restrict discussions to practical concerns in a Cable TV network. Let us now systematically examine the noise at each stage in a cable network.

Controlling Distortion in a CATV Network 3

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RULE OF THUMB CALCULATIONS
A Rule of Thumb is typically a quick approximation that is accurate enough to use in practice. We have developed such a "Rule of Thumb" for calculating amplifier distortion. The Cross Modulation of the first Amplifier is fairly simple to calculate. As indicated above
Cross Mod 1 = 58 + 2(108 - Actual Level)
We strongly recommend that for all practical purposes, users maintain a maximum output level from hybrid amplifiers of 100 dBU. Hence for all Hybrid Amplifiers utilizing the usual Motorola or Philips Hybrid IC, Cross Mod of the first Amplifier will be
= 58 + 2(108 - 100)
= 74 dB
The key "Rule of Thumb" for Amplifier distortion is:
Doubling the Number of Amplifiers Increase distortion by 6 dB Hence if one Amplifier generates Cross Mod distortion (for 100 dBU output level) of -74 dB, 2 Amplifiers will result in Cross Mod distortion of -68 dB, 4 Amplifiers will result in X-Mod distortion of -62 dB, 8 Amplifiers will yield -56 dB and 16 Amplifiers will yield -50 dB. However BIS specifications dictate that distortion should not be worse than -54 dB. Clearly we can therefore use between 8 and 16 amplifiers. The exact mathematical calculation yields 10 amplifiers. To calculate distortion follow these simple steps :

STEP 1
Calculate the distortion of the first Amplifier:
X-Mod = 74 + 2(Rated output - Actual output)

STEP 2
In the left hand column list down numbers from 1, that keep doubling viz. 1, 2, 4, 8, 16, etc On the right hand side column against 1, list the X-Mod of the first Amplifier that was calculated in Step 1. Keep reducing this by 6 dB as you progressively move down the column, till you reach -53 dB.
As an example, let us calculate the X-Mod distortion for Hybrid Amplifiers operating at an output level of 98 dBU.
X-Mod of First Amp = 58 + 2(108 - 98)
= 78 dB
1 78 dB 2 72 dB
4 66 dB 8 60 dB
16 54 dB 32 48 dB
This indicates that more than 16 Amplifiers can be used in cascade, if the maximum output level is maintained below 98dBU. All these calculations are of course, only considering distortion. Actually, the noise contribution should also be reviewed. The method for estimating & controlling amplifier noise has already been outlined in the previous article, in this series of articles.

DISTORTION LIMITS
The Bureau of Indian Standards vide their specification IS-14264 dictates that the customer should receive a signal which is at least 54 dB higher than the total distortion on the signal. Assuming that the headend does not generate a significant amount of distortion, it implies that all cascaded amplifiers on the network from the headend to the subscriber at the furthest point, should not generate more than -54 dB of distortion. Table 1 indicates the effect of various types of distortion on the picture quality. It is interesting to note that CTB also generates a grainy picture which can sometimes be mistaken for a noisy picture.

WIDEBAND AMPLIFIER DISTORTION
The Wideband or Broadband Amplifier, offers poorer performance than a Hybrid Amplifier and is suitable for small distribution networks carrying approximately 24 channels. Such Amplifiers typically yield higher distortion. A good working rule is to operate the Wideband Amplifier at a maximum output level of 95 dBU. Operated at this level, the Wideband Amplifier gives satisfactorily low distortion for use in delivering a quality CATV signal.

MINI HYBRID AMPLIFIERS
The Mini Hybrid Amplifier, as the name suggests, uses a "Mini" hybrid circuit. Various such hybrid circuits are available. Some of these are manufactured in India while others are of Chinese origin.
The Mini Hybrid Amplifier offers better performance than a Wideband Amplifier but is not as good as a regular Amplifier utilizing the Motorola 5342 / 6342 Hybrid IC or its Philips equivalent. To obtain good performance from a Mini Hybrid Amplifier, it is recommended that the Amplifier be operated at a maximum output level of 97 dBU.

POWER DOUBLER ICs
As we have seen earlier, increasing the output level by 3 dB increases the distortion by 6 dB i.e. distortion rises very rapidly. An increase of 3 dB in the output level may not seem much to the non technical user but actually implies double the output power level. A new type of Hybrid IC ( Philips BGD 502 or 504 ) that uses a dual output stage offers double or 3 dB higher output, for the same distortion. This family of Hybrid ICs has been called "Power Doublers". As indicated, increasing output level by 3 dB increases distortion by 6 dB. On the other hand, if the Power Doubler (Hybrid IC) Amplifier output level is not increased by 3 dB, the Power Doubler Hybrid IC will provide 6 dB lower distortions than the Hybrid IC, for the same output level.
Again, reviewing the earlier formula which dictated that distortion increased by 6 dB when the Number of Amplifiers used is double. Hence, twice as many Power Doubler Amplifiers can be used in a cascade, compared to regular Hybrid IC Amplifiers, for the same output level of say 100 dBU. Of course, the Power Doubler Hybrid IC and Amplifier are more expensive than the regular Hybrid IC equivalent.

DISTORTION & NUMBER OF CHANNELS
Just as a truck engine would strain to pull a full load compared to an empty truck, similarly a CATV Amplifier would generate higher distortion if it is used to amplify 67 channels instead of 1 channel. Actually there is a formula that links the distortion to the number of channels being amplified. However an easier method is to remember that amplifier distortion increases by 3 dB if the number of channels is doubled. Similarly a Hybrid IC Amplifier that would operate well with 69 channels and 100 dBU output level can be operated with an output level of 103 dBU if used on an 18 channel CATV network. However it is apparent that changing the number of channels from 69 to 35 permits the output level to be increased from a maximum of 100 dBU to 101.5 dBU. The difference is not much and 1.5 dB could easily be the calibration error of your signal level meter measuring a 100 dBU output signal. Hence it best to adhere to the recommended maximum output level of 100 dBU for a Hybrid IC based CATV Amplifier.

CONCLUSION
The article presents a simplified overview of distortion. It also provides an easy, non mathematical method to determine the distortion generated in a chain of CATV Amplifiers. Further, a simple practice of maintaining a maximum output level of 100 dBU for CATV Hybrid Amplifiers is recommended. This will provide good performance and sufficiently low distortion for use in almost any practical CATV network.

CROSS MODULATION DISTORTION (X-Mod) :
Diagonal bars usually running from the top right hand corner to the lower left corner of the screen. This is often termed as "Wind Shield Wiper" effect since it resembles the Wind Shield Wiper across the screen. A faint picture of another channel is superimposed. This is most prominent when the screen goes blank in between programmed or when texts such as titles are scrolled against a dark background.

COMPOSITE TRIPLE BEAT DISTORTION (CTB) :
The picture is "Wormy". Appears as if tiny worms are crawling across the screen! Grainy picture. The picture will appear with good contrast and colour but consists of grains or tiny dots all over the screen. A grainy picture due to noise will often lack colour and Contrast.

Controlling Distortion in a CATV Network 2

2009-06-28T08:46:00.000-07:00

THE WIDEBAND / BROADBAND AMP
The most basic CATV Amplifier is the Wideband Amplifier sometimes also referred to as the Broadband Amplifier. This is typically constructed using 5 or 6 transistor stages of amplification. Each of these transistor stages uses several discreet components. As a result of this, the characteristics of the amplifier can vary quite significantly from piece to piece. However, these amplifiers are relatively low cost and have proved popular for use in small networks carrying 24 or less CATV channels over relatively short distances.

THE HYBRID AMPLIFIER
Most modern CATV Amplifiers now utilize a Hybrid IC manufactured either by Motorola or Philips.These Hybrid ICs provide all the essentials of a good CATV Amplifier in a easy to use, hybrid module. The manufacturer needs to arrange a few basic components around the Hybrid IC to obtain a quality CATV Amplifier. Of course, considering the very high frequency of operation, the actual component layout, quality of components as well as even the material used for the PCB, effects the performance of the finished product. Nevertheless, Amplifiers manufactured from Hybrid ICs provide very consistent specifications that do not vary significantly from piece to piece. Further, the Hybrid ICs are designed to provide a significantly higher output level for the CATV signal, with low distortion.

POWER DOUBLER AMP
As we will see later in the article, the distortion generated by an Amplifier depends largely on the output signal level. Just as a car would strain to run at a higher speed, an Amplifier generates more distortion if operated at a larger output level. Circuit designers then thought of using 2 amplifier output stages to share the output. As a result, each output stage bears only half the load. Alternatively, for the same distortion, the power doubler amplifier can provide twice the output (i.e. 6 dB more) than a conventional Hybrid IC.

LINE LENGTH & DISTORTION
In a CATV network, the output of 1 amplifier, after being attenuated by the distribution cable feeds the input of another CATV Amplifier. This is referred to as "Cascading" of Amplifiers. In large networks, it is not uncommon to encounter a cascade of 10 to 12 Amplifiers before the signal reaches the end subscriber from the control room. In a cascade, not only the output signal but also the distortion of one Amplifier is feed into the input of the next Amplifier which amplifies both the signal and distortion!

One can very easily appreciate that unless the distortion; particularly in the first few amplifiers; is kept at a minimum, the picture at the customer end will be significantly deteriorated. Because of this larger networks need to pay greater attention to minimizing distortion from the Amplifiers.
Again, it is for this reason that large networks require to deploy superior performance "Trunk Amplifiers" on the trunk line. Somewhat lower performance and of course cheaper "Line Extender Amplifiers" can be used on the branch and distribution routes.

ESTIMATING DISTORTION
The distortion of an Amplifier can be calculated quite accurately from the data sheets. Since most Hybrid Amplifiers use similar Hybrid IC modules, the data sheet of the Hybrid IC module is an excellent starting point to calculate and predict amplifier distortion. The Cross Modulation (X-Mod) distortion in any amplifier is given by the formula
X-Mod
= X-Mod Spec + 2(Rated output - Actual output)
Let us try and understand what this formula conveys. The Cross Modulation of the Hybrid IC is specified for a particular output level (usually the maximum output, e.g. 108 dBU) and number of channels on the system (usually 69 channels for 550 MHz). As one can expect the amplifier would generate less distortion for a lower output level. How much lower, is what the formula will reveal? The formula actually indicates that Distortion deteriorates by 2 dB of every dB increase in the output level!
Note: While referring to the Maximum output level, we refer to the Channel with the highest level.As an example, if the output of the amplifier is 102 dBU at Channel 2 & 92 dBU on channel s-20, the max output for the calculations should be taken as 102 dBU.
Let us assume that the Motorola Hybrid IC 5342 is rated to provide -58 dB distortions with an output level of 108 dBU. Let us assume that all amplifiers in the network are set so that their maximum output is 100 dBU putting these values in the formula, we obtain the X-Mod distortion of the first Amplifier as
X-Mod 1 = 58 + 2(108 - 100)
= 58 + 16
= 74 dB
Thus the first Amplifier generates -74 dB of distortion.
The BIS specifications dictate that the customer should not receive more than -54 dB of distortion. Hence more amplifiers can be cascaded till the distortion reaches -54 dB. The formula that yields the maximum number of Amplifiers that can be used before distortion exceeds permissible limits is :
System X-Mod = X-Mod of first Amplifier - 20 log (n) where n is the number of Amplifiers.
Plugging in the values we have obtained above:
54 = 74 - 20 log (n)
Using a scientific calculator yields n = 10 i.e. a maximum of 10 Amplifiers can be used in cascade if the output level of each amplifier is kept within 100 dBU. A cascade of 10 amplifiers is fairly reasonable for most networks. If the distribution network requires more than 10 Cascaded Amplifiers, then the output level of all amplifiers should be suitably reduced. A max output level of 95 dBU will permit a 40 Amplifier cascade!
The use of a scientific calculator may prove daunting for many readers hence we have formulated a simple method for calculation of distortion without the use of anything more than a paper and pencil.

Controlling Distortion in a CATV Network 1

2009-06-28T08:41:00.000-07:00

INTRODUCTION
While most of us have a general idea of distortion and know that it deteriorates the signal, it may be useful to re-state what distortion is. When a signal is processed through an electronic circuit, the output of the circuit often contains unwanted signals which were not part of the input. This unwanted output signal’s frequency is usually related to the input signal's frequency. As a simple example, if a 100 MHz signal is fed into a CATV Amplifier, the output signal may contain unwanted signals at 200 MHz, 300 MHz, 400 MHz etc. These signals are referred to as distortion. It is important to note that only active electronic circuits generate distortion. Passive elements or circuits such as coaxial cables or even traps and filters do not generate any distortion.

TYPES OF DISTORTION
Several types of Distortion exist. The example given above with a 100 MHz input signal is demonstrated graphically.
The original 100 MHz input signal is referred to as the "Fundamental". The 200 MHz distortion is the "Second Harmonic" since it is twice the frequency of the original. Similarly the 300 MHz distortion is the "Third Harmonic". This example of a single frequency input signal yielding multiple harmonics is the simplest case of distortion and is termed as "Harmonic Distortion" since the distortion products are harmonics or multiples of the original (fundamental) signal. In the real world, it is unlikely that any amplifier would be fed a single frequency input. Even a single
TV channel contains 2 carriers via the video and audio carriers. Let us assume that 2 frequencies f1 and f2 are fed into the amplifier. Besides simple, harmonic distortion, other types of distortion such as f1 + 2xf2, f2 + 2xf1 and various other multiples of f1 and f2 will be created. These are shown in Figure 2. Lets assume that f1 = 50 MHz and f2 = 80 MHz. Distortion products will be generated at 210 MHz, 180 MHz, etc. These distortion products are collectively termed as "Intermodulation Distortion" since they are the result of the interaction or modulation of both the input frequencies. Readers will realize that a Cable TV system with even 10 to 12 channels can very quickly generate a large variety of intermodulation distortion products. It is therefore crucial to maintain a low level of intermodulation distortion.
There are various types of intermodulation products depending on the harmonic multiples that are selected. These are CSO (Composite Second Order) distortion and CTB (Composite Triple Beat) distortion. In typical CATV networks, particularly those with a large number of channels, CTB is the distortion which rises very rapidly and is the predominant distortion. Therefore, usually the CTB specifications of an Amplifier are to be considered since this is usually the worse case distortion.
Fortunately for technicians involved in the installation (rather than the design) of CATV networks, the measures that reduce CTB are also effective for reducing most other types of distortion such as CSO or of course the Grand Daddy of all distortions - Harmonic Distortions. As indicated above, harmonic distortion is the simplest form of distortion. The second harmonic of the 450 MHz signal will be at 900 MHz. However, the bandwidth of most cable networks does not exceed 890 MHz. Hence the harmonic distortion of any channel above 450 MHz will not affect the picture quality on a CATV network.
However intermodulation distortion products occur both above and below the input frequencies.
Cable networks have now realized that improper setting up of channel X, Y or Z modulators often creates distortion in the picture on channels E2 to E4. Switching off the offending channel x Modulator magically clears the picture on channel E2 ! Given this basic primer on the different types of distortion, lets take a look at the various types of amplifiers typically used in a cable network.

Network Dimensioning

2009-06-12T07:43:00.000-07:00

To support the transport of video, IPTV distribution networks need to feature high bandwidth carrying capacities. The amount of bandwidth required to carryIPTV services is generally a multiple of the bandwidth required to support Voice over IP(VoIP) and Internet access services. The total bandwidth required to implement

IPTV services depends on a couple of factors:
The number of IPTV multicast channels on offer As noted a single copy of each channel is sent from the IPTV data center on to the distribution network. Once the channel is streamed onto the networking infrastructure the multicast process handles the copying of channels and routing to individual IPTV subscribers. Consider an example of a service provider who is offering its 10,000 subscribers a package of 100 standard definition (SD) IP broad cast TVchannels. If we assume that the provider is using H.264 to compress thechannels, this generally translates to a bandwidth requirement of at least 2 Mbps for each broadcast channel. In the scenario where at least one subscriber is accessing each channel at a particular instance in time then the Next Generation Network (NGN) core distribution network will require200 Mbps of bandwidth capacity. This is the bandwidth requirement from the IPTV data center to each of the regional offices. At these offices techniques such as IGMP snooping can be used to reduce the bandwidth required over the local access section of the network. Note that H.264 and 1GMP will be described in greater detail later in the book.

Inclusion of IP-VoD services The dimensioning of the network is further complicated by the addition of IP-VoD applications. These types of applications use the unicast transport mechanism to provide communications between the IPTV consumer devices and the on-demand video server. This mode of operation consumes a large amount of bandwidth and the network needs to accommodate this level of network traffic. Consider the same network of 10,000 end users and assume that at a particular instance in time there is 5% of the subscriber base accessing IP-VoD titles. By assuming again that the H.264 compression standard is used, this translates to a peak usage on the network of 1 Gbps (10,000 5% 2 Mbps). This is a significant
requirement for the core network.

Reliability
The IP networking infrastructure needs to be reliable in the event of device failures. There should be no single point of failure that could interrupt the delivery of IPTV services, both multicast or unicast applications. Redundant links should be used wherever possible.

Fast Responsiveness
The network needs to support minimum response times associated with channel zapping (refers to changing from one channel to another during a TV viewing experience).

Predictable Performance
The nature of video bit rate streams is variable due to the differing scene complexities, which are delivered to an IPTV access device on a frame-by-frame basis. Therefore, it is difficult to predict the exact requirements of a video transmission until the service is operating in real-time. IPTV operators have to bearthis in mind and assign appropriate network resources to cope with variable bit ratestreams.

5 Level of QoS
Owing to the fact that most IPTV services operate over a private IP broadband network, it is advisable to implement a QoS policy when delivering video contentto paying subscribers. A QoS system preserves a video signal and lessens theprobability of impairments as it gets transmitted over long distances. It allowsoperators to provide services that require strict performance guarantees such as IP-VoD and IP Multicast. It comprises of a number of network techniques and supporting protocols that guarantee IPTV subscribers a specific level of viewing quality. The subject of IPTV QoS is covered in greater detail later in the book.

Core of an IPTV

2009-06-10T10:49:00.000-07:00

The backbone or core of an IPTV networking infrastructure is required to carry large volumes of video content at high speed between the IPTV data center and the last mile broadband distribution network. There are several different types of backbone transmission standards that provide multipath and link protection capabilities that are necessary to ensure high reliability capabilities. Each standard has a number of specific features including data transfer speed and scalability.
Three of the major backbone transmission technologies used in IPTV network infrastructures are ATM over SONET/SDH, IP over MPLS and metro Ethernet. As FIGURE 2.13 IPTV core-networking infrastructure.

IPTV BACKBONE TECHNOLOGIES
illustrated in Fig. 2.13 these core networking technologies provide connectivity between the centralized IPTV data center and various types of access networks.

ATM and SONET/SDH
As described, ATM can support demanding applications such as IPTV that require high bandwidth and low transmission delays. ATM will operate over different network media including coaxial and twisted pair cables, however, it runs at its optimum speed over fiber optic cable. A physical layer called Synchronous Optical Network (SONET) is typically used by a number of telecommunication carriers to transport ATM cells over the backbone network.

SONET is a protocol that provides high speed transmission using fiber optic media. The term Synchronous Digital Hierarchy (SDH) refers to the optical technology outside the United States. The SONET signal rate is measured by optical carrier (OC) standards. Table 2.7 illustrates the available transmission rates(called OC levels).

SONET uses time division multiplexing (TDM) to send multiple data streams simultaneously. With TDM, the SONET network allocates bandwidth to a certain portion of time on a specific frequency. The preassigned time slots are active regardless of whether there is data to transmit.
In the context of an IPTV networking environment the SONET equipment receives a number of bit streams and combines into a single stream, which then is sent out onto the fiber network using a light emitting device. The rates of the combined input rates will equal the rate outputted from the SONET device. For example, four input streams carrying IPTV traffic at 1 Gbps will be combined by the SONET device and a 4 Gbps stream is forwarded onto the fiber network.

IP and MPLS
A number of larger telecommunication companies have started to deploy the Internet Protocol in their core networks. Although IP was never originally designed TABLE 2.7 SONET Optical Carrier Standards
OC level Signal Transmission Rate
OC-1 (base rate)51.84 Mbps
OC-3 155.52 Mbps
OC-12 622.08 Mbps
OC-24 1.244 Gbps
OC-48 2.488 Gbps
OC-192 10 Gbps
OC-256 13.271 Gbps
OC-768 40 Gbps

with features such as QoS and traffic segregation capabilities the protocol works quite well in these environments when combined with a technology called Multiprotocol Label Switching (MPLS). An MPLS enabled network supports the efficient delivery of various video traffic types over a common networking platform.
An MPLS platform is designed and built using advanced Label Switch Routers (LSRs). These LSRs are responsible for establishing connection-oriented paths to specific destinations on the IPTV network. These virtual paths are called Label Switched Paths (LSPs) and are configured with enough resources to ensure the smooth transition of IPTV traffic through an MPLS network. The use of LSPs simplifies and speeds up the routing of packets through the network becau se deep packet inspection only occurs at the ingress to the network and is not required at each router hop.
The other main function of LSRs is to identify network traffic types. This is achieved by adding a MPLS header onto the beginning of each IPTV packet.
As illustrated in Fig. 2.14 the header is added at the ingress LSR and removed by the egress LSR as it leaves the MPLS core network. While the IPTV traffic traverses across MPLS enabled routers a number of local tables called Label Information Bases (LIBs) are consulted to determine details about the next hop along the route. In addition to examining the table, a new label is applied to the packet and forwarded onward to the appropriate router o utput port.
Other added benefits of MPLS networks include their support for high levels of resilience when a failure occurs.

Metro Ethernet
Another technology, which may be deployed in the core network, is Metro Ethernet. An alliance of leading service providers, equipment vendors, and other TABLE 2.8 MPLS Header Format Field Name Field Length (Bits) Description of Functionality Label 20 Contains specific next hop routing details that are specific to each IPTV packet.
Experimental bits 3 Reserved for other uses.
Stacking bit 1 A header can contain one or more labels. Once the stacking bit is set to one, the LSR will identify the last label in the packet.
Time to live (TTL) 8 This value is copied from the TTL field in the IP header.

MPLS core network topology
prominent networking companies called the Metro Ethernet Forum (MEF) isresponsible for establishing specifications for integrating Ethernet technologies intohigh capacity backbone and core networks. In addition to developing specificationsthe MEF also certifies Ethernet equipment for use in service provider’s corenetworking infrastructures. The key technical and operational characteristics of
Metro Ethernet based core networks include:
. It meets the various requirements that are typical of a core networkingtechnology, namely, resilience, high performance, and scalability.
. Some of the modern Metro Ethernet networking components can operate atspeeds up to 100 Gbps across long geographical distances. This providesservice providers with an ideal platform for efficiently delivering newvalue-added services such as IPTV to geographically dispersed regionaloffices.
. It implements a sophisticated recovery mechanism in the event of a networklink failure thereby, ensuring that services such as IPTV are unaffected by theoutage.
. Metro Ethernet technologies support the use of connection orientated virtualcircuits that allow IPTV service providers to guarantee the delivery of highquality video content within the network core. These dedicated links are calledEthernet Virtual Connections (EVCs). Figure 2.15 shows how four EVCs areused to provide connectivity between the IPTV data center and a number ofregional offices.
In addition to the above characteristics, the low delay and packet loss features ofMetro Ethernet make it an ideal core networking technology for carrying IPTVservices.

IPTV Internet Downloads

2009-06-10T10:30:00.000-07:00

As the name suggests this avor of IPTV allows consumers to download and watchon demand content. Most Internet download services are subscription based or pay-per-download and can include a mix of local news and weather, movies, local films and music, an entertainment guide, and classified advertising. A number of the major online Internet based portal sites have recently started to offer their own libraries of downloadable IPTV programming content to Internet users. In most cases standard or media center PCs are used to view Internet downloads; however,some companies have started to provide broadband enabled IP set-top boxes for TABLE 2.6 Technical Characteristics of an Internet Download IPTV Service Characteristic Description.
Networking protocols Standard file transfer protocol (FTP) and hypertext transfer protocol (HTTP) standards are generally used to transfer the IPTV content from the server to the client device. The use of these protocols minimizes the possibility that the IPTV content will be blocked by firewalls.
Server technology Standard Web server software is generally used to service requests for on-demand video content.
Network speed The time taken to download an Internet movie depends on the speed of the broadband connection and quality of the video content. Standard definition movies and programs download relatively quickly in comparison to HD based video content. Although broadband is the preferred connection type it ispossible to also use slower dial-up links to access Internet download services.
Network errors Standard TCP error correction techniques are used to correct problems during the downloading process. Storage requirements Both server(s) and client(s) require advanced storage capabilities to support the processing of large IPTV files. Some Internet download applications allow IPTV subscribers to burn a copy of the downloaded video content to a DVD and play in a DVD player.

consumers who do not want to watch videos on their PCs. The technical characteristics of an end-to-end IPTV based Internet download service are shown in Table 2.6.
Peer-to-Peer (P2P) Video Sharing A peer-to-peer video sharing application allows users to watch, share, and create online video content. Using a peer-to-peer video sharing application is quite straightforward and typically involves the download and installation of specialized software. Once the software is operational on the PC, the user simply clicks on a link to download a particular video file. Once the download process is initiated, the P2P video sharing application software establishes connections and starts to retrieve the requested video content from a variety of different sources. Once the video file is downloaded to the local hard disk, viewing can commence.

3G Networking Technologies

2009-06-05T07:42:00.000-07:00

Mobile networks based on 3G technologies such as EV-DO and HSDPA are also capable of delivering a range of mobile IPTV applications.

EV-DO Evolution-Data Optimized (EV-DO) is a wireless radio broadband data standard that boosts maximum data rates up to 4.9 Mbits/s.

HSDPA High-Speed Downlink Packet Access (HSDPA) supports rates up to approximately 14 Mbps on the downstream path, with higher speeds planned in the future.

Although not an ideal platform for delivering IPTV services, EV-DO and HSDPA do provide network operators with the ability to deliver IPTV services to consumers who live in areas that are poorly served by DSL and cable broadband systems.
Since the invention of television, a number of different distribution technologies have been developed to deliver signals to consumers around the world. Until recently, there were five primary networking platforms used to distribute TV content, namely, wireless off-air, satellite, DSL, fiber and cable TV networks. In more recent times a new platform has emerged that also allows consumers to view broadcast and on-demand video content the Internet.
Improved broadband speeds combined with advances in compression technologies and greater viewing choices are some of the reasons why consumers have started to increasingly turn to the Internet for video entertainment over the last couple of years. IPTV over the Internet is available in a number of formats.

Streamed Internet TV Channels
The delivery of TV channels over the Internet is a popular IPTV application and involves the streaming of video content from a server to a client device, which is capable of processing and disp laying the video content. The type of device used toview Internet TV channels is typically a PC or a media center PC. StreamedInternet TV channels can however also be accessed via a mobile phone or an IP set-top box. The content available via streamed Internet TV channels is delivered in real time, and the viewing experience mimics the traditional approach to TV viewing, namely, a channel is chosen and viewing commences immediately. The technical process of streaming an Internet TV channel usually starts at the streaming server where the video content is broken into multiple IP packets,compressed, and then transmitted or streamed across the Internet to the client PC.
The PC includes software, typically a browser, which decompresses the videocontent and generates a live video feed. The time period between selecting a TV channel and the commencement of viewing is generally quite short and depend s on the connection speed that is available between the client and the server. A highly simplified graphical view o f a networking architecture used to deliver a single streamed TV channel over the Internet is presented in (Fig. 2.12).In all Internet TV channel deployments a streaming server is required. Inaddition to playing out video content upon request from an IPTV subscriber, a standard streaming server will also support the following functions:

. Storing and retrieving source video content.

. Controlling the rate at which the IP video packets are delivered to the client
viewing device.

. Executing forward and back commands as requested by the Internet TV viewer.

A single streaming server works fine for delivering a small number of Internet TV channels to a limited number of users. To support the delivery of multiple channels to thousands and possibly hundreds of thousands of IPTV subscribers, a number of streaming servers need to be deployed in different parts of the network.
From a security perspective the streaming of video content is quite secure because the material is not stored on the client’s access device. Thus, unauthorized copying of content is prohibited. Other benefits of this avor of IPTV include itsability to operate effectively over low bandwidth connections and viewers have theability to start watching content at any point in the IPTV stream.
What separates the delivery of Internet TV with the other delivery mechanisms discussed in this chapter is the fact that Internet portal sites do not own or control the underlying infrastructure used to stream IP video services to Internet users. The networking infrastructure is generally owned by either cable TV providers or telecommunication companies.

Wireless Municipal Mesh Networks

2009-06-05T07:30:00.001-07:00

Municipal Mesh Networks, also known as muni networks are another wireless platform that promises to support the delivery of IPTV services to end users. A number of these network types have been deployed in various cities and towns around the world.
FIGURE 2.11 Municipal mesh network architecture Municipal networks operate in an outdoor environment in either the unlicensed 2.4 or the 5 GHz spectrum range. Wi-Fi, also known as the 802.1x family ofwireless products has been the technology of choice for building mesh networks,because most if not all notebooks and handhelds manufactured nowadays comewith in-built Wi-Fi interfaces. Constructing Wi-Fi networks in an outdoor environment requires the use of a number of access points interconnected to each other and to a wired connection that provides backhaul to the broadband service provider (Fig. 2.11).
Mesh Wi-Fi Access Points (APs) APs used by municipal networks cover a much greater area compared to conventional indoor APs. They are generally attached or mounted onto fixed physical structures that provide a goodline of sight and easy access to power. Suitable locations for mounting outdoor APs include light posts, tall buildings, and communication towers. Interconnecting outdoor APs back to a central point using some type of physical cable is cost prohibitive for most deployments. Therefore, all APs in a municipal wireless networking architecture dynamically connect wirelessly with each other and a gateway AP in a cluster type configuration. The gateway aggregates the 802.11xsignals within the cluster and interfaces via an Ethernet port with the broadbandbackhaul link. The number of APs in a cluster varies between implementations.
Each AP in the cluster is ruggedized and uses a mesh routing protocol to interconnect with other APs and back to the backhaul point of presence. The main responsibility of the routing protocol is to provide the most efficient route for IP packets through the mesh from each AP to and from the backhaul link. It does this by continuously monitoring the wireless network and identifying wireless paths that provide the greatest bandwidth throughput capabilities. Mesh APs come in two variants single and multiple radio configurations:

. Single Wi-Fi radio-mesh APs use one channel to support access from various client devices in addition to carrying interconnectivity traffic to and from the mesh network. To minimize constraints and improve performance, these types
of APs are typically configured into clusters that operate at different frequencies to their neighboring clusters.

. Dual Wi-Fi radio-mesh APs use separate channels for carrying mesh traffic and providing access to client Wi-Fi devices in the 2.4 GHz frequency band.
The use of two separate channels that operate in different frequency bands offer improved performance levels and reduced latency levels that make dual channel APs more suitable to carrying time-sensitive applications such as IPTV.

Wired Backhaul Connectivity A wired backhaul is required to provide connectivity to the IP data center and onwards to the public Internet. A technology called virtual LANs (VLANs) is often used to segment the different types of traffic that traverse a wireless municipal network. Note that VLAN technology will be described in greater detail later in the book.
At the time of writing the average downstream data rate of a municipal wireless network is approximately 1 Mbps, which is more than adequate for public Internet access applications. However, IPTV has more demanding throughput needs, therefore, deployments of video centric applications over these types of networks are typically confined to specialized functions such as streaming IPTV content from Wi-Fi cameras.

WiMAX MAC Layer Properties

2009-06-05T07:22:00.000-07:00

WiMAX MAC Layer Properties The MAC layer is subdivided primarily into three sublayers:

(1) Service specific convergence sublayer (CS) The main purpose of this sublayer is to interface with the higher layers in the WiMAX communication model.

(2) MAC common part sublayer (MAC CPS) This sublayer takes care of core MAC functionalities such as security, management of connections, and access to the physical network.

(3) Privacy sublayer As suggested by the name this manages authentication of IPTV subscribers and encryption of the v ideo content.

WiMAX Transport Layer Properties Standard TCP/IP is generally used at the network and transport layers to ensure delivery of IPTV services.
Transmission Ranges Geographic topologies combined with other factors such as equipment specifications and weather conditions can all have an impact on the distance between a IPTV consumer device and a WiMAX base station. WiMAX has a theoretical maximum speed of approximately 60 Mbps within a coverage area of 6 10 km. This varies between implementations and equipment vendors. Assuming that the contention ratios are correctly planned, these data throughput levels will comfortably allow subscribers within the WiMAX coverage area to access IPTV services. It is important to note that WiMAX can support both line of sight (LoS) and nonline of sight (NLOS).

Mobile WiMAX
IEEE 802.16 cannot be used to provide broadband services in a mobile environment. For this purpose, an amendment to the IEEE Standard 802.16 standard was developed called IEEE 802.16e. Also known as Mobile WiMAX, IEEE 802.16e was approved in 2005 and certified products were released to the market in 2006. It operates in several licensed spectrum bands: 2.5, 3.3, and 3.4 3.8 GHz. Mobile WiMAX incorporates a number of key features that are necessary to transport IPTV services and applications:
. The technology supports peak data speeds of around 32 46 Mbps. These types of speeds if deployed correctly allow delivery of compressed IP based high definition content to mobile handsets.
. It utilizes technologies such as OFDMA and optimized handoffs to allow IPTV viewers access multicast broadcast TV channels in geographical areas that are susceptible to the affects of multipath transmission paths.
. It integrates with multimedia subsystem (IMS), which simplifies interworking between IPTV applications and other IP based services such as high speed Internet access and VoIP. Note that IMS is an emerging technology architecture that allows network operators to accelerate and simplify the deployment of IP based services.
. Mobile WiMAX provides support for advanced quality of services (QoS)mechanisms, which are beneficial to real-time applications such as IPTV.
In addition to the above characteristics, it is important to note that at the time ofwriting the WiMAX forum continues its work on expanding the multicasting capabilities of Mobile WiMAX. This is expected to further enhance the ability of Mobile WiMAX to meet the stringent requirements associated with broadcasting live IP based TV channels to mobile devices.

IPTV Over Wireless Networks

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>
New broadband wireless networks provide telecom operators with another alternative distribution platform to deliver IPTV services to households. Various options are available and are described in the following sections.
Fixed WiMAX
Demands from consumers and the telecommunication sector to use WiMAX as a platform to carry IPTV content is growing at a steady pace. WiMAX (Worldwide Interoperability for Microwave Access) is a high capacity IP broadband wireless technology that is considered by the industry to be a close “relative” of the Wi-Fi family of wireless standards. It defines a number of services that conform to the IEEE 802.16 technical standard. The WiMAX Forum, an industry association, is responsible for developing WiMAX specifications, promoting the technology, and FIGURE 2.9 Simplified block diagram of a WiMAX System carrying IPTV traffic.FIGURE 2.10 WiMAX communications model managing the overall certification of WiMAX products. The organization boasts a membership of over 370 companies at the time of writing this book. Figure 2.9 shows a simplified diagram of two WiMAX broadcast cells connected together and delivering video content to a number of IPTV end users.The technical characteristics of fixed version of WiMAX follow.

Operating Frequencies WiMAX will operate within licensed and unlicensed frequency bands. These bands have been allocated by various communication regulation bodies around the world. The licensed bands are the preferred operating frequency option for real-time applications such as IPTV because there is less chance of interference occurring. Fixed WiMAX operates in frequencies of 3400 3600 MHz.

Physical and MAC Layer Protocols As depicted in Fig. 2.10, the 802.16 communications model defines three layers: physical, MAC, and transport.WiMAX Physical Layer Properties Under the WiMAX standard, equipment manufacturers have a choice between three different PHY options when building products:

(1) The Single Carrier physical layer option is intended for straightforward line of sight applications.

(2) The Orthogonal Frequency Divisio n Multiplexing (OFDM) option is the most popular physical layer choice for most WiMAX equipment manufacturers because of its ability to deal with the issue of multipath propagation. This feature means that OFDM based WiMAX technologiesare particularly suited to the delivery of IPTV services.

(3) Orthogonal Frequency Division Multiple Access (OFDMA) is the most sophisticated option and is capable of separating user connections on the upstream frequency channels.

IPTV Over a Satellite Based Network

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IP is also emerging as a preferred method of distributing video content via satellite links. Satellite links can provide higher bandwidth than terrestrial transmission networks and are starting to get used for IP based triple-play services that comprise of digital video content, VoIP, and high speed Internet access.
Many of the satellite network providers have started to use their satellite based networking platforms to deliver IP video content to cable and telecommunication headends and IPTV data centers. The networking infrastructure used to support this mechanism of IPTV distribution is shown in Fig. 2.8.
A fully rounded end-to-end satellite centric IPTV distribution system As shown the original content is received, aggregated, encoded in MPEG-2,MPEG-4, or Windows Media format and encrypted at the satellite operator’s video operations center. Once processed at the operations center, the content is uplinked to a satellite and relayed back down to various video hubs. These video hubs are generally operated by cable or telecommunication companies and use their own existing networking infrastructure to deliver IPTV services to residential subscribers.With regard to delivering IPTV services directly to consumers a number of options are currently available.
Deploy Hybrid Satellite IP Set-top Boxes The deployment of hybrid satellite IP set-top boxes allows consumers to access traditional satellite services via the satellite link and IPTV services via a standard broadband connection.
Utilize Standard IP Set-top Boxes This model involves the delivery of programming channels using standard satellite transmission techniques to large housing developments, converting the channels to IP, and streaming to IP set-top boxes.
Provide Subscribers with Set-top Boxes That Include a Hard Disk The large bandwidth capacity requirements of delivering on-demand content makes IP-VoD based applications for most satellite systems impractical. However, some satellite service providers have started to circumvent the limitations of satellite based delivery systems by deploying set-top boxes that incorporate a hard disk. Once installed at the subscribers home, the IP-VoD content is automatically downloaded to the hard disk. This allows IPTVend users to view the content at their own convenience.
Use Satellite Broadband Modems Broadband modems may also be used to deliver satellite IPTV services. These modems generally comply with one of three international standards:

(1) IP over satellite (IPoS) This standard has been ratified by U.S. based Telecommunication Industry Association (TIA) and ETSI. It utilizes a technology called DVB-S2 (please refer to Chapter 5 for specific details on DVB-S2)and supports data throughputs of up to 120 Mbps.

(2) DVB return channel via satellite (DVB-RCS) DVB-RCS was developed by European based DVB. It defines a forward-link data transmission rate of 40 Mbps and a return channel capacity of approximately 2 Mbps. DVB-RCS is officially defined in ETSI EN 301 790.

(3) DOCSIS over satellite This mechanism for transmitting IPTV content over a satellite link is based on an adapted version of the DOCSIS standard. The main difference between cable DOCSIS and satellite DOCSIS is the use of the Quadratare Phase Shift Keying (QPSK) modulation scheme instead of the QAM, which has been designed for HFC networks. Early versions of the protocol provide support for speeds of 1.5 Mbps, whereas newer versions can achieve much higher data rates. These newer rates mean that satellite operators can start to leverage IP based technologies to deliver video content across their networks.

ENCAPSULATION PROTOCOL

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AN ENCAPSULATION PROTOCOL
This protocol encrypts the IPTV packets as they traverse the cable TV network. In addition to encrypting packets that are destined for IPTV and triple-play users, the BPIþ encapsulation protocol is also used to encrypt other types of protocol information, which is used in the provisioning of cable modems, namely DHCP, Trivial File Transfer Protocol (TFTP), and various types of management messages that are transported via the MAC layer. Note that DOCSIS 3.0 provides stronger network traffic encryption compared to its predecessors through its support for the 128-bit Advanced Encryption Standard (AES) algorithm.
The protocol designed by CableLabs to secure A KEY MANAGEMENT PROTOCOL the distribution of keying data between the CMTS and cable modems is called Baseline Privacy Key Management (BPKM). Standard technologies such as digital certificates and public-key encryption algorithms are used by BPKM tosecure key communications across the HFC network. Note that MAC management messages discussed previously are used to transport the BPKM protocolinformation.
DOCSIS 3.0 uses BPIþ to secure initialization of a cable modem onto the network. The BPIþ security process commences when the modem identifies acommunications channel. At this stage the modem sends an authentication information message to the CMTS. The details contained in this message are described in Table 2.5.Once the authentication information message arrives at the CMTS, it is verifiedand the CMTS responds back to the cable modem using an authorization reply message. This message includes identification details and an encrypted key.
The BPIþ also uses a technique called source address verification to eliminateIP spoofing by in-home networking devices. To enforce this security policy DOCSIS 3.0 specifies that any network packets that originate from a device whoseIP source address has not been assigned by the IPTV service provider is discarded.
EuroDOCSIS The European cable industry has developed its ownstandards for high speed data transfer across a cable TV network. For the most partthe technical details follow the DOCSIS system very closely. The primary difference between the two standards is the difference in channel widths. European cable Structure of BPIþ Authentication Information Message Item of Information Purpose Identifiers The identifiers include the cable modems hardware address and details about the manufacturer.
Public key This security component is incorporated into the device during manufacturing.
Digital certificate The use of a digital certificate allows the modem to be authenticated by the CMTS and restricts network access to authorized devices. Digital certificates are supplied to “trusted vendors”who incorporate it into DOCSIS modems during manufacturing.
Cryptographic algorithm descriptor This provides the CMTS with information on the authentication and data encryption algorithms supported by the cable modem.
Security association identifier (SAID) This item of information identifies the security information, which is shared between theCMTS and one or more of its client cablemodems.

The DOCSIS 3.0 specification

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The DOCSIS 3.0 specification has
SUPPORT FOR MANAGEMENT MESSAGE TYPES
defined 61 messages for exchanging management data between cable modems and a CMTS. The specification also provides a provision to extend this number up to 255. Management messages that are particularly relevant to IPTV implementations include the dynamic bonding change request, response, and acknowledge commands. A detailed description of all 61 management messages types is available in the specification on CableLabs Web site.
Examining the DOCSIS 3.0 Operations Support System Interface (OSSI)
The CM-SP-OSSIv3.0 document provides details on the various OSSI technologies.
DOCSIS 3.0 OSSI specification defines the requirements to support fault, configuration, performance, security, and accounting management functions. In addition to the various mechanisms used by previousFAULT MANAGEMENTversions of DOCSIS to identify, monitor, correct, and record faults, DOCSIS 3.0 also provides for some new features:

. A diagnostic log that records operational problems such as repeated cable modem initialization and registration details.

. Enhancements that identify and resolve issues related to new functionality integrated into DOCSIS 3.0.

The management of faults in a DOCSIS 3.0 environment requires the use of protocols such as the simple network management protocol (SNMP) in combination with a number of different types of event logging and recording mechanisms.
Note that Chapter 11 on IPTV network administration provides a description of the SNMP protocol.
As the name suggests this function ensures that CONFIGURATION MANAGEMENT configuration parameters are kept current and are suitable for the smooth operation of the network. One of the key differences to this management function when compared to previous versions is an updated configuration file structure. The parameters within the file itself have been adjusted to support DOCSIS 3.0 technologies such as IPv6, multicasting, and channel bonding. CableLabs has adopted 44

IPTV NETWORK DISTRIBUTION TECHNOLOGIES
existing standardized protocols to support the execution of configuration management within a DOCSIS networking infrastructure.
The purpose of this function is to use either IPDR orPERFORMANCE MANAGEMENTSNMP protocols to gather event and error data at the PHY, MAC, and IP layers of the DOCSIS protocol stack. This information is used to help engineers determine the overall health of the networking infrastructure.
This function is responsible for protecting various types ofSECURITY MANAGEMENTdata in particular access to SNMP traffic. This helps to strengthen the security aspect of DOCSIS by minimizing the risk of hackers using SNMP as a mechanismto damage a cable providers networking infrastructure.
This function is executed by the CMTS and its coreACCOUNTING MANAGEMENTresponsibility is to gather network usage statistics for individuals and businesses who have subscribed to one of the available services, such as high speed Internet access.
Once collected this usage information is passed to the billing system. The mechanismfor delivering subscriber usage billing records is facilitated through a schema thatincludes the use of a streaming protocol developed by the IPDR organization. Thisnetworking protocol has been designed to efficiently transport high volumes of XMLbased usage billing records over a Transmission n Control Protocol (TCP) networkconnection. To reduce the size of these records, CableLabs has additionally specifiedthe use of a compression mechanism called IPDR/XDR. As the name suggests this encoding format was developed by the IPDR organization and uses the External DataRepresentation Standard (XDR) coding format defined in RFC 1832.
Examining DOCSIS 3.0 Security Mechanisms The security mechanisms required to support communication between different devices across a DOCSIS 3.0 enabled infrastructure is described in CM-SP-SECv3.0. These various mechanisms help to ensure that subscriber’s privacy is not compromised and preventing theft of services such as IPTV. The core portion of the DOCSIS 3.0 security system is based on the Baseline Privacy Plus (BPIþ) scheme. The architecture of this scheme is logically portioned into two protocols.

Initialization and Provisioning of DOCSIS Modems

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modem is authorized by the CMTS for use on the network, and configures itself according to parameters that are passed to it from a provisioning server. This sequence of events occurs automatically without user involvement. The process of initializing a DOCSIS 3.0 modem on a HFC network is divided into the followingphases:

(1) Once the cable modem is powered up, it scans the network for both a downstream and upstream channel that supports the transfer of data.

(2) After locating the channels is complete, the cable modem informs the headend of its presence on the broadband network by sending a variety of parameters back to the CMTS.

(3) Once the cable modem has been assigned to the network topology, it needs to be authenticated. This typically requires the cable modem to send a digital certificate, which is installed during manufacturing, to the CMTS for validation. Once validated a series of keys are generated and used to encrypt the data transferred across the network.

(4) After the modem is connected to the cable network, it must invoke DHCP mechanisms to obtain an IP address, which is part of the operators authorized address space. Note that DOCSIS 3.0 allows IPTV network administrators to configure cable modems for both IPv4 and IPv6 address formats.

(5) Once the modem receives an IP address, it downloads a configuration file that has parameters the cable modem needs to configure itself. Time of day information is also downloaded to the modem.

(6) Once the modem has been configured and authorized, the CMTS authorizes resources at the MAC layer and the cable modem can use the network like any standard Ethernet network device.

CM-SP-PHYv3.0

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CM-SP-PHYv3.0 This specification deals with physical layer aspects of the technology. 5 42 MHz or 85 MHz frequency band is used. Operating DOCSIS 3.0 frequencies varies between implementations. The physical media portion of the document defines the electrical characteristics and signal processing operations used between cable modems and CMTS(s) installed at the data center. With regard to gaining access to the physical network DOCSIS 3.0 defines two primary methods: time division multiple access (TDMA) and SCDMA (synchronous code divisionmultiple access). The modulation of IPTV data on to these channels are achieved through the use of a wide variety of modulation schemes. The modulation scheme is dependent on the access method and the direction of the network traffic, namely,upstream or downstream. These schemes deal with the hostilities that are often present on a HFC network when used to deliver real-time data traffic such as IPTV.
Examining the DOCSIS 3.0 MAC Layer DOCSIS 3.0 introduces a number of features that build upon what was present in previous versions of DOCSIS. These enhancements are detailed in the CM-SP-MULPIv3.0 document. These features are explored in the following subsections.
DOCSIS 3.0 has defined new ways of utilizing the MAC frame in order to support some of the new features incorporated in to the technology. A frame is the basic unit of transfer used for communication at the MAC layer between the CMTS and cable modems. The length of the frame is variable and consists of two parts a header and the video payload. The structure of a DOCSIS 3.0 packet header.

Note that a DOCSIS 3.0 packet also includes overhead details that are used by the upper layers of the protocol stack.

DOCSIS 3.0 MAC Layer Field Descriptions Field Name Size (Byte) Description of Functionality FC 1 Short for frame control, this field identifies the header type and identifies whether an extended header (EHDR) is present or not.
MAC_PARM 1 When an EHDR sequence field is present, this provides the EHDR length.
LEN (SID) 2 This defines the length of the frame.
EHDR 0 240 The extended MAC header is used to support a number of functions, including the tagging of Multicast IPTV packets and the sequencing of out-of-order packets. HCS 2 HCS stands for header check sequence and is used toidentify transmission errors that affect the header.
The ability of DOCSIS 3.0 devices to use a technique calledChannel Bonding.
Channel Bonding to increase the data throughput over a HFC network allows cable operators implement IPTV services. Under this mechanism, multiple smaller channels are bonded together to create a larger logical channel with high bandwidth capabilities. In addition to providing greater throughput when compared to a single channel, this mechanism also reduces the congestion delays associated with sending packets over a single channel. DOCSIS cable modems include multiple tuners,which are used to access the various channels available as part of the group ofbonded channels.
The mechanism operates by concurrently distributing IPTV packets across anumber of channels, which gets delivered to a cable modem. The distribution packets across multiple channels can cause difficulties for the cable modem as some of the channels suffer from jitter and latency. If this is the case the packets will arrive out of order. To mitigate against such problems, DOCSIS 3.0 introduces amethod of tagging or marking each packet with a particular sequence number,which is used by the cable modem to reassemble the original IPTV stream before forwarding onwards to an IP set-top box. The packet sequence number is 2 bytes in length and in the case of downstream traffic is included in the DS-EHDR fieldof the MAC header.
Overall management and allocation of bonded channels is the responsibility of the CMTS. The channel bonding mechanism can be used for transmitting data both upstream and downstream across a cable TV network.

DOCSIS 3.0 builds on the load balancing mechanisms used by LOAD BALANCING previous versions of the technology. Load balancing is a critical feature of large cable TV networks that support hundreds and in some cases thousands of cable modems. The load balancing section of the DOCSIS 3.0 specification makes provisions to cope with the large volumes of traffic that are generated by multimedia applications such as IPTV.
DOCSIS 3.0 allows cable operators to u se IP multicasting as a method of delivering IPTV sessions and services to their subscribers. The use of multicasting is particularly attractive to cable TV providers because it reduces the amount of network ban dwidth required to deliver a multichannel pay TV service.
To facilitate the use of multicasting DOCSIS 3.0 features an identifier called a multicast downstream service ID (MDSID). The MDSID allows cable modems that have been authorized to view a particular IPTV multicast stream to efficiently identify packets associated with the stream. It is 20 bits in length and is added by the CMTS to the packet header. Applying a MDSID to various packets allows cable modems to process only multicast and IP-VoD streams that are intended for transmission onwards to an IPTVCD connected to its home networking port. In addition to tagging network traffic and bonding multiple channels together DOCSIS 3.0 also accommodates support for multicast specific communications models and protocols such as:
. Source Specific Multicast (SSM)
. Version 3 of the Internet Group Membership Protocol (IGMP)
. Multicast Listener Discovery (MLD) versions 1 and 2 Chapter 4 presents further details on these technologies.

IPTV via DOCSIS

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Data over Cable Service Interface Specification (DOCSIS)
EuroDOCSIS, were originally designed to carry high speed Internet traffic over wide area networks. The specifications have evolved over the years and the latest version of DOCSIS provides enough capacity to support the delivery
of IPTV services across HFC networks. Both DOCSIS and EuroDOCSIS are discussed in the following sections.

Understanding DOCSIS Two-way cable TV networks deployed in the United States that support high speed broadband services are likely to use aspecification called DOCSIS. CableLabs, a research and development consortium of cable television system operators representing the Americas, developed thistechnology. The specification defines the protocols and modulation formats used fordelivering IP broadband services over a cable TV network.
The first revision of the technology, known as DOCSIS 1.0, was approved as astandard by the ITU in 1998 and there has been a family of DOCSIS specifications issued over the past 8 years. An overview of the technical features supported by each generation of DOCSIS technology is presented in Table 2.3. The DOCSIS versions through to DOCSIS 2.0 support a data rate of approximately 40 Mbps on the downstream. This bandwidth capacity is typically used to service a number of high speed Internet access subscribers. If this bandwidth was to be utilized for delivering IPTV multicast TV channels, then 10 or possibly 15 IPTV streams could be simultaneously provided on any individual downstream channel. This calculation is based on streaming 10 15 standard definition IPTV channels that each occupy between 2.5 and 4 Mbps of bandwidth.
Once the channels are configured, cable operators use a networking technique called multicasting (described in Chapter 3) to assign multiple users to each IPTVstream.
The latest release of the specification, DOCSIS 3.0, enhances cable TVoperator’s capacity for delivering IP-VoD and multicast IPTV services to theircustomers. DOCSIS 3.0 is a next generation wideband technology that allows cableoperators to bond multiple channels together to form high speed IP pipes capable ofoperating at hundreds of megabits per second. In North America 6 MHz channelsare bonded together whereas in Europe the channels bonded together have acapacity of 8 MHz. Note that the bonding feature applies to both upstream and downstream channels. Other features of DOCSIS 3.0 include its in-built support for next generation IPv6 management addresses. Advanced support for improved network security, IP multicasting and associated QoS mechanisms are all major features of DOCSIS 3.0.
Inside the DOCSIS 3.0 Technical Specification Before discussing the DOCSIS technical specification, it is first helpful to gain a high level understandingof how an end-to-end DOCSIS 3.0 system operates. A highly simplified graphical view of an end-to-end DOCSIS 3.0 system.
As illustrated the cable modem communicates via a two-way HFC network to adevice located in the headend called a Cable Modem Termination System (CMTS).
DOCSIS defines two variants of CMTSs an integrated CMTS and a modularCMTS. An integrated CMTS consists of a single unit with RF interfaces and upstream network interface(s). A modular CMTS (M-CMTS) implements the upstream RF interfaces and the network interface(s); however, the downstream traffic is processed separately via cable modulators. also includes a connection from the CMTS to network management system (NMS) and provisioning systems. The provisioning system comprises of a number of servers that provide different types of functionality including dynamic host configuration protocol (DHCP) IP address allocation, cable modem configuration parameters, and providing time services. The NMS provides monitoring andmanagement services to the end-to-end networking architecture. A connection to the Internet is also provided to facilitate subscriber’s access to high speed Internet access service. The DOCSIS 3.0 technology itself is a significant standard, which is broken down into four specifications.

Overview of HFC Technologies

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If a cable television network is available in a particular area, then consumers access IPTV from a network based on hybrid fiber/coax (HFC) technology. HFC technology refers to any network configuration of fiber-optic and coaxial cable that may be used to redistribute a variety of digital TV services. Most cable television companies are already using it. Networks built using HFC technology have many characteristics that make it ideal for handling the next generation of communication services:
. HFC networks are capable of simultaneously transmitting analog and digital services. This is extremely important for network operators who are rolling out digital and IPTV to their subscribers on a phased basis.

HFC meets the expandable capacity and reliability requirements of an IPTV system. The expandable capacity feature of HFC based systems allows network operators to add services incrementally without major changes to the overall network infrastructure.

. HFC is essentially a “pay as you go” architecture that matches infrastructure investment with new revenue streams, operational savings, and reliability enhancements.

. The physical characteristics of coaxial and fiber cables support a network operating at several gigabits per second.

The topology of an end-to-end HFC network is illustrated in Fig. 2.4. From the diagram we can see that the HFC network architecture consists of a fiber based backbone connected via an optical node to a coaxial network. The optical node acts as an interface that connects upstream and downstream signals that traverse the fiber optic network with the coaxial cabling. The coaxial portion of the HFC network uses tree-and-branch topology and is used to connect cable TV subscribers via specialized devices called taps to the HFC network. The digital TV signal is transmitted from the headend in a star-like fashion to the fiber nodes.
The fiber nodes, in turn, distribute the signals over coaxial cable, amplifiers, andtaps throughout the customer serving area.

Deploying IPTV Over a Cable TV Network
The debate within the cable TV industry to start carrying video traffic across an IPbased architecture continues as of this writing. Threats to their core pay TV FIGURE 2.4 End-to-end HFC network.
business from telecommunication operators combined with bandwidth efficienciesassociated with IP delivery mechanisms are two of the key factors drivingcable operators toward a more IP centric model of delivering video content to endusers.
Switching a radio frequency based network over to an IP based switched digitalvideo (SDV) environment however requires the installation of a number of newpieces of equipment ranging from routers to IP set-top boxes and high speednetworking switches. Some of the advantages associated with deploying an SDVenvironment include:

. A large amount of network bandwidth is freed up due to the fact that theoperator is only required to transmit a sing le TV channel to a subscribers IPset-top box. This contrasts quite sharply from traditional systems where allchannels on the operator’s lineup are broadcasted across the network andunused channels still occupy bandwidth.

. Spare bandwidth allows cable operators to deliver advanced IPTV content andservices to their subscribers.

. Cable operators can accurately measure and monitor what video content iswatched by each of its subscribers. This is an important feature for operatorswho want to generate revenue through advertising.

From a technical perspective a typical cable IPTV system constitutes a mix of IPand RF based hardware devices that are used to carry the video signal across the networking infrastructure. Figure 2.5 shows an example of a cable IPTV architecture that constitutes a mix of IP and RF technologies.
As shown the network consists of a variety of hardware devices, including

. GigE routers or switches Gigabit Ethernet (GigE) is emerging as thetransport protocol of choice for connecting IP networking components. GigE is typically used by higher capacity applications, such as VoD. The GigErouter aggregates the IPTV traffic and provides interconnectivity to the coreaccess network.

. Optical transport network The core network provides the network pathbetween the video servers in the headend and the modulators at the edge ofthe network.

Synchronous optical network (SONET), ATM, and dense wavelength division multiplexing (DWDM) are examples of technologies that maybe used in the core network.

. Edge modulators The modulators located at the regional offices receive theIPTV content from the core network, convert the video content from IP basedpackets to RF, and distribute it via an HFC network to the set-top boxes.

This is an example of a large scale cable IPTV deployment and makes use of ahierarchical architecture through the establishment of regional distribution headends.

VDSL

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VDSL (Very high speed Digital Subscriber Lines) is based on the same underlying technology as ADSL2þ. It is the newest and most sophisticated DSL technology at the time of writing and was developed to overcome the shortcomings of previous versions of ADSL access technologies. It eliminates last mile bottlenecks and supports huge data rate capacities that allow service providers to comfortably offer IPTV subscribers a whole range of services including video-on-demand and Multichannel high definition TV broadcasting. VDSL was also designed to support the transmission of ATM and IP based traffic over the copper loop that is very useful for providers who want to migrate their legacy ATM networks over to an IP based infrastructure. There are different members of the VDSL family:
VDSL1 This avor of DSL was ratified in 2004. It operates at an upper limit of 55 Mbps on the downstream and 15 Mbps over the upstream channel. It does however have a very short range and is typically installed inside multiple dwelling units (MDUs).
VDSL2 is an enhancement to VDSL1 and is defined in the ITU-T Recommendation G.993.2. It can be subclassified into VDSL2 (Long Reach)and VDSL2 (Short Reach)VDSL2 (Long Reach) Owing to the fact that DSL is highly dependent upon the length of the local loop, a version of VDSL was created to deliver VDSL services to as many customers as possible, while enjoying the high speed capacities of the VDSL broadband access platform. In fact, VDSL with enhanced long reach capabilities can provide IPTV subscribers with 30 Mbps broadband access at distances of between 4000 and 5000 ft (1.2 1.5 km) away from the CO. These long reach capabilities are enabled through theuse of relatively high power levels when transmitting the data. A frequency spectrum of 30 MHz is used to achieve the high throughput as opposed to the 12 MHz frequency range used by VDSL1. Advanced error correction mechanisms also help to improve the reliability of end-to-end VDSL2 connections.
VDSL2 (Short Reach) Based on the DMT modulation scheme, the technology uses 4096 tones that are separated by 4 and 8 KHz frequency bands. The VDSL2 standard uses channel bonding techniques to allow it to operate at over 12 times the speed of the original ADSL standard, namely, a massive 100 Mbps on the downstream path over relatively short distances approximately 1200 ft (350 m). Although the data rates do not typically reach 100 Mbps on the upstream path, the rates do exceed the upstream data rates of ADSL2þ based networks. These performance levels make the assumption that no interference exists on the copper cable and the quality of the cable is quite good. The ability to provide IPTV subscribers with 100 Mbps access service enable operators to start offering a wide variety of advanced interactive services to their customers.
New VDSL2 features such as advanced quality of service (QoS), dual latency(the ability to segregate delay sensitive data such as IPTV multicast traffic), andimproved coding techniques are all particularly suited to the delivery of triple-playapplications. One final benefit of VDSL that solidifies its position as the ultimateDSL technology is its backward compatibility and interoperability with previous versions of ADSL networks. This enables IPTV providers to smoothly andefficiently migrate to VDSL based next generation networks.
There are two main approaches used by IPTV service providers when integrating VDSL2 into their existing networking infrastructure. The first approach is to add new VDSL2 equipment to the regional office and allow the DSLAM run in parallel to the existing ADSL and ADSL2 DSLAM systems. The various avors of DSL will subsequently continue in operation. The second approach is to locate the VDSL2 equipment closer to the IPTV subscriber. Possible locations for the new euipment include street cabinets and underground chambers for new housingestates. Table 2.2 provides a feature comparison between the various avors of DSLtechnologies used to carry IPTV signals.
The main advantage of DSL for IPTV systems is the fact that it utilizes theexisting wires that already run into most houses around the world. On the negativeside, all DSL systems have to make a trade-off between distance and bandwidthcapacity. In other words the DSL access speed reduces as the distance between theIPTV subscriber and the CO increases.Cable TV operators have mad e significant investments in the recent past to upgradetheir networks to support advanced communication services such as IPTV. Tounderstand the delivery of IPTV content over a cable TV network and to put thetechnology in context, it is first necessary at least on a high level, to review thebasic principles of hybrid fiber networks and traditional digital TV transmissiontechnologies.

IP over ADSL equipment architecture

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DSLAM
DSLAM stands for Digital Subscriber Line Access Multiplexer. At the IPTV service provider’s regional office, the DSLAM receives the subscriber’s connections over copper cable, aggregates them, and connects back to the central IPTV data center through a high speed fiber based network backbone. For IPTV deployments it is typical that the DSLAM supports multicast transmission. This negates the need to replicate channels for each request originating from an IPTV viewer. The DSLAM has overall responsibility for distributing the IPTV content over the “last mile” to theIPTV subscribers. DSLAMs fall into two broad categories: Layer 2 and IP-aware.

(1) Layer 2 DSLAMs operate at level 2 of the OSI (Open Systems Interconnection) communications model and perform functions such as switching traffic between Ethernet and ATM, passing network traffic up-stream, and preventing interference between IPTV subscribers. The switching between ATM virtual circuits and upstream Ethernet packets is facilitated through the used of a bridging mechanism.

(2) IP-awa re DSLAMs include limited support for level 3 IP networking protocols. Advanced functions supported by this category of DSLAMs include replication of broadcast TV channels and executing channel changing instructions.
ADSL technology is ideal for a range of different interactive services; however,
it is not an optimal solution for delivering IPTV content due to the following
reasons:
Data rates An ADSL maxes out at around 8 Mbps, which supports two SD channels and some Internet traffic; however, it will not be able to meet the needs of IPTV providers who plan to deliver high definition programming to their subscribers.
Interactivity Because the technology is ADSL the upload data rate is lower than the download rate. This limitation means that ADSL is unsuitable for applications, such as peer-to-peer services that require as much upstream bandwidth as download bandwidth. Therefore, network service providers are starting to deploy more advanced DSL technologies that overcome these limitations.

ADSL2
The ADSL2 family of standards was created to address the increased demand for capacity to support bandwidth intensive applications such as IPTV. There are three different members of the ADSL2 family:
ADSL2 The initial version of ADSL2 was approved by the ITU in 2003 and included a number of enhancements to the original ADSL standard, namely, higher downstream data rates and longer reach from the CO to the subscriber’s modem.
ADSL2þ Soon after the standardization of ADSL2, a further avor of DSL was adopted by ITU known as ADSL2þ. This standard builds on ADSL2 and allows network service providers to offer speeds up to 20 Mbps to subscribers that live within a distance of approximately 5000 ft (1.5 km) away from the CO. ADSL2þ operates within the signal bandwidth of 138 kHz to 2.208 MHz.
ADSL-Reach Extended The deployment of ADSL2þ is of no use to subscribers who live over 5000 ft (1.5 km) away from the CO. Therefore, a technology called ADSL-Reach Extended, also known as RE-ADSL2 was standardized in 2003 to allow IPTV service providers to increase the range of their offerings to subscribers that are up to 19,000 ft (6 km) away from their nearest CO. It shows good performance in terms of reach and speed over long copper lines.

ADSL

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Asymmetric Digital Subscriber Line (ADSL) is currently the most popular avor of DSL deployed on telecommunication networks across the globe. It has made considerable in roads into the residential market, where it competes with cable modems for customers who are looking for high speed broadband connections.
ADSL is a point-to-point connection technology. This feature enables telecommunication providers to deliver high bandwidth services such as IP video over existing copper telephone lines. It is called “asymmetric” because the transmission of information from the central data center or regional office to an IPTVCD is quicker than information traveling from the IPTVCD to the central data center. Also the point-to-point characteristic of ADSL eliminates the variation in bandwidth capacities of a shared networking environment.By using specialized techniques, ADSL typically allows a downstream rate of 8 Mbps and an upstream rate of 1.5 Mbps. Therefore, one ADSL connection issufficient to simultaneously support two MPEG-2 standard definition broadcast TVchannels and a high speed Internet connection.

ADSL’s major drawback is that the availability is contingent upon distance from the service provider’s central office (CO). ADSL is a distance sensitive technology so subscribers near the central data center receive a better quality service than those further away. A basic ADSL service is limited to homes within 18,000 ft from the nearest telephone exchange or regional office. From a technical perspective, telephone lines were originally designed to support the transmission of low frequency voice traffic. Traffic that is sent over a telephone line at high frequencies normally experiences distortion and interference.
Apportioning the bandwidth of a telephone line helps to minimize interference and increase data rates. The frequency allocation of an ADSL circuit reserves the lower 4 kHz for the existing telephone service, while upstream and downstream data channels occupies 26 kHz to 1.1 MHz frequency range.

ADSL equipment provides a digital connection over the PSTN network;
however, the signal that is transmitted across this connection is modulated as an analog signal. ADSL circuits must use analog signaling because the local loop network is not capable of carrying signals coded in digital format. Thus, the modem at the IPTV data center is responsible for converting digital data into analog signals that are acceptable for transmission. The residential modem, which connects to the IPTVCD then converts the analogue signals that are received over the ADSL circuit back into appropriate digital signals.

The two primary techniques used to modulate digital IPTV data into an analog signal for ADSL transmission are carrier less amplitude and phase (CAP) and discrete multitone (DMT).
CAP was the original approach used to modulate digital data into an analog signal for ADSL transmission. While the name specifies that the modulation is “carrierless” an actual single carrier is used to transmit the data over the telephone network. CAP is closely related to the quadrature amplitude modulation (QAM) scheme. QAM is a very well understood andwidely used technique, common in satellite and cable TV applications formany years.
DMT Discrete multitone is now the preferred modulation alternative over CAP and is used in by modern DSL technologies. It separates the DSL signal frequency range into a number of small subchannels or frequency tones. During transmission, each one of these subchannels carries a portion of the total data rate. By dividing the transmission bandwidth into a collection of subchannels, DMT is able to adapt to the distinct characteristics of each telephone line and maximize the data transmission rate. DMT is closelyrelated to Orthogonal Frequency Division Multiplexing (OFDM) or C-OFDM(Coded OFDM). COFDM is used by Europe’s DVB television standards.

ADSL Equipment ADSL is a technology that can expand the useablebandwidth of existing copper telephone lines. The equipment used in the installation of a full rate ADSL service is shown in Fig. 2.3, and consists of thefollowing:
An ADSL modem In the subscriber’s home there is an ADSL transceiver or modem. The modem connects usually through an USB or Ethernet connection, from the home network or PC to the DSL line. Many modems nowadays also incorporate a router, which supports high speed Internet access and data services.
A POTS splitter Users getting connected to the Internet with an ADSL broadband connection use a device called a POTS splitter to separate the datasignals from the voice signals. The splitter divides the incoming signal into

IPTV Over An ADSL Network

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IPTV Over An ADSL Network
In the last couple of years a number of telephone companies (telcos) in differentparts of the world have entered the IPTV market. Their entrance into this markethas been driven by efforts to counteract the threat to their revenue streams fromcable television operators and wireless broadband providers who have started tooffer a variety of Internet access and telephone services. In response to thischallenge, the telcos are taking advantage of their DSL based networkinginfrastructure to start rolling out next generation television services to theirsubscribers. Note that DSL is a technology that enables telecommunicationproviders to deliver high bandwidth services over existing copper telephone lines. Ittransforms the existing telephone cabling infrastructure between a local telepho neexchange and a customer’s telephone socket into a high speed digital line. Thiscapability allows telepho ne companies to use their existing networks to providemultiple high speed Internet data services to their subscriber bases.Bandwidth is a key issue in the delivery of next generation IPTV services. Thisis particularly true for the local DSL loop. Many of the existing DSL basedbroadband networks are built around legacy DSL standards, which are simply notcapable of meeting the growing demand to support high speed video services. Mostof these networks are restricted to delivering one IP video stream to eachhousehold. In some cases it is impossible to send a standard quality TV signal overthese DSL access networks. The boost in performance required for IPTV can

however be achieved through the deployment of DSL technologies such as ADSL,ADSL2þ, and VDSL. An overview of the features of these technologies and howthey work is provided in the following sections.

IPTV FTTH Network using PON Technologies

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IPTV FTTH network using PON
technologies multiplexers (WDMs) are installed at the data center and inside the OLT that allow a PON to support the transmission of multiple parallel channels or wavelengths on the one piece of fiber. Thus, creating a number of virtual fiber channels over a single fiber pair. Under WDM, the capacity of the network is increased by assigning signals that originate from optical sources to specific wavelengths on the optical transmission spectrum.
There are various avors of PON technologies including BPON, EPON, and GPON that support both traditional RF based TV services and IPTV. Details of each of these technologies are provided in the following sections.

BPON Broadband PON or BPON is based on the ITU-T G.983 specification. This type of FTTx networking topology supports data rates up to 622 Mbps downstream and up to 155 Mbps on the upstream. Thus, it is anasymmetrical transmission method. In other words, the downstream communication speeds are higher than the upstream speeds. This is because downstream data ows in a point-to-point fashion between the OLT and each ONT whereas on the upstream each ONT is given a time slot for transmitting data. Assigning time slots reduces the possibility of traffic collisions between ONTs on the network; however,it reduces the overall data rates of the upstream communication channel. Note that BPONs can also be configured to support symmetrical data traffic.BPON uses Asynchronous Transfer Mode (ATM) as the bearer protocol. ATM based networks are quite popular for delivering h igh speed data, voice, and video applications. ATM is a cell relay technology capable of very high speeds. It dividesall information to be transferred into blocks called cells. These cells are fixed insize each has a header with 5 bytes and an information field containing 48 bytes of data. The information field of an ATM cell carries the IPTV content, whereas theheader contains information relevant to the functioning of the ATM protocol.
ATM is classified as a connection orientated protocol. In other words, aconnection between the receiver and the transmitter is established prior to sending the IP video data over the network. The ability to reserve bandwidth to time sensitive applications is another feature of ATM networks. This is particularly auseful feature for delivering IPTV services. The allocation of specific channels to different services helps to remove interference.

EPON Ethernet PON or EPON is an optical based access technology that was developed by a subgroup of the IEEE called the Ethernet in the first mile (EFM) group and adopted as a standard in 2004. As the name suggests this avor of PON uses Ethernet as its transport mechanism. The rates supported depend on the distances between the OLT and ONTs. Note EPONs only support Ethernet network traffic.

GPON Gigabit PON or GPON is an optical access system, which is based on the ITU-T G.984 specification. GPON is basically an upgrade to the BPON specification and includes support for
. higher transmission rates downstream rates of 2.5 Gbits/s and upstream rates of 1.5 Gbits/s
. This rate is achievable at distances up to 20 km.
. protocols such as Ethernet, ATM, and SONET are supported.
. enhanced security features.
The multiprotocol support provided b y GPONs allows network operators to continue providing customers with traditional telecommunication services, while also having the facility to introduce new services such as IPTV onto their networking infrastructure. Table 2.1 summarizes the characteristics of the various PON technologies used to carry IPTV signals.
In addition to their extensive use by FTTx networks, PONs can also serve as a backbone infrastructure for hybrid fiber coaxial and wireless broadcast networks.With regard to the future development of PON technology the Full Service Access Network (FSAN) consortium in conjunction with the IEEE continue to develop next generation PON technologies. At the time of writing exploratory work.
PON Technologies Comparison: BPON, EPON, and GPON
ITU-T Specification Data Rates Transmission Protocol
BPON G.983 622 Mbps downstreamPrimarily
ATM however IPand 155 Mbps upstream
over Ethernet will alsooperate on the network
GPON G.984 2.5 Gbps downstream
Ethernet and SONET
and 1.5 Gbps upstream
EPON P802.3ah 1.25 Gbps downstream
Gigabit Ethernet
and 1.25 Gbps upstream
has commenced on two possible successors to GPON wavelength division
multiplexing passive optical network (WDM-PON) and 10G-PON.

IPTV Over A Fiber Access Network

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Increasing demand for bandwidth combined with lower operating costs and immunity to electromagnetic interference are some of the factors that are driving deployments of optical fiber based access networks. Networks that utilize fiber have been used by multiple service operators (MSOs) to build networks for decades. Owing to a recent reduction in equipment and deployment costs over the last couple of years, interest in using fiber based networks to deliver emerging IP based services such as IPTV has risen dramatically. Additionally, fiber links provide end consumers with a dedicated connection, which is well suited to the delivery ofIPTV content. Bringing fiber technologies and higher bandwidth capabilities nearer to the user can be implemented using one of the following network architectures:
Fiber to the regional office (FTTRO) This refers to the installation of fiber from the IPTV data center to the nearest regional office owned by the telecommunication or cable company. Existing copper wiring is then usedto carry signals from the regional office to the IPTV end user.Fiber to the neighborhood (FTTN) Also known as fiber to the node, FTTN entails installing fiber from the IPTV data center to a neighborhood splitter.This node is generally located less than 5000 ft away from the subscriber.Another mechanism such as digital subscriber line (DSL) over copper wire is then utilized to make the final link to the customer. The deployment of FTTN allows end users to receive a complete bundle of pay services, including IP based TV, high definition TV, and video on demand (VoD). Fiber to the curb (FTTC) An FTTC networking infrastructure involves the installation of optical fiber to within a thousand feet of a home or a business.A coaxial cable or copper wire is typically used to establish the connection from the optical cable terminated in a cabinet located at a “curb” on a street to the residential gateway located in the IPTV subscriber’s premises. This configuration is typically installed during the construction of a new housing development.
Fiber to the home (FTTH) With fiber to the home, the entire route from theIPTV data center to within the home is connected by optical fiber. FTTH based optical networks are capable of delivering very high volumes of data toend users of the system. This architecture is also quite popular for new construction sites, since the trenches are cut and the cost of laying fiber is relatively similar to installing copper cables. FTTH is a full-duplex communication system and supports the interactive nature of IPTV services.
Fiber to the apartment (FTTA) The deployment of an FTTA network entails the installation of a number of fiber cables between a central gateway hub,typically located in the basement of an apartment block and each individual apartment.The delivery of these architectures is typically enabled through two different variants PON and AON.shows how the basic PON networking infrastructure could be built to support the delivery of IPTV and high speed Internet services to six different households.As shown, a single piece of optical fiber is run from the backend office to anoptical splitter, which is typically located in close proximity to the subscriber’s house. The bandwidth on this fiber is typically shared and is capable of supporting high bandwidth capacities ranging from 622 Mbps all the way up to several gigabytes of data per second.In addition to the physical components of a PON Fig. 2.1 also illustrates the transmission of three different light wavelengths (channels) over the network. The first wavelength is used to carry high speed Internet traffic. The second wavelength is allocated to carry IP video services and the third wavelength may be used to carry interactive traffic from the subscriber’s home network back to the service prov ider’sbackend equipment. As shown specialized filters called wavelength division.

IPTV Network Distribution Technologies

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IPTV Network Distribution Technologies

It looks like IPTV is well on its way to becoming a popular means for delivering digital TV services to consumers. Owing to the nature of IPTV, a high speed distribution networking platform is required to underpin the delivery of IP based content. The purpose of this network is to move bits of data back and forth between the IPTV consumer device and the service provider’s IPTV data center. It needs to do this in a manner that does not affect the quality of the video stream delivered to the IPTV subscriber, and it is up to each IPTV provider to decide on the type and sophistication of the network architecture required to support their IPTV services.

An IPTV network architecture consists of two parts the “last mile” broadband distribution and the centralized or core backbone. A wide variety of networks,including cable systems, copper telephone, wireless, and satellite networks may beused to deliver advanced IPTV network services over the last mile section of thenetwork. The delivery of video over all of these different types of networks comes with its own set of challenges. The majority of this chapter focuses on describing these key technology platforms. Once the key delivery platforms are covered, the chapter concludes with an analysis of the two primary core networking technologies and a brief overview of network factors that affect the deploymentof IPTV services.

“LAST MILE” Broadband Distribution Network Types.

One of the primary challenges faced by IPTV service providers is providing enough bandwidth capacity in the network segment that lies between the core backbone and the end-users’ home. A number of terms are used to describe this segment ranging from local loop and last mile to edge and broadband access network. There are six different types of broadband access networks that are scalable enough to meet the bandwidth requirements of IPTV:

. Through a network built with fiber
. Via an DSL network
. Via a cable TV network
. Via a satellite based network
. Via a fixed wireless broadband connection
. Via the Internet
Different service providers operate each system. The following sections give a technical overview of these platforms when used in an IPTV end-to-end networking infrastructure.

DVB-IPI Protocol Framework

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DVB-IPI
To develop standards for the transmission of digital TV services over IP broadband networks the DVB organization has formed a group called the DVB Technical Module Ad Hoc Group on IP Infrastructure (DVB-IPI). The goal of the IPI group is to specify technologies that allow consumers to purchase a DVB-IP set-top box in any shop, connect it to a broadband network, switch it on and, without further ado start to receive DVB services over IP based networks.

IPTV Principles Put Forward by the CEA and a Number of U.S. Telecom Operators.
Principle name Description Nationwide compatibility This principle aspires to defining a set of a nationwide (United States initially) common protocols that allow consumer electronics (CE) manufacturers to manufacture devices, which will interoperate with all home networks that run IPTV services.
Open standards The establishment of a forum that drives the adoption of open standards for the sector. Reasonable licensing terms This principle hopes to introduce reasonable and non-discriminatory licensing terms that allow CE manufacturers and video service providers toinclude improved features in their IP based products. Reasonable testing and As the name suggests the group is planning to establish certification procedures a testing and certification process for products that support IPTV services.

Reasonable terms of service
This fifth and final principle aims to provide consumers for a choice when deciding on a digital device to accesstheir IPTV services.

Digital TV technology offers fundamental improvements over analog services. For instance, where an analog signal degrades with distance, the digital signal will remain constant and perfect as long as it can be received. The advent of digital TV benefits the general public because of crystal clear pictures, CD quality sound and access to a range of new entertainment services. Depending on geographical location, analog television systems are based on either NTSC, PAL or SECAM standards. There are two main global digital TV standards, namely, DVB and ATSC. By using digital technologies to transmit television, service providers can carry more information than is possible with analog systems.

IPTV is a new method of transporting digital TV content over a network and is seen as part of the larger triple-play bundle that is typically on offer from network operators worldwide. IPTV is a term that describes a system that enables the delivery of real-time television programs, movies, and other types of interactive video content over an IP based network. Consumers often do not realize that behind the simple end-user IPTV environment is a series of powerful components that seamlessly work together to make the delivery of TVover broadband networks possible. These components or subsystems include the processing of video, security, and the delivery platform. An end-to-end IP video network infrastructure can include some or all of the following elements:

. The IPTV data center that is responsible for processing and preparing content for distribution across a broadband network.

. An IPTV distribution network consisting of a mix o f technologies that carry IPTV content from the data center to end users.

. IP digital set-top boxes or residen tial gateways that are installed at the subscribers home and provide connectivity between the TV and the IP based access network.

. A home network enabling the distribution of data, voice, and video between different consumer devices.

A recipe of increased broadband adoption combined with advancements in compressio n technologies and the need for telecommunication companies to offer video services to their customers is helping to grow the size of the global IPTV marketplace. A number of organizations are involved in developing technological standards and products to encourage consumers to adopt IPTV services, including SARFT, ITU, ATIS, DVB, IPDR, ISMA, and the CEA.

Industry Initiatives to Standardize IPTV Part.2

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Broadband Services Forum (BSF)
According to the organization’s Website “BSF is an international industry resource that provides a forum for dialogue and development, along with the tools and information to address the fundamental business and technology issues vital to the vgrowth and health of the broadband industry.” This consortium of companies has a particular focus with regard to IPTV and is promoting the industry through its participation in various industry conferences and trade shows.

WirelessHD Consortium
The WirelessHD Consortium is a group of technology and consumer electronics companies that were formed in 2006. At the time of writing the group had started towork on a wireless digital interface specification that sends uncompressed IP- and RF-based high definition (HD) TV to HD display panels. When completed the technology is to be incorporated into a range of audio video equipment types including IP set-top boxes and HD at panel displays.

State Administration of Radio, Film, and Television (SARFT)
The Chinese state run organization SARFT in conjunction with the Ministry of Information is responsible in China for issuing standards related to the deployment of IPTV technologies in the country.

ITU-T FG IPTV
ITU established a focus group on IPTV, known as theIPTV FG, to coordinate and promote the development of global IPTV standards.The group is concentrating its energies in five key areas:
. Architecture
. DRM
. Quality of Service (QoS) metrics
. Metadata
. Interoperability and test

The Alliance for Telecommunications Industry Standard (ATIS)
ATIS is a telecom industry organization that includes more than 350 companies including the major service providers. To further the standardization work for theIPTV industry sector, ATIS has launched a subgroup called the IPTV InteroperabilityForum (IIF). According to the group’s Web site, the primary remit of the IIF is to produce an overall reference architecture for deploying IPTV services, which focuses on four major areas, infrastructure equipment, content security, interoperability testing, and quality of service. The company recently published a number of guidelines in the areas of IPTV digital rights management and architecture:

. ATIS-0800001: This document defines the interoperability specifications associated with implementing IPTV DRM systems. The organization plans to use this document as a basis for creating an IPTV DRM/security interoperability specification in the future.
. ATIS-0800002: This document provides guidelines to content and service providers on the architecture required to deliver IPTV services.
. ATIS-0800003: Published in 2006, this document sets out a roadmap consisting of a number of phases for standardizing the architecture of IPTV systems.
. ATIS-0800004: This document defines a framework for monitoring QoS metrics for various types of IPTV services.
. ATIS-0800005: This technical document covers the topic of packet loss across IPTV networking infrastructures. In addition to identifying the various causes of packet loss the document also provides readers with a set of recommendations with regard to reducing the impact of packet losses in a live IPTV networking environment

The organization has agreed to share these documents with other IPTV standard organizations such as the ITU-T FG IPTV to ensure interoperability between the various technologies. The organization also has plans to establish a certification process for IPTV hardware and software vendors in the future.

The Internet Protocol Detail Record Organization (IPDR)
IPDR.org is an industry consortium of service providers, and equipment suppliers exclusively focused on developing and driving the adoption of next generation IP service usage exchange standards worldwide. This organization has taken on the responsibility of defining interoperability standards for IPTV billing, network management, and back-office systems.

Internet Streaming Media Alliance (ISMA)
Founded in the year 2000, ISMA is a nonprofit industry alliance of companies, and since its inception, it has received wide industry support. Its mission is to facilitate and promote the adoption of an open architecture for streaming audio and video over IP networks. The organization has developed a number of specifications ranging from improving the channel changing times for IPTV systems to synchronizing graphics and data with streaming video content. All of its specifications produced to date make extensive use of open Internet standards that have been produced by the IETF.

Industry Initiatives to Standardize IPTV

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Worldwide IPTV forecast subscriber forecast, TVMentors estimated that approximately 4.8 million households around the world subscribed to an IPTV service. TVMentors forecasts that the number of households around the world subscribing to IPTV services offered by network carriers will reach 37.4 million in 2010.Over the long term, the number of households adopting IPTV will grow at a steady pace. In fact, TVMentors believes that IPTV services will start to become a mainstream product in 2009.
Note that TVMentors publishes reports and databases, periodically and methodologically, to forecast the number of global IPTV subscribers on an ongoing basis. Readers who are interested in this level of information are encouraged to visit www.tvmentors.com for further details.
Similar to the cable and satellite pay TV sectors, the IPTV industry sector also requires a set of standards that will promote competition, lower costs for subscribers, minimize confusion in the market, and improve the delivery of compelling IPTV services. Standardizing IPTV is not an easy task because there are a whole range of components and systems from different vendors involved with building an end-to-end IPTV system. However, as with any emerging technology, a number of standard bodies and industry consortiums have got involved in standardizing IPTV.

Parts of MPEG-E Standard
Part Number Part (API) Description
ISO/IEC 23004-1 Architecture
ISO/IEC 23004-2 Multimedia API
ISO/IEC 23004-3 Component model
ISO/IEC 23004-4 Resource and quality management
ISO/IEC 23004-5 Component download
ISO/IEC 23004-6 Fault management
ISO/IEC 23004-7 System integrity management
ISO/IEC 23004-8 Reference software
number of recommendations, which are particularly relevant to the IPTV industry sector. For more information visit www.dslforum.org.
The Moving Picture Experts Group is a working group of ISO/IEC in charge of the development of international standards for compression, decompression, processing, and coded representation of moving pictures, audio, and their combination. The MPEG group is progressing a number of specifications that are relevant to IPTV. In addition to the various video coding standards, the group has also developed the multimedia middleware ISO/IEC 23004 (MPEG-E M3W) standard. MPEG-E comprises a number of application program interfaces (APIs), which are defined in eight separate parts.

European Telecommunications Standards Institute (ETSI) ETSI formed a group called Telecoms & Internet converged Services & Protocols for Advanced Networks (TISPAN) in 2003 to develop specifications for next generation wireless and fixed networking infrastructures. TISPAN in turn has structured itself into groups that work to deliver specifications on topics that are particularly important to the IPTV industry sector ranging from home networks and security to network management and addressing. For more information on the various TISPAN specifications visit www.etsi.org/tispan/.

Video on Demand (VoD)

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Video on Demand (VoD)
In addition to allowing telecommunication companies to deliver linear TV channelsto their subscribers, IPTV provides access to a wide range of downloadable andVoD based content. In contrast to traditional TV services where video programs arebroadcasted according to a preset schedule, VoD provides IPTV end users with theability to select, download, and view content at their convenience. The contentdelivered through an IPTV VoD application typically includes a library of on-demand movie titles and a selection of stored programming content.Facilitating access for VoD is a pretty major challenge for all telecommunicationcompanies. For a start, broadband subscribers that regularly access on-demandcontent consume huge amounts of bandwidth. On top of this the server architecturerequired to stream video content to multiple subscribers is quite large.

The Digitization of Television
Most satellite, terrestrial, and cable TV providers have started to switch their delivery platforms from analog over to digital. In addition, most if not all of the video production studios are using digital technologies to record and store content. These factors have negated the need to support legacy analog technologies and encouraged the adoption of IP based video content. Enhancements in Compression TechnologiesThe delivery of video content over an IP network is nothing new, with a number of Internet streaming video sites in operation for a number of years, at this stage. Traditionally, the quality of the material streamed over the Internet was poor due to limited bandwidth capacities. Increasing numbers of broadband subscribers combined with improvements in compression techniques for digital video content has in recent years changed the whole dynamic of sending TV content over IP connections.

Business and Commercial Drivers
Increased competition combined with declining revenue streams is forcing many telecommunication companies to start the process of offering IPTV services to their subscribers. These new IPTV services typically extend the current broadband, and telephony offerings to form a product bundle called a triple play. For both fixed andwireless telecommunication companies, the triple-play bundle of IP based productsis identified as being a key part of growing their businesses in the years ahead.

Growth in Broadband Use
The pervasiveness of the Internet has brought the need for high speed, always on Internet, access to the home. This need is being satisfied through broadband access technologies such as digital subscriber line (DSL), cable, fiber, and fixed wireless networks. The adoption of broadband Internet access by many househ olds in turn has become a very powerful motivation for consumers to start subscribing to IPTV services.

Emergence of Integrated Digital Homes
People’s homes and lifestyles are evolving and undergoing a number of positivechanges. Many of these changes are underpinned by a range of new technologiesthat are helping to make life easier in addition to keeping consumers entertained. Digital entertainment devices such as gaming consoles, multiroom audio systems,digital set-top boxes, and at screen televisions are quite common. In addition, thedramatic reduction in the costs of PCs is increasing the number of households thatown multiple PCs. All of these technologies have finally spawned the emergence ofa number of households that can be classified as “digital homes.” The increase inthese types of homes has started to drive demand for whole home media networking (WHMN) services such as IPTV.
A Wide Range of Companies Are Deploying IPTVIn addition to allowing traditional telepho ne companies to add video services to their product portfolios, IPTV also allows satellite and terrestrial companies to provide their customer bases with IP based pay TV services. The Migration of Standard Definition (SD) Television to High

Definition TV (HDTV)
HDTV has finally arrived and is here to stay. Demand from consumers is exploding, and improved adoption of digital networking technologies is enticing multiservice network operators to start offering HDTV channels to their program lineup. Additionally, the delivery of HDTV over IP broadband networks is now a required option for telecommunication providers. The simultaneous combination of all of these drivers has made IPTV a practical reality that is both commercially and technologically successful.

Benefits of Digital TV Transmissions

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Benefits of Digital TV Transmissions When compared to analogtechnology the broadcasting of television in computer data format provides digitalTV viewers and service providers with a number of benefits.Improved Viewing Experience The viewing experience is improved throughcinema quality pictures, CD quality sound, hundreds of new channels, the powerto switch camera angles, and improved access to a range of exciting new entertainment services, additionally, any of the picture saw that are present in analogsystems are absent in the new digital environment. Improved Coverage Both analog and digital signals get weaker with distance. However, while the picture on an analog TV system slowly gets worse for viewersthat live long distances away from the broadcaster, a picture on a digital system willstay perfect until the signal becomes too weak to receive.Increased Capacity and New Service Offerings By using digital technologies totransmit television, service providers can carry more information than is currentlypossible with analog systems. With digital TV, a movie is compressed to occupyjust a tiny percentage of the bandwidth normally required by analog systems tobroadcast the same movie. The remaining bandwidth can then be filled withprogramming or data services such as

. Video on demand (VoD)
. E-mail and Internet services
. Interactive education
. Interactive TV commerce

Increased Access Flexibility Traditionally, it was only possible to view broadcastquality analog content on a TV set. With the introdu ction of digital technologies,video is accessible on a whole range of devices ranging from mobile phones tostandard PCs.
Note that eventually, all analog systems will be replaced with digital TV. Thetransition from analog to digital will be gradual to allow service providers toupgrade their transmission networks and for manufacturers to mass produce digitalproducts for the buying public. In development for more than a decade, the digitalTV system that has evolved today is the direct result of work by scientists,technologists, broadcasters, manufacturers, and a number of international standardbodies. Till a couple of years ago it was only practical to use radio frequency (RF)based signal technologies to deliver digital TV to consumers. Recent advancementsin compression and broadband technologies are however changing this situation,and many service providers have started to use IP based networks to deliver broadcast digital TV services to their customers.

IPTV: The Evolution of TV

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The evolution of TV
Some of the best-known international organizations that contribute to thestandardization of digital television include:
. ATSC
. DVB
. Association of Radio Industries and Businesses (ARIB)

ATSC The ATSC is an organization that was formed to establish a set of technical standards for broadcasting television signals in the United States. ATSC digital TVstandards cover a number of different key broadcasting techniques including the delivery of high definition, standard definition, and satellite direct-to-home signals to homes acro ss the United States. The ATSC was formed in 1982 by the member organizations of the Joint Committee on Intersociety Coordination (JCIC):

(EIA)Electronic Industries Association
(IEEE)Institute of Electrical and Electronic Engineers
(NAB) National Association of Broadcasters
(NCTA)National Cable and Telecommunications Association
(SMPTE)Society of MotionPicture and Television Engineers

Currently, there are approximately 200members representing the broadcast, broadcast equipment, motion picture, consumer electronics, computer, cable, satellite, and semiconductor industries. ATSChas been formally adopted in the United States where an aggressive implementationof digital TV has already begun. Additionally, Canada, South Korea, Taiwan, andArgentina have agreed to use the formats and transmission methods recommendedby the group. For more information on the various standards and specificationsproduced by this organization visit www.atsc.org.

DVB The DVB project was conceived in 1991 and was formally inaugurated in 1993 with approximately 80 members. Today DVB is a consortium of around 300 companies in the fields of broadcasting, manufacturing, network operation, andregulatory matters that have come together to establish common international standards for the move from analog to digital broadcastin g. The work of the DVB project has resulted in a comprehensive list of standards and specifications that describe solutions for implementing digital television in a variety of different environments. The DVB standards cover all aspects of digital television from transmission through interfacing, security and interactivity for digital video, audio, and data.

Because DVB standards are open, all the manufacturers making compliant systems are able to guarantee that their digital TV equipment will work with other manufacturers’ equipment. To date, there are numerous broadcast services around the world using DVB standards. There are hundreds of manufacturers offering DVB compliant equipment, which are already in use around the world. DVB has its greatest success in Europe; however, the standard has its implementations in North and South America, China, Africa, Asia, and Australia. For more information on the various standards and specifications produced by this organization visit www.dvb.org.
ARIB As per the organization’s Web site, ARIB conducts studies and researchand development, establishes standards, provides consultation services for radio spectrum coordination, cooperates with other overseas organizations, and provides frequency change support services for the smooth introduction of digital terrestrial television broad casting. The organization has produced a number of standards that are particularly relevant to the digital TV sector, including the video coding, audio coding, and multiplexing specifications for digital broadcasting (ARIB STD-B32).
For more information on the various standards and specifications produced by this organization visit http://www.arib.or.jp/english/.

Key IPTV Applications And Services - Part 2

2009-05-01T11:13:00.000-07:00

The first color televisions with integrated digital signal processing technologies were marketed in 1983. At a meeting hosted in 1993, the Moving Picture ExpertsGroup (MPEG) completed a definition of MPEG-2 Video, MPEG-2 Audio, andMPEG-2 Systems.

Also in 1993, the European Digital Video Broadcasting (DVB) project was born. In 1996, the FCC established digital television transmission standards in the United States by adopting the Advanced Television Systems Committee (ATSC) digital standard. As of 1999, many communication mediums have transitioned to digital technology. In recent years, a number of countries have started to launch standard definition and high definition TV services and are acting as the primary driving force behind a new type of television systems liquid crystal display (LCD) panels and plasma display panels (PDPs). A summary of significant historical TV developments is shown in Table 1.1 and illustrated in Fig. 1.2.

DTV Formatting Standards The standard for broadcasting analogtelevision in most of North America is NTSC. The standard for video in otherparts of the world are PAL and SECAM. NTSC, PAL, and SECAM standards willall be replaced over the next 10 years with a new suite of standards associated withdigital television. Making digital television a reality requires the cooperation of avariety of industries and companies, along with the development of many new standards. A wide variety of international organizations have contributed to thestandardization of digital TV over the past couple of years. Most organizationscreate formal standards by using specific processes: organizing ideas, discussing theapproach, developing draft standards, voting on all or certain aspects of thestandards, and then formally releasing the completed standard to the general public.

TV Development HistoryYear Historical Event
1884 Paul Gottlieb, patented the first mechanical television system.
1923 Vladimir Kosma Zworykin replaced the Nipkow disc with an electronic component.
1925 The first TV electronic system was patented.
1935 The first electronic television system was demonstrated by EMI.
1941 The NTSC developed a set of guidelines for the transmission of electronic television.
1956 The era of black and white television commenced.
1993 The European DVB project was founded.
1996 The FCC established digital television transmission standards in the United States.
1999 Implementation of digital TV systems across the globe.

Key IPTV Applications And Services

2009-05-01T11:02:00.000-07:00

The two key IPTV applications typically deployed by service providers are broadcast digital TV and content on demand (CoD).

Broadcast Digital TV
Before going into the world of ones and zeros it is important to take a perspective ofwhere television has come from over the past number of years. The history of television started in 1884 when a German student, Paul Gottlieb, patented the firstmechanical television system. This system worked by illuminating an image via alens and a rotating disk (Nipkow disk). Square apertures (small openings) were cutout of the disk, which traced out lines of the image until the full image had beenscanned. The more apertures there were, the more lines were traced and hence thegreater the detail.

In 1923, Vladimir Kosma Zworykin replaced the Nipkow disk with an electronic component. This allowed the image to be split into many more lines, which allowed a higher level of detail without increasing the number of scans per second. Images could also be stored between electronic scans. This electronic system was patented in 1925 and was named the Iconoscope.

J.L. Baird demonstrated the first color (mechanical) television in 1928. The first mechanical television used a Nipkow disk with three spirals, one for each primary color (red, green, and blue). At the time, very few people had television sets and the viewing experience was less than impressive. The small audience of viewers was watching a blurry picture on a 2- or 3-in. screen.

In 1935, the first electronic television system was demonstrated by a company called Electric Musical Industries (EMI). By late 1939, sixteen companies were making or planning to make electronic television sets in the United States. In 1941, the National Television System Committee (NTSC) developed a set of guidelines for the transmission of electronic television. The Federal Communications Commission (FCC) adopted the new guidelines and TV broadcasts began in the United States. Television benefited from World War II, in that much of the work done on radar was transferred directly to television set design. One area that was improved greatly was the cathode ray tube. The 1950s were an exciting time period and heralded the golden age of television. The era of black-and-white television commenced in 1956 and prices ofTV sets eventually dropped. Toward the end of the decade, U.S. manufacturers were experimenting with a range of different features and designs. The 1960s began with the Japanese adoption of the NTSC standards. Toward the end of the 1960s, Europe introduced two new television transmission standards:

(1) Systeme Electronique Couleur Avec Memoire (SECAM) is a television broadcast standard in France, the Middle East, and parts of Eastern Europe.

(2) Phase Alternating Line (PAL) is the dominant television standard in Europe.

Overview of an IPTV Network Infrastructure

2009-05-01T10:50:00.000-07:00

IPTV Data Center
Also known as the “headend,” the IPTV data center receives content from a varietyof sources including local video, content aggregators, content producers, cable,terrestrial, and satellite channels. Once received, a number of different hardware components ranging from encoders and video servers to IP routers and dedicatedsecurity hardware are used to prepare the video content for delivery over anIP based network. Additionally, a subscriber management system is required tomanage IPTV subscriber profiles and payments. Note that the physical location of the IPTV data center will be dictated by the networking infrastructure used by theservice provider.

Broadband Delivery Network
The delivery of IPTV services requires a one-to-one connection. In the case of a large IPTV deployment, the number of one-to-one connections increases significantly and the demands in terms of bandwidth requirements on the networking infrastructure can be quite large. Advancements in network technologies over the past couple of years now allow telecom providers to meet this demand for large amounts of bandwidth networks. Hybrid fiber and coaxial based cable TV infrastructures and fiber based telecommunication networks are particularly suited to the delivery of IPTV content.

IPTVCDs
IPTV consumer devices (IPTVCDs) are key components in allowing people to accessIPTV services. The IPTVCD connects to the broadband network and is responsiblefor decoding and processing the incoming IP based video stream. IPTVCDs supportadvanced technologies that minimize or completely eliminate the effect of networkproblems when processing IPTV content. As broadband starts to become a mainstream service, the functionality of IPTVCDs continues to change and increase insophistication. The most popular types of IPTVCDs (residential gateways, IP set-topboxes, game consoles, and media servers).

A Home Network
A home network connects a number of digital devices within a small geographical area. It improves communication and allows the sharing of expensive digital resources among members of a family. The purpose of a home network is to provide access to information, such as voice, audio, data, and entertainment, between different digital devices all around the house. With home networking, consumers can save money and time because peripherals such as printers and scanners, as well as broadband Internet connections, can be easily shared. The home networkingmarket is fragmented into a range of different technologies.

Defining IPTV

2009-04-30T20:07:00.000-07:00

There is a lot of buzz and excitement at the moment with regard to IPTV. The technology is growing in importance and is starting to have a disruptive effect on the business models of traditional pay TV network operators.But what does the IPTVacronym mean and how will it affect TV viewing? For a start, IPTV, also called Internet Protocol Television, Telco TV, or broadband TV, is about securely delivering high quality broadcast television and/or on-demand video and audio content over a broadband network. IPTVis generally a term that is applied to the delivery of traditional TV channels, movies, and video-on-demand content over a private network. From an end user’s perspective, IPTV looks and operates just like a standard pay TV service. The official definition approved by the International Telecommunication Union focus group on IPTV (ITU-T FG IPTV) is as follows: IPTV is defined as multimedia services such as television/video/audio/text/graphics/ data delivered over IP based networks managed to provide the required level of quality of service and experience, security, interactivity and reliability. From a service provider’s perspective, IPTV encompasses the acquisition, processing, and secure delivery of video content over an IP based networking infrastructure. The type of service providers involved in deploying IPTV services rang from cable and satellite TV carriers to the large telephone companies and private network operators in different parts of the world.

IPTV has a number of features:
. Support for interactive TV The two-way capabilities of IPTV systems allow service providers to deliver a whole raft of interactive TV applications. The types of services delivered via an IPTV service can include standard live TV, high definition TV (HDTV), interactive games, and high speed Internet browsing.
. Time shifting IPTV in combination with a digital video recorder permits the time shifting of programming content a mechanism for recording and storing IPTV content for later viewing.
. Personalization An end-to-end IPTV system supports bidirectional communications and allows end users personalize their TV viewing habits by allowing them to decide what they want to watch and when they want to watch it.
. Low bandwidth requirements Instead of delivering every channel to every end user, IPTV technologies allows service providers to only stream the channel that the end user has requested. This attractive feature allows network operators to conserve bandwidth on their networks.

DIFFERENCES BETWEEN IPTV AND INTERNET TV
. Accessible on multiple devices Viewing of IPTV content is not limited to televisions. Consumers often use their PCs and mobile devices to access IPTV services.

1.2 DIFFERENCES BETWEEN IPTV AND INTERNET TV
IPTV is sometimes confused with the delivery of Internet TV. Although both environments rely on the same core base of technologies, their approaches in delivering IP based video differ in the following ways.

1.2.1 Different Platforms
As the name suggests Internet TV leverages the public Internet to deliver video content to end users. IPTV, on the contrary, uses secure dedicated private networks to deliver video content to consumers. These private networks are managed and operated by the provider of the IPTV service.

1.2.2 Geographical Reach
Networks owned and controlled by the telecom operators are not accessible to Internet users and are located in fixed geographical areas. The Internet, on the contrary, has no geographical limitations where television services can be accessed from any part of the globe.

1.2.3 Ownership of the Networking Infrastructure
When video is sent over the public Internet, some of the Internet Protocol packets used to carry the video may get delayed or completely lost as they traverse the various networks that make up the public Internet. As a result, the providers of video over the Internet content cannot guarantee a TV viewing experience that compares with a traditional terrestrial, cable, or satellite TV viewing experience. In fact, video streamed over the Internet can sometimes appear jerky on the TV screenand the resolution of the picture is quite low. The video content is generally delivered to end users in a “best effort” fashion. In comparison to this experience, IPTV is delivered over a networking infrastructure, which is typically owned by the service provider. Owning the networking infrastructure allows telecom operators to engineer their systems to support the end-to-end delivery of high quality video.

1.2.4 Access Mechanism
A digital set-top box is generally used to access and decode the video contentdelivered via an IPTV system whereas a PC is nearly always used to access Internet.

IPTV: The Ultimate Viewing Experience

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Digital Television, also known as Digital TV, is the most significant advancement intelevision technology since the medium was created over a century ago. Digital TVoffers consumers more choice and makes the viewing experience more interactive.

The analog system of broadcasting television has been in place for well over60 years. During this period, viewers experienced the transition from black-and-whitesets to color TV sets. The migration from black-and-white television to colortelevision required viewers to purchase new TV sets, and broadcasters had toacquire new transmitters, pre, and post production equipment. Today, the industry isgoing through a profound transition, migrating from conventional TV to a new eraof digital technology. Most TV operators have upgraded their existing networks andhave deployed advanced digital platforms in an effort to migrate their subscribersaway from traditional analog services to more sophisticated digital services. A new technology called Internet Protocol-based television (IPTV), has started to grabheadlines across the world with stories about several large telecommunication,cable, satellite, terrestrial, and a slew of Internet start-ups delivering video over anIP based service. As the name suggests, IPTV describes a mechanism fortransporting a stream of video content over a network that uses the IP networkingprotocol. The benefits of this mechanism of delivering TV signals vary fromincreased support for interactivity to faster channel changing times and improvedinteroperability with existing home networks. Before describing the varioustechnologies that make up an end-to-end IPTV system, this chapter will start bydefining IPTV. The growth drivers for the industry sector are then examined, andNext Generation IPTV Services and Technology.

IPTV Multicasting

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Internet Protocol (IP) multicast is a bandwidth-conserving mechanism for reducing data network traffic by simultaneously delivering a single stream of information to thousands of recipients. Multicasting is fundamental to the implementation of IPTV. This is how it works.
In diagram 1, none of the network switches have IGMP snooping or querying turned on and so the network is not multicast enabled. The backbone switch has all the streaming traffic coming into it from the MPEG IP Encoders. If we assume each stream is 4 Mbps the backbone switch will carry 4 Mbps x 3 streams = 12 Mbps of streaming traffic.
As the switches don’t have IGMP turned on, the streaming traffic will flood the entire network regardless of whether the user requests a particular stream or not.

In diagram 2, all switches have IGMP snooping turned on and the backbone switch has IGMP query turned on. Again, the backbone switch has a total of 12 Mbps of streaming traffic coming into it from the MPEG IP Encoders.

Switch 1 will only have 8 Mbps of traffic reaching it as a result of the two users who have requested the same red stream (4 Mbps) and a third user who has selected the green stream (4 Mbps). This is the distinct advantage of multicasting - the bandwidth used is per stream and not per user.

Switch 2 will only have 4 Mbps of traffic reaching it since only the blue stream has been requested by a user. There will be no streaming traffic on the port where there is no request for a stream.

Switch 3 will have no streaming traffic since none of the users connected to the switch have requested any streams

In calculating the bandwidth requirement for IPTV, it should be assumed that all streams will be present on the backbone but, at most, only one stream will be present on a user port.

DISEqC Switch

2009-02-27T07:45:00.000-08:00

DiSEqC
Digital Satellite Equipment Control - developed by EUTELSAT and Phillips. The purpose was to define a standard enabling satellite receivers to control add-on equipment like rotors, LNB switches, etc.

The most common use would be to switch, for example, between two different satellites that are received by 2 different LNB's or dishes, and hook it up to one single receiver.

Why DiSEqC?
Older satellite receivers started to create their own little standard and solution for several switching purposes. This started to confuse customers (which work with what?) so it was time for a decent standard.

Some satellite receivers use a 14/18 Volts currency to switch between the Horizontal and Vertical polarization.

Another thing to switch, often done by using a 22Khz tone-burst, is switching satellites (2 LNB's either mounted on 1 or 2 dishes).

Now-a-days the upper frequency range (11,7 to 12,75 GHz) becomes more and more of interest for use in digital programs. The LNB's used for this purpose are universal LNB's (10,7-11,7 and 11,7-12,75 GHz) which use the 22kHz-Signal to switch between these two frequency ranges. This however is in conflict with the original use of the 22Khz Tone-Burst: switching between two satellites.

Using DiSEqC changes this for the better. In our digital world, we can send digital commands to the add-on equipment. DiSEqC is downwards compatible, so older equipment should work just fine with newer DiSEqC equipment. It's true that a DiSEqC 2.0 compatible receiver can work just fine with a DiSEqC 1.0 LNB-switch.

The other way around might or might not work, it all depends on the switch. Most of them do listen to DiSEqC 1.0 commands, some don't.

TIP: Make sure that you can return the switch if it's not working properly.

The concept of DiSEqC continues to use 14/18 Volts and 22Khz tone-burst concept. DiSEqC additionally uses digital commands to control equipment (naturally, both receiver and switch must be DiSEqC compatible).

DiSEqC is of use for both analog, digital and analog/digital systems.

DiSEqC variants
Mini-DiSEqC

Uses the 22Khz Tone-Burst (= Mini-DiSEqC), being able to switch between two individual universal LNB's (for both digital and analog satellite reception), where the switch always has only one of the two LNB's active. This switch is a specific DiSEqC compatible switch using both the 14/18 Volts currency and the 22 kHz Tone-Burst for control purposes.

DiSEqC 1.0
DiSEqC Version 1.0 allows you to connect up to 4 receivers to your receiver, where the receiver (master) controls the switch (slave) by sending digital commands for selecting the desired LNB.

These signals are used:
- Low or high band frequency
- Vertical or horizontal polarisation
- Which LNB should be activated.

DiSEqC 1.0 uses:
- Reception of 1 satellite (using 1 LNB) - 14/18 Volt
- Reception of 1 satellite for both digital and analog reception - 14/18 Volt
- Reception of 2 satellites (using 2 LNB's) - 14/18 V + Tone-Burst
- Reception of 4 satellites (using 4 LNB's) - 14/18 V + Tone-Burst + Loop-Through

Special LNB's allow you to use the Loop-Through signal, where the 2nd LNB's signal is being routed through the 1st LNB. This works with DiSEqc 1.0.

DiSEqc 1.2
Can additionally be used for automatically control of rotor-based dishes.

DiSEqC 2.0
DiSEqC 2.0 adds a return channel, used for getting information on the connected add-ons. It will inform the reciever, after sending a particular signal, about the amound and type of LNB's available and how they are interconnected.

Particular 2.0 applications (Next to the 1.0 applications):
- Reception of up to 4 sattelites
- Two way channel (info on what's connected to the receiver)

DiSEqC 2.1
Adds the ability to control up to 64 LNB's, I can't imagine what my house would look like with that many dishes, but hey,... you can if you want to (sent me a picture!). Which DiSEqC version do I need?

When using only one dish, with 1 LNB for only one satellite: basically any equipment will do just fine.

If you want to receive 2 satellites, for example the Astra and the Hotbird (EutelSat), your reciever should at least be able to control one switch, using any DiSEqC version. Actually a Tone-Burst ability should be sufficient.

When using 3 or more dishes, you will definitly need at least DiSEqC 1.0 or higher (depending on what your setup looks like).

When using a rotor for your dish to change position, you should go for DiSQeC 1.2 or higher.

Note that almost ALL LNB switches support DiSEqC 2.0, but often they are able to work with lower versions aswell. At least that is what is said... only experimentation will show if it works, so make sure you can return the switch if it doesn't work.

Most LNB switches sold in Europe are inteded to work with the Astra and the Hotbird (Eutelsat). This implies they should work with the receivers for these satellites aswell. Most modern receivers support at least DiSEqC 1.0 anyway.

Preventive Maintenance for Trunk and Feeder Lines

2009-02-20T09:49:00.000-08:00

Keeping your CATV system in top condition requires a preventive maintenance program that is easy to perform, uses common test equipment, and does not affect the subscriber. The following steps are critical to a good maintenance program:

1) Keep accurate records.
2) Set up a maintenance schedule that fits into the work flow.
3) Make consistent checks on signal levels, voltage, distortions, and picture quality.

The first step is to keep accurate records for each amplifier in your system. By recording and keeping the data, you can compare current performance measurements to those you made in the past. Significant changes in the readings you take indicate problems in your system. Not only should you be careful to record measurements, but you need to maintain these records so they are easy to find.

Second, you need to set up a weekly maintenance schedule. You cannot check every end-of-line or last amplifier every day, but you can create a program that fits into your work week. For example, take the number of last amplifiers in the system and divide by the number of work weeks per year you can do some maintenance. This is the number of amplifiers you should check per week. Then, divide this number by the number of technicians available. This is the number of amplifiers each technician should check per week.

Then, organize your last amplifiers by geographical are so you can assign technicians to amplifiers in the same area, saving drive time between amplifiers. When setting up you schedule, keep in mind that you should check every last amplifier at least once a year, you should check different amplifiers in the same geographic area at least once a quarter, and you can expect to spend about 10-30 minutes at each amplifier (unless you uncover a problem).

Finally, at every last amplifier you'll need to measure and record signal level at all channels, voltages, carrier-to-noise ratio (C/N), composite triple beat (CTB), and composite second order (CSO). To perform these tests you'll need, at minimum, a signal level meter (SLM), graph paper (or use an SLM with memory and a printer) and a voltmeter. For more accurate measurements, consider using a spectrum analyzer with a printer and a tunable bandpass filter instead of the SLM.

Once you've finished each test, record your results and compare them with your records for that amplifier. Do the levels you've just measured match those previously taken? If you uncover changes that could cause problems for your subscribers, backtrack and find the source. To pinpoint a problem, try the "binary" method of troubleshooting. Determine which amplifier is halfway between your location and where the cascade begins. For example, if you're at Amplifier 16, check Amplifier 8. Continue to halve the distance until you find the amplifier you need to repair.

heck and record the levels of all channels You can make a simple sweep trace using an SLM to measure the level at each channel and graph paper to record the measurements. A broadband sweep system is the best approach to evaluating the full spectrum because an SLM won't show what's going on between carriers.

Although you can expect small changes in signal levels over time, significant changes can indicate one of these problems: suck-outs, low signal levels, high signal levels, incorrect tilt in the signal, cracked cables, or bad connectors. When recording the measurements, note the outside temperature as well, so you can see how it affects signal levels.

MEASURE THE VOLTAGE
Although you need a true RMS voltmeter for an accurate voltage reading, you can use a standard voltmeter for these maintenance tests because you are only looking for changes. But, you must use the same standard voltmeter whenever you test that amplifier. Even small changes in voltage should be checked. Voltage changes indicate cracked cable, corroded connectors or a line power supply in need of repair.

CHECK THE C/N
To measure the C/N, you need a reference signal that is at leas +20 dBmV and an SLM. Then, follow the instructions shipped with the meter's manual, or try this method. Tune the SLM to a channel that has no lower adjacent channels and is not next to the bandsplit; record the carrier level. Then, setting the SLM to the space where there is no channel, tune back and forth for the lowest reading you can find. Reducing the attenuation, tune back and forth again, until you get a constant reading, this the noise floor. The difference between the carrier level and the noise floor, minus a correction factor, is the C/N value. For best results, subtract the correction factor published in the meter's manual. A tunable bandpass filter probably will be necessary to avoid overloading the SLM when measuring noise.

A bad C/N reading indicates you have a low input to one or more amplifiers in the cascade. Low input to an amplifier can have several sources, including water in the cable, water in the splitter, a corroded connector, or an improperly set or defective amplifier.

CHECK THE CTB
With the same reference signal you used for the C/N measurement, tune the SLM to a vacant channel, where you would find the carrier frequency. The signal you find there is the CTB. A low CTB ratio indicates you have too high of an output from one or more amplifiers, which can be caused by an AGC error in the amplifier or an improperly set or defective amplifier. A tunable bandpass filter probably will be necessary when measuring the CTB.

CHECK THE CSO
Using the same reference signal, tune the SLM 0.75 or 1.25 MHz from the vacant carrier. (CSO is found ±0.75 and ±1.25 MHz from the video carrier.) This is second order. If you subtract this reading from the video carrier reading at the adjacent channel, you have a good indication of the CSO. A poor CSO measurement often indicates a defective or improperly set amplifier or problems with a fiber link. You can perform these distortion tests with a spectrum analyzer, but you need a tunable bandpass filter set to a vacant channel. When using a spectrum analyzer, remember that since you are working with active video, you need to take measurements when sync is high. You can use a slow sweep and wider resolution bandwidth setting to see when sync is high, and you may want to set the spectrum analyzer in "max hold"
mode.

Now that you've made all these measurements, make sure that you record the levels and keep you records. You'll need to refer to them when you check the amplifier again.

PICTURE QUALITY
The final check is picture quality. Use a good quality portable TV set and a converter to tune the channels. (This will minimize direct pickup from affecting the pictures.) Make sure the signal levels at the input to the converter do not exceed +5 to +10 dBmV. Record comments about the picture quality along with the measurement results from you other tests.

CATV Design Formula

2009-02-07T09:10:00.000-08:00

Noise Calculations (C/N)

Single Amplifier Carrier/Noise - CN0=Output-(-59+NF+G), CN1=Input-(-59+NF)
To Sum Identical C/N CNS=CNO-10log10(N)
To Sum Different C/N ratios CNS=-10log10[10(-CN1/10)+10(-CN2/10)+10(-CNN/10)]
C/NS = System Carrier / Noise Ratio
N = Number of Amplifiers in Cascade
NF = Noise Figure
G = Gain
-59 dBmv = Thermal Noise in 4 MHz Bandwidth
NOTE: For every increase in input signal level, the C/N improves by 1 dB and Cross modulation worsens by 2 dB.
________________________________________

Carrier to Cross Modulation (C/XM)

[Composite Triple Beat calculation: replace XM with CTB]
To sum Identical Cross Modulation Ratios XMS=XM-20log10(N)
To sum different Cross Modulation Ratios XMS=20log10[10(-XM1/20)+10(-XM2/20)+10(-XMN/20)]
XMOD of One Amplifier at Operating Level XM=XMREF+2(Output Level-Reference Level)
XMS = System Cross Modulation
XM = Cross Modulation Distortion
NOTE: For every double number of amplifiers with identical crossmod there is a 6 dB degradation total crossmod. For every 1 dB reduction in amplifier output level, the XM improves 2dB.
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2nd Order Distortions

To Sum Identical 2nd Order SOS=SOO-10log10(N)
To Sum Different SOS=-10log10[10(-SO1/10)+10(-SO2/10)+10(-SON/10)]
SOS = System 2nd Order Distortions
N = Number of Amplifiers in Cascade

System Frequency vs. dB Loss - LF2 = LF1 (sqRt[F2/F1])
LF2 = Unknown Cable Loss at a desired frequency (dB)
LF1 = Cable Loss at a Known Frequency (dB)
F2 = Desired Frequency (MHZ)
F1 = Known Frequency (MHZ)
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Tilt and Equalizer Calculations

TILT = (1-FR(1/2)) x cable
Cable = Electrical cable length in dB at design frequency.
FR (1/2) = cable loss ratio ~ sqRt [f1/f2]
TILT = Difference in loss vs. frequency.
Example: Calculate the tilt provided by 20 dB of 0.750 cable from 54 MHz to 450 MHz.
TILT = (1-FR(1/2)) x cable = (1-0.3330) x 20 = 13.4 dB
Equalizer Value = TILT(dB)/(1-FR(1/2))
Example: Calculate the value of the equalizer to compensate for 12 dB of slope between 54 and 450 MHz.
Equalizer Value = TILT/(1-FR(1/2)) = 12/(1-0.3330) = 17.9 dB
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Field Intensity vs. Dipole Level - V(dBmv)=20log10E(microvolts/m / 0.021 x f (MHZ) / 1000
Maximum Leakage Levels - Determine the maximum leakage levels by using the following equation: L=20log10(E/21f)
L = Maximum leakage level (dBmv)
E = Voltage (microvolts/m)
f = Visual carrier frequency (MHZ)
Cumulative leakage index -
10log10I 3000 is than or equal to -7 (Method 1)
10log10I infinity is less than or equal to 64 (Method 2)
Equations for dB and dBmv For Equal impedance: Z1=Z2
Power Gain can be expressed as follows:
dB=10log10(P1/P2)
dB=10log10(V1/V2)2 = 20log10(V1/V2)
dB=10log10(I1/I2)2 = 20log10(I1/I2)
dB's for CATV
0dBmV = 1mV across 75 = 1,000 microvolts across 75
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Distance to the Horizon

Optical D = 1.23(sqRt[H])
Radio D = 1.41(sqRt[H])
Assume smooth earth
D = Distance statute miles
H = Height, Feet

Design & Operation of CATV Powering Part.4

2009-02-07T06:21:00.000-08:00

Efficiency Calculation and Operating Cost
Understanding the principle function of CATV power supplies and how they are rated for operation can be of benefit of both the CATV operator and the utility company. The power supply most commonly in use is a 15 Amp unit. The attached document supplies information on the power consumption of the unit. This model utilizes a ferroresonant transformer that is 90% efficient. The basic operation of this unit is to transform the incoming utility AC to 60 VAC RMS to supply the equipment. The ferroresonant transformer is used because of its regulation characteristics and ability to withstand shorts on the 60 Volt side for long durations. The efficiency figure is lower than a normal transformer is utilized in a “tank” circuit that provides the regulation. Therefore, if the total output power [Volts x Amps or 60 x 15] is multiplied by the efficiency [1/0.90] the input power can be calculated.

(60 VACx15 Amp)x(1/0.90 Efficiency) = 1000 Watts

As the required load on the output side decreases, the energy to the tank circuit stays constant such that the efficiency of the transformer drops off. Because of this fact, Antec Power supply of several output loading levels are designed to minimize to cut operating cost for the CATV operator. It is therefore recommended that, wherever possible the power supplies be utilized at full load.

In most cable systems, it is difficult to provide a system design that can utilize the power supply to its full load rating. Such parameters as physical location of the power supply, allowances for RF line gear that is not fully loaded with optional modules, outdated design principles that de-rated the load to increase power supply life and the possibility of future expansion to the cable system all combine to present an approximate real world loading of 75%.

When a customer requests recommendations on powering for use in determining utility billing, our first recommendation is to meter the power supplies. If this is not economically feasible our next suggestion is to sample 10% or more of the power locations with a phase factor correcting power meter on the input side. If this is not practical then the calculation listed above should be used with some factor of underloading being applied to the output load. [((.75x15)x60)x(1/0.90)=833 Watts]

The underloading factor can sometimes be determined by examining the systems “as built” powering maps or by mutual agreement between the utility company and the engineers of the local system.

Design & Operation of CATV Powering Part.3

2009-02-06T02:40:00.000-08:00

Typical Construction
In use in the communication industry are two types of ferroresonant transformers. These are the non-standby ferroresonant transformer and the common ferroresonant transformer. Battery back-up systems or UPS (un-interruptible power supply) power supplies can be of either type. A redundant system utilizes a separate ferroresonant transformer and inverter transformer. The common ferroresonant transformer design combines the winding from the inverter input side into the ferroresonant transformer input. Thus the inverter is “common” to the ferro. The redundant inverter transformer supplies its output directly to the line. A common ferro inverter suppliesits output to the ferroresonant transformer. The redundant unit will typically run 2+hrs, on a set of three batteries while the common ferro inverter will run 1.5hrs. from the same set of three batteries. This difference in battery operation time is due to the losses incurred by passing through the ferroresonant transformer (90% efficiency). An inverter circuit that is 85% efficient is applied to a ferro transformer that is 90% efficient. The resulting battery to output line efficiency is 76%.

Momentary Input Inrush Currents in Ferroresonant Transformers
When power is first to applied to a ferroresonant transformer, there is an immediate inrush of current to energize the capacitor and tank circuit. This momentary condition can sometimes be 5 times greater than the operating current in common ferroresonant transformers and 2-3 times greater in ferroresonant transformers utilized in non-standby and redundant standby systems. This momentary current surge can cause circuit breakers installed on the input line to trip open if they are under rated or not classified to handle this type of inrush. While a 15 ampere ferro will draw only 8.3 ampere in normal operation under full load, this inrush characteristics demands the use of 20 ampere circuit breakers for the standard ferroresonant transformers and 20 ampere high magnetic or high inrush circuit breakers for the common ferroresonant transformers. A high magnetic circuit breakers is similar in function to a slow blow fuse. It will allow a momentary high current condition to pass but trip open during a sustained over current condition. The common ferroresonant transformer has a larger mass core and thus has a higher inrush current that requires the use of a high magnetic circuit breaker.

Design & Operation of CATV Powering Part.2

2009-02-04T18:07:00.000-08:00

Why use a Ferroresonant Transformer?
When a source of power is introduced in a coaxial cable, transmission loses are incurred based on the circuit resistance to the load. The FR amplifiers can draw from 0.3 amperes current to as much as 1.75 amperes of current dependent on the design and function of the device. These amplifiers are distributed on CATV coaxial network to distance of 3000’+ (914 meters+) in a branching configuration. Ohms law dictates that there will be voltage loss when a current passes through a resistance. In this example, the resistance is the coaxial cable between the power supply and the amplifier. Each amplifier has a power pack or module that converts the incoming 60-90VAC to 24VDC for use by the amplifier circuitry. The amplifiers combine to give the highest amount of current draw on the coaxial cable that connects the power supply with the nearest amplifier location.

An example would be:
Coaxial cable 0.750”, 1650’ length 0.76 ohms loop resistance per 1000’
Combined amplifier loading or current on this segment 4.2 amperes
Output voltage 60VAC
1650 feet X 1.254 ohms = 5.27 VACTRMS (loss)
60VAC – 5.27 VACTRMS = 54.73 VACTRMS at that amplifier location

Each segment of cable with load passing on it will further reduce the voltage or a higher current draw will further reduce the voltage. By the time the designer gets to the end of line, they must stop when the voltage drops to 42VAC the typical minimum input to the amplifiers. A pictorial representation of a typical network with a loop resistance of 1ohm/1000ft system designed for a minimum of 40 volts is shown in Figure 2 below:

If a standard transformer were to be used in this application, the output voltage of the transformer would follow the input voltage (2:1 transformer converting 120 VAC to 60 VAC). If the end of line were at 42 VAC and the input line dropped to 110 VAC, the resulting voltage at the end of line would be less than 37 VAC causing all the devices in this area to turn off. In addition, most of the power modules in the amplifiers today are switchmode regulators or “switching power supplies”. These units are constant power meaning that they will draw higher amperage at lower voltages (27 VAC / 60 VAC = 0.45 amperes or 27 VAC / 42 VAC = 0.64 amperes). If a voltage drop occurs, the amplifiers will draw more current which increases the voltage drop in the coaxial cable in which turn reduces the voltage available to the amplifier’s power module which causes it to draw more current, and the cycles continuous until the units at the end of line start switching off which reduces the current and allows the voltage to rise again. Hence, if a standard transformer were utilized, a momentary voltage drop could cause interruption of service to the customer.
A ferroresonant transformer will regulate its output voltage typically to within +/- 2% of its rated output voltage. This characteristic allows it to function typically from 90VAC to 140 VAC. This prevents wide voltage fluctuations at the end of line of the CATV coaxial system.

Design & Operation of CATV Powering Part.1

2009-02-04T08:50:00.000-08:00

Ferroresonant power supplies have been utilized in the CATV industry for many years. The inherent short-circuit protection that they provide protects both equipment and technicians from surges caused by accidental shorting of the cable center conductor to ground. A ferroresonant transformer composed of two main components, a saturating transformer and a resonant capacitor. When voltage is applied to the main winding, the magnetic flux path becomes excited and sets up a resonance in the “tank circuit” or capacitor winding. The resonant capacitor and winding generates high currents that saturate the transformer and act as a “flywheel” which resists change. Once this has happened, any variations in the input line voltage will be resisted and provide a wide range of input voltage. The load on the secondary is regulated by the use of shunts as shown on Fig. 1 below

These shunts limit the magnetic path base on the air gap that is designed for the transformer. If the load on the secondary or output winding is increased, the resonance of the magnetic circulating paths decreases. To achieve load regulation, the effect of the shunts on the main magnetic path also decreases allowing more power from the primary to pass to secondary. If a short is applied to the output, the transformer will “fold-back” or current limit because the resonant circuit will collapse similar to breaking drive belt from the flywheel. Fold-back normally occurs at 125% of the rated load of the transformer. Most modern ferroresonant transformers have a higher fold-back current to allow them to operate into highly capacitive loads in use in today’s coaxial networks.

All power sources utilized in the communication corridor must be “inherently current limited as per the NESC (National Electrical Safety Code). Most Municipalities adopt the NESC guidelines as an enforceable code. In addition to the power characteristics of the transformer, it also has good RF isolation characteristics separating the input from the output.

CATV DESIGN FUNDAMENTALS Part. 5

2009-01-26T10:32:00.000-08:00

Now let us estimate the total losses in the system above.

1. Total Cable length is 190 feet. Therefore Cable Loss is 4.0 x 1.9 = 7.6 dB

2. There are 5 Tapoffs in the branch. Since the losses must be estimated, we will use the median tap value 17 dB with insertion loss of 0.5 dB per tapoff. Therefore total tapoff loss is 0.5 x 4 = 2.0 dB.(Note that last tapoff will not be considered)

3. Since 12 dB is the lowest tap value we can have at the last tapoff, we shall use this tap value.
Now system losses tally as follows:
Cable Loss 7.6 dB
Splitter Loss 4.0 dB
Insertion Loss 2.0 dB
Tap Loss 12.0 dB
Signal required 10.0 dBmV
Signal level required 35.6 dB

Therefore, the headend must supply at least 35.6 dBmV signal to overcome the system’s losses, and deliver a minimum of 10dBmV to the last TV receiver on the line.

Headend amplifier
Since the system’s losses are 35.6 dB, we will base our calculations on the assumption that we will use an amplifier with a signal output level of +37 dBmV which will provide enough signal to overcome the system losses.

The amplifier sends +37 dBmV of signal to the 2-way splitter, which incurs a 4.0 dB loss.
37.0 dBmV
- 4.0 dB
33.0 dBmV being sent to each branch of the distribution system.

NOTE: In instances where it is necessary to locate the amplifier at a distance
from the splitter, the cable loss from the amplifier to the splitter will have to be calculated.

FIRST TAPOFF
The splitter sends 33.0 dBmV of signal to the system. This signal must pass through 50 feet of cable to reach for first tapoff. 50’ cable at 4.0 dB signal loss per 100 feet equals 2.0 dB cable loss.
33.0 dBmV Input Signal
- 2.0 dB Cable Loss
31.0 dBmV Input to First Tapoff

Now, using an Tap value at the first tapoff of 17 dB, we deduct the tap value from the input signal to determine the signal being fed to the set.
31.0 dBmV Input Signal
-17.0 dB Isolation Value
14.0 dBmV Signal being fed to the set. This provides us with more than the required 10 dBmV signal input to the set.

To figure the input signal to the second tapoff, we have 31.0 dBmV input to the first tapoff, minus    .5 dB insertion loss of that tapoff.
31.0 dBmV Input to the First Tapoff
- 0 .5 dB Insertion Loss
30.5 dBmV Signal Level at the Output of First Tapoff
The signal must now pass through 30 feet of cable to reach the second tapoff. 30’ of cable at 4.0 cable per 100 feet equals 1.2 dB cable loss.
30.5 dBmV Output from First Tapoff
   - 1.2 dB Cable loss
29.3 dBmV Input to Second Tapoff

SECOND TAPOFF
We now have 29.3 dBmV of signal being fed to the input of the second tapoff. This requires us to use a tapoff value of 17 dB, with an insertion loss of .5 dB. To determine the signal being fed to the set at the second tapoff, we have +29.3 dBmV going into the tapoff, minus the 17 dB tap loss value.
+29.3 dBmV Input Signal
-17.0 dB Isolation Value
+12.3 dBmV Being fed to the set

To determine the input signal to the third tapoff, we have +29.3 dBmV going into the second tapoff, minus .5 dB insertion loss.
   +29.3 dBmV Input to Second Tapoff
-0.5 dB Insertion Loss
+28.8 dBmV Output from Second Tapoff

We now have +28.8 dBmV of signal which must pass through 30’ of cable before entering tapoff number 3. 30’ of cable at 4.0 dB loss per 100 feet equals 1.2 dB cable loss.
+28.8 dBmV Output from Second Tapoff
-1.2 dB Cable Loss
+27.6 dBmV Input to third Tapoff

THIRD TAPOFF
We now have +27.6 dBmV of signal at the input of the third tapoff, allowing us to use a 17dB tap value with a .5 dB insertion loss. To determine the signal being fed to the set at the third tapoff:
+27.6 dBmV Input Signal
-17.0 dB Isolation Value
+10.6 dBmV Being fed to the set

To determine the signal level at the input to the fourth tapoff, we have +27.6 dBmV input to the third tapoff with a .5 dB insertion loss.
+27.6 dBmV Input to Third Tapoff
-0.5 dB Insertion Loss
+27.1 dBmV Output from Third Tapoff
The signal now passes through 40 feet of cable. 40 feet of cable at 4.0 dB cable loss per 100 feet equals 1.6 dB cable loss.
   +27.1 dBmV Output from Third Tapoff
- 1.6 dB Cable Loss
+25.5 dBmV Input to Fourth Tapoff

FOURTH TAPOFF
We have +25.5 dBmV input signal coming to tapoff number 4, which requires our using 12dB tap value at the tapoff, again with a .7 dB insertion loss. To determine the signal to the set:
   +25.5 dBmV Input Signal
-12.0 dB Isolation Value
+13.5 dBmV Fed to Fourth Set

To determine the signal being fed to the input of the fifth tapoff, we have +25.5 dBmV input to the fourth tapoff, less the .7 dB insertion loss.
   +25.5 dBmV Input to Fourth Tapoff
   -0.7 dB Insertion Loss
   +24.8 dBmV Output from Fourth Tapoff

We have 24.8 dBmV coming out of the fourth tapoff and passing through 40 feet of cable to reach the fifth tapoff. At 4.0 dB loss per 100 feet gives us 1.6 dB cable loss.
   +24.8 dBmV Output from Fourth Tapoff
   -1.6 dB Cable Loss
   +23.2 dBmV Input to Fifth Tapoff

FIFTH TAPOFF
We now have +23.2 dBmV of signal fed to the fifth tapoff, requiring the use of 12 dB
Tap value. To determine the signal being fed to the set:
+23.2 dBmV Input to Fifth Tapoff
-12.0 dB Isolation Value
+11.2 dBmV Being fed to the Set

Since this is the last tapoff in the line, we must terminate the output of the tapoff. Inplanning your systems, always remember to include a terminator at the end of eachbranch line to maintain impedance match.
Since the other branch of the system has the same number of tapoffs and the same cable lengths, it is a mirror image of the branch we have just calculated. Therefore, the same tap values can be applied. Our system calculations are now complete.

CATV DESIGN FUNDAMENTALS Part. 4

2009-01-26T10:10:00.000-08:00

TAPS AND DIRECTIONAL COUPLERS.

Directional couplers, or Taps as they are sometimes called, are used to extract a small portion of the signal from the distribution cable to feed subscribers taps, while maintaining the proper characteristic impedance of the ditribution cable. A directional coupler has three important parameters to check: insertion loss, isolation, and tap loss.

The most important parameter of a directional coupler is the tap loss. Tap loss is how much lower the signal level at the tap output is, compared to the signal level at the input. Common tap loss values range from 3 dB to 28 dB. Directional couplers are placed at various locations throughout a distribution system based upon the required tap loss and signal level needed. The isolation of a directional coupler becomes greater as the tap loss increases, with a typical isolation of 20 dB for a 3 dB directional coupler. The table shows the isolation and insertion loss of different taps.

Tap Values 6 dB 9 dB 12 dB 16 dB    20 dB    24 dB 27 dB 30 dB
Insertion
Loss Max 2.2    1 1       1    .5 .5          .5       .5
Isolation(Output)
to Tap 26 28    35    35    40       40       40       40

Design Procedure

While designing the distribution system following points should be kept in mind.

Design the distribution system with highest frequency to be carried by the cables.
Design the distribution system for the branch that runs longest and having the maximum number of splitters, taps and other passive devices.

For design of distribution system given in diagram below, we will assume the following.

Cable is used that has a loss of 4.0 dB per hundred feet at highest frequency under consideration.
Two way splitter is used that has insertion loss of 4.0 dB.
Signal level required at each tap output is 10dBmV.
Tapoffs are available with the following specifications.

TAP LOSS (dB) INSERTION LOSS (dB)
23 0.3
17 0.5
12 0.7

CATV DESIGN FUNDAMENTALS Part. 3

2009-01-26T10:05:00.000-08:00

SPLITTERS, TAPS and DIRECTIONAL COUPLERS.

Every CATV distribution system contains splitters, taps/ directional couplers and other passive components. These components may develop excessive signal attenuation and losses, or poor isolation between inputs and outputs.

SPLITTERS

Splitters are used in distribution systems to divide an input signal into two or more equal output signals. RF Splitters are rated in two basic ways: The frequency range they are designed to handle and the insertion loss incurred when a signal passes from the input to any of the outputs. The insertion or the through loss which is the amount of attenuation the signal receives as it passes from input to output. The lower the insertion loss of a given splitter, the better it is.

A typical two-way splitter has a through loss of about 3.5 dB from the input to each output, and an isolation of 20 dB or more. Four-and eight-way splitters are also common, having typical through losses of 7 and 11 dB typically. The table below shows insertion and isolation losses of some common type of CATV splitters.

Types of Splitters Insertion Loss (dB) Isolation loss(dB)
2 Way 4.0 25
3 Way 5.5 23
4 Way 7.0 21
6 Way 9.5 20
8 Way 12 18
16 Way 15 20

CATV DESIGN FUNDAMENTALS Part. 2

2009-01-26T10:00:00.000-08:00

Resistance loss is by far the largest contributor of losses in coaxial cable. Losses caused by the resistance of the inner conductor vary with the cross sectional area of the conductor. Most of the loss, however, is frequency related, a condition called "skin effect." Skin effect describes the condition where, as the frequency of the signal increases, the signal is carried through the conductor further and further away from the center. Thus, the resistance loss in any given cable type varies in direct proportion to the frequency of the RF signal-the higher the frequency the greater the loss. The losses also very with the temperature of the cable. For cables exposed to the weather, when the temperature is colder, the losses of the cable are less. On the hottest day in the summer, the cable losses are the greatest. Cables buried in the ground or the indoors cable usually remain at the same temperature year round.

Important Note:

Cable loss varies with the type of cable
Losses are greater at higher frequencies
Losses very with the temperature of the cable

Table above lists some commonly used RF distribution cables and the typical losses for each. This chart is used as a guide to determine the normal amount of attenuation expected in a piece of cable.

CATV DESIGN FUNDAMENTALS Part. 1

2009-01-26T09:54:00.000-08:00

In designing a cable television system it must be remembered that the headend must supply enough signal level to meet the various losses encountered in the distribution system. The most important losses in a distribution system are the cable loss, splitter loss and the tap loss. Before designing a cable television system we shall examine these losses.

Cable Loss
The largest single passive device in a Radio Frequency (RF) distribution system is the coaxial transmission cable itself. A piece of coaxial cable is nothing more than a piece of round wire surrounded by an insulating material that is then surrounded with an outer metal jacket which may consist of foil layers or a braided wire. The object of the coaxial cable is to keep electromagnetic RF carrier waves inside the cable from getting out and similar waves from the outside world from getting inside the cable. The term coaxial means that the center conductor must stay in the center. If the cable is flattened, it will create reflections and it will radiate and absorb signals to and from the outside world. The purpose of the tansmission cable is to carry the RF signal with a minimum amount of loss. At the RF frequencies involved in CATV distribution systems, characteristics of the cable and losses in the cable must be taken into careful consideration.

One of the losses associated with coaxial cable is signal leakage. Signal leakage occurs when the coaxial cable can not contain the whole RF signal, and allows some of it to leak out into free space.

Two other types of cable loss, dielectric loss and resistance loss. All coaxial cables have a specific amount of dielectric and resistance loss. These losses are taken into account when the distribution system is designed and built. Any changes in these parameters after the system is operating, however, may severely affect the performance of the distribution system.

About CATV Interference Part. 8

2009-01-21T12:00:00.000-08:00

Part #8 Closing And Summary

In closing, I would like to point out a few things that may help you in this endeavor.

Anything in your house that is conductive can be an effective antenna for RFI. This includes water pipes and grounds. Remember if your ground wire is more than a half wave long in respect to your fundamental frequency, it may be an effective safety ground, but, it also may be a very effective long wire antenna for RFI. How long and where are the water pipes in your house? How long is your station ground wire before it gets to the ground rod? If your station ground is shared with the cable ground, or the electrical ground (shame on you!), this could be a real problem.

Do not modify or fix your neighbors equipment. If something should happen coincidentally to his television, they are going to blame you!

Terminating your Amateur Radio activities is not an acceptable (permanent) solution to the problem. As long as your station is in order, and the interference is not caused by spurious emissions from your station, the FCC will allow you the privilege to continue operating.

But if you decide to be diplomatic about it, there are other ways to enter the packet network, such as 440 Mhz or other bands depending on your location. It also might be a good idea to refrain from operating if you are in conflict with the Super Bowl, or a community-access viewing of your neighbor's grand-daughter's dance recital.

There are many sources where you can find information on the RFI phenomenon. I would recommend first contacting The American Radio Relay League. They have several free informational packages, and a new book on interference; "Radio Frequency Interference - How to Find It and Fix It." You can also contact The Federal Communications Local Field Office in your area. They too have information on the subject. You can contact your local cable company or The National Cable TV Association, Director of Engineering, Science and Technology Department, 1724 Massachusetts Ave., Washington, DC 20036.

Now if you find an unusual solution to any type of RFI (EMI), I would urge you to fill out an EMI Report Form, which is available from the ARRL. Hopefully your discovery and knowledge can be passed on to other amateurs who may be experiencing the same type of interference.

I've seen more than a few bulletins of late concerning CATV RFI on bothcable channels 18 and 24 (and I'm sure on others and believe this articlewill be of help.

About CATV Interference Part. 7

2009-01-21T11:08:00.000-08:00

Part #7: How the RFI is getting in.

Let's eliminate all unnecessary components of the cable system, such as old video games that you never use anymore, and temporarily eliminate all but one TV (and its converter box, if necessary). Tighten all connections. Cable connectors should be finger tight, plus a quarter of a turn with a wrench. Check to see if the cable connectors are properly secured to the cable. All unused inputs or outputs of the cable system should be properly terminated with a terminating resistor. These are available in most any electronics supply store. Eliminate any 300 Ohm twin lead antenna wire that for whatever reason may be around and check to make sure that all cable coax is of the 100% shielded type.

Now, it would be of some help if you had a fellow ham with a H/T to help you in this endeavor. This way you can operate the packet station and he or she can tell you if the RFI is still present at the television. If you use the H/T on low power, you will probably find that it is about the same signal strength as the packet station. If it causes the same type of interference, it will let you do a lot of troubleshooting right in the neighbor's house.

These steps can be approached from many different directions. Choose which is easiest for you. But for an example:

Take the antenna leads off of your TV or cable converter if this is what receives cable channel 18. Terminate the antenna input on the equipment with the proper resistor. 300 ohm for a 300 ohm set and 75 ohm for a 75 ohm set. Keep the leads as short as possible. Transmit on 2 meter packet. If you still have RFI, there is a good chance that the RFI is entering through the A/C line, or being directly picked up by the television circuitry. Wrap the A/C line around the toroid. Transmit again. If the RFI is eliminated or reduced, you have found the path and cured the problem. If the RFI is still there, check to see if there are any other wires or conductors attached to the TV. Remember, any thing that can transform itself into an antenna, can be the source of the RF current. If this is the case, more experimenting with more toroids may be necessary. Now after these areas have been eliminated and the RFI is still present, the RFI is either stronger than the toroids and more turns or toroids are needed or the RFI is being induced through the case of the TV.

Laws prohibit you from fixing your neighbor's TV without a state electronic repair license. Besides, if anything goes wrong when you have the back off the set, you will be assuredly (and expensively) blamed! Contact the set manufacturer through the Electronic Industries Association (EIA), 2001 Eye St NW, Washington, DC 20006. The EIA maintains a database of EMI/RFI contact persons at each of their member companies.

On the other hand, if after you properly terminate the cable input with the resistor, and the RFI disappears, then you know that the RFI is entering via the cable before your TV. This can either be via common-mode on the shielding of the cable coax, or by the RFI penetrating the shield of the coax itself and entering via the center conductor. The latter is a more difficult problem to fix and you may need the help and cooperation of your cable company, and a lot of perseverance.

A quick check to see if there is leakage into the cable system is to determine if there is leakage out. Tune your H/T receiver to 145.250 and see if there is a carrier present. If there is not, or only a very weak one, then the cable system itself is "clean". You will have to look elsewhere.

This method of locating the path of the RFI current and eliminating it can be used with most all types of appliances and radios. You just have to use the process of elimination.

Now there are a number of factors that we really can't do too much about. Some televisions have only a 300-ohm balanced input. For these TVs, we need a 300 - 75 Ohm transformer in line. These pieces of equipment have been known in many instances to be a major contributor to the problem. In many other TVs, you have a coaxial 75 ohm input, but your TV converts it right back to 300 ohm inside the case! And in the cases where the RFI is induced through the case of any essential piece of equipment, depending on what piece of equipment is suspect the only things that you can do are:

Contact a licensed television repair technician if your television is suspect and see if the television can be modified to prevent RFI. Some manufacturers have good information about these types of modifications.

About CATV Interference Part. 6

2009-01-21T10:41:00.000-08:00

Part #6: Ideas For Solutions

First of all, if you contact your cable company, have them make sure that you have a good strong signal at all of your TV outlets. Then, the most useful piece of "equipment" that I have seen is the ferrite choke. They come in a number of shapes and sizes. They come in the form of rods, split cores, toroidal cores, beads and many others. They also come made in many different mixes, each having a certain capacity to choke out different frequency ranges. Obviously they are not a miracle cure for all purposes, but they do work great on most types of common-mode and some of the differential-mode interference. For the frequencies that we are working with (VHF), the most effective material to use is the #43 mix.

These ferrite chokes can be used on just about every wire in your house. Make certain that you choose a ferrite mix that will function on the frequency you are trying to suppress. In a cable coaxial antenna the desired signal is restricted to the center conductor and a large toroid can be placed over the coaxial antenna, thus suppressing any common-mode current flowing down the shield without affecting the desired VHF signal inside the cable. Test have shown that 300 ohm twin lead wire can be protected in the same manner, as long as you run both leads through the same toroid. If you put a toroid on each lead, the primary VHF signal will be attenuated also. Obviously this would not be desired.

Since the object is to interrupt the common-mode rf current path, thus breaking that RFI circuit, sometimes it is much easier to wind your A/C power cord through the toroid, rather than trying
to wind the stiff coaxial wire through it.

Let's say that you take your A/C power cord and run it through a toroidal core or wrap it around a ferrite rod. This will attenuate the given frequency for that ferrite mix (X) amount. If you wrap it two times it will attenuate it, (X) times 2. Three times, (X) times 3, and so on. The usual way to use these devices is to take your cable coax or A/C power cord that is leading into your TV and wrap it around a ferrite 14-15 turns, as close to the set as possible. Now, if you don't have enough coax, just make up a section out of new coax, wrap it, put end connectors and a barrel connector on it and connect it to your cable connector. (I like to bring a few of these with me when I do a troubleshooting "service call".) With the A/C power line, you can use an extension cord to wind the toroid if you don't have enough free line.

* I really don't think this is a good idea. It will form a transformer, of sorts. This could couple unwanted energy from the AC line to the coaxial line, and vice versa. I recommend separate cores in all cases - KA1CV. If you have a big enough ferrite, you can wind your A/C line and Coax through it. This may quadruple the effect of the toroid. But, you must wind the wires in opposite directions so that the coupling of the rf current from the common-mode interference is maximized. If you are not sure which direction to wind them, try it both ways. The difference will be fairly obvious.

These toroids can be found in many locations. There are advertisements in most amateur magazines, you can find them in electronic supply stores.

In a pinch, you can even use discarded TV reflection yokes. After the wiring has been removed from these yokes, just tape the two ferrite sections together to form a large toroid. This MAY work, but keep in mind that the ferrite material was designed for 15 kHz. Who knows what it may do at VHF!

There are also other types of cures that we can try, like A/C line filters and tuned stubs at the television antenna lead.

About CATV Interference Part. 5

2009-01-21T10:34:00.000-08:00

Part #5: The Detective Work

Previously I explained in very simple terms the difference between differential and common mode signals. I went into this area because there are different ways to eliminate the two different types of interference. Understanding the difference between the two will assist you in locating the path of the interference into the television! The interference may be entering the system by one method, or by any combination of different methods. Finding the pathway of the interference can sometimes be a very complex, and at times very frustrating piece of detective work.

There are many areas or pieces of equipment where this interference can enter your system. Let me mention a few that I have heard of: (I am sure that there are many more.) Bad, loose, corroded or improper size cable connectors; improperly grounded cable systems; poorly-shielded (cheap) patch cords between cable boxes, VCRs, TVs and the like; poor 300 - 75 Ohm cable transformers; any twin lead wire; distribution amplifiers (either in your house or in the cable company distribution system); splitters; A/B Switches; video games; TVs; FM broadcast radios.

In fact, anything that is connected to your cable antenna system, cable ground, or A/C source to any equipment that is connected to any part of your cable system, or any combination of them all can be the culprit!! Well, that kind of narrows it down! Are we having fun yet?

But do not despair yet. With the right amount of perseverance and a little luck, you might be able to solve the problem.

First of all, let's focus on the simple and most obvious solutions. Sometimes, we as amateurs, as soon as we think of RFI, we think of either low pass filters, high pass filters, baluns, SWR, proper grounds or the like, when in fact the #1 influencing factor is our fundamental frequency radiating out of our best radiator, the antenna! Let's use the least amount of power necessary to accomplish the communication. Try to avoid putting our antennas or transmission lines right next to cable installations.

Maybe you will be lucky and find a leaky connection with your H/T as described in Part 2! If this is the fact, then eliminate that and you are all set! If not, we have a little detective work to do using the process of elimination.

In an ideal typical cable installation, the cable will be brought in from the pole, parallel to the ground with the shortest run possible, connected to a lightning arrestor which is connected to a good low resistance earth ground. This ground should be exclusively for the cable system, although more often than not, the earth ground is shared with the telephone system. This in itself is not really a problem, as long as the cable ground is connected directly to the ground source and not some kind of terminal block in the telephone system. The cable then should continue downward to ground level and run under the house to the appropriate rooms. All runs of coax up the walls and in the attic or ceilings should be avoided if at all possible. In many locations this is not possible. The people in apartments with multiple units and distribution systems are really at a disadvantage because of the amount of cable run all over the place. All of this excess cable can act as a long wire antenna system for both differential and common mode interference.

About CATV Interference Part. 4

2009-01-21T10:25:00.000-08:00

Part #4: Who's Is To Blame

There is a lot of misunderstanding between the cable subscriber, the cable-company service personnel and the amateurs. The subscriber often feels that the amateur is to blame. After all, if the amateur wasn't operating, there wouldn't be a problem. The amateur often feels that the problem is always the responsibility of the cable company, and that it can always be fixed if the cable system is better shielded. The cable-company repair personnel are often stuck in the middle. Sometimes they tell the subscriber that the ham must be transmitting signals outside of the ham bands, thus causing the problem (he really ought to know better!), or, just as bad for him, believes that the interference problem is always due to some defect in the cable system.

In reality, any, or all, of the parties may have some responsibility. Let's take a close look at what is really going on!

Although it is not usually related to cable TVI to channel 18 or channel 24, I must point out that the amateur is, by law, required to ensure that any spuriousemissions from his or her station do not cause interference to other services. Th ham must, and will, make any necessary adjustments to the station equipment to ensure that it is in compliance with FCC regulations.

In the case of channel 18, or channel 24, the cable company makes use of amateur frequencies for these channels. The problem is being caused by the amateur's fundamental signal. Any leakage, anywhere in the system, can allow that signal to get inside the cable. Once this happens, the interference cannot be filtered out.

If the leak is in the cable company's wiring, it is their responsibility to fix it. Keep in mind that they are not legally bound to fix leakage INTO the cable, only leakage OUT of the cable, but most operators will take whatever steps are necessary to ensure that their customers enjoy top-notch service.

If the channel-18 video carrier can be readily heard on the test 2-meter receiver, the cable company will probably need to locate and repair a leak. If this carrier is nearly inaudible, it may be best to try some of the following cures first:

There are several other forms of "leaks" that are not the responsibility of thecable company, although they are usually willing to help if they can. Some cable ready TVs and VCRs, and even some set-top converters, can be affected by the strong amateursignal present on the OUTSIDE of the coaxial shield (this signal is a common-mode signal). Many a cable operator has spend days changing perfectly good wiring when the real problem was common-mode interference. They, the ham or the subscriber can put a common-mode choke on the incoming cable line. In many cases, this will now allow for interference-free viewing.

The house AC wiring may also pick up a fair amount of signal. A common-mode choke, sometimes in conjunction with an AC-line filter, may help here. In extreme cases, it may be necessary to install both types of filters in the TV, the VCR and the set-top converter.

If the cable system doesn't leak (as evidenced by your 2-meter receiver) and the application of the common-mode filters and ac-line filters do not effect any improvement, you may be dealing with a case of direct pickup by the television set circuitry. There is (almost) nothing the cable company or the amateur can do in this case. Contact the manufacturer of the TV for assistance.

I say almost, because after the common-mode and ac-line filters are installed, it may now be possible to get good reception by using a set-top converter or VCR to tune in the desired channel. Keep in mind, when you are using an external tuner, the TV will not be tuned to the amateur frequency, so it may not be susceptible. Most set-top converters and modern VCRs are pretty well shielded, so after you solve the common-mode problem, they may now function just fine. The result is a happy subscriber, amateur and cable repair person.

In summary, all parties concerned are responsible for conducting themselves in a courteous and neighborly manner. The amateur is responsible for the proper operation of the amateur equipment, the cable operator is responsible for leakage within the cable wiring, and the manufacturers of the TVs and VCRs are responsible for ensuring that their equipment will continue to function properly near strong radio transmitters. Remember, if you have a hole in your roof, you don't blame the rain when your furniture gets wet!

About CATV Interference Part. 3

2009-01-21T10:19:00.000-08:00

Part #3: Now for a little theory!

There really are only three ways an unwanted signal can interfere with wanted television signal:

Radiated Interference - This type of interference is usually the fundamental frequency and is usually radiated directly out of your antenna! But this method can also include signals radiated through your shielding, off of your shielding, off of your ground wire or anything else that could directly radiate your signal.

Conducted Interference - This interference is propagated directly via a conductor such as A/C wiring or a common ground system. There are two different types of conducted interference, differential and common mode. I will explain these shortly.

And Induced Interference - Induced interference starts out as radiated interference and is picked up or received by some internal or external part of the affected television or its associated wiring. This type of interference is usually common mode.

Now to explain in more detail the distinct differences between differential mode and common mode interference. In any electrical circuit there must be two paths, the forward path and the return path.

In the differential mode signal, it is conducted via a two wire pair, such as your A/C power cord or your antenna lead in. In this case the circuit is created between the two wires in the system. In one wire and out the other! It is called differential because under normal circumstances these two wires are out of phase with each other. This circuit occurs without the necessity of an earth ground.

With the common mode signal, the unwanted signal can either overpower the normal out of phase signal and make the multi-wire appear to be a single phased long wire antenna with the return path being through the internal circuitry (IE: Capacitors) of the television to the earth ground, thus completing the circuit. Or, the signal can ride the outer coaxial shield and find the earth ground through the same internal circuitry to chassis ground.

It is important to make the distinction between the two, because leakage into or from the cable system, a differential-mode phenomenon, is clearly a cable company responsibility, but a common-mode signal conducted only via the coaxial shielding
clearly is not.

About CATV Interference Part. 2

2009-01-21T10:05:00.000-08:00

Part #2: Searching For Leaking CATV Signals

With an FM Handi-Talkie or scanner receiver the audio frequencies will seem to be distorted and over deviated, but obviously audio! On either the FM or AM the video carrier will sound just like an unmodulated carrier.

If leakage from the cable system caused harmful interference, it is the responsibility of the cable operator to eliminate the interference, regardless of the cause. If they determine that the leakage is coming from a subscriber's home, they must, if necessary, disconnect that home until the cause of the interference is found and corrected.

(a) Harmful interference is any emission, radiation or induction which endangers the functioning of a radionavigation service or of other safety services or seriously degrades, obstructs or repeatedly interrupts a radio communication service operating in accordance with this chapter.

(b) The operator of a cable television system that causes harmful interference shall promptly take appropriate measures to eliminate the harmful interference. There is much more to be read out of this part of the FCC Rules and Regulations and can be seen at your local Field Operations Bureau if you are interested.

In these bulletins, I am going to focus mostly on the type of interference caused to the Cable Channel 18, 24, and 64-68. The reason for this is two-fold. One, is because these are the channels that most commonly experience fundamental overload interference from amateur stations, and, two, because any interference caused by amateur stations to these cable television channels is clearly not the legal responsibility of the amateur. Depending on the ultimate cause of the problem, it may be the responsibility of the cable operator, the TV manufacturer or the subscriber.

Now if you are causing interference to your or your neighbors cable system there are many causes, suspected pieces of equipment, and possible cures that can be found.

But first I would like to address the situation of fielding your neighbors complaints. Your average neighbor probably does not understand the simplest aspects of radio theory. All they know is that they just spent a lot of money on a brand new cable television system and you are messing it up! Well, it is hard to make this person understand that the cable company decided to put their channel right on top of our amateur frequency! And quite possibly, he may never be able to understand. Well there are many areas of the country that have local interference committees comprised of amateurs to help you deal with this situation. It is a great advantage if possible to have a neutral third party meet with the offended neighbor and attempt to assist him in isolating the problem. This third party is most often your ARRL section Technical Coordinator (TC). To find your TC, contact your ARRL Section Manager (SM). A list of SMs appears on page 8 of any recent QST.

Above all, and this cannot be stressed strongly enough, be polite, courteous and as civil as possible when you are dealing with your neighbors! This same cooperative, friendly attitude should be used when communicating with your local cable company. There are all kinds of "big" problems that can be caused by undiplomatically just telling someone "It's your problem!"

And there may be no practical way to cure any specific problem! It may be that the only possible cure is for the local cable company move their premium channels to another frequency. And I have been told by other amateurs that this is exactly what the cable company in their areas have done.

About CATV Interference Part. 1

2009-01-21T09:53:00.000-08:00

Part #1: Overview and Channels Effected

The problem exists because the cable-television spectrum covers a wide frequency range. It starts down in the low end of the VHF range and continues on through the UHF range. This spectrum, as you know, also includes a number of amateur bands! In theory, the cable system is supposed to be a "closed" system. By "closed", I mean entirely enclosed in shielded cable. No cable leaks out, and no amateur radio leaks in! But in reality we find that this is not true. There are many areas of improper shielding, loose connectors, substandard coax and even thepossibilities of RFI or EMI (Electro-Magnetic-Interference) radiating through the cabinets of your TVs or being introduced into your TVs by the cable coax shielding itself!

As amateurs, before we start any investigation into the causes of a case of cable TV, we should first make sure that our shack is clean. Make sure your equipment is properly grounded. Make sure that your TNC is not over-deviating your radio. Check that your transmission lines are in proper condition. If necessary, apply the needed cures to your own cable-connected television system. This demonstrates that the cures used do work and that they cause no harm.

The television channel is 6 Mhz wide! Any signal transmitted in this 6 Mhz spectrum will cause noticeable interference to a television signal! Even a signal that is 40 dB weaker than the television signal will result in perceptible interference.

There are a number of cable channels that we as amateurs can have problems with. These cable channels roughly, if not exactly, correspond with these Amateur Bands:

Cable 2 [ 2] (55.55 Mhz)    = 6 Meters
Cable 17 [ D] (139.25 Mhz) = 2 Meters
Cable 18 [ E] (145.25 Mhz) = 2 Meters
Cable 23 [ J]    (217.25 Mhz)    = 1.25 Meters
Cable 24 [ K]    (223.25 Mhz)    = 1.25 Meters
Cable 64 [UU] (421.25 Mhz)    = 70 Centimeters
Cable 65 [VV]    (427.25 Mhz)    = 70 Centimeters
Cable 66 [WW]    (433.25 Mhz) = 70 Centimeters
Cable 67 [XX]    (439.25 Mhz)    = 70 Centimeters
Cable 68 [YY]    (445.25 Mhz)    = 70 Centimeters
Cable 69 [ZZ]    (451.25 Mhz) = 70 Centimeters

Be advised that there are no stipulated rules as to what channel number designator has to be on what frequency. (This is supposed to be a closed system!) These channels and frequencies may vary from one cable area to the next, although most cable systems use one of three "standard" systems. (The nice thing about standards is that there are so many to pick from!)

This interference can go both ways! In the majority of the cases reported, the interference was caused to the cable by the amateur. But there have been instances where there have been 1large enough leaks from the cable system to interfere with the amateur! From what I have seen, these instances have been created by either an improperly terminated cable ends (unconnected!), or by customer-installed "illegal" hookups. Rarely, the problem can be caused by a bad cable distribution amplifier or shield break somewhere on the pole cable.

As an example, I was receiving a S9 carrier on 145.250 on my packet radio. My radio was an Icom 28H and I had an 11 element vertical beam at 60 feet. I reported this to my cable company. A few days later they sent out their Engineer and DF Vehicle. I brought him up in my shack and showed him the interference. We used my beam antenna to determine that the interference was coming from a beam heading of 315 degrees (NW). We went to our vehicles and found on that heading, the offending equipment 500 yards away! It was an improperly terminated connector inside someone's house! An illegal hook up! So you can imagine what a bad distribution amplifier outside your window will do! Most of these leaks can be easily located with your HT or a hand held scanner. These are a few frequencies to listen for:

Cable Channel 17 (D) --- Audio at 143.75 Mhz.
Cable Channel 18 (E) --- Video at 145.25 Mhz.
Cable Channel 18 (E) --- Audio at 149.75 Mhz.
Cable Channel 23 (J) --- Audio at 221.75 Mhz.
Cable Channel 23 (J) --- Video at 223.25 Mhz.

CATV Upgrade Summary

2009-01-16T03:14:00.000-08:00

RE-LOCATION
Given the practical difficulties in relocating amplifiers, Scientific Atlanta, USA, a leading manufacturer of CATV hardware has proposed an alternate technique for system upgrade. Scientific Atlanta have designed their LE3 Amplifier for 750MHz operation and their Gainmaker Amplifier for 870MHz operation. Both these designs have been optimised for easy upgrade for existing 550MHz systems.

BREAKING THE RULES
Both the LE3 and Gainmaker Amplifiers provide a higher gain. The LE3 offers a total gain of 34 dB at 750MHz and the Gainmaker provides 36 dB gain at 870MHz. At first sight, providing a somewhat higher gain may seem simple, almost trivial. However, consider the higher gain, in light of what has been said earlier.A higher gain of approximately 6 dB would imply that the amplifier would be capable of delivering a 6 dB higher output level without higher distortion, or handling 6 dB lower input signals, without adding significant noise, or handling a 3 dB lower input signal & a 3 dB higher output level, without deterioration in the amplifier specifications. The new SA amplifiers adopt this third alternate. Keep in mind that these superior specifications are to be achieved along with the additional burden of the number of channels to be carried. The "Gainmaker" amplifier needs to deliver almost twice the number of channels at 890MHz, compared to a 550MHz product. This increased channel loading itself carries with it a 3 dB additional penalty on the specs.

REQUIRED SPEC
To summaries, the required specifications for a drop in upgrade amplifier, would demand 6 dB higher gain, a lower noise figure, a higher output, all with the additional burden of 106 Channel loading, instead of 60 Channel loading at 550 MHz. The new products offer a Noise figure of 7.5 dB at 750MHz for the LE III and 6.5 dB at 870 MHz for the Gainmaker, as compared to approx. 9 dB at 750MHz for conventional trunk amplifiers. A comparison of output levels & distortion (Composite Triple Beat - CTB) shows that the LE III achieves -67 dB at an output level of 106 dBU, and the Gainmaker offer -68 dB at 106 dBU, compared to conventional products that offer -59 dB at 106dBU @ 750 MHz or -65 dB at 106 dBU output @ 550 MHz.

SUMMARY
All put together, the specifications required for a simple drop-in replacement are very demanding & have been achieved using state-of-the art components & design techniques. Just simple additional gain will not provide a solution, but can lead to very poor performance in terms of both, noise & distortion.The solution provided by the new range of Scientific Atlanta amplifiers is very elegant & provides a smooth & simple upgrade path to 750MHz or even 890MHz systems, by using the amplifiers as drop-in upgrades. However, there is a huge amount of new technology that has gone into the making of the advanced products. As with all good ideas, imitations are sure to emerge quickly. However, the customer needs to be wary of any quick solutions that are offered, that simply provide additional gain, without addressing the associated issues such as lower noise & higher output levels that are essential for proper implementation of the solution.