Introduction to MPEG-1

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.