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.
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.
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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.
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.
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