MPEG - applications of compression

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.