Information Technology Reference
In-Depth Information
(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
1.3 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 The resultant Video-CD was quite successful but has now been superseded by DVD.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.
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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.
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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.
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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)
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LeGall, D., MPEG: a video compression standard for multimedia applications.
Communications of the ACM
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34
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No.4, 46-58 (1991)
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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))
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MPEG-4 Standard: ISO/IEC 14496-2: Information technology - coding of audio-visual objects: Amd.1 (2000)
1.4 Some 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 speech-grade 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.
