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 The resultant Video-CD was quite successful but has now been superseded by DVD.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)
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)
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