The Moving Picture Experts Group (MPEG) is a small group charged with the development of video and audio encoding standards. Since its first meeting in 1988, MPEG has grown to include approximately 350 members from various industries and universities. MPEG's official designation is ISO/IEC JTC1/SC29 WG11.
MPEG has standardized the following compression formats and ancillary standards:
- MPEG-1: Initial video and audio compression standard. Later used as the standard for Video CD, and includes the popular Layer 3 (MP3) audio compression format.
- MPEG-2: Transport, video and audio standards for broadcast-quality television. Used for over-the-air digital television ATSC and DVB, digital satellite TV services like DirecTV, digital cable television signals, and (with slight modifications) for DVD video discs.
- MPEG-3: Originally designed for HDTV, but abandoned when it was discovered that MPEG-2 was sufficient for HDTV.
- MPEG-4: Expands MPEG-1 to support video/audio "objects", 3D content, low bitrate encoding and support for Digital Rights Management. A new (newer than MPEG-2 Video) higher efficiency video codec is included (an alternative to MPEG-2 Video), see H.264.
- MPEG-7: A formal system for describing multimedia content.
- MPEG-21: MPEG describes this future standard as a Multimedia Framework.
How MPEG works
The MPEG codecs use lossy data compression using transform codecs. In lossy transform codecs, samples of picture or sound are taken, chopped into small segments, transformed into a frequency space, and quantized. The resulting quantized values are then entropy coded.
The moving picture coding systems such as MPEG-1, MPEG-2, and MPEG-4 add an extra step, where the picture content is predicted from past reconstructed images before coding, and only the differences from the reconstructed pictures, and any extra information needed to perform the prediction, are coded.
MPEG standardizes only the bitstream format and the decoder. The encoder is not standardized in any way but there are reference implementations available for members that produce valid bitstreams. That means that any MPEG-4 decoder can decode any MPEG-4 material (of the same type) regardless of the encoder which produced that material.
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