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Acronym for Motion Picture Experts Group

Name for a number of different multimedia container formats and associated audio/visual CoDecs.

  • MPEG-1 -- original spec from the early 1990s, allowing for video at around 320*240 resolution (roughly comparable to VHS quality). Used in Video CD discs, which became quite popular in Asian markets, though they were supplanted by DVD before they could take off in the West.

The spec allowed for three different ways of encoding audio, called layers 1, 2 and 3. MPEG-1 audio layer 3 became very popular in the late 1990s as a way of distributing music on the Internet, because its compression allowed a music piece of several minutes' duration to occupy just a few megabytes, which was tolerable to distribute at dial-up speeds while still offering decent audio quality. These files, commonly known as "MP3" files, are essentially MPEG-1 files containing audio but no video.

  • MPEG-2 extended the MPEG concept to handle a greater variety of resolutions (both low and high) and newer video and audio codecs. MPEG-2 files are commonly found on DVDVideo discs.
  • MPEG-3 was supposed to be a standard for use in HDTV. It was abandoned when it was realized that these needs could be easily met by either MPEG-2 or MPEG-4. Hence there is no MPEG-3.
  • MPEG-4 was apparently based on Apple's QuickTime technology, and includes more interactive features beyond basic audio and video playback.

An MPEG file contains one or more "streams". Thus, video is one stream, and audio is another stream; even if the audio is stereo with two or more channels, that is still one stream. MPEG files on DVD-video discs can contain multiple audio streams for soundtracks in different languages, as well as "private" streams (in formats not defined by the original MPEG specifications, but by the DVD-video specification) for holding subtitles and trick-play data. Streams are multiplexed, which means that, as the file is read sequentially, you encounter blocks of data belonging to each stream in turn, which are meant to be played at the same time. This allows a player to process all the streams concurrently, without having to continually jump around the file.

GOP, I-Frame, B-Frame, P-Frame

Most video codecs rely heavily on interframe as well as intraframe compression to reduce data sizes. An I-Frame is a frame of video compressed by itself, without looking at other frames. The encoding scheme used is similar to JPEG compression. However, subsequent frames are quite likely to look similar (think of the common case of something or someone moving against a still background); therefore, instead of compressing them on their own as additional I-Frames, it makes sense to encode them as P-Frames which are encoded as differences from the preceding reference frame (which can be an I-Frame or a P-Frame) or as B-Frames which are encoded as differences from both preceding and following I- or P-frames. The following reference frame is the closest following reference frame (I or P).

The drawback with this is, if you try to start playback from some arbitrary point that is not at the beginning of the file, the player has to seek backwards until it hits an I-frame before it can start sensibly decoding the video. Thus, using fewer I-frames improves compression, at the expense of quick random access into the video stream. The DVD-Video specification requires at least one I-frame in just over every second of video.

The sequence of frames starting from an I-frame until the last frame before the next I-frame (in other words, containing all the frames depending in some way on the starting I-frame) is called a Group of Pictures (GOP).

More esoteric details can be found in MPEGTerminology.

Licensing

MPEG-1 doesn't seem to be subject to any licensing requirements. MPEG-2 and MPEG-4 are licensed by MPEG-LA. It looks like licensing for MPEG-4 is less onerous than for MPEG-2, if only because Microsoft's Windows Media Player doesn't include support for MPEG-2 or DVD-Video playback, unless you pay for a third-party codec or upgrade to Vista Home Premium or Ultimate.

See Also


CategoryStandards