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. 

  

 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|DVDVideo] 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). 

  

 !Licensing 

 MPEG-1 doesn't seem to be subject to any licensing requirements. MPEG-2 and MPEG-4 are licensed by [MPEG-LA|http://www.mpegla.com/]. 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 

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 CategoryStandards