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7.3. MPEG-4 File Format

Historically, prior to the MPEG-4 standard, MPEG has not had an explicit file format. MPEG-1 and MPEG-2 content typically is exchanged as files that represent, in a sense, a stream ready to be delivered. These files usually have embedded absolute time stamps, and the data often has been fragmented with a particular transport in mind (e.g., MPEG-2 Systems transport stream). These characteristics can make random access difficult and editing or reuse of the streams hard without decoding, demultiplexing, and then rebuilding the stream after editing. In MPEG-4, no single transport protocol is preferred, so fragmenting the data to suit the packet size of a specific protocol was not acceptable.

The MPEG committee sought a life-cycle file format—one in which the files could be used when capturing media, editing it, and combining it; when serving the media as a file download or as a stream; and when exchanging partial or complete presentations. This need for a life-cycle format is not met in many simple file format designs. For example, as noted previously, the design approach of MPEG-2, in which a stream is simply recorded to a file, makes editing hard. A more flexible design and approach were needed.


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