Operation where the bit rate changes with time during the decoding of a compressed bit stream. Although variable bit rate is acceptable for plain linear playback, one important consideration not to use variable bit rate is that reasonably quick random access becomes nearly impossible. There is no table of contents or index in MPEG. The only tool the play back system has for approximating the correct byte position is the requested play back time stamp and the bit rate of the MPEG stream.

MPEG streams do not encode their play back time. To approximate an intermediate position in a variable bit rate stream, the play back system must grope around near the end of the stream to calculate the playback time, and assume the stream is approximately constant bit rate. The groping around for the correct position can take several seconds. This is not appropriate for an interactive presentation or game. This groping around is, at least, annoying when trying to view a portion of a movie but it is not even possible for video streams because there are no time stamps (the SMPTE. time codes in video streams need not be continuous or unique).

Audio streams are always fixed bit rate.