What is variable bit rate (VBR) in MPEG?
VBR is an 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 for not using 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 playback system has for approximating the correct byte position is the requested playback time stamp and the bit rate of the MPEG stream. MPEG streams do not encode their playback time. To approximate an intermediate position in a variable bit rate stream, the playback system must grope around near the end of the stream to calculate the playback time, and assume the stream has an 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 to be continuous or unique). Audio streams are always fixed bit rate.
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