Variable bit rate
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The term variable bit rate (VBR) is a term in telecommunications Quality of Service. Compare with constant bit rate.
An *.mp3 file can be recorded with a variable bitrate from 8 - 320 Kbit, a lower rate will save storage.
When referring to codecs, variable bit rate encoding varies the amount of output data in each time segment based on the complexity of the input data in that segment. The goal is to maintain constant quality instead of maintaining a constant data rate. VBR is preferred for storage (as opposed to streaming) because it makes better use of storage space: More space is allocated to more complex segments while less space is allocated to less complex segments.
Codecs such as Vorbis and almost all video codecs are inherently VBR. *.mp3 files can optionally be encoded in this way.