Variable bitrate (VBR) is a term used in telecommunications and computing that relates to the bitrate used in sound or video encoding. As opposed to constant bitrate (CBR), VBR files vary the amount of output data per time segment. VBR allows a higher bitrate (and therefore more storage space) to be allocated to the more complex segments of media files while less space is allocated to less complex segments. The average of these rates can be calculated to produce an average bitrate for the file.
MP3, WMA, Vorbis, and AAC audio files can optionally be encoded in VBR. Variable bit rate encoding is also commonly used on MPEG-2 video, MPEG-4 Part 2 video (Xvid, DivX, etc), MPEG-4 Part 10/H.264 video, Theora, Dirac and other video compression formats.
Advantages and disadvantages of VBR
The advantages of VBR are that it produces a better quality-to-space ratio compared to a CBR file of the same data. The bits available are used more flexibly to encode the sound or video data more accurately, with fewer bits used in less demanding passages and more bits used in difficult-to-encode passages.
The disadvantages are that it may take more time to encode, as the process is more complex, and that some hardware might not be compatible with VBR files. VBR may also pose problems when streaming over a dial-up or broadband Internet connection, because the instantaneous bitrate, if unconstrained, may rise above the maximum data transfer speed of the web connection. These problems can be avoided by specifying a maximum instantaneous bitrate during the encoding process.
Also, encryption of VBR-encoded speech (or other signals including video) gives only limited privacy, as the patterns of variation of the bit rate allow recognition of many words and phrases.
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