Variable Bitrate

In Variable Bitrate (VBR) coding, the user chooses the desired quality level and/or a range of allowable bitrates. Then the encoder tries to maintain the selected quality during the whole stream by choosing the optimal amount of data to represent each frame of audio. The main advantage is that the user is able to specify the quality level and conserve as much space as possible, but the inconvenience is that the final file size is quite unpredictable.

Most modern encoders are able to perform VBR encoding, including (but not limited to) nearly all popular MP3, AAC, Ogg Vorbis, Musepack, and WMA encoders.

Some codecs limited to CBR include AC3 (in theory it can encode to VBR, but no high quality encoder offers that feature), DTS (same thing as AC3, no publicly available VBR encoder), Real Audio and ATRAC3.

All lossless codecs are, by nature, VBR.