As opposed to constant bitrate (CBR), VBR files vary the amount of output data per time segment.
[citation needed] The advantages of VBR are that it produces a better quality-to-space ratio compared to a CBR file of the same data.
[2][4] 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.
In the first pass of two-pass encoding, the input data is being analyzed and the result is stored in a log file.
[8] The disadvantage of single pass ABR encoding (with or without Constrained Variable Bitrate) is the opposite of fixed quantizer VBR — the size of the output is known ahead of time, but the resulting quality is unknown, although still better than CBR.
[9] The multi-pass ABR encoding is more similar to fixed quantizer VBR, because a higher average will really increase the quality.
In the last pass, the encoder distributes the available bits among the entire video to achieve uniform quality.