Inside Macintosh: QuickTime Reference

Background

The fundamental difference between constant bitrate (CBR) and variable bitrate (VBR) audio is related to the rate at which audio data is presented to the sound decoder to generate sound.

In CBR audio, the rate is constant. If one second of audio requires 40 K bytes, then 5 seconds will require 200 K bytes (= 40 Kbytes/sec * 5 sec). Moreover, given a stream of 3 minutes of audio compressed like this, to start playing at 2:30, you would advance 6,000K into the stream.

With VBR audio, the data rate varies depending upon the complexity of the encoded sound. For example, a very quiet passage of a score could be compressed much more than a very exciting passage. A VBR encoder will analyze the audio and use the appropriate number of bits, varying its usage in the process. This means that the amount of data for a complex passage is greater than for a less complex passage. This also complicates locating data in the stream because the "road map" is located within the stream.

By way of analogy, video encoding formats are typically VBR in nature. A more complex image requires more bits than a less complex image. As different images are encoded, the number of bits required for each will vary. The analog to CBR audio in the video space is raw RGB or uncompressed YUV.

Inside Macintosh: QuickTime Reference