Average bitrate (ABR)
refers to the average amount of data transferred per unit of time, usually measured per second. This is commonly referred to for digital music or video. An MP3 file, for example, that has an average bit rate of 128 kbit/s transfers, on average, 128,000 bits every second. It can have higher bitrate and lower bitrate parts, and the average bitrate for a certain timeframe is obtained by dividing the number of bits used during the timeframe by the number of seconds in the timeframe. Bitrate is not reliable as a standalone measure of audio/video quality, since more efficient compression methods use lower bitrates to encode material at a similar quality.
Average bitrate can also refer to a form of variable bitrate (VBR) encoding in which the encoder will try to reach a target average bitrate or file size while allowing the bitrate to vary between different parts of the audio or video. As it is a form of variable bitrate, this allows more complex portions of the material to use more bits and less complex areas to use fewer bits. However, bitrate will not vary as much as in variable bitrate encoding. At a given bitrate, VBR is usually higher quality than ABR, which is higher quality than CBR (constant bitrate).ABR encoding is desirable for users who want the general benefits of VBR encoding (an optimum bitrate from frame to frame) but with a relatively predictable file size.Two-pass encoding is usually needed for accurate ABR encoding, as on the first pass the encoder has no way of knowing what parts of the audio or video need the highest bitrates to be encoded.
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.
Constant bitrate (CBR)
is a term used in telecommunications, relating to the quality of service. Compare with variable bitrate.
When referring to codecs, constant bit rate encoding means that the rate at which a codec's output data should be consumed is constant. CBR is useful for streaming multimedia content on limited capacity channels since it is the maximum bit rate that matters, not the average, so CBR would be used to take advantage of all of the capacity. CBR would not be the optimal choice for storage as it would not allocate enough data for complex sections (resulting in degraded quality) while wasting data on simple sections.
The problem of not allocating enough data for complex sections could be solved by choosing a high bitrate (e.g., 256 kbit/s or 320 kbit/s) to ensure that there will be enough bits for the entire encoding process, though the size of the file at the end would be proportionally larger.
Most coding schemes such as Huffman coding or run-length encoding produce variable-length codes, making perfect CBR difficult to achieve. This is partly solved by varying the quantization (quality), and fully solved by the use of padding. (However, CBR is implied in a simple scheme like reducing all 16-bit audio samples to 8 bits.)
