LAME - Average Bit Rate (ABR)
Suggested usage:
lame --abr 128 input.wav output.mp3
ABR is a mix between CBR and VBR.
Like CBR, the files will have the (approximate) bitrate specified in the
commandline, and uses the CBR algorithm to compute the number of bits needed
to encode each frame.
Like VBR, the files will use different frame bitrates so instead of relying on bit reservoir like CBR, each frame just uses the smallest possible bitrate that can encode it.
The difference between ABR and true VBR is in how the desired number of bits is chosen. The true VBR mode determines the number of bits based on the quantization noise. VBR figures out how many bits are needed so that the quantization noise is less than the allowed masking.
ABR mode uses the CBR formula to determine the desired number of bits. This formula is based on the perceptual entropy, which is a rough measure of how difficult the frame is to encode.
The majority of frames in a VBR MP3 produced with an ABR method is normally at or near the target bitrate chosen by the user, but each frame can still potentially vary within the normal range of 8 to 320 kbps. The ABR encoder will typically limit the range of bitrates it can choose from, or will greatly favor certain bitrates, in an effort to ensure that the average comes out near the target.
Who should use ABR
ABR encoding is desirable for users who want the general benefits of VBR (an optimum bitrate from frame to frame) but with a relatively predictable file size like they would get with constant bitrate (CBR), and a greater preference for bitrates that are near a desired target. Inevitably, some frames will be encoded with more bits than necessary, but the result will always be equal to or better than that of CBR for the target bitrate.
