GPSYCHO

Variable Bit Rate



OVER = number of scalefactor bands with distortion that exceeds the allowed distortion given by the masking thresholds.   OVER is computed by outer_loop, and the masking thresholds are computed by the psycho-acoustic model.

VBR_q = the minimum value of OVER which is to be allowd.   Default = 4.  Can be set from 0..9 with -V.  Setting VBR_q = 0 is the highest quality, but will produce larger files.  VBR_q = 9 will produce small files and poor results.

VBR_min = the minimum allowed bitrate.  Default = ?   Can be set with -b.

VBR_max = the maximum allowed bitrate.  Default=256kbs, unless the minimum bitrate is 256kbs or higher, in which case VBR_max=320kbs.  (the highest allowed by MP3)


The LAME VBR algorithm is at present very simple:

LAME will find the smallest allowed bitrate such that OVER  <=  VBR_q.

If the frame contains short blocks, then the minimum bitrate is made much larger since the OVER will not measure distortion casued by pre-echo.  We use bitrates of at least 160kbs for short blocks to make sure they sound good.


Usage notes:

If you want to use VBR to improve the quality of 128kbs encodings while only making the file slightly larger, use:

% lame -v -b 112  input.wav output.mp3

If you want to ge t the maximum compression, try:

% lame -v -b 32 input.wav output.mp3

If the encoded file is much too big, add the option -V n, and try values of n>4.   To improve the quality (and make the file bigger) try -V n with n<4.