>
> >>>>> "S" == Sergey A Maslyakov <[EMAIL PROTECTED]> writes:
>
> MT> The 16kHz cuttoff in LAME is only used at 128kbs.
>
> S> By the way, Mark, if VBR falls back, let's say from 192kbps
> S> to 128kbps or lower, does it impose this cutoff?
>
> See the sourcecode, and you will find it depends on VBR quality
> setting (-V option).
>
> and more exactly speaking, 16kHz cutoff is used when compression ration
> is bigger than 9.
>
> converting 44.1kHz 16bit stereo PCM into 128kbps mp3, the ratio is about
> (44100 * 16 * 2) / (128 * 1024) = 10.7
> so 16kHz cutoff is used.
>
> converting 32kHz 16bit stereo PCM into 128kbps mp3, the ratio is about
> (32000 * 16 * 2) / (128 * 1024) = 7.8
> so 16kHz cutoff is not used.
>
> Maybe these matter should be on the man page, but is not. I hope someone
> good at English writing add this mechanism to the manual or document.
> ---
> [EMAIL PROTECTED] // may the source be with you!
> --
Also, the above formulas are just a first guess. I would like
to replace the scalefactor band 21 cutoff with a true lowpass filter,
and have the frequency where the filter is applied depend on the
compression ratio (or any other formula that involves bitrate and
sampling rate).
As for VBR: we decided a while ago that switching the cutoff
on and off (as well as jstereo - jstereo is also not used at high bitrates)
as the bitrate changed from frame to frame would probably
be a bad idea, since this switching can make the noise more noticable.
Since VBR doesn't have a pre-defined compression ratio, the
various choices have to be made based on the -V setting.
Mark
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