[vorbis-dev] PlusV algorithm

gtgbr at gmx.net gtgbr at gmx.net
Wed Jan 29 05:38:07 PST 2003



"Rany ." wrote:
> Important (imho)!
> I found this site in the internet:
> http://www.plusv.org/

We already had this, search the archives.

> P.S. what about some long-awaited features in Ogg Vorbis?
> 1) ability to turn off the frequency filter in the encoder (especially for high bitrates)

According to vorbis/lib/modes/psych_44.h, there's no lowpass for q6 and
higher for 44.1kHz input, a lowpass at 48kHz for q5 and 20.5kHz for q4,
etc. The complete line is

tatic double _psy_lowpass_44[11]={
  15.1,15.8,16.5,17.9,20.5,48.,999.,999.,999.,999.,999.
};

So, why do you want to set the lowpass, as it's very high, i.e.
nonexistant already in high bitrates? If you'd raise it for those modes
that actually use a lowpass, you'd just decrease overall quality in many
other aspects. I also see no point in lowering it, as Vorbis does a
pretty good job encoding the frequencies the current filters let
through.

Anyways, libvorbis has a function "vorbis_encode_ctl" where you can set
those parameters, and oggenc already has this (undocumented) feature. I
haven't used it, yet, and never will - so if you want to figure out how
it works, look at oggenc.c and encode.c in vorbis-tools/oggenc for
parameter names and arguments.

> 2) CBR encoding

You can do something close to that already with the bitrate management
engine, e.g. limit the bandwidth to a certain bitrate within a 2-seconds
window. An oggenc command line would look like `oggenc --managed -m
<bitrate> -b <bitrate> -M <bitrate> input.wav`. I'm not quite sure why
you'd want this, though - quality suffers and CBR is only required in
pretty exotic environments.

<p>Moritz
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