[vorbis-dev] Just to dispel any hopes -- RC3 really low bitrate

[Kenneth Arnold]

I've just done some rudimentary testing to see how Vorbis degrades at
absurdly low bitrates without downsampling. In summary, don't hope for
anything decent below -q 0 for now. I tried oggenc -b <bitrate> -M
<bitrate> for the below and a few in between:

24k - spectral energy "floor" captured decently, but many pure-tone
blips (think old computer movie sound effects) in mid to high
range. Totally untolerable.

...

35k - blips mostly gone, audio present but very muffled (high
frequency basically gone). Occasionally it will "open up" a little and
get some high-end, and stay like that for maybe a second (definately
more than one block), then close up again. I might be able to live
with this, it sort of sounds like Real at 16k or so.

...

45k - now most of the time it sounds like 35k when it opened up, and
like 35k there are times when it'll pull in another block of
higher frequencies. The average quality level seems about consistant
with what I'd expect a stream coming down from my 28.8 modem (~24k) to
sound like, if the rough edges were smoothed over a little.

...

64k - just to try what (ideal) single-channel ISDN would sound like;
generally quite tolerable. Still munges high frequency, but nearly an
order of magnitude less perceptably than lower bitrates. Note that
this is not the VBR 64k mode; I've essentially locked the codec at 64k
and below. It still should sound roughly like 64k normally (which I
can live with for e.g. background music), but it has more obviously
bad spots.

Definately some improvement in tuning elsewhere too. Quality around 2
gets me roughly the same result I've come to expect from 128k --
pretty much transparent unles listening very closely -- but at around
30kb/s less than RC2. Those with more "golden ears" than I may contest
that as a general statement, but I think I'll encode at around 2.5 for
most things.

For future Vorbis development, I think the quality scale should be
shifted lower, or perhaps made logrithmic -- the marginal benefit per
unit bitrate is much more at lower quality settings than higher (I
doubt I would be able to hear any difference at all for 5 and above,
while 0 through 3 were obvious step-ups). Also, if possible lower
bitrates should degrade more gracefully. Gradually add
high-frequencies, not in chunks. Does the bitrate limiting engine have
any linkup with psycoacoustics? -- it doesn't sound like it, at least
not for lower bitrates. I'd experiment with adding in harmonics of the
highest frequency coded to fill in the high end; it may be better to
have a wrong high-end than no high-end, psycoacoustically speaking.

Note that the Vorbis internals have changed significantly since I
last really tried to study them, so pretend that I don't know what I'm
talking about and you'll probably be right.


-- 
Kenneth Arnold <ken at arnoldnet.net>
- "Know thyself."


-------------- next part --------------
A non-text attachment was scrubbed...
Name: part
Type: application/pgp-signature
Size: 233 bytes
Desc: not available
Url : http://lists.xiph.org/pipermail/vorbis-dev/attachments/20020101/873399bb/part.pgp