[vorbis-dev] Low bitrate high-band coding...

[Jean-Marc Valin]

Hi,

I'd like to contribute to Vorbis and I think this may be of some interest for
low bitrate coding. I have been experimenting with low bit-rate coding for the
high-band (11 kHz to 22 kHz) and, though I haven't yet started quantizing my
coefficients (a gain and an LPC filter), I expect to be able to approximate the
whole 11-22 kHz band with around 1000 bits/s per channel (maybe even 500 bps).
Now, I don't know what is the normal bit-rate allocated for this band, but I
expect it is greater than that. Am I right? (can anyone give me numbers for
this?)

The technique I use to do this is inspired from an acticle I published recently
(http://panoramix.dyndns.org/jm/scw2000.pdf) and is based on the fact that at
these frequencies, the ear is totally insensitive to the spectral fine
structure. The processing also has relativly low complexity (most of it is two
LPC analysis in the encoder and one in the decoder).

I have tested it with some files (including harpsichord, which is supposed to be
hard to code) and the difference with the original (CD rip) is hard to hear. You
can find demo files of this at:
ftp://freespeech.sourceforge.net/pub/freespeech/

There are 6 files:
bach10-ref.sw        : Original file (right channel from Bach's Chromatic
Fantasia)
bach10-ext.sw        : Resulting file from my experiment.
bach10-lp.sw         : Low-passed at 11 kHz
bach10-lame-ref.sw   : Encoded with lame (128 bps), but original in the low band
bach10-ogg-ref.sw    : Encoded with vorbis (160 bps), but original in the low
band
bach10-ogg128-ref.sw : Encoded with vorbis (128 bps), but original in the low
band

For the last 3 files, I put back the original low band (0-11 kHz) so that only
the high band differences are present. The files are PCM 16 bits/sample, little
endian, 44.1 kHz.

Anyone thinks this could be useful? Any interesting audio file you'd like me to
process?

        Jean-Marc


-- 
Jean-Marc Valin
Universite de Sherbrooke - Genie Electrique
valj01 at gel.usherb.ca

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