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

[Monty]

> 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?)

Depends.  It varies from zero to a few kilobits depending on what the
psychoacoustics model says.

> 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. 

Correct, however, the ear is extremely sensitive to preecho and
time-localization of high frequency energy.  You don't hear the pitch
in the high frequencies, you hear the fact that a sharp edge was
smeared (what aggressive quantization in the high end will cause).

> 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/

Harpsichord (like voice) is well suited to this technique because of
regular harmoncs.  Try it on violin, cymbals, and nonmusical sources.

I hear a brief, glassy preecho ... what block size were you using for
your experiment?  I'm guessing very short.... The results might be
more if not used in situations where ogg/lame would be using short
blocks and used over lapped 2048 sample blocks like ogg.

Monty

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