[speex-dev] Optimizing speex for 44.1kHz

[Jean-Marc Valin]

Le ven 10/01/2003 à 14:39, John Hayes a écrit :
> I've been playing with speex for use in a VoIP application between PC's. One
> thing I've found (correlating to the documentation) it that speex runs much
> faster and produced much better output when it's fed a 32kHz signal instead
> of a 44.1kHz sample rate. This is whether I tell it a 44.1kHz sample rate
> and feed it 44.1kHz or tell it 32kHz and feed it 44.1kHz.

First, note that the sampling rate you *tell* it is only used for
playing back at the right speed and doesn't influence the algorithm
(yet). Speex treats 32 kHz the same way as 44.1 kHz so the speed factor
is just 44.1/32

> What part of the algorithm is optimized for 32kHz (my initial guess was the
> codebooks and/or the codebook search) - would it be possible to modify it to
> take a 44.1kHz signal?

It's still not clear yet to me, but it's probably a combination of the
number of LSP's (LPC order) used and the tuning of the "perceptual
weighting filter" (analogous to psycho-acoustic models in other audio
codecs). In the first case it's not possible to change it without
changing the bit-stream, but in the second case, it would be possible to
do something about it.

> I'm interested in avoiding the sample-rate conversion on record and playback
> since it costs me latency and CPU time.

The cost of down-sampling, if done efficiently, is probably less then
the cost difference between 32 kHz and 44.1 kHz so it's probably worth
it. If you don't care about standard sampling rate, you could even to a
2/3 conversion which would get you 29.4 kHz...

        Jean-Marc


-- 
Jean-Marc Valin, M.Sc.A.
LABORIUS (http://www.gel.usherb.ca/laborius)
Université de Sherbrooke, Québec, Canada

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