[speex-dev] Higher Bandwidth at lower quality settings

[Jean-Marc Valin]

Le jeu 27/02/2003 à 09:43, Trevor Yensen a écrit :
> Hi Jean-Marc,
> 	I thought at quality 3 (wideband) - wb_submode1 that the 4-8k band was not
> using a codebook table.  From the code I can see that some sort of "lsp"
> encoding is performed.  What exactly does this encode?  (I assume lsp means
> line-spectral pairs)

You're right that there's no codebook being used below a certain
quality. What happens is that for low bit-rates, only the shape of the
high-band spectrum is encoded and the excitation (the part usually
encoded with a codebook) is simply "copied" from the low-band part with
the proper gain. 

> 	The reason I am asking is I'm comparing the "effective" spectral bandwidth
> of Speex against the AMR-WB (G.722.2) codec.  (Available from 3GPP.org)
> Speex performs very comparably to AMR-WB until the quality setting drops
> below around quality 5.  At approximately equivalent bitrates AMR-WB
> maintains the spectral bandwidth better, whereas Speex doesn't seeem to
> match above 3.5kHz.  I was wondering if it would be possible to tweak Speex
> to drop the narrowband quality at the expense of increasing the bandwidth?

You can try playing with the settings in mode.c if you like. Please
report anything interesting you find.

        Jean-Marc


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

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