[speex-dev] reommended settings for low bitrate voicecom codec ?

Jean-Marc Valin jean-marc.valin at hermes.usherb.ca
Mon May 12 19:00:34 PDT 2003

> What I did to find this out:
> I comprared a speex AVB with 6.3 KBit/sec (total, overhead for packets and 
> stuff included) and the 6.3 Kbit/sec Celp Codec vom hawkvoice (
>  http://www.hawksoft.com/hawkvoice) and the result is quite favorable for 
> hawkvoice.
> I cant believe that this is *normal* speex behavior, please correct me if that 
> hawkvoice thing just performs better on that bandwidth.

First, what do you mean by AVB (average bit-rate?)? Also, when you say
6.3 kbps with overhead how much of that is the Speex bandwidth (helps me
get an idea of the expected quality). 

> Could you please tell me what options are best for what codec bandwidth ? Im 
> figuring something is wrong since speex sounds worse than hawkvoice on the 
> low bandwidth...I would appreciate if youd post the *recommended* settings 
> for 5, 10, 15 and 20 KBit narrowband AVB speex (used in a voicecom that 
> shouldnt take too many cpu cylces of the game).

First, in the decoder make sure that you turn perceptual enhancement
*on* (it is off by default when using the libspeex API). To do that:
int tmp=1;
speex_decoder_ctl(decoder_state, SPEEX_SET_ENH, &tmp);

For the rest, all you need to do is decide whether you want constant
bit-rate or variable bit-rate and whether you want discontinuous
transmission (DTX). You might also want to play with the "complexity"
value, but the default is usually OK.


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

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