[Speex-dev] Re: [Iaxclient-devel] iaxclient & speex

[Steve Kann]

David Brazier wrote:
> Hi
>
> The latest SVN trunk for speex has changed the SpeexPreprocessState to
> an opaque structure, for jolly good software engineering reasons.
> However, the Analogue AGC (AAGC) feature of iaxclient (in audio_enode.c)
> relies on some members of this.  It uses speech_prob to detect when
> there is enough speech to consider AAGC and then loudness2 to decide how
> to adjust the input mixer.  We want to use the latest speex, for echo
> cancellation and other reasons, so I tried just making
> SpeexPreprocessState visible again, but it turns out neither of these
> members are present in the new SpeexPreprocessState_ structure anyway.
> I was able to re-create the speech_prob member by setting it from Pframe
> before speex_preprocess_run returns, and I've used agc_gain as the basis
> for adjusting the mixer (i.e. if agc_gain is high, the input is low, so
> turn the mixer up and v.v.).  This appears to be effective.
>
> So is there a way to make these items available to speex clients?  The
> speex_preprocess_ctl interface could be extended, but it's not really
> what it is intended for, is it?
>
> Also, can anyone clarify the new use of speex_preprocess_ctl for
> SPEEX_PREPROCESS_SET_PROB_START/CONTINUE?  It used to have a float
> parameter, and it now seems to be int, but I couldn't get it to set the
> value correctly and have had to change speex_preprocess_ctl back to
> using a float.
>   


Hi, David,

    As you can tell, the AAGC integration with speex was really a 
classic hack.  Instead of re-creating the hack, what's probably best 
here is to integrate AAGC back into speex, and have a proper API.

    For those of you just tuning in, what I call "AAGC" is an AGC 
implementation where analog gains are manipulated instead, or in 
addition to the AGC within speex (where levels are normalized via 
multiplication).  The benefits of AAGC are:  (1) (most important), 
reducing the analog gain can prevent clipping, which can't be done with 
speex' current AGC, and (2) when raising levels, you get better quality 
by raising the mixer levels, as opposed to just multiplying.

(1) is really the most important reason.

    Now, the API I'd envision for this would be one where you could tell 
speex that you would like to use AAGC, and then register some callbacks 
that speex_preprocess() could call to query or set the input or mixer 
level.  Further, a more intellegent implementation within speex could 
consider the requested changes in the rest of the preprocessor chain 
(i.e. it would know that if it asked for a 3dB increase in input gain, 
to expect that input levels would rise by 3dB within a few frames).  The 
hacky implementation I did inside of iaxclient gave speex no such 
information.

-SteveK