[Speex-dev] Speech detection in preprocessor with echo

[Tom Grandgent]

agc_gain seemed to fit with the idea of what I wanted to do, it was 
easy to understand its units and behavior, and freezing it produced 
the desired results.  Also I wanted to cap it, so that's done at the 
same place, and that definitely works.

All I want to do is be able to freeze AGC adaptation and put an 
upper bound on the AGC (for example, 2x amplification).  Both of 
these things seem necessary in a real-world app because:

1) AGC gain should not increase when speech is not detected.  If it 
does, then it will inevitably rise during periods of inactivity on 
the part of the speaker, and then background sounds will be end up 
being amplified too much and detected as speech.  This is a problem 
regardless of echo.

2) The upper bound is necessary in some situations when VAD is not 
sufficient to distinguish between desired and undesired sounds.  
For example, consider a person using a headset and communicating 
infrequently while constantly using a nearby and noisy peripheral 
such as a force-feedback steering wheel.  Noises from the wheel are 
going to get picked up and detected as speech, but they usually 
won't be as loud as speech.  By capping the AGC at the right level, 
it's possible to prevent the AGC from amplifying the wheel noises 
too much while still allowing it to do its job for the speech.

I see now that st->loudness2 is also used in the VAD.  Maybe this 
explains some problems I was having. :)  I'll have to give the 
preprocessor's VAD another try now that I'm aware of this.

So, do you think it's better to use st->loudness2 for both freezing 
and capping the AGC?

Tom

Jean-Marc Valin <Jean-Marc.Valin at USherbrooke.ca> wrote:
> 
> Just curious, why are you freezing agc_gain instead of freezing
> st->loudness2 ?
> 
> Jean-Marc
> 
> 
> Le lundi 20 juin 2005 à 14:40 -0400, Tom Grandgent a écrit : 
> > I think you'll have to modify Speex to get the functionality you're 
> > looking for.  I've made a few simple modifications to the AGC to prevent 
> > it from 1) exceeding a specified level of amplification and 2) enable 
> > and disable adaptation, so I can freeze it at a certain level while 
> > speech is not detected.  It's mostly just a matter of doing this at the 
> > end of speex_compute_agc():
> > 
> >    if (!st->agc_frozen)
> >    {
> > 	   agc_gain = st->agc_level/st->loudness2;
> > 	   /*fprintf (stderr, "%f %f %f %f\n", active_bands, st->loudness, st->loudness2, agc_gain);*/
> > 	   if (agc_gain>st->agc_max_gain)	/* was 200 */
> > 		   agc_gain = st->agc_max_gain;	/* was 200*/
> >    }
> >    else
> > 	   agc_gain = st->agc_gain;
> >    st->agc_gain = agc_gain;
> > 
> > and adding a few items to speex_preprocess_ctl() and the state struct.  
> > (I control these things at the application level.. you may wish to 
> > control them from within the preprocessor if you're using the 
> > preprocessor's VAD.)
> > 
> > Anyway, if you can figure out what's going on with the variables you 
> > named, I'm sure you can make the necessary modifications to do what 
> > you've asked for.  I think the preprocessor in general needs a little 
> > tweaking like this to work well in various real-world situations, but 
> > I'm not sure how much of this Jean-Marc wants to incorporate into 
> > Speex vs. leave to application developers.
> > 
> > Tom
> > 
> > Thorvald Natvig <speex at natvig.com> wrote:
> > > 
> > > 
> > > Echo cancellation works like a charm, but it seems to confuse the 
> > > preprocessor a bit.
> > > 
> > > If listening to background music (properly fed through the echo 
> > > cancellator), the music is removed but the result is still detected as 
> > > speech even if almost silence remains in the signal.
> > > 
> > > Also, the AGC keeps adjusting to the minute remains in the signal, meaning 
> > > that sooner or later it will amplify the remains enough that it's clearly 
> > > audible on the other side. If I cough or say a word, the AGC readjusts and 
> > > all is fine.
> > > 
> > > Looking at the members of the speex_preprocess structure, I see that 
> > > during these long periods of "silence" (only the background music or 
> > > only the other end talking while I shut up):
> > > 
> > > - Zlast (which looks like a SNR variable) is at 0.05-0.2, but jumps up
> > >    above 1.0 if I actually say something.
> > > - loudness2 keeps decreasing from the "normal" of ~6000 to 1000 or so, at
> > >    which point the residual echo is amplified enough that it's clearly
> > >    audible at the other end. If I say something, it adjusts.
> > > - speech_prob is at 0.999 or 1.000 as long as the other end talks.
> > > 
> > > This is all with up-to-date SVN version of speex, and in a fairly noisy 
> > > environment (it's hot, so I have the window open, so passing cars on the 
> > > nearby road are quite audible, as is my air cleaner).
> > > 
> > > Is there something I can do to tune this away, a way to tell the AGC to 
> > > never go that low, and a way to tell the speech detector that echo remains 
> > > are not speech?
> > > 
> > > _______________________________________________
> > > Speex-dev mailing list
> > > Speex-dev at xiph.org
> > > http://lists.xiph.org/mailman/listinfo/speex-dev
> > 
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