[Speex-dev] Echo cancel

Jean-Marc Valin Jean-Marc.Valin
Tue Jul 20 10:12:58 PDT 2004

<0I0W00GUFF02LV at tid.hi.inet> <20040716084029.GA1732 at kwaak.net>
Message-ID: <1090343579.3194.6.camel at idefix.homelinux.org>

> With a lot of fiddling and some good hints from jean-marc, I
> actually got good reasonable good cancellation.
> One of the biggest problem I had was that the dial-tone somehow
> influenced the adaption-process that it continued singing along
> for more than 30 seconds. By stopping the adaption process at the
> moment that nothing comes into the mic (I am cancelling the echo
> from the telephone line connected to the modem), it was greatly
> improved.

Did you try with the echo canceller in CVS (SVN actually), I think it
should be better. Also, it's not when there's nothing in the mic that
you should cancel adaptation, but when there's nothing in the speaker.

> Second improvement was using auto-gain to get a constant "volume"
> that needs to be cancelled.

I think that's a bad idea, because it means you're changing the echo
transfer function all the time and forcing the echo canceller to adapt
all the time.

> Third "improvement" was slowing down the adaption process.
> Fourth "improvement" was adding "comfort noise" to give the users
> a base noise to focus on. Without this noise, users tend to
> increase the headset volume, which increases the amount of
> feedback they hear, which makes them want to increase the volume
> to hear the other side better, etc...

Comfort noise always helps adaptation, so it's good.

> BTW: I tend to increase the volume of the incoming signal by 2,
> and divide the echosignal by 10 before I fed it into the
> echocancel process.

The echo cancellation is normalized internally, so what you're doing
here has no effect.


Jean-Marc Valin <Jean-Marc.Valin at USherbrooke.ca>
Universit? de Sherbrooke
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