Speex dev - Sep 2005 - Help needed regarding Echo Cancellation

Dear all, 

I need to use echo cancellation in a VOIP application
using speex narrowband codec.I am facing some problems
with echo cancellation. It will be very helpfull if
someone can put some light on the following issues.

1: 
==================
-- When call is started EC works fine for first few
seconds. Bur after a certain period [usually 20 sec or
more] it does not work or works very bad [produces
distorted or noisy echo].

-- If no one talks for some time [around 4/6 mins] EC
works again for some period and after that period it
fails again. 

-- I used echo tail 200ms. Using long echo tail
[2000ms], EC works very fine but in that case CPU
usage becomes very high. 

What should be the solution in this situation? I mean
what can be the reason that EC fails after some time.


2: 
===================
-- I read the played packets from DirectSound Buffer
and use it as echo frame. I have noticed that my echo
frame is of low amplitude and many values of it are 0
[zero]. 

-- Since the values are small, I applied AGC on echo
frame with another Speex preprocessor before passing
it to EC. With this approach and echo tail 400ms, I
get very good echo cancellation. But some times we get
distorted audio, sounds like someone speaking in pipe
or underwater.
-- Is there any possibility to get distorted audio if
we apply speex AGC on echo frame before passing it to
EC? 

3:
=============-- Is there any tradeoff that if echo cancellation
becomes more aggressive, the voice quality will fall
or will be distorted?
-- What are the ways to make EC more aggressive?
 

Any help is greatly appreciated. Thanks in advance.
 

Shantanu

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> -- When call is started EC works fine for first few
> seconds. Bur after a certain period [usually 20 sec or
> more] it does not work or works very bad [produces
> distorted or noisy echo].
The most common reason for that is skips (underruns/overruns) in the
audio, which change the (effective) echo impulse response abruptly. The
fix for that would be in the sound interface.
> -- I used echo tail 200ms. Using long echo tail
> [2000ms], EC works very fine but in that case CPU
> usage becomes very high. 
Define very high. If increasing the echo tail to 200ms makes it much
better, then it's probably just a problem of sync between far end and
near end (and the beginning of the echo tail being wasted because of
that)
> -- I read the played packets from DirectSound Buffer
> and use it as echo frame. I have noticed that my echo
> frame is of low amplitude and many values of it are 0
> [zero]. 
Can't help you with DirectSound.
> -- Since the values are small, I applied AGC on echo
> frame with another Speex preprocessor before passing
> it to EC. With this approach and echo tail 400ms, I
> get very good echo cancellation. But some times we get
> distorted audio, sounds like someone speaking in pipe
> or underwater.
NOOOOOOOOOOOOOOOOOOO! Never, ever apply any kind of processing on any of
the AEC inputs (output doesn't matter), unless it's linear and constand
(e.g. a filter).
> -- Is there any possibility to get distorted audio if
> we apply speex AGC on echo frame before passing it to
> EC? 
It's not a possibility, it's a guarantee.
> -- Is there any tradeoff that if echo cancellation
> becomes more aggressive, the voice quality will fall
> or will be distorted?
> -- What are the ways to make EC more aggressive?
Yes, it's got to do with the adaptation rate, but I wouldn't suggest you
play with it unless you know what you're doing.
 
	Jean-Marc

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