[Asterisk-Users] echo cancellation

[Nicolas Bougues]

On Wed, Nov 19, 2003 at 06:56:04PM -0600, Rich Adamson wrote:
> > I've got an X100P & a cisco 7960. if i call from an analog line via the 
> > x100p to the cisco, there is an overly audible echo on the cisco. If i 
> > make a call from a cisco to cisco, there is no echo.  zapata.conf has 
> > echocancel=yes & echocancelwhenbridged=yes  set.   Any ideas?
> > 
> > I'm currently using the default implementation of echo 
> > cancellation...which one should I try next?
> > 
> 
> Take a look at:
>  http://www.voip-info.org/wiki-Asterisk
> 
> Most echo problems are not actually related to the X100P or the software,
> but rather the pstn line (including house wiring, analog phones, and other
> crap left hanging on the pstn line). It's a technical issue that can't be
> resolved with non-technical approaches.
> 

It's called line echo. Basically, the hybrid that converts the 4 wires
(2 TX/ 2 RX) into a 2-wire analog phone line intrinsically generates
echo.

It's not noticeable (you hear your own voice while you talk, just like
your ear hears your mouth in a plain talk) when the switching is done
at the byte level in the TDM mux. But it does exist. That's why faxes
are half duplex devices, and full duplex modems incorporate full
duplex echo cancellers. 

Digital cell phones, digital phones (either ISDN or connected to
proprietary PBXs) or IP phones don't generate line echo. And they are
usually clever enough to avoid acoustic echo as well.

But then, in the case of line echo, removing it is not an easy
task. It involves quite complex signal processing. Look at the various
tries in the zaptel source.

An no, POTS phones don't have echo cancellers.

--
Nicolas Bougues
Axialys Interactive