[asterisk-users] B410P echo cancellation

[Stefano Arata]

We own an Asterisk box with the following configuration:

	Linux Debian 4.0
	Kernel 2.6.18
	Asterisk 1.2.23
	Zaptel 1.2.19

We installed a Digium B410P card, and we are using 3 ports in te_ptp mode
(it is connected to BRI adapters on Italian ISDN lines, provided by Telecom
Italia; in Cisco language, they should be "isdn switch-type basic-net3").
Configuration strictly complies the documentation/manual.
Phone models used are:
- GrandStream BudgeTone 200;
- GrandStream GXP 2000;
- Thomson ST2030.

All the system is up and working.
However, users often hear an annoying noise during conversations.
Such noise appears suddenly, has a short duration, and happens in particular
when local users talk while people at the other end are silent.
Expecially it happens when remote person stops talking and we (local users,
on Asterisk) start to talk.

Note that people at the remote end do not hear any noise at all:
on their, remote side, communication is felt as perfect.

We run many tests, and we got that this problem is in some way bound to the
echo cancellation. 
If we disable it (echocancel=no in misdn.conf), this annoying non-continuous
noise disappears. But in that case we hear our echo.
If we set a lower rx gain (rxgain=-2 in misdn.conf), noise is is lower and
less annoying, but it does not disappear. And rx gain cannot be set too much
low...

Are there any fixes to this issue?
How can rx gain be fine-tuned in an optimal way?

We know that ztmonitor can be used with zaptel interfaces; but it does not
work with our Digium B410P card.

Are there any ways to avoid such annoying non-continuous noise?

Thank you.
We are available, if you need more information.
Regards,
             Stefano Arata