[Asterisk-Users] E1 Echo (was: Small explanation of txgain rxgain statement please)

[Steve Davies]

On 12/15/05, Rich Adamson <radamson at routers.com> wrote:
>
> > I was just looking at:
> >
> http://www.asteriskdocs.org/modules/tinycontent/content/docbook/current/docs-html/x1695.
> html
> > regarding echo canceller tuning, and I noticed the statement
> >
> > "Most people find that they need an rxgain level around 8.0 to have
> > good echo cancellation. The txgain setting varies from installation to
> > installation."
> >
> > Which feels a bit wrong :) Could someone explain why increasing the
> > gain on the inbound zap leg (rxgain) would improve echo cancellation?
> > Of have I misunderstood the roles and meanings of rxgain and txgain?
>
[snip]

Many thanks for clearing that up for me :) the largest part of my
misunderstanding was caused by not noticing that that article was
referring to the tuning of an "FXO" line. I am in fact trying to find
information on the tuning of an E1 to reduce echo. (Doh!)

In theory of course an E1 should work with rxgain=0.0, txgain=0.0
(assuming there is no digital messing going on in the network) and the
echo canceller should have a relatively easy job of cancelling echo
given that the large majority of the UK phone network is digital, and
only the last leg at the far end is usually analogue.

I am running Asterisk 1.0.9, and have backported the KB1 canceller
into Zaptel 1.0.9.2, which does not seem to have caused any problems.
Nor has it really caused any improvement though :)

I am beginning to wonder whether what echo IS heard is being caused by
packetisation delays "in the network" - The default tap length is 128,
or I believe 16ms. If something in the PSTN causes a delay more than
that length (no idea what might cause that) then echo would still be
heard.

Does anyone have any experience in this area? Any ideas? How "heavy
handed" would it be to increase the tap length to 256? I have not seen
anyone suggest that this might be a good idea.

Thanks,
Steve