[Asterisk-Users] Echo calibration with ztmonitor and a test linefrom a telco

[Rich Adamson]

> I have already seen this page but since the head version of ztmonitor is
> able to show the real number value of the rx and tx (ztmonito  -vv), I
> was thinking that maybe someone could confirm to which value we want the
> rx of ztmonitor  when we try to calibrate the system with a test line
> from a Telco and a TDM card.
> 
> The only information that I have found is that I have to setup my rxgain
> to get a 14800 value with ztmonitor but that is not working.
> 
> Any others suggestions ?

The bottom line is that regardless of what tool you use to adjust the
gains, you will still have to play with the settings to balance echo and
transmission loss. 

The gain settings (in a non-asterisk telephony environment) typically are
about 2db less then whatever the pstn cable loss happens to be for your
particular implementation. However, if you find out that your 7db from the
central office (as an example only) and you set your gains to +5 db (-2db
less then the loss), you _will_ end up with echo. The majority of us that
have digium analog cards (regardless of whether its the old X100P or the
newer TDM) end up with a transmission loss that is less then optimal, and
very little echo. Most of the echo that is left is heard only during the
first few seconds of a call.

Assuming you're in the US, I'd suggest starting with something like this:
echocancel=yes
echotraining=800
echocancelwhenbridged=yes
rxgain=5.0
txgain=0.0
callprogress=no
channel => 1

and adjust the rxgain & txgain upwards until echo is objectionable, and
then back downwards a small amount.

If the gain settings are too far off, dtmf will fail as well, so keep
that in mind during your testing.

If you still can't get rid of the echo, there are some options that can
be configured in one of the zaptel files to change the echo cancellation
method to Agressive, etc. Some folks have to play with those options while
others don't. It is 100% dependent on _your_ specific pstn connection
(eg, cable loss & quality) and your specific asterisk system (eg, distro,
motherboard).