[asterisk-users] TDM analog cards, volume, echo, fxotune, ztmonitor and HPEC

[Stephen Bosch]

Hi, everyone:

I am developing a system using Asterisk, TDM-400 analog cards, analog
lines, and Polycom SIP phones for internal extensions.

Initially there was bad echo but after a series of efforts, I've managed
to reduce it to a negligible level (it only happens when both parties
speak simultaneously, and even there, only for a few hundred
milliseconds). From an echo standpoint, things are very satisfactory.
The HPEC has been very effective.

The remaining problem is extremely variable volume, particularly as
perceived by our internal callers. The remote party is often, but not
always, too quiet. I've determined that it depends heavily on where that
remote party is.

- If the remote party is on a circuit on the same local exchange, the
volume is perfect or nearly perfect.

- If the party is on a remote exchange, the volume can vary, from barely
audible, to audible but still too quiet to be really comfortable. In
these cases, the users will crank the handset or headset volume on their
Polycoms to make the remote party audible, but that ends up causing
distortion which is bad enough to be irritating. The volume level does
seem to depend on the exchange. For instance, all calls to a specific
exchange are at the same general volume level.

Now, standard analog sets have a varistor circuit to compensate for
these variations in signal level, but it would appear that the TDM cards
don't incorporate this kind of dynamic gain control.

Zaptel allows you to control the transmit and receive gain at the
interface card, but isn't that what fxotune is supposed to do? Tune
those gains so that the echo is minimized? I don't want to play with
gain only to undo the echo tuning done by fxotune.

Does tinkering with the gain undo the tunings done by fxotune?

Similarly with ztmonitor: Is tinkering with the gain using ztmonitor
going to undo the tuning done by fxotune, or can I do both?

Have you encountered gain issues like this on analog lines, and if so,
how did you address them?

Is cranking the rx gain on the Polycom phones a viable solution, or is
it likely to make echo worse? I've tried a few adjustments, and they
seemed to aggravate echo; once even so badly that the remote party could
hear echo.

-Stephen-