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

[Tzafrir Cohen]

Just a partial answer:

On Sat, Mar 24, 2007 at 10:24:18AM -0600, Stephen Bosch wrote:
> 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.

No, this is not Zaptel. This is Asterisk's chan_zap (Asterisk zaptel
channel). 

fxotune, however, runs directly with zaptel. Thus it cannot be affected
by whatever gain settings you have in zapata.conf .

> 
> 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?

tinkerring with the gain with ztmonitor?

> 
> 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.

-- 
               Tzafrir Cohen       
icq#16849755                    jabber:tzafrir at jabber.org
+972-50-7952406           mailto:tzafrir.cohen at xorcom.com       
http://www.xorcom.com  iax:guest at local.xorcom.com/tzafrir