[Asterisk-Users] Calibrating both RX and TX gain?

[Shaw Terwilliger]

On Wed, Oct 12, 2005 at 12:05:32PM -0400, tmassey at obscorp.com wrote:
> I am in the middle of trying to get a milliwatt test line to calibrate the 
> rxgain properly.  However, this won't help me with the txgain, will it? 
> How can I properly calibrate the txgain?  By ear?  Or is there a more 
> scientific method?

Maybe I can help.

I had a similar problem.  I was using a Digium TE205P card and two
Rhino channel banks, and every call that was bridged from a phone on
an FXS interface to a PSTN line on an FXO interface was (1) loud and
(2) had an echo with a tiny delay (maybe 30ms).  The echo sounded almost
like excess sidetone, but was delayed enough to phase shift the speech
and make things sound hollow.  I could verify that what was being
transmitted was coming back on the RX channel of the PSTN interface
(using ztmonitor).  I'm using Nortel analog, wall-powered phones (pretty
nice models).

I had echo cancellation on, and had tried all possible configuration 
settings for taps, etc.  Nothing killed my echo.

I had tried adjusting all the gains down in Asterisk for all the interfaces,
but that didn't work.

I contacted Rhino to see if they had any suggestions, and they were
able to give me a few.  What finally worked was setting the Asterisk gains
back to 0 for all channels, then adjusting the gains down on the channel banks
themselves for the phone (FXS) interfaces only.  A huge improvement!  My
current adjustements are the following:

On the Rhino channel banks:

  For FXS (phones) interfaces:

    rx -4 dB
    tx -4 dB

  For FXO (PSTN lines) interfaces:

    rx 0 dB (default)
    tx 0 dB (default)

In Asterisk's zaptel.conf:

  context=phones
  rxgain=3.0    ; This is to compensate for the drop in volume because of
                ; the -4 dB setting on the channel bank for rx.
  txgain=3.0    ; This is to compensate for the drop in volume because of
                ; the -4 dB setting on the channel bank for tx.

  context=pstn
  rxgain=1.4    ; This was bumped up last, as a result of a milliwatt test.
                ;
  txgain=1.4    ; This was also bumped up, because it makes the outbound
                ; calls a bit louder, and doesn't seem to overdrive the 
                ; line.  I figure the gain loss on rx (which was calibrated
                ; with the milliwatt test) should be similar to tx gain lost,
                ; although I couldn't directly test this.

Now, when I turn on echo cancellation for all my interfaces, the echo is
completely gone.  After compensating for the gain drop on the channel banks
with asterisk boost, the call volumes sound good too.

-- 
Shaw Terwilliger <sterwill at sourcegear.com>
SourceGear LLC
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