asterisk users - Oct 2005 - Calibrating both RX and TX gain?

Hello!

I'm having an echo problem with a TDM card.  The TDM card is being fed by 
a channel bank just 12 or so feet away.  When you put an analog handset on 
the line, both the RX and TX volume seem to be just fine.  However, when I 
use the TDM card, I have to have an rxgain of 13.5, and even then, the 
audio is relatively quiet.  I'm also getting echo on these lines, so I 
have turned the txgain down as low as I can and still be heard.  Right 
now, it's at -6, but it will have to come up some because that is too 
quiet.  But I still have echo.

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?

For example, once I have the rxgain calibrated for all of the lines, could 
I then call into, say, Zap/3 from Zap/4 and run Milliwatt() on Zap/3 and 
use ztmonitor on Zap/4 to calibrate it?  I'm sure it's not perfect, but 
would it be close enough?

A second question:  doesn't it seem wrong that my rxgain and txgain are so 
far off when I'm just talking to a channel bank 12 feet away?  I sure 
don't have cable loss.  It sure seems like the impedance is way off or 
something.  Is there a way to test this further, rather than just cranking 
up the gain?  My guess is that using the milliwatt line will just tell me 
to make the rxgain higher, which will probably just make the echo issues 
worse... 

Tim Massey
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Hello :)
> For example, once I have the rxgain calibrated for all of the lines, 
> could I then call into, say, Zap/3 from Zap/4 and run Milliwatt() on 
> Zap/3 and use ztmonitor on Zap/4 to calibrate it?  I'm sure it's
not
> perfect, but would it be close enough?
That's exactly what you do.  Once I had adjusted my rxgains to calibrate 
them to the signal that the phoneco gave me, I just dialed out of one 
line and into another.  Everything's supposedly digital on the phoneco 
side, so no loss should occur.  (because with the rxgains you've already 
compensated for what will happen on the inbound trip through the 
copper).  So by then adjusting your txgains on each channel, you can 
feel confident that the phoneco is accurately representing to you how 
you sound from its point of view.
> A second question:  doesn't it seem wrong that my rxgain and txgain are
> so far off when I'm just talking to a channel bank 12 feet away?  I
sure
> don't have cable loss.  It sure seems like the impedance is way off or 
> something.  Is there a way to test this further, rather than just 
> cranking up the gain?  My guess is that using the milliwatt line will 
> just tell me to make the rxgain higher, which will probably just make 
> the echo issues worse...
It does seem like something else is wrong.  You shouldn't require such 
high rxgains in my opinion, but I have no idea what could be causing 
this need.

Mojo

On Wed, Oct 12, 2005 at 12:05:32PM -0400, tmassey@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@sourcegear.com>
SourceGear LLC
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> -----Original Message-----
> From: asterisk-users-bounces@lists.digium.com
> [mailto:asterisk-users-bounces@lists.digium.com]On Behalf Of Mojo with
> Horan & Company, LLC
> Sent: Friday, October 14, 2005 4:07 PM
> To: Asterisk Users Mailing List - Non-Commercial Discussion
> Subject: Re: [Asterisk-Users] Calibrating both RX and TX gain?
> 
> 
> can one use a milliwatt test line from a telco in a different 
> areacode/prefix?  does the long-distance transmission destroy levels 
> unpredictably?
> 
{clip}

The call may pass through echo cancellation or conditioning equipment along the
way so your mileage may vary however, for the purposes people use them, here I
suspect it would work just fine. It's always best to try to use the
milliwatt source on the local switch to eliminate any additional variables.

Also note that on some CO switches, such as the Telus GDT5 in this area, the
milliwatt generator is an analog source so it's barred from being dialed
from PRI B channels as the resulting signal levels are not calibrated. Try
troubleshooting that one through the repair service...

Hope that helps.

Kris Boutilier
Information Services Coordinator
Sunshine Coast Regional District