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

[tmassey at obscorp.com]

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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