[asterisk-users] fxotune vs rxgain/txgain

[Matthew Fredrickson]

Noah Miller wrote:
> Hi Matthew -
> 
>> These techniques are not mutually exclusive, I usually want people to
>> use gain modification as the last step in trying to eliminate echo
>> (after balancing the hybrid and making sure you are using a good echo
>> canceller).
>>
>> In the case of running fxotune, your zapata.conf software gain levels
>> should not affect its operation.  If you are using any of the hardware
>> gain settings (wctdm24xxp module parameters) you should normalize those
>> to 0 beforehand so that they do not interfere with the calibration process.
> 
> Thanks for your responses!
> 
> I actually didn't realize there are hardware gain settings available
> for wctdm24xxp (is there any documentation on this?  I can't seem to
> find any).  I assume the hardware gains default to 0 if left unset?

Correct.  They are set as module parameters, and actually only apply to 
fxo modules.

> Just two more questions:
> 1) I think we were experiencing ECFO with an rxgain setting of +10db
> (after having balanced the hybrid using fxotune).  I'm guessing this
> is because that rxgain value amplifies the echo a bit too much.  I
> know this is a bit of a loaded question, but is there a certain range
> of values for rxgain/txgain that we should stay within in order to
> avoid exacerbating any echo issues?

I couldn't give you exact numbers off the top of my head.  It's not hard 
to notice though if it's happening :-)

> 2) Are rxgain/txgain values applied before or after hardware echo cancellation?

rxgain is pre-hardware echo canceller and txgain is post hardware echo 
canceller. (zapata.conf rxgain and txgain).

-- 
Matthew Fredrickson
Software/Firmware Engineer
Digium, Inc.