[asterisk-dev] Tuning Software Echo Cancellers

[Rich Adamson]

Inline... general comments

> 	I am working on trying to track down and eliminate echo on a PRI. 
> I'm using Gen 2 TE-405P cards, w/ the MG2 echo canceller. I cannot enabnle 
> AGGRESSIVE_SUPPRESSION because that causes dropouts in the speech that are 
> unacceptable to my customers needs. I am awaiting the RMA of a pair of 
> TE-405s for upgrade to Gen 3 so that I can put the VPM-450 hardware echo 
> can card on them, but in the meantime, I am trying to tune the software 
> echo cancelling routines to lessen the impact of the echo to specific 
> local central offices where the Telcos just do not have any edge 
> supression going on.
> 
> I am using MG2 w/ the following settings in zapata.conf
> 
> echocancel=256
> echotraining=800
> rxgain=+0.21
> txgain=-0.0

The echotraining parameter was originally created for analog pstn lines, 
adding a delay (in the above example, 800 milliseconds) to allow analog 
central office equipment to settle down before the s/w echo can routines 
pulsed the line. The reflective energy from the pulse was then used to 
preload the echo can routines.  Doubtful this parameter should be 
anything other then =yes on a PRI.

The rxgain and txgain values need only be expressed in units; the 
numbers after the decimal point have no practical value.

> The rxgain was set using a local milliwatt number (line side). 

Exactly how did you measure that? (What type of transmission test set 
and what did you attach the test set to?)

> I'm still 
> having trouble getting a long enough tone from the local C/O to do the 
> txgain, hence the 0.0 setting. Apparently, X/O has their DMS 100 set to 
> only provide 6 seconds of Millwat before it ends. No one seems to be able 
> to figure out how to extend it on the local switch.

The above statements don't make any sense. When you dial into a CO 
milliwatt generator, it generates a 1k tone allowing you to measure the 
level of that tone at your site. You then adjust rxgain. Since this 
happens to be a PRI, there is no pstn loss and therefore no reason to 
try and compensate for a pstn loss. How did you jump to a conclusion 
that you could use a CO milliwatt generator to set txgain?

> Using a value of 256 should give me a 64ms echo tail (from what I have 
> read), but it does not seem to be enough to qwell the echo coming back 
> from the particular trouble CO, which appears to have a 189 ms echo.

Highly unlikely that "the particular trouble CO" involves 189 ms echo, 
unless that CO is half way around the world or is accessed via satellite 
facilities. I'd suggest going back to 128 and compare echo with 256 to 
the exact same distant tel number. Suspicion is you can't tell the 
difference and 128 is just fine.

The sangoma folks created a utility that could be used to measure the 
echo and plot it in an excel graph. I've not used it since roughly 12 
months ago and don't actually remember the utility's name. (Hopefully 
someone else on the list will remember and post it.)

Think I'd start with parameters like these:
  echocancel=yes
  echotraining=yes
  rxgain=0
  txgain=-3
and adjust txgain downward (more negative) by two or three units (eg, 
-3, -6, -9) to see if that impacts echo. If one of those values is 
better then another, then adjust by units around that point (forget the 
decimal point). Be sure to call the same pstn tel number for each test 
(and not a cell phone), and restart asterisk after each parameter change 
(don't use reload).