[Asterisk-Users] Fun with Echo

[Brian Swan]

I've spent the last week or so troubleshooting echo problems at my  
Wife's business, and I've been able to clear up about 99% of the  
echo, but there is still a little residual echo that I can't seem to  
"tweak out".  The users describe it as "buzzing or crackling", but  
what it sounds like to me is a slight echo, but just of one syllable  
of the word.

I've followed the numerous suggestions in the mailing list archives  
which is what has enabled me to get this far.  After trying all the  
echo cancelers, and all the settings on each I settled on:

- KB1 (with AGGRESSIVE_SUPRESSOR)
- echocancel=128
- echotraining=600
- rxgain=0
- txgain=0

If I turned up the echocancel to 256 I'm able to eliminate the  
"buzzing" echo, however, on inbound calls I get a HUGE very loud echo  
that takes a good 20 seconds to go away (which I don't get on  
echocancel=128).  Does anyone have any suggestions on how  
troubleshoot/tune this to eliminate the "buzz"?

Here's my setup:

Asterisk 1.2.9.1
Zaptel 1.2.6
TDM400P (3xFXO, 1xFXS)
6 Cisco 7960's running chan_sccp2

Along the same lines, in my hunt for echo, I was rather disappointed  
that I couldn't really find any tools to help troubleshoot the  
issue.  It seemed like the general consensus was "Play with all the  
settings until it sounds good".  I did manage to compile the Zaptel  
driver with the Zaptel "preload" patch, which helped me visualize my  
echo problems a bit, but preloading didn't get rid of the buzz.   
Also, I've eliminated inside wiring by moving the Asterisk box to my  
demarc to test, I also tried the TDM400P on different hardware to  
ensure it's not a motherboard issue.  But, at the end of the day,  
those are all just stabs in the dark...

If anyone has any suggestions, I'd sure appreciate it!

Thanks!
Brian