[asterisk-users] TDM-400, Polycom SIP phones, and echo problems

[Stephen Bosch]

Hi:

I am working on a new system with a TDM-400P card with three FXO modules
and one FXS module.

The system has been in place for a week. Users are complaining of echo
problems. I have noticed this echo myself. It varies in severity. It is
sometimes bad enough to make it difficult to converse, but the users
find it generally unacceptable. They miss their old phones, which just
worked. As you can imagine, this is not the way to get them excited
about this technology.

I have used the 1.4 branch version of fxotune to tune the card. These
are the echo statistics I get on the affected channels.

> asterisk1 asterisk # fxotune -d -b 1 -w 1004
> Dumping module /dev/zap/1
> echo ratio = 0.0077 (85.5 / 11145.0)
> Done!
> asterisk1 asterisk # fxotune -d -b 3 -w 1004
> Dumping module /dev/zap/3
> echo ratio = 0.0296 (330.2 / 11145.0)
> Done!

According to the wiki, both those echo ratios should be more than
sufficient to let the echo canceller handle the echo. Residual echo is
present, however. It doesn't interfere with conversation, but I could
see how it would be irritating, especially if you never had it before.
The call experience should get better, rather than worse -- especially
if we're using Polycom IP 650s.

Questions:

- What settings might I tweak to eliminate this remaining echo?
- Is there a hardware or software echo canceller that will do a more
thorough job of it?
- How is the HPEC in dealing with this echo?