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

[Matthew Fredrickson]

On Mar 12, 2007, at 7:12 PM, Stephen Bosch wrote:

> 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?

It's as good (if not better) than using Octasic hardware echo 
cancellation.  I would definitely download it and install it.  And IIRC 
it comes free with the purchase of your TDM card from Digium.

Matthew Fredrickson