[Asterisk-Users] Re: Incoming echo cancel

[Peter Svensson]

On Fri, 11 Mar 2005, Jon Bebeau wrote:

> I'm having echo too - ISDN-PRI using a Sangoma card to the PSTN.  My SIP 
> outgoing calls have an echo about 80% of the time, but only on a local T1. 
> It only an echo to the SIP caller; the called party never hears the echo.  I 
> have a second T1-PRI (port 2 of the same card) to a long distance carrier 
> with no echo problem, ever.  Played around with all the echo stuff, you 
> identified below.  Seems there is much ambiguity on the guidance as much of 
> the echo suggestions apply only to analog lines.  It's unclear how there can 
> be an echo on a T1-PRI (digital) anyway.

There is no echo on the t1-sip line. The echo you are hearing is caused by 
analog devices connected through the pstn. You hear the echo generated by 
the far end. The good news is that the remote end (usually your customers 
etc) should net hear any echo.

The echo is always present when calling one of these devices. Normally the 
delay is so short that the human ear does not perceive it as an echo but 
rather "sidetone". Your sip link introduces a lot of delay which moves the 
reflected energy form nice sidetone to obnoxious echo.

On the long distance lines delays are expected to be longer and the 
provider may automatically use echo supressors and echo cancellers. This 
is good news for you since this will handle the echo for you.

The echo cancellers in Asterisk seem to need a lot of voodoo to tune. 
Recently there has been some movement on the bug tracker to clean up and 
analyse that part of the code.

Peter