[Asterisk-Users] Echo Cancelaltion in Zaptel Changes

[Stephen Davies]

On Sun, 4 May 2003, Tilghman Lesher wrote:

> If you are hearing really bad echo, it signals a problem with
> the echo canceller on the other end of the call.  (i.e. the other
> device is not cancelling out the sound coming in through the
> microphone, what is heard on the speaker).  If your callers are
> hearing no echo, then your echo cancellation is working fine
> (it would be THEIR echo cancellation that needs to be fixed).

Its not quite so simple, though.  Quite likely there is NO echo
canceller at the other end of the call.  Many national PSTNs don't
have any, because in their design parameters the echo simply doesn't
matter.

For echo to be a problem you need first the echo (whether electrical 
- generated in the hybrids where 2 to 4 wire conversions are done - or
acoustic), and then the latency.

A phone network may not have echo cancellation because the network
doesn't have enough latency for the echo to be noticed.  Any echo
generated just adds a little more sidetone for the caller.

Just because you start to hear that echo when you interpose a
high-latency network doesn't mean the phone network becomes
responsible to cancel it.

Even if you do believe its their job its still not going to happen.

So high-latency networks (eg VOIP) need echo cancellation for
audio they send INTO the pstn.

Generally, they don't need echo cancellation for audio coming FROM the
pstn. Because digital telephony keeps receive and transmit separate,
no electrical echo is generated to be cancelled.  Acoustic echo with
handset phones is pretty low (-40dB?)  And speakerphone designs will
usually include cancellers for the acoustic echo.

So if you call from a VOIP (high-latency) network to a PSTN user and
hear echo - then the echo cancellers at your gateway into the PSTN are
the problem.

The PSTN user is unlikely to hear echo.  In the event that they do,
most probably its acoustic and you should stop trying to use a
softphone with no acoustic cancellation with your PC speakers and
microphone.

Regards,
Steve