[asterisk-dev] Queries regarding Echo Cancellation in Zaptel-1.2.6

[Chan Kwang Mien]

Currently, my test-bed to verify echo cancellation of zaptel-1.2.6 is :

    Analog Phone A <------> Asterisk IP PBX <---> Analog Phone B

Phone A and B are directly connected to the Digium FXO/FXS card. Phone A
is a basic Speaker(Handsfree) Phone with no acoustic echo cancellation.
Phone A and B are also located far apart. 

Calls to Phone A is answered by using its speaker. When I spoke into
Phone B, echo cannot be heard at Phone B. I think the reason is because
there wasn't much delay when the echo travels from Phone A to B. Hence,
the echo wasn't audible at Phone B.

My questions is :

Does anyone know how I could add delay at the Asterisk PBX such that the
echo would be delayed so that it becomes audible at Phone B.

Thank you.


> Hello,
> 
> Chan Kwang Mien wrote:
> > IP Phone A <------> Asterisk IP PBX <---> Analog Phone B
> >
> > In my tests, echos were generated by IP Phone A when I turned on the
> > speaker. As was pointed out, zaptel could not cancel these echos because
> > the echo was not received at the zaptel interface, rather on the SIP
> > interface.
> 
> Correct.
> 
> > I have a question :
> >
> > Does it mean that the SIP interface has to cancel the echo from IP Phone
> > A ?
> 
> No. VoIP part always uses different receive and transmit pathes, so there
> (theoretically) is no way to mix receive and transmit signals to produce echo.
> 
> > This should be a real problem since it is possible that echo could be
> > generated by IP Phones when they are in the handsfree mode (Speaker
> > mode) or echo could be generated from an old IP Phone.
> 
> A phone (analog or VoIP) implementing speakerphone mode should care about strong
> acoustic echo cancellation in this mode. Regular handset operation for best
> results also requires some sort of acoustic echo cancellation (to resolve signal
> "connections" between speaker and microphone embedded into single handset), but
> with correct handset design it's not so important (residual echo through
> accurate designed handset less than -20 dB).
> 
> 
> WBR,
> Paul.
> 
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