[asterisk-users] Question about echo cancelation

[Olivier]

2008/10/13 Tilghman Lesher <tilghman at mail.jeffandtilghman.com>

> <snip>
>
> Pray tell, how do you echo cancel in both directions?  Wouldn't that
> necessitate cancelling echo before it occurs on the line (sort of a white
> noise/pink noise kind of operation)?  Seems like modelling a projectile
> such
> that when it reaches its target, the atmospheric stresses during its flight
> turn it into a perfect sphere (and with about the same likelihood of
> success).
>
> --
> Tilghman
>
>
Hi,

Thanks for replying.

So, bottom line is "you can't cancel echo in both directions", right ?

I read from http://lists.digium.com/pipermail/asterisk-dev/2006-January/017774.html
: <http://lists.digium.com/pipermail/asterisk-dev/2006-January/017774.html>
"Generally speaking there is only one direction of echo cancellation needed:
towards the 4-wire (digital) party. Any echo heard by the 2-wire party is
normally acoustic echo, which we don't try to alleviate."

It seems it could also have been written this way :
"Echo cancellation in both directions can't be done and generally speaking
there is only one direction of echo cancellation needed: towards the 4-wire
(digital) party. Any echo heard by the 2-wire party is normally acoustic
echo, which we don't try to alleviate."


Anyway, excuse me if my question seems naive but, when using a media gateway
between PRI (digital) and VoIP, which are respectively the 2-wire and the
4-wire sides as both technologies use 4-wire connections ?
If Alice is on the VoIP side and Bob on the ISDN/PSTN side, which one will
benefit from EC ?
Will Alice stop to hear echo of its own voice or Bob's voice (I've never met
this case but I'm not a reference on this) ?
Will Bob stop to hear echo from its own voice or Alice's voice (I've never
met this case but I'm not a reference on this) ?

Regards
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