[asterisk-users] Question about echo cancelation

[Olivier]

2008/10/10 Eric ManxPower Wieling <eric at fnords.org>

> All calls with a 2-wire analog piece have echo.  You cannot perceive the
> echo because it happens so fast on non-VoIP connections.  On VoIP calls
> you have significant extra latency while causes you you to perceive the
> echo.

Do you mean "generated locally" or "generated distantly" ?

I understand that VoIP extra latency sometrimes renders perceivable what was
unperceivable before.
What suprises me is to hear that media getways "filter one-way only" : as
2-wires analog devices produce echo, and every phone has 2-wires analog
audio, in every call you've got at least 2 sources of echo : one in each
endpoint.


>  Echo must be removed before the call is converted to VoIP -- in
> your case the Media Gateway is the device that must remove echo.


So, if Alice is hearing its own voice,
1. where does it most probably come from ?
2. where should it be removed ?

For both, I would reply :
1. it most probably comes from Bob's phone (as other devices in-between are
digital so voice can't leak from there),
2. Alice voice echo should canceled at every location: Bob's PBX, PSTN
network (ISDN in the case I had in mind) and Alice's Media gateway

Do you agree ?

>
>
> Olivier wrote:
> > Hi,
> >
> > I'm using the following setup :
> > Alice ---- IPPhone ------<LAN>----- Media gateway ----<PSTN> -------
> Phone
> > ---- Bob
> >
> > For certain calls, users complains about echo : they can ear their own
> voice
> > in their handset, though media gateway echo cancel is turned on.
> >
> > I'm wondering how this echo cancelation engine is supposed to work.
> > My understanding of echo is that most probably, when users complains
> about
> > earing their own voice, that means that distant phone or nearby equipment
> is
> > "leaking" : Bob's phone is sending Alice's voice signal back to Alice.
> >
> > So, to properly cancel, I would say Media gateway should substract from
> > incoming signal the signal that left the media gateway few ms before.
> >
> > Discussing here and there, some say that Media Gateway never work this
> way :
> > it would only filters out locally generated echo.
> > Do you agree with that ?
> > If positive, then what can you do, if Bob's phone generate much echo ?
> >
> > Regards
> >
> >
> >
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