[asterisk-users] Question about echo cancelation

[Eric "ManxPower" Wieling]

Olivier wrote:
> 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.

Where did you hear that media gateways "filter one-way only"?

Any 2-wire analog leg will be a source of echo.  Many, many, many calls 
do not have a 2-wire leg.  Think cell/mobile or endpoints with PRI or T-1.

> 
>>  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

If you (Alice) are hearing echo then the echo canceling can be done any 
time after it leaves Bob's 2-wire circuit but before the audio is 
converted to VoIP on your end.

Telcos echo cancel cell/mobile phone calls (also a high latency path) 
and long distance calls, but almost never do EC on local calls.   This 
is why you seldom get echo when calling a mobile phone or a long 
distance number -- you mostly get it on local calls.

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