[asterisk-dev] Adding Octastic Soft-Echo to external SIP adapters

[Paul Cadach]

Looks like Cisco does some heuristics to try to "compensate" echo in VoIP path and this option could not be switched off by Cisco configuration (but it eliminates VoIP this "phantom" echo when you turn echo cancellation on). I seen this issue when turned echo cancellation off on Cisco, and I got a much higher echo than I have from Zaptel with echo cancellation disabled on the same call path (PSTN->PRI->{Asterisk/Cisco}->IP phone).

Probably, Cisco's echo cancellation scheme is next:
TX <---- * --- * ---<-
TDM      X     Y
RX ->--- + --- - --->-

So, when you disable echo cancellation, only Y transformation is disabled, but X is still active and produces additional echo which comes through VoIP path. When you enable echo-cancellation, Y compensates X together with residual echo comes from TDM side, so VoIP path gets clear audio.

Looks like Cisco implemented interesting idea by splitting echo canceller into two parts - with such scheme you can compensate much wider range of echo signals than with single stage. But they forget to turn BOTH parts of echo-canceller off when echo cancellation configured as disabled. As a result, you cannot use Cisco VoIP gateway with echo canceller disabled. :(


Ideally each system should handle echo-cancellation/suppression when it does:
1) 2-wire <-> 4-wire transformation should perform echo-cancellation which could come from 2-wire side;
2) TDM->VoIP transformation should perform residual echo cancellation/suppression in case 1) is not done well.

Thus, we will have echoless signal in VoIP system which have time-variable and long delays and difficult for echo cancellation.


WBR,
Paul.

----- Original Message ----- 
From: "Vazir" <anton.vazir at gmail.com>
To: "Asterisk Developers Mailing List" <asterisk-dev at lists.digium.com>
Sent: Monday, May 07, 2007 10:23 PM
Subject: Re: [asterisk-dev] Adding Octastic Soft-Echo to external SIP adapters


> On 7 May 2007 20:57, Andrew Kohlsmith wrote:
>> Echo is not generated by VOIP. VOIP is known as a 4-wire
>> system. Transmit and receive paths are along totally
>> separate (albeit logical) paths.
> 
> True, But in my case 90% calls are PSTN <-> VOIP <-> PSTN so 
> it could be echo coming from other side AFTER VOIP-PSTN 
> conversion... and some cases shows that it's so - that if I 
> switch off echo cancelling I can hear my echo in ~1 second 
> time difference.
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