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

[critch]

On Tue, 2007-05-08 at 10:23 +0500, Vazir wrote:
> 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.

Okay, simple thought experiment. You are talking at a wall, you stand
real close and don't notice the echo. You stand back a ways, and you
start to notice the echo. Where did the echo come from that you hear? It
came from the bounce off of the wall. Telephony is very similar. The
VOIP is just the equivalent to standing back. It didn't cause the echo,
it just made it more pronounced. You echo canceller needs to sit at the
conversion point and filter out the audio it sent to the PSTN from the
audio it receives from the PSTN.
-- 
critch <critch at basesys.com>