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<DIV><FONT face=Arial size=2>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).</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Probably, Cisco's echo cancellation scheme is
next:</FONT></DIV>
<DIV><FONT face="Courier New" size=2>TX <---- * --- * ---<-</FONT></DIV>
<DIV><FONT face="Courier New"
size=2>TDM X
Y</FONT></DIV>
<DIV><FONT face="Courier New" size=2>RX ->--- + --- -
--->-</FONT></DIV>
<DIV><FONT face="Courier New" size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>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.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>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. :(</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Ideally each system should handle
echo-cancellation/suppression when it does:</FONT></DIV>
<DIV><FONT face=Arial size=2>1) 2-wire <-> 4-wire transformation
should perform echo-cancellation which could come from 2-wire side;</FONT></DIV>
<DIV><FONT face=Arial size=2>2) TDM->VoIP transformation should perform
residual echo cancellation/suppression in case 1) is not done
well.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Thus, we will have echoless signal in VoIP system
which have time-variable and long delays and difficult for echo
cancellation.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>WBR,</FONT></DIV>
<DIV><FONT face=Arial size=2>Paul.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>----- Original Message ----- </FONT>
<DIV><FONT face=Arial size=2>From: "Vazir" <</FONT><A
href="mailto:anton.vazir@gmail.com"><FONT face=Arial
size=2>anton.vazir@gmail.com</FONT></A><FONT face=Arial size=2>></FONT></DIV>
<DIV><FONT face=Arial size=2>To: "Asterisk Developers Mailing List"
<</FONT><A href="mailto:asterisk-dev@lists.digium.com"><FONT face=Arial
size=2>asterisk-dev@lists.digium.com</FONT></A><FONT face=Arial
size=2>></FONT></DIV>
<DIV><FONT face=Arial size=2>Sent: Monday, May 07, 2007 10:23 PM</FONT></DIV>
<DIV><FONT face=Arial size=2>Subject: Re: [asterisk-dev] Adding Octastic
Soft-Echo to external SIP adapters</FONT></DIV></DIV>
<DIV><FONT face=Arial><BR><FONT size=2></FONT></FONT></DIV><FONT face=Arial
size=2>> On 7 May 2007 20:57, Andrew Kohlsmith wrote:<BR>>> Echo is not
generated by VOIP. VOIP is known as a 4-wire<BR>>> system. Transmit and
receive paths are along totally<BR>>> separate (albeit logical)
paths.<BR>> <BR>> True, But in my case 90% calls are PSTN <-> VOIP
<-> PSTN so <BR>> it could be echo coming from other side AFTER
VOIP-PSTN <BR>> conversion... and some cases shows that it's so - that if I
<BR>> switch off echo cancelling I can hear my echo in ~1 second <BR>>
time difference.<BR>> _______________________________________________<BR>>
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