[asterisk-dev] Zaptel Echo Cancellation

[Steve Davies]

On 8/3/06, Andrew Kohlsmith <akohlsmith-asterisk at benshaw.com> wrote:
> On Thursday 03 August 2006 11:09, Eric "ManxPower" Wieling wrote:
> > > Is there not an inherient delay of several ms due to CPU and encoding
> > > overhead etc or is this not a factor in the calculations? I'm just
> > > wondering what kinds of delays play into this or is the only thing that
> > > matters the delay on the PSTN before the signal hits the card?
>
> > The PSTN delay is what matters.  Echo must be removed at the PSTN
> > interface.
>
> When the echo cancellation is being done in software on the PC (i.e. in
> zaptel), you need to account for the PCI delay and processing delays as well,
> do you not?  That is the advantage of the hardware echo cans; they work on
> the echo "closer" to the source.

There are tools out there to allow you to dump waveform data from
Zaptel to "see" the echo. When I did this on a UK E1 circuit that I
knew had analogue echo at the far end, the total echo latency was
between 8ms and 10 ms.

Sangoma have some sample graphs on their site which show similar results.

The VoIP packetisation latency does not get involved except where echo
fails to be cancelled, and then it will contribute to the delay that
is heard.

Cheers,
Steve