[Asterisk-Users] Fun with Echo

[Andrew Kohlsmith]

On Monday 12 June 2006 17:55, shadowym wrote:
> Believe me, you can drive yourself insane trying to come up with some
> magical formula that JUST works because it usually won't happen that way.
> Software echo cancellers are simply not good enough for many situations.

Actually that's untrue.  I think (hope) that Steve Underwood will jump in here 
and correct me, but it's my understanding that the only real reason why the 
software echo cancellers available in Zaptel don't work as well as the 
hardware echo cancellers from Tellabs and the Octasic chips in the Sangoma 
and Digium hardware echo cancellers is because of implementation.  

There is a spec for echo cancellation on PSTN called g.168.  I believe it's a 
suite of tests which put the echo canceller through its paces and if you pass 
them you are certified to conform to g.168. None of the echo cancellers in 
zaptel conform to this, whereas the Octasic, Tellabs and other hardware echo 
cancellers all do.  If someone were to put the effort and energy into making 
the software echo cancellers compliant, you should find similar results to 
the hardware echo cans.

The echo cancellers in Zaptel are far better than anything I could throw 
together myself, and there's a lot of heavy math and dark juju hiding inside 
that optimized code, but they're all still very much proof of concept and 
test code compared to a true g.168-compliant echo can.

Basically they're there for free and might get you what you need, but they're 
certainly not a reflection of all that is possible with a general CPU echo 
canceller.

-A.