[Asterisk-Users] Tweaking AGGRESSIVE_SUPPRESSOR

[Steve Underwood]

Steve Kann wrote:

> What he describes is echo suppression.  Because an echo canceller can, 
> generally, only remove some part of an echo, not the entire echo, 
> systems are generally designed to suppress the residual echo in some 
> circumstances.  Old speakerphones had poor on no echo cancellation, so 
> the suppression kicked in like that, because it was the only choice.  
> In modern systems, the echo cancellation is much better, so 
> suppression is not needed as much, and when it is used, it's probably 
> done much more imperceptibly (with comfort-noise and stuff like this).

Only a very few high end conferencing speakerphones have ever used echo 
cancellation. Even most expensive digital phones on PBXs merely do echo 
suppression in speakerphone mode.

The nature of A-law/u-law limits the performance of an echo canceller 
across the PSTN to about 30dB of echo improvement. If you look at the 
behaviour of those codecs, you will see they give a roughly contant 30dB 
of instantaneous dynamic range, and the echo cancellation enhancement 
will never exceed that dynamic range. There is still enough residual 
echo that good quality cancellers have to perform non-linear suppression 
to eliminate it, and substitute comfort noise. 30dB, on top of the 
minimum of at least 12-15dB of echo suppression the hybrids give, means 
the echo should be rather quiet. It is still enough to annoy people, 
though, and suppression is standard practice. It is specified in G.168.

Regards,
Steve