[Asterisk-Users] chan_capi-cm-0.5 release announcement

[Armin Schindler]

On Thu, 23 Jun 2005, Klaus-Peter Junghanns wrote:
> > > > Yes, that should be possible. But I don't think a channel driver (and each
> > > > channel driver) should do that on its own. Software echo cancelling
> > > > belongs in a common part of Asterisk.
> > > > 
> > > I strongly agree. But asterisk doesn't seem to work this way. Zap channel has
> > > it's own echo cancel engine. Other channels don't.
> > > This is so sad :-(
> > > Why not implement a really common echo cancel api usable from any channel ??
> > 
> > Exactly!
> > I'm not familiar with the Asterisk API, but it could be some
> > plugin like res_* ... 
> > 
> > Maybe this belongs to the Asterisk-Dev list.
> > 
> > Armin
> 
> I strongly disagree. :-) You dont want to do echo cancelation in
> userspace. Especially not on a non-realtime operating system.
> To make echo cancelation work it has to be as close to the line
> interface as possible. Also the frames have to be as small
> as possible. This rules out capi pretty much.

If you don't want echo-canceling in user-space, then neither Asterisk nor
any chan_* plugin should do it.

I don't know the zap channel code, but does the zap echo-cancel-code is 
inside a kernel module?
If yes, then I have to disagree here. Something like 'playing' with 
audio-data is nothing the kernel should be concerned with.
This belongs in user-space and if you need realtime, then you should use a 
realtime OS or use RT-scheduling. Just putting such a code into kernelspace 
is a bad idea.

So the correct way is either the hardware supports it or the 
application knows what to do with the data received, like DTMF.

Armin