[Asterisk-Dev] High resolution timers using POSIX clocks instead of zaptel

Andrew Kohlsmith akohlsmith-asterisk at benshaw.com
Tue Jun 7 13:27:35 MST 2005

On Tuesday 07 June 2005 15:57, Steven wrote:
> > The zaptel ec happens in the kernel driver.  It's totally separate and
> > unrelated to "timers" in asterisk..
> It isn't unrelated when you deal with the problems related to the added
> delay. I'll point you to all the zaptel to VoIP echo problems people
> deal with because of the added delay of just packetization.

... but the echo can is done in the 1ms timer from the zaptel code.  You don't 
echocancel on the far end so 50ms or 100ms shouldn't matter...

> So a timer running every 20ms or so and then the 20ms from the SIP
> device before it packetized the audio plus 20-200ms of transit time
> means a good chance of quarter second or more of lag before the echo
> starts to come back. Do you think the kernel ec will handle a half
> second delayed echo without having trouble?

... it's a 4-wire circuit, so no echo cancellation is necessary.  Each end is 
handling the echo cancel on its own hybrid (loud earpiece getting picked up 
by the mic), so whatever delay you're introducing in the 4-wire system should 
be irrelavent.

> Take a VoIP to VoIP conference. 20ms to first packetize, if you use a
> 20ms timer to mix, you may be up to 40ms from start till you mix and
> transmit. Okay, now it gets to the otherside and causes echo and is sent
> back 20ms later and mixed up to 40ms later yet. Now we are getting the
> echo locally around 80ms late as it is.

That's a failure of the far point echo canceller, isn't it?

I know I'm not an expert in this by any means but if nothing else, this 
sticking my foot in my mouth should provide for good list traffic.  :-)


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