[asterisk-dev] Re: Asterisk timing without hardware

[James Harper]

> 
> Zaptel (and without hardware, ztdummy) does more than just provide
timing.
> For MeetMe, for instance, the mixing engine for the conferencing is
within
> the zaptel module. It is this module that needs invoking at
device-driver
> level 1000 times every second.

I didn't know that. Thanks.

> 
> > Failing that, it would be nice if ztdummy could use the HPET. HPET
would
> > allow ztdummy to get timing spot on rather than the 1024Hz -> 1000Hz
> > approximation that it does now.
> 
> It was I who submitted the RTC-based 1024->1000 patch for ztdummy,
based
> on the old zaprtc from Junghanns.
> If you have no zaptel hardware, you must
> be using VoIP, which uses 20ms frames, so the small amount of jitter
> caused by the 1024->1000 conversion is not usually significant. I have
not
> found it to be a problem in practice.

Not all that is not Zaptel is VoIP :) Do you know how CAPI, mISDN, vISDN
etc fare if there is no zaptel hardware in the system? Do you know how
they fare if there is zaptel hardware in the system, given that the
timing source could be +/-1ms out of sync, and there is potential for
drift. Or maybe Asterisk and the zaptel stuff you mentioned above can
derive timing from CAPI et al anyway...

So much to learn...

> 
> I wasn't aware of HPET at the time, and it looks worth investigating.
> 

It does, it seems very very flexible and is available from user and
kernel space. The only thing that may limit its usefulness is that it
may be limited to newish hardware and then only certain APIC
configurations thereof. Having said that, I haven't looked too closely
yet so I could be completely wrong.

Thanks

James