[asterisk-users] ignorant question about Digium cards and MeetMe

[Kevin P. Fleming]

On 07/29/2010 03:35 PM, David Backeberg wrote:
> So historically I've done one of two things on systems where I've
> needed to use MeetMe
> 
> * used a real Digium card, and I've only ever used a TE400 or a TE420
> for that purpose, and I know they have the timing chip
> * used dahdi_dummy, which works well with light load, but I had it
> running on a very overloaded server and had audio quality issues. I
> may have had quality issues even with a real Digium card, but it was
> impossible to isolate the problem on an overloaded server
> 
> So here's my actual question...
> 
> When I go through the Digium phone cards on the website,
> 
> * TE420 mentions 'synchronization' between channels
> * TE220 mentions 'synchronization' between channels
> * TE121P does not mention 'synchronization'
> 
> Does this mean that TE220 is the 'minimum' Digium phone card that
> still provides the timing / synchronization circuitry that MeetMe uses
> for mixing?

No. Any Zaptel/DAHDI supported device, will provide 'timing' to
Zaptel/DAHDI to be used for various purposes. There are no 'timing'
chips to speak of, it's not a specific function provided by the card,
it's just a side effect of the way that Zaptel/DAHDI cards function
normally. DAHDI can also provide timing using kernel-based timers (which
used to be provided by dahdi_dummy, but is now a core part of DAHDI and
more reliable).

Timing in modern versions of Asterisk can also be provided by other
sources (the pthreads library and Linux kernel timers), but MeetMe
requires both timing and mixing, which are provided by DAHDI itself.
DAHDI devices are not involved in conference mixing at all, except as
members of the conference.

So... if you are going to be using MeetMe, you should be using the most
recent release of DAHDI, which can provide both timing and mixing
without the need for a physical device (using core timing in DAHDI
itself). It is possible that in some heavily-loaded server environments,
or in virtualized environments, that a hardware device providing timing
might be able to maintain proper timing better than DAHDI core timing
can, but there's no way to know that without testing the specific
environment.

-- 
Kevin P. Fleming
Digium, Inc. | Director of Software Technologies
445 Jan Davis Drive NW - Huntsville, AL 35806 - USA
skype: kpfleming | jabber: kfleming at digium.com
Check us out at www.digium.com & www.asterisk.org