[asterisk-users] DTMF detection problems on PRI channels?

Michelle Dupuis
2007-03-02T09:22:07Z

Sounds like the DTMF tones are too far from spec, or noisy.  Is the DTMF
being transcoded somewhere along the way?

If you have time to kill....try to separate the two frequencies in your
software (I don't know goldwave) - are both present and clean and same
amplitude and on freq?  Remove the two frequencies and what's left?  If
there's a lot of noise, then the other party is doing a bad job encoding the
DTMF.  Otherwise we can start to chase your machine causes

Michelle Dupuis
Technical Support Specialist

Generation Software - Linux and Asterisk solutions and support.  Visit us at
www.generationd.com




-----Original Message-----
From: asterisk-users-bounces@lists.digium.com
[mailto:asterisk-users-bounces@lists.digium.com] On Behalf Of Tony
Mountifield
Sent: Friday, March 02, 2007 10:56 AM
To: asterisk-users@lists.digium.com
Subject: [asterisk-users] DTMF detection problems on PRI channels?

I am using Asterisk 1.2 with a TE410P connected to E1 PRI trunks.
The application relies on a DTMF digit string sent by the phone after the
call has connected. This DTMF is detected by Asterisk under the control of
WAIT FOR DIGIT commands send from an AGI processor over a FastAGI
connection.

Usually the DTMF is detected without error, but on a significant minority of
calls, Asterisk is missing digits.

In order to diagnose this, I modified chan_zap to save the received Alaw
audio direct to a file, BEFORE the dsp is called for DTMF detection.
I needed to do this because the detection routines do not pass the DTMF
audio on, so using the standard recording or monitoring commands from the
dialplan does not actually capture the tones as received from the wire.
This capturing is turned on and off by an AGI command, so that my AGI
program can turn it on before waiting for the DTMF string and off again
afterwards.

Examining this captured audio in an audio editor such as Goldwave does not
provide any clue why the digits might have been missed. On most occasions
the digits are clear, long enough and well spaced. Yet Asterisk still misses
them.

The system does not seem to have been heavily loaded at the time either.

Can anyone offer any clues as to why this might be the case, and what I
could do to solve it? Hacking the code doesn't bother me, although I know
very little about DSP.

Last I knew, the TE411P board could do on-board DTMF detection, but that the
newer TE412P could not. Is that still the case?

Cheers
Tony
--
Tony Mountifield
Work: tony@softins.co.uk - http://www.softins.co.uk
Play: tony@mountifield.org - http://tony.mountifield.org
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