[asterisk-dev] DTMF detection in dsp.c intolerant?
zoachien at securax.org
Wed Aug 22 08:33:37 CDT 2007
Relaxdtmf should give more false positives instead of less detections. ?
Eric "ManxPower" Wieling wrote:
> relaxdtmf=yes is the most common cause of this issue.
> Tony Mountifield wrote:
>> I'm posting this to -dev because I believe the answer will relate to the
>> code in dsp.c for DTMF detection.
>> I have a system connected to the PSTN using EuroISDN over E1 with aLaw
>> encoding. When it has answered a call, it listens for a DTMF string which
>> contains command codes for the specific application.
>> Asterisk seems to be very intolerant of slight discrepancies in the DTMF
>> tones. We are finding that with calls from certain locations, it will
>> often miss digits, even though to the ear the audio sounds clear. Calls
>> from other locations will be completely reliable.
>> I have the system capturing pre-DSP aLaw audio, so I can decode by hand
>> and see which digits are missing (using Goldwave's spectrum display
>> Here are two examples of separate calls sending the same string:
>> The string sent in both cases was *82779736*91856360**
>> In both cases, Asterisk failed to detect the third 7, and returned the
>> string *8277936*91856360**
>> The audio sounds good to me, so I am at a loss as to what Asterisk didn't
>> like. Particularly as it failed in the same way on both occasions, which
>> suggests it is not something random such as a system load peak.
>> The string is always originated by a GSM mobile, and my understanding is
>> that DTMF is sent as out-of-band signalling over GSM and then re-generated
>> by the GSM to PSTN gateway. Is that correct? So the tones ought to be good.
>> If anyone has the time and interest to check out these files and offer
>> any insights, I would be very grateful indeed. DSP is not my field!
>> I have a number of other recordings where the tones sound ok but Asterisk
>> doesn't like them.
>> Thanks in advance!
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