[asterisk-users] DTMF detection on dahdi with b4xxp (again, some more details)

[Karsten Wemheuer]

Hi,

Am Dienstag, den 05.01.2010, 15:38 +0100 schrieb Christian Theune:
> Hi,
> 
> I tried again getting DTMF detection on my ISDN devices with dahdi going 
> again. I used the channel debug to see whether asterisk sees the frames 
> and detects them as DTMF.
> 
> Interestingly here's what works:
> 
> 1. GSM phone -> chan_dahdi g1 -> asterisk -> can_sip -> SIP phone
> 
> Both the GSM phone and the SIP phone can issue DTMF that will be 
> detected as features (transfer)
> 
> 2. GSM phone -> chan_dahdi g1 -> asterisk -> chan_dahdi g4 -> ISDN phone
> 
> The GSM phone can issue DTMF that will be detected. The ISDN phone 
> won't. (That's my issue.) I don't see any messages of asterisk 
> recognizing the DTMF frames when pressing the keys. I do hear the DMTF 
> sound on both phones.
> 
> 3. ISDN phone -> chan_dahdi g4 -> asterisk -> chan_dahdi g1 -> GSM phone
> 
> The ISDN phone can issue DTMF that will be recognized and so does the 
> GSM phone.
> 
> So. When the ISDN phone is receiving a call on g4 its DTMF sounds won't 
> be recognized. OTOH when the GSM phone on g1 is being called it's sounds 
> are recognized.

I *think* there are two possibilities to transfer DTMF on ISDN:
- as audio on B-Channel
- as Key-Press events (Info-Elements) on D-Channel

DTMF on GSM can not be signalled as audio (because of codec with high
compression). I guess in case GSM => asterisk via "chan_dahdi g1" in
Your example, the DTMF is signalled as Info-Elements on D-Channel.

I guess in the cases where Your DTMF is not working, audio path is used.
In this case DTMF detection is done by DSP-Software. Look for the
relaxdtmf statement (in case of zaptel this worked for me in a simmilar
scenario).

HTH,

Karsten