[Asterisk-Users] DTMF information?

[Steve Underwood]

Chris Lee wrote:

> Steve Underwood wrote:
>
>> Chris Lee wrote:
>>
>>> I am looking at building an IVR product with a few interesting 
>>> features and need some more information about how asterisk and VoIP 
>>> work and what I can get from them.
>>>
>>> As far as I can tell when I use ISDN/GSM telephone networks the DTMF 
>>> information travels as data representing 'start tone' and 'stop 
>>> tone' for each button pressed, it is then generated at the other end 
>>> if an audio representation is required.
>>> I am interested to know if I can get access to these events 'start 
>>> tone' and 'stop tone' through the dialplan or an AGI or by acting as 
>>> a VoIP device. Or of course if I am completely off track and should 
>>> give up now.
>>>
>>> I am looking to get the length of time a button was held down rather 
>>> than that it was pressed.
>>>
>>> Thanks for any help
>>
>>
>>
>> ISDN never does this. GSM only does this between the handset and the 
>> base-station. You only see DTMF tones from outside the GSM network 
>> itself. For fancy IVRs, beware that the timing of DTMF from a GSM 
>> handset has nothing to do with the timing of the user's keypresses. 
>> Because the base-station generates the tones, it controls their 
>> timing, and always generates rather long slow pulses of DTMF tones.
>>
>> Regards,
>> Steve
>>
> Thanks for the info, I will have to re-think my plans.
>
> With GSM the tone lasts until the user releases the key, this I have 
> tested GSM to GSM and GSM to Land Line (pots).

Not quite. The tone usually lasts as long as you press, if you press for 
a long time. If you give a short press, the GSM network will extend it 
to a fairly long tone. This screws up any rhythmic entry methods you 
might devise (been there, got screwed by that :-) ). Other cellular 
systems are similar.

> I assume then that the ISDN carries DTMF as audio and this must be 
> decoded by something? hardware or software depending on the card in 
> the PC.
> I have noticed with analogue modems that the hardware decoders provide 
> a 'DTMF tone received' indicator and then ignore the length of the tone,
>
> Does any one know if tone length is acquired by any of the asterisk 
> drivers? and retained/passed on within asterisk?
>
> It seems like throwing away information, which could be useful, if the 
> length of the key press is ignored and only the fact it was pressed is 
> retained.
> I dont know what the standards say but I have noticed that all phones 
> I have tested (except DECT phones) send a continuous tone until the 
> button is released.

Most cordless phones play around with the length of the tone, so they 
also tend to screw up rhythmic entry methods. Key phone type PBXes, and 
other digital phones attached to PBXes also screw around with the digit 
timing in various ways - these also generate the tones at the PSTN 
interface, rather than the phone.

Regards,
Steve