Detection OF DTMF signals

[midhun78]

which is the most
appropriate way to detect the DTMF signals using a
modem ?

 

[BrianG]

You can't detect DTMF tones with a modem as it will only recognise either of two specific tones for FSK signalling (i.e. "0" and "1" tones) and DTMF uses different tone pairs for each digit 0-9, etc.

What you need is a purpose-made DTMF tone decoder chip. These are made by Motorola and others.

 

[Heydave]

Many modern modems use DSP processors to MODulate and DEModulate data on a voice channel. Many such modems run DSP processors from ram or flash memory that can be reprogrammed. These usually cover the old FSK schemes as well as the more complex schemes for high data rates. It is no problem for them to detect DTMF as long as they areprogrammed to do so. A few of these support DTMF detection as well but you will have to look for them as the market does demand this feature by itself. Look for modem units that support voice mail features with mail boxes. If you are trying to monitor a line to capture numbers dialed without drawing current like an extension phone, you may have trouble finding one unless it also supports caller ID. The caller ID signal is sent using an old FSK scheme (bell 202) sent in a burst between the first and second rings if no loop current is drawn. The programming of the modem may not allow it to do anything but caller ID while "on hook".
Good luck.

 

[twistdpr]

Although modem hardware is capable of detecting almost any kind of analogue signal, manufacturers of modem chips do not put all of this functionality into the modem chip firmware. Modems are not designed to detect DTMF signals, they have no need to detect DTMF signals. However they will produce DTMF signals generally for signalling purposes such as dialling. Your request is not an appropriate use of a modem. If your need to detect DTMF signals is for a hobby project then its easy to make a small circuit on veroboard using a DTMF detection chip. Otherwise contact the modem chip manufacturer for the source code to the firmware and make the changes yourself.

 

[RichardMid]

I have another question concerning DTMF and modem:

I want to dial a nr using DTMF, and the transmit data using DTMF to a unit that can decode it. Can I do it using the AT command set? What command do I use? Otherwise what will be the most appropriate way? I am doing all this from a C# app.

 

[IRstuff]

Why can't you use the AT dial command?

TTFN

 

[RichardMid]

I found the VTS and VTD commands in another AT command set table. They were not in the table I used initially . It looks like that is what I am looking for. I will try it and come back to you

Thanks
Richard