Analogue DTMF (dial tone) Decoder help

[george2525]

Hi

I am interested in making an analogue DTMF decoder that will take the dialling signal from a phone cable and then sort the tones into their corresponding phone number(s)

All the info I can find uses computers to do this however im aware that analogue methods were used previously. Unfortunately I have found nothing whatsoever regarding this!

my basic guess is that i would need 8 bandpass filters (there are 8 frequencies used to indicate 16 symbols) which trigger some kind of sample and hold thing that when triggered lights up an 8 seg display with the number corresponding to the filters that allow signal through them.

like i said theres 0 articles on this unless I have missed something (eg - the term for the old decoders)

if anyone has any ideas/links about this stuff I would be very happy.

Thanks

 

[Harald Kapp]

Would you care to use a dedicated DTMF decoder IC?

Why would you use a sample and hold circuit? DO you need to store the last coded number? Tell us a bit more about your application. This will help us help you.

 

[AnalogKid]

I think he means sampling the decoded output so it is displayed after the tone pulse ends - a data latch.

An old-school way to do touch-tone is with 8 phase-locked loops. Signetics developed the NE567 for this back in the 70's.

www.ti.com/lit/ds/symlink/lm567.pdf

Page 13, figure 12.

ak

 

[Harald Kapp]

I think he means sampling the decoded output so it is displayed after the tone pulse ends - a data latch.

Sounds plausible - and much simpler than a sample and hold.

 

[Colin Mitchell]

It is very difficult to read DTMF via a program. It has been done by an absolutely brilliant programmer.
It is much easier to use a DTMF chip.