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Analogue DTMF (dial tone) Decoder help

george2525

Jan 30, 2015
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Jan 30, 2015
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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

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Nov 17, 2011
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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

Jun 10, 2015
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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
 

Colin Mitchell

Aug 31, 2014
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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.
 
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