dtmf decoding

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Hi,

anyone got source code for this one they would share ?

I really wanted to do a hardware solution, but damned if I can't find a source in North America for any of the ICs for this purpose.

On the surface this seems simple so maybe everyone is doing it in software now and hence the lack of hardware solutions.

I figure if I AC couple the audio to the analog comparator in one of the AVRs, and use that to detect zero crosses, that is all the hardware needed. I would use a couple clamp diodes as well just to make sure the levels stayed reasonable.

Then as far as software I am not quite sure - I can figure out how to detect the sum frequencies if they were the only thing present, not sure how the difference frequencies will affect the zero crosses. Possibly I should incorporate a highpass filter as well ....

Suggestions appreciated.

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Hi Jon,

there are of couple of DTMF receivers that should be available in the US. Ever tried these?

MT8870 mitelsemi.com
CM8870 calmicro.com
MC145740 mot-sps.com

Hope that helps

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If you know a source for them, care to share it ?
I spent a few hours looking for the first two, and some others with no success.

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Co-incidentally I am working on a telephony application, and have just acquired samples on the Mitel 8870. Dirt-cheap (the 8870 is _really_ industry standard, you should get them everywhere).

OK, before I put my foot in it again, I have the chips, and the AVR circuit, but I haven't made them work together though ;) That's tomorrows baby. Looking at the datasheets for MT8870, it really should be a breeze. Phone in one side, binary digits and a 'valid tone flag' on the other side (5 pins AVR).

If you do some research on DTMF, you'll see that going the ADC route is a no-go regarding overheads and accuracy. You would effectively have to do FFT and DCT / filtering all in code... no thanks; not on a micro ;)

The "original" way of decoding was analog band pass filters preset to the possible high and low frequencies. Nowadays that's done in a single DIP... much easier ;)

Mitel's HQ is in Canada, here's there North America contact: +1 (770) 631-8213

http://www.mitelsemi.com

ciao
Andi

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The 8888 would be the prefered device for AVR (bus) interfacing.

Datasheet here:- http://www.calmicro.com/prod/dat...

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Hi Jon,

I don't know if you have a Dick Smith Electronics store where you are, but if so, they stock the MC145436 DTMF decoder.
their part # Z-5460

Cheers Jack

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I've taken a look at the 8888, the specs are virtually identical to the 8870, except that it is also a DTMF generator, and costs about 8 times as much :(

I only need to read a PIN code from a handset, and the 'added benefits' of the 8888 are an overkill. I do realise that the IRQ support is nifty, but hooking the 8870 STD to INT0 should do the trick in my application - alternatively I'll just poll it in the idle loop.

The 8888 requires (for full functionality) 9 pins on the I/O Bus, as opposed to 5 in my application.

Anyway, like I say, it's for my personal application - the 8888 does look like a better option for a generic telephony solution. If you don't pay in Dollars (like our stuffed up currency) every dollar adds R8 to the device. I buy the 8870 for R9.00 one-shots - that's about $1.05. If I had to pay $2.50 or more that triples the price ;)

Damn import duties and hassles of living in a Banana Republic.

Anyway, thanks for the reference, I have saved the pdf in the library ;)

Ciao
Andi

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Hallo John,

im looking in internet for DTMF encoding and decoding. I found 2 interesst Sites:

http://gallery.uunet.be/micro-te... (nederlands) and

http://members.optushome.com.au/...

The first site has an assemblercode !!!

I hope this Sites can help you

Peter

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Hallo Jon,

I look at the optushome-Site again. An good overview you get under

http://members.optushome.com.au/...
2000FEB/subject.html#457

Peter

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