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Decoding DTMF tones at the speaker connections of a GSM modem

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Nikunj Tanna

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Hi all,

Actually I need to decode DTMF tones from the speaker of the local telephone or GSM modem. My main requirement is that I want to detect the key pressing using this. I have tried with 8870 decoder. But i found circuits which comes directly from the telephone lines where as my ultimate application has GSM modem in lead. Like in simple language i want to detect the tones at the speaker connections of modem or a local side telephone.. If you have any idea, just share your thoughts and suggestions.

NIKS
 

Re: DTMF decoding.

The 8870 is just an audio tone decoder, it doesn't care where the audio comes from.

Just copy the telephone line circuit but feed your signal in instead of making connection to the phone line, it will work. I use the 8870 for remote control using radio links for example. I just take the audio from a radio receiver.

Brian.
 

Re: DTMF decoding.

hi
u can achieve ur purpose by using a condensor mic. now connect a LM358 as an audio amplifier. finally connect it to 8870. now u get ur required dtmf tones
 

Re: DTMF decoding.

yes its possible


regards
kj
 

Re: DTMF decoding.

possibly your using 8870 in differential input method
this is direct input sample:
https://www.qsl.net/oe3mzc/mt8870.gif
make sure your speaker output, rests at gnd, in else case you have to use differential one line input(8870's opamp) 0 level biased for your speaker output rest level (example 2.5v of input)
 

Re: DTMF decoding.

Eclosed is a simple block diagram for ur better underastand.



Part 1:(a) POLARITY CONVERTER

The first most block diagram is the interfacing with telephone line .there are two wire in the telephone line known as ring & tip. These line are generally analog lines that provide adequate response over the frequency range from 300 hz to 3400 hz ,that is suitable for voice communication. the technique used for communication between two exchange is D.T.M.F. (DUAL TONE MULTI FREQUENCY) ,in which two frequency combination is used .these two frequency are of two different band which lies into frequency range supported by telephone line. by considering all this point , there is voltage of range from -48 volt to -60 volt available on the line & this is corresponds to D.T.M.F. signal. So to provide a constant voltage of 6 volt regardless of connection of wire .here we are use POLARITY CONVERTOR CIRCUIT. From circuit diagram we there is pair of resistance & capacitor on both line for blocking of D.C. LEVEL. and then this is connect with back to back connected ZENAR DIODE which work as a polarity converter that provide constant 6 volt regardless of connection,& then there is resister divider which connect to the inputs of D.T.M.F. DECODER.

(b) D.T.M.F. DECODER

after receiving 6 volt from polarity converter is fed into the resister divider for proper biasing of the operational amplifier that is inbuilt into the D.T.M.F. decoder. The output obtain from the op-amp is fed to two the different band pass filter along with high and low group frequencies. Which followed by zero crossover detector for converting a sine wave into a square wave of same frequency. These square waves are fed to input of digital detection algorithm ckt, in which equivalent BCD code is generated correspond to the input signal frequencies and this BCD code is latched through next block. The output of latched ckt is available on the output pins of DTMF decoder from pin no.11 to 15. In DTMF decoder there is pulse steering logic for controlling of digital detection algorithm and code converter logic.


Regards
Chanchal
 

Re: DTMF decoding.

Hi,

Thanks a lot for the replies. Now trying with your suggestions. I will tell you the results very soon. Thanks..

NIKS
 

Re: DTMF decoding.

Hi.....

I dont know your problem solved yet or not ... If not you can try a single input dtmf decoding circuit... It will work.. I did that.........

---------- Post added at 07:35 ---------- Previous post was at 07:33 ----------

Hi.....

I dont know your problem solved yet or not ... If not you can try a single input dtmf decoding circuit... It will work.. I did that.........
 

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