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Figure 0 - This device will decode the DTMF tones from the phone line or audio source
This device will decode the DTMF tones from the phone line or audio source

When you press the numbers on your phone to make a call, the tones you hear are called Dual Tone Multi Frequency tones (DTMF), and they represent one of 16 possible numbers on the keypad. The DTMF tones are made up of frequency pairs ranging from 697 Hertz to 1633 Hertz. There are actually eight frequencies that form a 4x4 matrix of 16 possible tones by mixing the pairs together. DTMF signaling has proven to be a very robust method of sending data over the telephone system, and it has become a universally excepted standard.

In order to decode the tones back into a 4 bit (16 possible combinations) signal, some type of high speed and accurate signal processor is needed. A fast microprocessor can be programmed to decode DTMF signals, but it will take a lot of processing power and careful analog signal control in order to make a reliable DTMF decoder in software. There are, however, inexpensive single IC solutions for touch tone decoding. This project is based on one called the CM8870 DTMF decoder chip. Several methods of deciding DTMF tones back into digital pulsed will be shown here, allowing devices to be controlled over the phone line or allowing you to spy on numbers being dialed live over the phone or from pre-recorded audio clips.



Figure 1 - The CM8870 DTMF decoder chip is a standalone DTMF decoding solution
Figure 1 - The CM8870 DTMF decoder chip is a standalone DTMF decoding solution

The CM8870 DTMF decoder IC shown in Figure 1 only needs an external 3.579 MHz crystal oscillator and a few passive components in order to decide DTMF signals on the fly. There are actually several makers of this chip, so when you are souring this part, the name "8870 DTMF Decoder" will be a close enough description. The reason for the odd 3.579MHz crystal is because this value is the same as the NTSC color burst frequency used in analog television and is very common and easy to source. The 8870 DTMF decoder does all of the difficult work for you, taking an analog audio signal at its input and then converting the decoded tones back into a 4 bit binary value that can represent 1 of 16 possible values. Most phones only have nine buttons on the keypad, but there are 16 DTMF combinations.

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