PROJECT 11Phone Number Decoder
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 1 of 16 possible numbers on the keypad. The DTMF tones are made up of frequency pairs ranging from 697 to 1633 Hertz (Hz). There are actually eight frequencies that form a 4 × 4 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.
Figure 11-0 This device will decode the DTMF tones from the phone line or audio source.
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