DUAL TONE MULTI FREQUENCY (DTMF)

The main principle of the DTMF is that a number code drom the number pad converts it to DTMF ( Dual Tone Multi Frequency) signal and a DTMF decoder converts the DTMF signal to a digital code that can be fed to a microcontroller. A DTMF generator generates two frequencies corresponding to a number or code in the number pad which is tranamitted through tge communication networks, constituting the transmitter section which is simply equivalent to a mobile set. In the reciever part, the DTMF dectector IC, for example MT8870 dectects the number code represented by DTMF back, through the inspection of the two transmitted frequencies. The DTMF frequencies representing the number codes as shown below

    1                2                 3                 A             697Hz

    4                5                 6                 B             770Hz

    7                8                 9                 C             852Hz

    *                 0                 #                D             941Hz

1209Hz. 1336Hz. 1477Hz. 1633Hz

DTMF generation is a composirion of two audio signal or two tones between the frequency 6971Hz and 1633Hz. In a DTMF each row has its own unique frequency and also each column will have its own unique tone. The tone frequencies are selected such that harmonics and intermodulation products will not cause any unreliable signal. Each and every tone falls within a proper band pass before valid decoding takes place. If one rone falls outside the band pass spectrum, the decoder will become unreliable. A DTMF decoders main purpose is to dectect the sinusoidal signals in the presence of noice.