3/6/2023 0 Comments Touch tone telephoneHow is it possible to determine a phone number by listening to the signal generated? This involves the FFT, the Finite Fourier Transform, or, more likely, some specialized version of the FFT adapted to this task. The top subplot depicts the two underlying frequencies and the bottom subplot shows a portion of the signal obtained by averaging the sine waves with those frequencies. If your computer is equipped with a sound card, the statementįor example, this is the display produced by touchtone for the "1" button. The tone generated by the button in position (k,j) is obtained by superimposing the two fundamental tones with frequencies fr(k) and fc(j). If s is a character that labels one of the buttons on the keypad, the corresponding row index k and column index j can be found withĭ = s- '0' j = mod(d-1,3)+1 k = (d-j)/3+1 Īn important parameter in digital sound is the sampling rate.Ī vector of points in one-fourth of a second at this sampling rate is These frequencies were chosen so that none is an integer multiple of any other. Associated with each row and column is a frequency. The telephone dialing pad acts as a 4-by-3 matrix. The program touchtone, which is available here, or which is included with my NCM App, demonstrates both these aspects of DTMF. Analysis is the decoding of these tones to retrieve the digits. Synthesis is the generation of analog tones to represent digits in phone numbers. The basis for touch-tone dialing is the Dual Tone Multi-Frequency (DTMF) system. Touch-tone telephone dialing is an example of everyday use of Fourier analysis.
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