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Spectrogram View of Dial Tone Signal

Create a signal, sampled at 4 kHz, that resembles dialing all the keys of a digital telephone. Save the signal as a MATLAB® timetable.

fs = 4e3;
t = 0:1/fs:0.5-1/fs;

ver = [697 770 852 941];
hor = [1209 1336 1477];

tones = [];

for k = 1:length(ver)
    for l = 1:length(hor)
        tone = sum(sin(2*pi*[ver(k);hor(l)].*t))';
        tones = [tones;tone;zeros(size(tone))];

% To hear, type soundsc(tones,fs)

S = timetable(seconds(0:length(tones)-1)'/fs,tones);

Open Signal Analyzer and drag the timetable to a display. Click Time-Frequency to add a spectrogram view. On the Spectrogram tab, under Time Resolution, select Specify. Enter a time resolution of 0.5 second and zero overlap between adjoining segments.

The spectrogram view shows that each key is dialed for half a second, with half-second silent pauses between keys. The first tone has its frequency content concentrated around 697 Hz and 1209 Hz, corresponding to the digit '1' in the DTMF standard.

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