Splitting a DTMF signal from a wav file using Matlab

Question

Here is the context of the problem: I have a DTMF signal in wav format, I have to identify the number sequence it has encoded. I must do so using fast fourier transform in Matlab, implying that I read the wav file using wavread and to identify each number that is seperated by 40ms silence or more.

Here is my code so far:

[signal, fs] = wavread( 'C:\Temp\file.wav' );  % here, fs = 8000Hz

N = 512;                    
T = 1/fs;                   
L = length( signal )        
samples = fs / 1000 * 40    
windows = floor(L / samples) 
t = (1:L)/fs;

figure(1), plot(t, signal);

Here is what the figure 1 looks like, that is the signal read from the wav:

How can I effectively split the signal into pieces so that I can then do an FFT on each of the 10 pieces seperately to decode the corresponding numbers?

Answer 1

This worked for me:

windowSize = 256;   
nbWindows = floor(L / windowSize);

for i=1:nbWindows
    coeffs = fft(signal((i-1)*windowSize+1:i*windowSize));    
    plot(abs(coeffs(1:N)));
    waitforbuttonpress
end;

This way it is possible to shift the window until the end of the input signal

Answer 2

I would recommend the following approach:

• Find the envelope of the signal in the time domain (see Hilbert transform).
• Smooth the envelope a bit.
• Take the diff and find peaks to get the onsets of the tones.
• Use the onsets to pick frames and find the spectrum using fft.
• Find the index of the max in each of the spectrums and convert them to a frequency.

The tricky part in this is to get a robust onset detector in point 3. The peaks in the difference you pick, has to be of a certain size in order to qualify as on onset. If your tones are of varying strength this might pose a problem, but from your image of the time signal it doesn't seem like a problem.

Regards