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I am totally new in Matlab. Can anyone please explain in details how I can extract pitch contour of a speech? I would like to observe how the speaker's pitch varies during his speech to know whether he is speaking in a flat tone.

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I recommend computing the spectrogram of the audio recording using the MATLAB function spectrogram().

The fourth return value (ps in the documentation page linked above) of the spectrogram() function is a matrix of the power spectral density estimates. This can be used to investigate how much energy there is at each frequency at each time instant. A tone of constant frequency results in a straight horizontal line, a short impulse in a vertical one. Most signals have much more complex spectrograms.

Looking at the matrix as a two-dimensional image reveals a lot about the signal at hand. As the range of coefficient values in the spectrogram of an audio signal is usually quite large, it is a good idea to compress this range by computing the logarithm before plotting. The following code snippet will hopefully yield the image you had in mind - with x your (single channel) audio signal and fs the sampling rate:

L_block = 4096;
L_overlap = floor(L_block/2);
L_DFT = L_block;
[~,f,t,ps] = spectrogram(x, ...
    hann(L_block, 'periodic'), ...
    L_overlap, ...
    L_DFT, ...
    fs, ...
    'xaxis');
imagesc(t, f, 10*log10(ps));
axis xy;
ylim([20 600]); % zoom to the fundamental frequency range
xlabel('Time in s');
ylabel('Frequency in Hz');

To find out how much the pitch of the speaker varies, analyze the horizontal lines that you see. Experiment with the block length (L_block), overlap length (L_overlap) and the window (here: hann()) to obtain optimum results.

By the way: Here is another example on how to compute and display the spectrogram in MATLAB.

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