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i wrote a function in MATLAB which will divide a audio waveform (a speech) into smaller windows with overlap .The output will be a matrix X, each column of which contains the windows which i export. Then i calculate the FFT of each column and i visualize the measure of each FFT frame as an image, using the "imagesc" command. I saw a shape that is symmetric to the horizontal axis. Why i have this symmetry?.enter image description here

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Very simple:

The Fourier transform of every real-valued signal is (hermitian) symmetric.

You can thus, for your real-valued signals, simply not visualize half of the FFT bins.

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