stem function and fft

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Eman Ahmed Elsayed 2011-12-16

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i used stem function to convert a continuous signal to discrete signal.

and then i want the discrete function to get the fast Fourier transform of it.

stem return a vertical lines only so what should i do with this vertical lines???

I hope that my question was clear.

Thanks,

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Answer 1

Wayne King 2011-12-16

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Hi Eman, stem() is not converting the continuous signal to discrete time. stem() just plots. If you have the sequence (vector) in MATLAB, then it is already a discrete-time sequence.

You want to operate on the vector with the function fft()

% an example
x = cos(pi/4*(0:95));
stem(x);
% now obtain the discrete Fourier transform
xdft = fft(x);
stem(abs(xdft));

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Eman Ahmed Elsayed 2011-12-16

Ok. but what's the difference between stem(x) and stem (abs(x))

Wayne King 2011-12-16

The discrete Fourier transform of a vector is (in general) complex-valued, so you want to plot either the magnitudes (abs()), or the phases (angle() )

In the simple example I gave you, the imaginary parts are all zero, but that is not going to be the case with any real-world signal

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