Question regarding FFT MATLAB demo

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Martin 2012-10-23

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Hi,

I am computing the frequency response function of 1 input and 1 output data sample. I am using the demo found at http://www.mathworks.se/products/matlab/examples.html?file=/products/demos/shipping/matlab/fftdemo.html to calculate each power spectral density. I have a simple question regarding the demo. In this, they use a data sample of 251 measurements and use the command;

Y = fft(y,251);

This is perhaps a stupid question, but shouldn't the number 251 be replaced by 2^nextpow2(251) = 256?

Hope someone out there can answer me. Thanks in advance.

Kind regards, Martin Ulriksen.

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Answer 1

Honglei Chen 2012-10-24

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编辑：Honglei Chen 2012-10-24

Hi Martin,

In theory, Fourier transform can be performed using any number of data points.

In old days, people prefer to have the number of FFT points to be power of 2 to take advantage of some computation benefit. Nowadays, at least in MATLAB, doing so no longer brings significant performance improvement so many people just use the length of data samples.

HTH