Question regarding FFT MATLAB demo

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.

Answer 1

[Honglei Chen]

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

Answer 2

[Martin]

Hi Honglei Chen,

Thank you very much for the answer - I really appreciate it.

//Martin.