Forums

FFT pad or no pad

Started by kctu...@gmail.com August 21, 2009
I have 300sec data collected at 1Hz. I want to look at power spectrum of this time series, so first I need to do FFT(). But I am not sure if I need to pad it with zeros to make it 512. Matlab FFT() lets me do it either way, but I don't really understand which way I should do. Please advise. Thanks.
KC-

> I have 300sec data collected at 1Hz. I want to look at power
> spectrum of this time series, so first I need to do
> FFT(). But I am not sure if I need to pad it with zeros to
> make it 512. Matlab FFT() lets me do it either way, but I
> don't really understand which way I should do. Please advise. Thanks.

A power spectrum implies more than one frame; i.e. averaging of some type in the frequency domain. But you have only
one 300 pt. frame.

I think at this stage in your analysis, either DFT or zero-padded FFT would be fine. The main reason to use FFT is in
a real-time or product implementation where there are constraints on your available processing resources.

Also, if you do use a zero-padded FFT, be sure to take into account "edge noise" artifacts, due the cut-off at the end
of your 300 pts. You can mitigate this by applying a window prior to the FFT.

-Jeff