Using FFT for selected data

Question

Nani C 2013-1-5

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Hi,

   i have time data for say t=50s. i am trying to take FFT of this data using limited data only from say t=10 to t=20s. what is the change in the fft command i should be doing ?
   i have tried changing the sample length n and NFFT accordingly, but it does not match with the FFT of the 50s data.

Thank you.

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Answer 1

Matt J 2013-1-5

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编辑：Matt J 2013-1-5

If you take the DFT of the same signal samples over a truncated time horizon, you will get coarser sampling in the frequency domain. There is therefore no reason the output should match in the two different cases.

However, as long as your time sampling rate is good and as long as the 10s horizon still covers the non-zero region of the signal, you should be getting a pretty accurate spectrum. What were you trying to accomplish by truncating the signal?

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Image Analyst 2013-1-5

Make it easy for him/us to help you by posting your code. For example, we have no idea how many samples are between your 10 second element and your 20 second element. Is it (unfortunately) only 10? Or is it a million? Also post your initial data. I don't see any reason to run out of memory especially for a 1D signal.

Nani C 2013-1-5