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Forum: DSP What is the effect of reducing nfft less than the signal length


von Usman A. (Company: LUMS) (usmanashraf678)


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Dear members,
I am analyzing a signal in matlab by taking its fft. The singal has 200 
samples and I am currently taking nfft = 256. I understand that this 
would result in zero padding. Now, I want to try to reduce the nfft to 
128, 64 etc. I wanted to ask what should I expect to happen with the 
fft.

Matlab says
for Y = fft(X,n)
If X is a vector and the length of X is greater than n, then X is 
truncated to length n. What would this actually result in? Can anyone 
explain, please?

Thanks in advance.

von Andreas S. (andreas) (Admin) Flattr this


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You will get a reduced frequency resolution, and, depending on your 
input vector, windowing artifacts (you can consider truncation as 
equivalent to applying a rectangular window).

von Martin B. (Guest)


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You may also use Nfft = 200

I think in matlab you can use any number Nfft which is a factor of 
2,3,5.
It is not neccesary to use Nfft = 2^n, You may also use 
Nfft=200=2*2*2*5*5

Most fft librraries are able to use these numbers of Nfft (fftw, kissFFT 
..)

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