Explanation of FFT calculation

[Alex65111]

Let's assume at us is available 1024 discrete samples. Further it is carried out FFT. Whether probably from received 1024 spectral samples to receive 256 spectral samples which are equivalent as if we have executed FFT only on 256 input discrete samples?

 

[Aya2002]

Calculation FFT

i did not understand you

 

[Communications_Engineer]

Re: Calculation FFT

Let's assume at us is available 1024 discrete samples. Further it is carried out FFT. Whether probably from received 1024 spectral samples to receive 256 spectral samples which are equivalent as if we have executed FFT only on 256 input discrete samples?

Perhaps your first language isn't english but you still need to re-phrase your question to make it more understandable

 

[Alex65111]

Re: Calculation FFT

It turns out so that at the decision of one stage of a problem to have to
count FFT the large size, and at the second stage it is required on the
same data FFT of the smaller size. Accordingly it would be desirable to
count not anew FFT, and to use result from the first stage.

Let's assume there are 1024 samples X = [X1...X1024].
Y=FFT(X), Y =[Y1...Y1024].
It is required from Y to receive Z, where Z=FFT(X1...X256)
or
Z=FFT((X1...X256 + X257...X512 + X513...X768 + X769...X1024)/4).

(X1...X256 + X257...X512 + X513...X768 + X769...X1024=[X1+X257+X513+X769, X2+X258+X514+X770, ... , X256+X512+X768+X1024])

 

[JoannesPaulus]

Calculation FFT

No, if I understand what you mean, you cannot do that unless you are interested on only 1/4 of the spectrum.

I have to confess though that it is still difficult to understand what you exactly need.

 

[Alex65111]

Re: Calculation FFT

Problem. At the first stage it is necessary signal to look at the good spectral resolution, and at the second stage the same signal should be looked at more rough spectral resolution.
One obvious approach - at first to take FFT the big size, and then to take FFT the smaller size.
However it would not be desirable to carry out the second FFT (the small size), and to obtain more rough spectral resolution on the basis of spectral samples of the good spectral resolution.