FourierTransform - extrapolating the results?

(Otakar Trunda) #1

I have two questions about FourierTransform:

How can I extrapolate the results? I have a serie with - lets say - 100 elements, I compute its Fourier Forward Transform - this gives me also an array of 100 elements. Now when I compute Fourier Inverse of the result, I get the original serie. So far it works as expected. (So far I’m working with a simple sinusoid)
Now I need to use the Fourier Transform to estimate how the serie will continue, i.e. I need to evaluate it on indices larger than 100, lets say on interval [100, 200].
I tried to add additional zeros to the result of the Fourier Forward and then use the Inverse, but that will only rescale the original serie to the larger interval (i.e. it will have 200 elements, but there will be the same number of peaks as in the original. I need it to have twice as many peaks.)

Is there any difference between using ForwardComplex and only feeding it real data, and using ForwardReal? I want to use FF to a serie of real numbers. I guess I should create a serie of complex numbers with complex parts equal to zero, then use ForwardComplex, and then replace complex numbers in the result by their absolute value. Is it correct? Is this what I will get when I use ForwardReal and InverseReal?

Thanks a lot a to anyone who can help me

(Peter Vanderwaart) #2