[Numpy-discussion] FFT usage / consistency

Stéfan van der Walt stefan@sun.ac...
Sat Jul 26 04:56:49 CDT 2008


Hi Felix

This doesn't look quite right:

    # Re-Transform to frequency domain
    fftdata = fftpack.fft(ifftdata)
    fftdata = fftpack.fftshift(ifftdata) # not the "i"

You probably want fftshift(fft(ifftdata))?

As an aside, you also don't need vectorise, since those functions are
all "vectorised" by themselves.

Cheers
Stéfan

2008/7/25 Felix Richter <felix@physik3.uni-rostock.de>:
> Hi all,
>
> I found myself busy today trying to understand what went wrong in my FFT code.
> I wrote a minimal example/testing code to check the FFT output against an
> analytic result and also tried to reverse the transformation to get the
> original function back. Most curiously, the results depend on whether I first
> do the fft and then the ifft or the other way round.
>
> For the test function, I use the Lorentz function 1/(x^2+1). The exact FT is
> exp(-|t|)*sqrt(pi/2), the IFT yields the same.
>
> 1) First FFT and then IFFT: The real part of FFT oscillates, the imaginary
> part is not zero, and the magnitudes do not match. All this should not be,
> but the IFFT reproduces the original function just fine.


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