Re: fft question

A last idea:

in order to visualize the spectrum of a "complex" Fourier transform
(unlike FFT), you need to calculate the module of the result, which
implies squaring the result.
[Caution: I think it is called "module" in English.]

Maybe this is the reason for getting only half the image - maybe
"complex data" implies the generic Discrete Fourier Transform (DFT),
and the result is a spectrum containing complex numbers, which needs
a further transformation in order to be displayed - the modulus
being displayed instead of the actual DFT.



-------- Original-Nachricht --------
Datum: Sun, 03 Oct 2010 21:08:00 +0200
Von: "Leonard Mada" <discoleo gmx net>
An: Jean Brefort <jean brefort normalesup org>, gnumeric-list gnome org
Betreff: Re: fft question

Dear Jean,

FFT generates symmetric spectra - basically symmetric mirror images.
[The frequencies are mirrored against the central 0.]

This seems to be the case in the "real" image.

While the "complex" image seems to include only one half,
which is the usual way to represent the NMR spectrum.
[and more generally any frequency-power spectrum in real life]

I do not have though access to go_fft, and I am unsure what
"complex data" in your post means.



-------- Original-Nachricht --------
Datum: Sun, 03 Oct 2010 08:41:51 +0200
Von: Jean Brefort <jean brefort normalesup org>
An: Gnumeric List <gnumeric-list gnome org>
Betreff: fft question


I'm a bit puzzled about the output of the go_fft function (actually the
same code as what we have in gnumeric, may be gnumeric might use the
goffice version?). I'm trying to use it to transform an NMR FID to a
spectrum. When I use only the real data, things give the expected
result, but I do not understand the output when I use the complex data
input (see attached images). Is there some FFT expert around?


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