Re: fft question

["Leonard Mada" <discoleo gmx net>]

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.

Sincerely,

Leonard


-------- 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.
>
> Sincerely,
>
> Leonard
>
>
> -------- 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
>
> > Hi,
> >
> > 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?
> >
> > Regards,
> > Jean
>
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