| Re: FFT example. Help! [message #19893 is a reply to message #19879] |
Mon, 01 May 2000 00:00  |
Paul van Delst
Messages: 364
Registered: March 1997
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Senior Member |
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Peter Brooker wrote:
>
> I am trying to understand the FFT routine IDL uses. Part of my problem
> is that though I am familiar with Fourier transforms, I am somewhat
> unfamiliar with the fast Fourier transform.
>
> Has anybody written a program that works through a known transform using
> the FFT procedure? In particular, I want to be able to plot out F(u) vs
> u and have it "make sense".
Umm, I'm not quite sure what exactly it is you are asking but I have
code I use to transform spectral data (i.e. down- or up-welling measured
atmospheric radiance) into interferograms and vice versa. Check out:
http://airs2.ssec.wisc.edu/~paulv/#IDL Spectral
and look at the functions fft_to_interferogram.pro and
fft_to_spectrum.pro. The actual FFT'ing is performed in one line (of
course) with all the rest of the code for input checking and "x"-value
(abscissae?) calculation (e.g. from frequency in cm^-1 to optical delay
in cm and back).
If, on the other hand, you want FFT references, the online help gives a
good general description and, I believe, a reference. For a comparison I
did of the IDL FFT with the (Fortran) NR FFT, check out
http://airs2.ssec.wisc.edu/~paulv/fft/fft_comparison.html
It details some of the intricacies of organising the input and output
data (mostly for the Fortran FFT) so you get what you expect on the
ass-end of the problem. :o) The test code and data files are at the
bottom of the page.
Hope this is helpful.
paul
--
Paul van Delst Ph: (301) 763-8000 x7274
CIMSS @ NOAA/NCEP Fax: (301) 763-8545
Rm.202, 5200 Auth Rd. Email: pvandelst@ncep.noaa.gov
Camp Springs MD 20746
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