Numerical Recipes FFT routine bugs ???

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2 total messages Started by Ed Croteau Fri, 24 Jul 1992 19:26

#138

Author: Ed Croteau
Date: Fri, 24 Jul 1992 19:26

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Does anyone have any information on the "FOUR1.C" routine in the
Numerical Recipes in C book? I believe that the routine as printed
(and on a source disk) does not properly compute the FFT (try a 
simple real SIN wave and examine the imaginary).  Please let me know
if I am wrong about this but it seems like the imaginary part is
frequency reversed or something similar.  Please respond via email
if you have information or would like to hear of the results of this
posting.  Thanks.

P.S. How can I get in touch with the company? Their Cambridge address
is no longer valid (or at least no phone listing).

Re: Numerical Recipes FFT routine bugs ???

#139

Author: Dr. Steven L. Eddins
Date: Fri, 24 Jul 1992 20:37

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croteau@ccwf.cc.utexas.edu (Ed Croteau) writes:

>Does anyone have any information on the "FOUR1.C" routine in the
>Numerical Recipes in C book? I believe that the routine as printed
>(and on a source disk) does not properly compute the FFT (try a 
>simple real SIN wave and examine the imaginary).  Please let me know
>if I am wrong about this but it seems like the imaginary part is
>frequency reversed or something similar.

Perhaps your difficulty is in their definition of the DFT (Discrete
Fourier Transform).  See equations (12.1.7) and (12.1.9), and note
that the signs in the complex exponential terms are reversed from the
engineering norm.  Or maybe I shouldn't say "norm"---it's reversed
from the definition *I* use.  ;-)

-- 
Steve Eddins	
eddins@holst.eecs.uic.edu 	(312) 996-5771 		FAX: (312) 413-0024
University of Illinois at Chicago, EECS Dept., M/C 154, 1120 SEO Bldg,
Box 4348, Chicago, IL  60680

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