Re: Numerical Recipes FFT routine bugs ???

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Article #149

#149

From: cpyy
Date: Tue, 28 Jul 1992 13:32

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In article <$=c1Hsr*p8@atlantis.psu.edu>,
mek@guinan.psu.edu (Mark E. Kotanchek) writes: 
> In article <1992Jul24.203717.2527@bert.eecs.uic.edu>  
> eddins@bert.eecs.uic.edu (Dr. Steven L. Eddins) writes:
>> 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.

I also tried to implement the Numerical Recepie's algorithm for
speech processing algorithms.  Strangely, if you do a forward
transform followed by a reverse transform, you don't get back
your original input.  I had to go through the Cooley-Tukey
derivation to produce the correct algorithm.  It seems that
the one in the book is bass-ackwards in such a way that it will
produce a meaningful magnitude result, with a quasi-meaningless
reversed phase result.

-- 

Jon C. Russo
internet : cpyy@vax5.cit.cornell.edu
phone    : 607.277.3295

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Path: arc1-ca.pugleaf.net!sparky!uunet!newsstand.cit.cornell.edu!vax5.cit.cornell.edu!cpyy
From: cpyy@vax5.cit.cornell.edu
Newsgroups: comp.dsp
Subject: Re: Numerical Recipes FFT routine bugs ???
Message-ID: <1992Jul28.093232.14073@vax5.cit.cornell.edu>
Date: 28 Jul 92 09:32:32 EDT
References: <1992Jul24.203717.2527@bert.eecs.uic.edu> <$=c1Hsr*p8@atlantis.psu.edu>
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Organization: Cornell University
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