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Re: [Help-gsl] mixed-radix fft

From: Brian Gough
Subject: Re: [Help-gsl] mixed-radix fft
Date: Wed, 14 Jan 2009 16:28:49 +0000
User-agent: Wanderlust/2.14.0 (Africa) Emacs/22.2 Mule/5.0 (SAKAKI)

At Thu, 8 Jan 2009 19:06:11 +0000 (GMT),
Sean Parsons wrote:
> However I noticed that the "halfcomplex" output of gsl_fft_real_transform is 
> different according to whether the no. of samples (nsample) is a power of two 
> or not. With nsample a power of 2, I get a sensible result, but with nsample 
> not a power of two I get rubbish.
> I have illustrated this with the program below (also attached). I input a sum 
> of three sine waves. With nsample = 4096 I get three nice peaks at the 
> correct frequencies, but with nsample = 5000, I get just a sequence of small 
> values.
> Could you help me? I realise that upon the reverse transform, I get my 
> original data, whether nsample = 4096 or 5000. But I need to know how the 
> forward transformed data is represented so I can apply filters.


Your program is missing the imaginary components (real components are
at index 2*i-1 and imaginary at 2*i).  If you look at these you will
find the three peaks.  The manual has a description of the layout in
the section "Mixed-radix FFT routines for real data"

Brian Gough
(GSL Maintainer)

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