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FFT problems


From: John W. Eaton
Subject: FFT problems
Date: Sat, 25 Aug 2001 17:56:11 -0500

On 25-Aug-2001, Roberto Hernandez <address@hidden> wrote:

| The sequence a is non-radix 2. So what algorithm did the fft() function use? 
| Is it the DFT: X(k) = sum(1, N) x(i) * exp(-j*(i-i)*(k - 1)*2*pi / N) ?
| Does anyone know?

One of the good things about Octave is that all the source code for it
is available, so things like this are not hidden from view and anyone
can find the answers to questions about details like this.  If you
take a look at the source distribution, you will find that, depending
on how it was compiled, Octave's fft function uses either fftw or
functions from fftpack.  I believe that fftw is well-documented (check
www.fftw.org for more information).  The functions from fftpack also
come with some documentation, and Octave's source distributions
include the file fftpack.doc in the directory libcruft/fftpack.

So the answer to your first question may depend on how your version of
Octave was compiled, but once you know that (check the output from
octave_config_info; if the output from that command includes the
string -DHAVE_FFTW=1, then you are using fftw), it can probably be
determined by reading the documentation for the appropriate package.

jwe



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