From: Adam Shelly <adam.shelly@...>
Date: 2006-06-03T04:47:33+09:00
Subject: Re: Time domain to frequency domain with FFT from GSL

On 6/2/06, Alex Polite <notmyprivateemail@gmail.com> wrote:
> OK, so say I'm using the ruby wrapper round FFTW3.
>
> na is my time domain data. Then I can get my freq domain data by this:
>
> FFTW3.fft(na, -1).real[0..na.length/2-1]
>
> Right?
>
What do you want to do with the result?  FFTW3.fft(na, -1) gives you
an array of complex numbers. You probably can't just throw away the
imaginary part.

For example, I just used FFTW in a C application to plot the spectrum
of a signal.

I plotted something equivalent to
  n = na.length
  fd = FFTW3.fft(na, -1)
  plotdata = fd.real.zip(fd.imag).map {|re, im|
    r = re/n; i = im/n;
    Math::sqrt(r*r + i*i)
 }

I'm lucky enough to have a math genius in the office to consult for
these things.  The book Dido recommended may be your next best bet...