[Octave-bug-tracker] [bug #49436] fftshift: incorrect frequency scale in help text

[Martin Kunz]

URL:
  <http://savannah.gnu.org/bugs/?49436>

                 Summary: fftshift: incorrect frequency scale in help text
                 Project: GNU Octave
            Submitted by: mkunz
            Submitted on: Tue 25 Oct 2016 08:54:50 AM GMT
                Category: Documentation
                Severity: 3 - Normal
                Priority: 5 - Normal
              Item Group: Incorrect Documentation
                  Status: None
             Assigned to: None
         Originator Name: mkunz
        Originator Email: 
             Open/Closed: Open
         Discussion Lock: Any
                 Release: 4.2.0-rc2
        Operating System: Microsoft Windows

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Details:

The help text for fftshift states:


If X is a vector of N elements corresponding to N time samples
spaced by dt, then 'fftshift (fft (X))' corresponds to frequencies

   f = [ -(ceil((N-1)/2):-1:1)*df 0 (1:floor((N-1)/2))*df ]

where df = 1 / dt.


I think this is wrong. Let N = 10 and dt = 1, then f would range from -5 to
+5, whereas the Nyquist frequency would be 0.5.
To my mind the correct formula is


   f = [ -(ceil((N-1)/2):-1:1)*df 0 (1:floor((N-1)/2))*df ]/N





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