[Octave-bug-tracker] [bug #64899] decimate with FIR filter incompatible with Matlab

[Geert van Boxtel]

URL:
  <https://savannah.gnu.org/bugs/?64899>

                 Summary: decimate with FIR filter incompatible with Matlab
                   Group: GNU Octave
               Submitter: geertvanboxtel
               Submitted: Wed 15 Nov 2023 09:03:35 PM UTC
                Category: Octave Forge Package
                Severity: 3 - Normal
                Priority: 5 - Normal
              Item Group: Matlab Compatibility
                  Status: None
             Assigned to: None
         Originator Name: 
        Originator Email: 
             Open/Closed: Open
                 Release: stable
         Discussion Lock: Any
        Operating System: Any
           Fixed Release: None
         Planned Release: None


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Follow-up Comments:


-------------------------------------------------------
Date: Wed 15 Nov 2023 09:03:35 PM UTC By: Geert van Boxtel <geertvanboxtel>
The decimate() function in the Octave signal package produces a result which
is incompatible with the Matlab implementation if a FIR filter is used.
Example:


decimate(1:100, 2, 'fir')


I think this is due to the fact that the decimate function calls the fftfilt()
function when a FIR filter is specified, which produces a phase shift. When an
IIR filter is specified, decimate() calls filtfilt(), which corrects for the
phase shift. Changing line 88 in the code to


y = filtfilt (b, 1, x);


produces the same output as the Matlab function.

Hope this helps,
Geert








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