bugGNU Octave - Bugs: bug #64899, [octave forge] (signal) decimate...


bug #64899: [octave forge] (signal) decimate with FIR filter incompatible with Matlab

Submitter:  Geert van Boxtel <geertvanboxtel>
Submitted:  Wed 15 Nov 2023 09:03:35 PM UTC
Category:  Octave Package Severity:  3 - Normal
Priority:  5 - Normal Item Group:  Matlab Compatibility
Status:  None Assigned to:  None
Originator Name:  Open/Closed:  * Open
Release:  * stable Operating System:  * Any
Fixed Release:  None Planned Release:  None
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Wed 15 Nov 2023 09:03:35 PM UTC, original submission:  

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 van Boxtel <geertvanboxtel>


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    Follow 2 latest changes.

    Date Changed by Updated Field Previous Value => Replaced by
    2023-11-16 mmuetzel Carbon-CopyRemoved 102357 -
    2023-11-16 mmuetzel Summarydecimate with FIR filter incompatible with Matlab [octave forge] (signal) decimate with FIR filter incompatible with Matlab

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