bugGNU Octave - Bugs: bug #54293, freqz plot differs from MATLAB


bug #54293: freqz plot differs from MATLAB

Submitter:  Roland Baudin <roland65>
Submitted:  Thu 12 Jul 2018 01:32:32 PM UTC
Category:  Octave Function Severity:  3 - Normal
Priority:  5 - Normal Item Group:  Matlab Compatibility
Status:  None Assigned to:  None
Originator Name:  Open/Closed:  * Open
Release:  * 4.2.2 Operating System:  * GNU/Linux
Fixed Release:  None Planned Release:  None
* Mandatory Fields

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Thu 12 Jul 2018 01:32:32 PM UTC, original submission:  

Plotting the frequency response of a filter gives different plots in MATLAB and in Octave.

To test:

n = [0 : 127];
h = sinc(0.2*(n-128/2)); % Simple test filter
freqz(h, 1, 128, 'whole', 1); % Frequency response on the whole Nyquist

Plots in MATLAB (tested in versions 2014a and 2017b) and in Octave differs because in this case Octave doubles the number of points used in the FFT, while MATLAB does not. Thus MATLAB uses a 128 points FFT (as requested) while Octave uses a 256 points FFT.

The same happens if we plot the frequency response on half the Nyquist band, like this:
freqz(h, 1, 128, 'half', 1);

I found that commenting out the following lines in freqz.m gives the expected result:

lines 154 to 157

    %~ if (k > n/2 && nargout == 0)
      %~ ## Ensure a causal phase response.
      %~ n *= 2 .^ ceil (log2 (2*k/n));
    %~ endif

lines 168 to 170

      %~ if (plot_output)
        %~ n += 1;
      %~ endif

Roland Baudin <roland65>


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