[Octave-bug-tracker] [bug #49318] Signal Pack zplane generate wrong zero-pole plot

[Yuzhou Cheng]

URL:
  <http://savannah.gnu.org/bugs/?49318>

                 Summary: Signal Pack zplane generate wrong zero-pole plot
                 Project: GNU Octave
            Submitted by: yc2986
            Submitted on: Tue 11 Oct 2016 09:27:51 PM GMT
                Category: Octave Function
                Severity: 3 - Normal
                Priority: 5 - Normal
              Item Group: Incorrect Result
                  Status: None
             Assigned to: None
         Originator Name: 
        Originator Email: 
             Open/Closed: Open
         Discussion Lock: Any
                 Release: 4.0.3
        Operating System: Microsoft Windows

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Details:

Matlab version of zplane:
https://www.mathworks.com/help/signal/ref/zplane.html

Tested in Octave 4.0.3 using Matlab's example of ellip filter

[z,p,k] = ellip(4,3,30,200/500);
zplane(z,p);


zplane in Octave generate pole out of unit circle...

According to this thread:
http://octave.1599824.n4.nabble.com/Stable-poles-handed-to-quot-filter-quot-can-result-in-unstable-filter-td4634447.html

Probably Octave is calculating Poles using:

poles = abs(roots(b))


Which generate this bug. The correct pole should be just:

poles = abs(b)


By the way, the freqz function plot unstable filter magnitude response for
butter. I think maybe it is due to this issue as well.




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