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

From: Yuzhou Cheng
Subject: [Octave-bug-tracker] [bug #49318] Signal Pack zplane generate wrong zero-pole plot
Date: Tue, 11 Oct 2016 21:27:53 +0000 (UTC)
User-agent: Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/53.0.2785.143 Safari/537.36


                 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



Matlab version of zplane:

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

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

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

According to this thread:

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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