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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

```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...

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

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