[Octave-bug-tracker] [bug #51936] zp2sos wrong results for single zero

[fietew]

URL:
  <http://savannah.gnu.org/bugs/?51936>

                 Summary: zp2sos wrong results for single zero
                 Project: GNU Octave
            Submitted by: fietew
            Submitted on: Mon 04 Sep 2017 04:30:20 PM UTC
                Category: Octave Forge Package
                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: 4.0.0
        Operating System: GNU/Linux

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Details:

Try:


zp2sos([1],[-1; 1], 1)
zp2sos([1; -1],[-1; 1], 1)
zp2sos([1; 0],[-1; 1], 1)


The input describes the following Systems:
1st Line: H(z) = (z-1)/((z-1)*(z+1)) = (z^(-1) - z^(-2))/(1-(z^-2))
2nd Line: H(z) = ((z-1)*(z+1))/((z-1)*(z+1)) = (1-z^(-2))/(1-z^(-2))
3rd Line: H(z) = ((z-1)*z)/((z-1)*(z+1)) = (1 - z^(-1))/(1-z^(-2))

Output:

ans =

   1  -1   0   1   0  -1
ans =

   1   0  -1   1   0  -1
ans =

   1  -1   0   1   0  -1


The third output is equal to the first one, which should not be the case,
since the systems are not the same. The documentation
(https://octave.sourceforge.io/signal/function/zp2sos.html) says "The b0 entry
must be nonzero for each section.", which explains the behavior, but does not
preserve the properties of the first system.




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