[Octave-bug-tracker] [bug #55098] wrong result from filtic when a(1) != 1

[Tony Richardson]

URL:
  <https://savannah.gnu.org/bugs/?55098>

                 Summary: wrong result from filtic when a(1) != 1
                 Project: GNU Octave
            Submitted by: amrichardson
            Submitted on: Sun 25 Nov 2018 06:55:42 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: 4.4.1
        Operating System: Any

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Details:

The filtic function from the signal package gives incorrect results when the
first element of the 'a' vector is not equal to 1.  Here is a simple example
to illustrate the error:


>> filtic(1, [2 1], 1)
ans = -1


The correct result (and also the result returned by MATLAB) is -0.5.

Octave returns the correct result when the b and a vectors (the first two
arguments to filtic) are normalized so that the first element of the a vector
is 1:


>> filtic(1/2, [2 1], 1)
ans = -0.5000


Theoretically, the result should be the same regardless of the normalization. 
That is filtic(1/anorm, [2 1]/anorm, 1) should return -0.5000 regardless of
the value of anorm (MATLAB does this).

A simple fix is to divide the return vector (zf) by a(1) at the bottom (just
before the endfunction line) of the filtic function:


zf = zf/a(1);







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