From bug-octave-request at bevo dot che dot wisc dot edu Tue Dec  3 14:03:05 2002
Subject: Re: odd order butterworth filters
From: Paul Kienzle <pkienzle at jazz dot ncnr dot nist dot gov>
To: Shaun Jackman <sjackman at shaw dot ca>
Cc: bug-octave at bevo dot che dot wisc dot edu
Date: Tue, 3 Dec 2002 15:02:54 -0500

Actually, it does support odd order filters.  Try:

octave:4> [z,p,k] = butter(1,0.5)
z = -1
p = 5.5511e-17 + 6.1230e-17i
k = 0.50000

The problem is that Octave's zp2tf gets confused by p.  You
could instead define zp2tf as follows, then butter will
work without change:

## usage: [b, a] = zp2tf(z, p, g)
##
## Convert to transfer function f(x)=sum(b*x^n)/sum(a*x^n) from
## zero-pole-gain form f(x)=g*prod(1-z*x)/prod(1-p*x)

function [b, a] = zp2tf(z, p, g)

  if nargin != 3 || nargout != 2
    usage("[b, a] = zp2tf(z, p, g)");
  endif

  b = g*poly(z);
  a = poly(p);

  # check if the system should be real
  if ( max(imag([a(:);b(:)])) ./ max(abs([a(:);b(:)])) < 100*eps )
     b = real(b);
     a = real(a);
  endif

endfunction

I don't know if this definition of zp2tf is correct for the
Octave control systems toolbox.

I will change butter, cheby1, cheby2, ellip to use the following
form so they will work even with Octave's broken zp2tf:

	b = real(g*poly(z)); 
        a = real(poly(p));

Paul Kienzle
pkienzle at users dot sf dot net

On Tue, Dec 03, 2002 at 03:33:43AM -0600, Shaun Jackman wrote:
> The butter command doesn't support odd order filters.
> 	>> [b a] = butter( 1, 0.5);
> 	error: invalid matrix index = 1
> 	error: evaluating assignment expression near line 43, column 9
> 	error: evaluating if command near line 34, column 3
> 	error: called from `__zp2ssg2__' in file
> 	`/usr/share/octave/2.1.36/m/control/system/__zp2ssg2__.m'
> 	error: called from `zp2tf' in file
> 	`/usr/share/octave/2.1.36/m/control/system/zp2tf.m'
> 	error: called from `butter' in file
> 	`/usr/share/octave/2.1.36/m/signal/butter.m'
> 
> Thanks!
> Shaun
> 
> 
> 
> -------------------------------------------------------------
> Octave is freely available under the terms of the GNU GPL.
> 
> Octave's home on the web:  http://www.octave.org
> How to fund new projects:  http://www.octave.org/funding.html
> Subscription information:  http://www.octave.org/archive.html
> -------------------------------------------------------------
> 



-------------------------------------------------------------
Octave is freely available under the terms of the GNU GPL.

Octave's home on the web:  http://www.octave.org
How to fund new projects:  http://www.octave.org/funding.html
Subscription information:  http://www.octave.org/archive.html
-------------------------------------------------------------