### Copyright (C) 1996 John W. Eaton
###
### This file is part of Octave.
###
### Octave is free software; you can redistribute it and/or modify it
### under the terms of the GNU General Public License as published by
### the Free Software Foundation; either version 2, or (at your option)
### any later version.
###
### Octave is distributed in the hope that it will be useful, but
### WITHOUT ANY WARRANTY; without even the implied warranty of
### MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
### General Public License for more details.
###
### You should have received a copy of the GNU General Public License
### along with Octave; see the file COPYING.  If not, write to the Free
### Software Foundation, 59 Temple Place - Suite 330, Boston, MA
### 02111-1307, USA.

## Usage: zr = tzero (a, b, c, d, bal)
##
## Compute the transmission zeros of a, b, c, d.
##
## bal = balancing option (see balance); default is "B".
##
## Needs to incorporate mvzero algorithm to isolate finite zeros.

## Author: A. S. Hodel <scotte@eng.auburn.edu>
## Created: August 1993
## Adapted-By: jwe

function zr = tzero (a, b, c, d, bal)

  if (nargin == 4)
    bal = "B";
  elseif (nargin != 5)
    error ("tzero: invalid number of arguments");
  endif

  [n, m, p] = abcddim (a, b, c, d);

  if (n > 0 && m > 0 && p > 0)
    if (m != p)
      warning ("tzero: number of inputs,outputs differ.  squaring up");
      if (p > m)
	warning ("       by padding b and d with zeros.");
	b = [b, (zeros (n, p-m))];
	d = [d, (zeros (p, p-m))];
	m = p;
      else
	warning ("       by padding c and d with zeros.");
	c = [c; (zeros (m-p, n))];
	d = [d; (zeros (m-p, m))];
	p = m;
      endif
      warning ("This is a kludge.  Try again with SISO system.");
    endif
    ab = [-a, -b; c, d];
    bb = [(eye (n)), (zeros (n, m)); (zeros (p, n)), (zeros (p, m))];
    [ab, bb] = balance (ab, bb);
    zr = -qzval (ab, bb);
  else
    error ("tzero: a, b, c, d not compatible");
  endif

endfunction