# Copyright (C) 1993, 1994, 1995 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.

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

# 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.

# Written by A. S. Hodel (scotte@eng.auburn.edu) August 1993.

  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