## Copyright (C) 1999 Paul Kienzle
##
## This program 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 of the License, or
## (at your option) any later version.
##
## This program 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 this program; if not, write to the Free Software
## Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA

## 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