## Copyright (C) 1996, 2000, 2004, 2005, 2007
##               Auburn University.  All rights reserved.
##
## 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 3 of the License, 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, see
## <http://www.gnu.org/licenses/>.

## -*- texinfo -*-
## @deftypefn {Function File} {[@var{num}, @var{den}] =} ss2tf (@var{a}, @var{b}, @var{c}, @var{d})
## Conversion from transfer function to state-space.
## The state space system:
## @iftex
## @tex
## $$ \dot x = Ax + Bu $$
## $$ y = Cx + Du $$
## @end tex
## @end iftex
## @ifinfo
## @example
##       .
##       x = Ax + Bu
##       y = Cx + Du
## @end example
## @end ifinfo
##
## is converted to a transfer function:
## @iftex
## @tex
## $$ G(s) = { { \rm num }(s) \over { \rm den }(s) } $$
## @end tex
## @end iftex
## @ifinfo
## @example
##
##                 num(s)
##           G(s)=-------
##                 den(s)
## @end example
## @end ifinfo
##
## used internally in system data structure format manipulations.
## @end deftypefn

## Author: R. Bruce Tenison <btenison@eng.auburn.edu>
## Created: June 24, 1994
## a s hodel: modified to allow for pure gain blocks Aug 1996

function [num, den] = ss2tf (a, b, c, d)

  ## Check args
  [n,m,p] = abcddim(a,b,c,d);
  if (n == -1)
    num = [];
    den = [];
    error("ss2tf: Non compatible matrix arguments");
  elseif ( (m != 1) | (p != 1))
    num = [];
    den = [];
    error(["ss2tf: not SISO system: m=",num2str(m)," p=",num2str(p)]);
  endif

  if(n == 0)
    ## gain block only
    num = d;
    den = 1;
  else
    ## First, get the denominator coefficients
    den = poly(a);

    ## Get the zeros of the system
    [zz,g] = tzero(a,b,c,d);

    ## Form the Numerator (and include the gain)
    if (!isempty(zz))
      num = g * poly(zz);
    else
      num = g;
    endif

    ## the coefficients must be real
    den = real(den);
    num = real(num);
  endif
endfunction