## Copyright (C) 1996 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 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 USA. 
 
## -*- texinfo -*-
## @deftypefn {Function File } {[@var{num},@var{den},@var{tsam},@var{inname},@var{outname}] =} sys2tf (@var{sys})
## Extract transfer function data from a system data structure
## 
## See @ref{tf2sys} for parameter descriptions.
## 
## @strong{Example}
## @example
## octave:1> sys=ss2sys([1 -2; -1.1,-2.1],[0;1],[1 1]);
## octave:2> [num,den] = sys2tf(sys)
## num = 1.0000  -3.0000
## den = 1.0000   1.1000  -4.3000
## @end example
## @end deftypefn

function [num,den,tsam,inname,outname] = sys2tf(Asys)
## Written by R. Bruce Tenison (June 24, 1994) btenison@eng.auburn.edu
## modified to make sys2tf by A. S. Hodel Aug 1995
## modified again for updated system format by John Ingram July 1996

  if(nargin != 1)
    usage("[num,den,tsam,inname,outname] = sys2tf(Asys)");
  endif

  if( !is_struct(Asys))
    error("Asys must be a system data structure (see ss2sys, tf2sys, zp2sys)");
  elseif (! is_siso(Asys) )
    [n, nz, m, p] = sysdimensions(Asys);
    error(["system is not SISO (",num2str(m)," inputs, ...
        ", num2str(p)," outputs"]);
  endif

  Asys = sysupdate(Asys,"tf");		# just in case

  num = Asys.num;
  den = Asys.den;
  
  tsam = sysgettsam(Asys);
  inname = sysgetsignals(Asys,"in");
  outname = sysgetsignals(Asys,"out");

endfunction