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+# Copyright (C) 1996 A. Scottedward Hodel 
+#
+# 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, 675 Mass Ave, Cambridge, MA 02139, USA. 
+ 
+function [num,den,tsam,inname,outname] = sys2tf(Asys)
+# function [num,den,tsam,inname,outname] = sys2tf(Asys)
+# Conversion from a system data structure format to a transfer function.  The 
+# transfer function part of ASYS is returned to the user in the form:
+#
+#                num(s)
+#          G(s)=-------
+#                den(s)
+#
+# The user can also have the sampling time (TSAM), the name of the input 
+# (INNAME), and the output name (OUTNAME)
+
+# 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
+# $Revision: 1.2 $
+
+  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 = Asys.tsam;
+  inname = Asys.inname;
+  outname = Asys.outname;
+
+endfunction
+