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+## Copyright (C) 1996, 1998 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{mag}, @var{phase}, @var{w}] =} bode(@var{sys}@{,@var{w}, @var{out_idx}, @var{in_idx}@})
+## If no output arguments are given: produce Bode plots of a system; otherwise,
+## compute the frequency response of a system data structure
+##
+## @strong{Inputs}
+## @table @var
+## @item   sys
+## a system data structure (must be either purely continuous or discrete;
+## see is_digital)
+## @item   w
+## frequency values for evaluation.
+##
+## if @var{sys} is continuous, then bode evaluates @math{G(jw)} where
+## @math{G(s)} is the system transfer function.
+##
+## if @var{sys} is discrete, then bode evaluates G(@code{exp}(jwT)), where
+## @itemize @bullet
+## @item @var{T}=@code{sysgettsam(@var{sys})} (the system sampling time) and
+## @item @math{G(z)} is the system transfer function.
+## @end itemize
+##
+## @strong{ Default} the default frequency range is selected as follows: (These
+## steps are NOT performed if @var{w} is specified)
+## @enumerate
+## @item via routine bodquist, isolate all poles and zeros away from
+## @var{w}=0 (@var{jw}=0 or @math{@code{exp}(jwT)}=1) and select the frequency
+## range based on the breakpoint locations of the frequencies.
+## @item if @var{sys} is discrete time, the frequency range is limited
+##               to @math{jwT} in
+## @ifinfo
+## [0,2 pi /T]
+## @end ifinfo
+## @iftex
+## @tex
+## $[0,2\pi/T]$
+## @end tex
+## @end iftex
+## @item A "smoothing" routine is used to ensure that the plot phase does
+## not change excessively from point to point and that singular
+## points (e.g., crossovers from +/- 180) are accurately shown.
+##
+## @end enumerate
+## @item out_idx
+## @itemx in_idx
+## the indices of the output(s) and input(s) to be used in
+## the frequency response; see @code{sysprune}.
+## @end table
+## @strong{Outputs}
+## @table @var
+## @item mag
+## @itemx phase
+## the magnitude and phase of the frequency response @math{G(jw)} or
+## @math{G(@code{exp}(jwT))} at the selected frequency values.
+## @item w
+## the vector of frequency values used
+## @end table
+##
+## @strong{Notes}
+## @enumerate
+## @item If no output arguments are given, e.g.,
+## @example
+## bode(sys);
+## @end example
+## bode plots the results to the screen.  Descriptive labels are
+## automatically placed.
+##
+## Failure to include a concluding semicolon will yield some garbage
+## being printed to the screen (@code{ans = []}).
+##
+## @item If the requested plot is for an MIMO system, mag is set to
+## @math{||G(jw)||} or @math{||G(@code{exp}(jwT))||}
+## and phase information is not computed.
+## @end enumerate
+## @end deftypefn
+
+## Author: John Ingram <ingraje@eng.auburn.edu>
+## Created: July 10, 1996
+## Based on previous code by R. Bruce Tenison, July 13, 1994
+## Modified by David Clem November 13, 1994
+## again by A. S. Hodel July 1995 (smart plot range, etc.)
+## Modified by Kai P. Mueller September 28, 1997 (multiplot mode)
+
+function [mag_r, phase_r, w_r] = bode (sys, w, outputs, inputs, plot_style)
+
+  ## check number of input arguments given
+  if (nargin < 1 | nargin > 5)
+    usage("[mag,phase,w] = bode(sys[,w,outputs,inputs,plot_style])");
+  endif
+  if(nargin < 2)
+    w = [];
+  endif
+  if(nargin < 3)
+    outputs = [];
+  endif
+  if(nargin < 4)
+    inputs = [];
+  endif
+  if(nargin < 5)
+    plot_style = "dB";
+  endif
+
+  if (strcmp (plot_style, "dB"))
+    do_db_plot = 1;
+  elseif (strcmp (plot_style, "mag"))
+    do_db_plot = 0;
+  else
+    error ("bode: invalid value of plot_style specified");
+  endif
+
+  [f, w] = bodquist(sys,w,outputs,inputs,"bode");
+
+  [stname,inname,outname] = sysgetsignals(sys);
+  systsam = sysgettsam(sys);
+
+  ## Get the magnitude and phase of f.
+  mag = abs(f);
+  phase = arg(f)*180.0/pi;
+
+  if (nargout < 1),
+    ## Plot the information
+    if(gnuplot_has_multiplot)
+      oneplot();
+    endif
+    gset autoscale;
+    if(gnuplot_has_multiplot)
+      gset nokey;
+    endif
+    clearplot();
+    gset data style lines;
+    if(is_digital(sys))
+      xlstr = ["Digital frequency w=rad/sec.  pi/T=",num2str(pi/systsam)];
+      tistr = "(exp(jwT)) ";
+    else
+      xlstr = "Frequency in rad/sec";
+      tistr = "(jw)";
+    endif
+    xlabel(xlstr);
+    if(is_siso(sys))
+      if (gnuplot_has_multiplot)
+        subplot(2,1,1);
+      endif
+      title(["|[Y/U]",tistr,"|, u=", nth(inname,1),", y=",nth(outname,1)]);
+    else
+      title([ "||Y(", tistr, ")/U(", tistr, ")||"]);
+      disp("MIMO plot from")
+      disp(outlist(inname,"     "));
+      disp("to")
+      disp(outlist(outname,"    "));
+    endif
+    wv = [min(w), max(w)];
+    if(do_db_plot && max(mag) > 0)
+      ylabel("Gain in dB");
+      md = 20*log10(mag);
+      axvec = axis2dlim([vec(w),vec(md)]);
+      axvec(1:2) = wv;
+      axis(axvec);
+    else
+      ylabel("Gain |Y/U|")
+      md = mag;
+    endif
+
+    grid("on");
+    if (do_db_plot)
+      semilogx(w,md);
+    else
+      loglog(w,md);
+    endif
+    if (is_siso(sys))
+      if (gnuplot_has_multiplot)
+        subplot(2,1,2);
+      else
+        prompt("Press any key for phase plot");
+      endif
+      axvec = axis2dlim([vec(w),vec(phase)]);
+      axvec(1:2) = wv;
+      axis(axvec);
+      xlabel(xlstr);
+      ylabel("Phase in deg");
+      title([ "phase([Y/U]", tistr, ...
+         "), u=", nth(inname,1),", y=",nth(outname,1)]);
+      grid("on");
+      semilogx(w,phase);
+      ## This should be the default for subsequent plot commands.
+      if(gnuplot_has_multiplot)
+        oneplot();
+      endif
+    endif
+  else
+    mag_r = mag;
+    phase_r = phase;
+    w_r = w;
+  endif
+endfunction