Add a useful package (from Source forge) for octave

[CreaPhase.git] / octave_packages / signal-1.1.3 / impz.m

diff --git a/octave_packages/signal-1.1.3/impz.m b/octave_packages/signal-1.1.3/impz.m
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+## Copyright (C) 1999 Paul Kienzle <pkienzle@users.sf.net>
+##
+## 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 3 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, see <http://www.gnu.org/licenses/>.
+
+## usage: [x, t] = impz(b [, a, n, fs])
+##
+## Generate impulse-response characteristics of the filter. The filter
+## coefficients correspond to the the z-plane rational function with
+## numerator b and denominator a.  If a is not specified, it defaults to
+## 1. If n is not specified, or specified as [], it will be chosen such
+## that the signal has a chance to die down to -120dB, or to not explode
+## beyond 120dB, or to show five periods if there is no significant
+## damping. If no return arguments are requested, plot the results.
+##
+## See also: freqz, zplane
+
+## TODO: Call equivalent function from control toolbox since it is
+## TODO:    probably more sophisticated than this one, and since it
+## TODO:    is silly to maintain two different versions of essentially
+## TODO:    the same thing.
+function [x_r, t_r] = impz(b, a = [1], n = [], fs = 1)
+
+  if nargin == 0 || nargin > 4
+    print_usage;
+  end
+
+  if isempty(n) && length(a) > 1
+    precision = 1e-6;
+    r = roots(a);
+    maxpole = max(abs(r));
+    if (maxpole > 1+precision)     # unstable -- cutoff at 120 dB
+      n = floor(6/log10(maxpole));
+    elseif (maxpole < 1-precision) # stable -- cutoff at -120 dB
+      n = floor(-6/log10(maxpole));
+    else                        # periodic -- cutoff after 5 cycles
+      n = 30;
+      
+                                # find longest period less than infinity
+                                # cutoff after 5 cycles (w=10*pi)
+      rperiodic = r(find(abs(r)>=1-precision & abs(arg(r))>0));
+      if !isempty(rperiodic)
+        n_periodic = ceil(10*pi./min(abs(arg(rperiodic))));
+        if (n_periodic > n)
+          n = n_periodic;
+        end
+      end
+      
+                                # find most damped pole
+                                # cutoff at -60 dB
+      rdamped = r(find(abs(r)<1-precision));
+      if !isempty(rdamped)
+        n_damped = floor(-3/log10(max(abs(rdamped))));
+        if (n_damped > n)
+          n = n_damped;
+        end
+      end
+    end
+    n = n + length(b);
+  elseif isempty(n)
+    n = length(b);
+  end
+
+  if length(a) == 1
+    x = fftfilt(b/a, [1, zeros(1,n-1)]);
+  else
+    x = filter(b, a, [1, zeros(1,n-1)]);
+  end
+  t = [0:n-1]/fs;
+
+  if nargout >= 1 x_r = x; end;
+  if nargout >= 2 t_r = t; end;
+  if nargout == 0
+    unwind_protect
+      title "Impulse Response";
+      if (fs > 1000)
+        t = t * 1000;
+        xlabel("Time (msec)");
+      else
+        xlabel("Time (sec)");
+      end
+      plot(t, x, "^r;;");
+    unwind_protect_cleanup
+      title ("")
+      xlabel ("")
+    end_unwind_protect
+  end
+
+endfunction