Add a useful package (from Source forge) for octave

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

diff --git a/octave_packages/signal-1.1.3/besselap.m b/octave_packages/signal-1.1.3/besselap.m
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+## Copyright (C) 2009 Thomas Sailer
+##
+## 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/>.
+
+## Return bessel analog filter prototype.
+##
+## References: 
+##
+## http://en.wikipedia.org/wiki/Bessel_polynomials
+
+function [zero, pole, gain] = besselap (n)
+
+  if (nargin>1 || nargin<1)
+    print_usage;
+  end
+
+  ## interpret the input parameters
+  if (!(length(n)==1 && n == round(n) && n > 0))
+    error ("besselap: filter order n must be a positive integer");
+  end
+
+  p0=1;
+  p1=[1 1];
+  for nn=2:n;
+    px=(2*nn-1)*p1;
+    py=[p0 0 0];
+    px=prepad(px,max(length(px),length(py)),0);
+    py=prepad(py,length(px));
+    p0=p1;
+    p1=px+py;
+  endfor;
+  % p1 now contains the reverse bessel polynomial for n
+
+  % scale it by replacing s->s/w0 so that the gain becomes 1
+  p1=p1.*p1(length(p1)).^((length(p1)-1:-1:0)/(length(p1)-1));
+
+  zero=[];
+  pole=roots(p1);
+  gain=1;
+
+endfunction