--- /dev/null
+## 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