1 ## Copyright (C) 2009 Thomas Sailer
3 ## This program is free software; you can redistribute it and/or modify it under
4 ## the terms of the GNU General Public License as published by the Free Software
5 ## Foundation; either version 3 of the License, or (at your option) any later
8 ## This program is distributed in the hope that it will be useful, but WITHOUT
9 ## ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
10 ## FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more
13 ## You should have received a copy of the GNU General Public License along with
14 ## this program; if not, see <http://www.gnu.org/licenses/>.
16 ## Return bessel analog filter prototype.
20 ## http://en.wikipedia.org/wiki/Bessel_polynomials
22 function [zero, pole, gain] = besselap (n)
24 if (nargin>1 || nargin<1)
28 ## interpret the input parameters
29 if (!(length(n)==1 && n == round(n) && n > 0))
30 error ("besselap: filter order n must be a positive integer");
38 px=prepad(px,max(length(px),length(py)),0);
39 py=prepad(py,length(px));
43 % p1 now contains the reverse bessel polynomial for n
45 % scale it by replacing s->s/w0 so that the gain becomes 1
46 p1=p1.*p1(length(p1)).^((length(p1)-1:-1:0)/(length(p1)-1));