--- /dev/null
+## Copyright (C) 2000 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: y = decimate(x, q [, n] [, ftype])
+##
+## Downsample the signal x by a factor of q, using an order n filter
+## of ftype 'fir' or 'iir'. By default, an order 8 Chebyshev type I
+## filter is used or a 30 point FIR filter if ftype is 'fir'. Note
+## that q must be an integer for this rate change method.
+##
+## Example
+## ## Generate a signal that starts away from zero, is slowly varying
+## ## at the start and quickly varying at the end, decimate and plot.
+## ## Since it starts away from zero, you will see the boundary
+## ## effects of the antialiasing filter clearly. Next you will see
+## ## how it follows the curve nicely in the slowly varying early
+## ## part of the signal, but averages the curve in the quickly
+## ## varying late part of the signal.
+## t=0:0.01:2; x=chirp(t,2,.5,10,'quadratic')+sin(2*pi*t*0.4);
+## y = decimate(x,4); # factor of 4 decimation
+## stem(t(1:121)*1000,x(1:121),"-g;Original;"); hold on; # plot original
+## stem(t(1:4:121)*1000,y(1:31),"-r;Decimated;"); hold off; # decimated
+
+function y = decimate(x, q, n, ftype)
+
+ if nargin < 1 || nargin > 4
+ print_usage;
+ elseif q != fix(q)
+ error("decimate only works with integer q.");
+ endif
+
+ if nargin<3
+ ftype='iir';
+ n=[];
+ elseif nargin==3
+ if ischar(n)
+ ftype=n;
+ n=[];
+ else
+ ftype='iir';
+ endif
+ endif
+
+ fir = strcmp(ftype, 'fir');
+ if isempty(n)
+ if fir, n=30; else n=8; endif
+ endif
+
+ if fir
+ b = fir1(n, 1/q);
+ y=fftfilt(b, x);
+ else
+ [b, a] = cheby1(n, 0.05, 0.8/q);
+ y=filtfilt(b,a,x);
+ endif
+ y = y(1:q:length(x));
+endfunction
+
+%!demo
+%! t=0:0.01:2; x=chirp(t,2,.5,10,'quadratic')+sin(2*pi*t*0.4);
+%! y = decimate(x,4); # factor of 4 decimation
+%! stem(t(1:121)*1000,x(1:121),"-g;Original;"); hold on; # plot original
+%! stem(t(1:4:121)*1000,y(1:31),"-r;Decimated;"); hold off; # decimated
+%! %------------------------------------------------------------------
+%! % The signal to decimate starts away from zero, is slowly varying
+%! % at the start and quickly varying at the end, decimate and plot.
+%! % Since it starts away from zero, you will see the boundary
+%! % effects of the antialiasing filter clearly. You will also see
+%! % how it follows the curve nicely in the slowly varying early
+%! % part of the signal, but averages the curve in the quickly
+%! % varying late part of the signal.