--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/main/signal/decimate.m	Wed Oct 10 19:54:49 2001 +0000
@@ -0,0 +1,83 @@
+## Copyright (C) 2000 Paul Kienzle
+##
+## 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 2 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, write to the Free Software
+## Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+
+## 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, 
+    usage("y=decimate(x, q [, n] [, ftype])"); 
+  endif
+  if q != fix(q), error("decimate only works with integer q."); endif
+
+  if nargin<3
+    ftype='iir';
+    n=[];
+  elseif nargin==3
+    if isstr(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, 1/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.