## 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, 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, 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 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.

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