/usr/share/octave/packages/signal-1.3.0/decimate.m is in octave-signal 1.3.0-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 | ## 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/>.
## -*- texinfo -*-
## @deftypefn {Function File} {@var{y} =} decimate (@var{x}, @var{q})
## @deftypefnx {Function File} {@var{y} =} decimate (@var{x}, @var{q}, @var{n})
## @deftypefnx {Function File} {@var{y} =} decimate (@var{x}, @var{q}, @var{n}, @var{ftype})
##
## Downsample the signal @var{x} by a factor of @var{q}, using an order @var{n}
## filter of @var{ftype} "fir" or "iir". By default, an order 8 Chebyshev
## type I filter is used or a 30 point FIR filter if @var{ftype} is "fir".
## Note that @var{q} must be an integer for this rate change method.
##
## Example:
## @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);
## stem (t(1:121) * 1000, x(1:121), "-g;Original;"); hold on; # original
## stem (t(1:4:121) * 1000, y(1:31), "-r;Decimated;"); hold off; # decimated
## @end example
## @end deftypefn
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.
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