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comparison main/signal/decimate.m @ 0:6b33357c7561 octave-forge

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Initial revision
author pkienzle
date Wed, 10 Oct 2001 19:54:49 +0000
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children a65fbfa12206
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1 ## Copyright (C) 2000 Paul Kienzle
2 ##
3 ## This program is free software; you can redistribute it and/or modify
4 ## it under the terms of the GNU General Public License as published by
5 ## the Free Software Foundation; either version 2 of the License, or
6 ## (at your option) any later version.
7 ##
8 ## This program is distributed in the hope that it will be useful,
9 ## but WITHOUT ANY WARRANTY; without even the implied warranty of
10 ## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
11 ## GNU General Public License for more details.
12 ##
13 ## You should have received a copy of the GNU General Public License
14 ## along with this program; if not, write to the Free Software
15 ## Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
16
17 ## usage: y = decimate(x, q [, n] [, ftype])
18 ##
19 ## Downsample the signal x by a factor of q, using an order n filter
20 ## of ftype 'fir' or 'iir'. By default, an order 8 Chebyshev type I
21 ## filter is used or a 30 point FIR filter if ftype is 'fir'. Note
22 ## that q must be an integer for this rate change method.
23 ##
24 ## Example
25 ## ## Generate a signal that starts away from zero, is slowly varying
26 ## ## at the start and quickly varying at the end, decimate and plot.
27 ## ## Since it starts away from zero, you will see the boundary
28 ## ## effects of the antialiasing filter clearly. Next you will see
29 ## ## how it follows the curve nicely in the slowly varying early
30 ## ## part of the signal, but averages the curve in the quickly
31 ## ## varying late part of the signal.
32 ## t=0:0.01:2; x=chirp(t,2,.5,10,'quadratic')+sin(2*pi*t*0.4);
33 ## y = decimate(x,4); # factor of 4 decimation
34 ## stem(t(1:121)*1000,x(1:121),"-g;Original;"); hold on; # plot original
35 ## stem(t(1:4:121)*1000,y(1:31),"-r;Decimated;"); hold off; # decimated
36
37 function y = decimate(x, q, n, ftype)
38
39 if nargin < 1 || nargin > 4,
40 usage("y=decimate(x, q [, n] [, ftype])");
41 endif
42 if q != fix(q), error("decimate only works with integer q."); endif
43
44 if nargin<3
45 ftype='iir';
46 n=[];
47 elseif nargin==3
48 if isstr(n)
49 ftype=n;
50 n=[];
51 else
52 ftype='iir';
53 endif
54 endif
55
56 fir = strcmp(ftype, 'fir');
57 if isempty(n)
58 if fir, n=30; else n=8; endif
59 endif
60
61 if fir
62 b = fir1(n, 1/q);
63 y=fftfilt(b, x);
64 else
65 [b, a] = cheby1(n, 0.05, 1/q);
66 y=filtfilt(b,a,x);
67 endif
68 y = y(1:q:length(x));
69 endfunction
70
71 %!demo
72 %! t=0:0.01:2; x=chirp(t,2,.5,10,'quadratic')+sin(2*pi*t*0.4);
73 %! y = decimate(x,4); # factor of 4 decimation
74 %! stem(t(1:121)*1000,x(1:121),"-g;Original;"); hold on; # plot original
75 %! stem(t(1:4:121)*1000,y(1:31),"-r;Decimated;"); hold off; # decimated
76 %! %------------------------------------------------------------------
77 %! % The signal to decimate starts away from zero, is slowly varying
78 %! % at the start and quickly varying at the end, decimate and plot.
79 %! % Since it starts away from zero, you will see the boundary
80 %! % effects of the antialiasing filter clearly. You will also see
81 %! % how it follows the curve nicely in the slowly varying early
82 %! % part of the signal, but averages the curve in the quickly
83 %! % varying late part of the signal.