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## 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
decimate <- function(x, q, n = if (ftype == "iir") 8 else 30, ftype = "iir") {
if (q != round(q))
stop("decimate only works with integer q.")
fir <- ftype == 'fir'
if (fir) {
b <- fir1(n, 1/q)
y <- fftfilt(b, x)
} else {
y <- filtfilt(cheby1(n, 0.05, 0.8/q), x)
}
y[seq(1, length(x), by = q)]
}
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