R/decimate.R

In signal: Signal Processing

## 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)]
}