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