octave-signal 1.3.2-5 / usr / share / octave / packages / signal-1.3.2 / downsample.m

## Author: Paul Kienzle <pkienzle@users.sf.net> (2007)
## This program is granted to the public domain.

## -*- texinfo -*-
## @deftypefn  {Function File} {@var{y} =} downsample (@var{x}, @var{n})
## @deftypefnx {Function File} {@var{y} =} downsample (@var{x}, @var{n}, @var{offset})
## Downsample the signal, selecting every @var{n}th element.  If @var{x}
## is a matrix, downsample every column.
##
## For most signals you will want to use @code{decimate} instead since
## it prefilters the high frequency components of the signal and
## avoids aliasing effects.
##
## If @var{offset} is defined, select every @var{n}th element starting at
## sample @var{offset}.
## @seealso{decimate, interp, resample, upfirdn, upsample}
## @end deftypefn

function y = downsample (x, n, phase = 0)

  if nargin<2 || nargin>3, print_usage; endif

  if phase > n - 1
    warning("This is incompatible with Matlab (phase = 0:n-1). See octave-forge signal package release notes for details.")
  endif

  if isvector(x)
    y = x(phase + 1:n:end);
  else
    y = x(phase + 1:n:end,:);
  endif

endfunction

%!assert(downsample([1,2,3,4,5],2),[1,3,5]);
%!assert(downsample([1;2;3;4;5],2),[1;3;5]);
%!assert(downsample([1,2;3,4;5,6;7,8;9,10],2),[1,2;5,6;9,10]);
%!assert(downsample([1,2,3,4,5],2,1),[2,4]);
%!assert(downsample([1,2;3,4;5,6;7,8;9,10],2,1),[3,4;7,8]);