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function fdowns = comp_downs(f,a,varargin)
%-*- texinfo -*-
%@deftypefn {Function} comp_downs
%@verbatim
%COMP_DOWNS Downsampling
% Usage: fdowns = comp_downs(f,a)
% fdowns = comp_downs(f,a,skip,L,'dim',dim)
%
% Input parameters:
% f : Input vector/matrix.
% a : Downsampling factor.
% skip : Skipped initial samples.
% L : Length of the portion of the input to be used.
% dim : Direction of downsampling.
% Output parameters:
% fdowns : Downsampled vector/matrix.
%
% Downsamples input f by a factor a (leaves every a*th sample) along
% dimension dim. If dim is not specified, first non-singleton
% dimension is used. Parameter skip (integer) specifies how
% many samples to skip from the beginning and L defines how many
% elements of the input data are to be used starting at index 1+skip.
%
% Examples:
% ---------
%
% The default behavior is equal to the subsampling performed
% in the frequency domain using reshape and sum:
%
% f = 1:9;
% a = 3;
% fupsTD = comp_downs(f,a)
% fupsFD = real(ifft(sum(reshape(fft(f),length(f)/a,a),2).'))/a
%
%@end verbatim
%@strong{Url}: @url{http://ltfat.github.io/doc/comp/comp_downs.html}
%@end deftypefn
% Copyright (C) 2005-2016 Peter L. Soendergaard <peter@sonderport.dk>.
% This file is part of LTFAT version 2.3.1
%
% 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 3 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, see <http://www.gnu.org/licenses/>.
if(nargin<2)
error('%s: Too few input parameters.',upper(mfilename));
end
definput.keyvals.dim = [];
definput.keyvals.skip = 0;
definput.keyvals.L = [];
[flags,kv,skip,L]=ltfatarghelper({'skip','L','dim'},definput,varargin);
% a have to be a positive integer
if(a<1)
a = 1;
end
if(a<0 || rem(a,1)~=0)
error('%s: Parameter *a* have to be a positive integer.',upper(mfilename));
end
% supported type are [0--a-1]
% if(type<0||type>(a-1))
% error('%s: Unsupported downsampling type.',upper(mfilename));
% end
if(ndims(f)>2)
error('%s: Multidimensional signals (d>2) are not supported.',upper(mfilename));
end
% ----- Verify f and determine its length -------
[f,Lreq,Ls,~,dim,permutedsize,order]=assert_sigreshape_pre(f,L,kv.dim,upper(mfilename));
if(skip>=Ls)
error('%s: Parameter *skip* have to be less than the input length.',upper(mfilename));
end
if(~isempty(L))
if(Lreq+skip>Ls)
error('%s: Input length is less than required samples count: L+skip>Ls.',upper(mfilename));
end
Ls = Lreq+skip;
end
% Actual computation
fdowns = f(1+skip:a:Ls,:);
permutedSizeAlt = size(fdowns);
fdowns=assert_sigreshape_post(fdowns,dim,permutedSizeAlt,order);
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