function y = expander(x,th,ratio)
%  expander --> Used to expand the dynamic range of a signal.
%
%  <Synopsis>
%    y = expander(x,th,ratio)
%
%  <Description>
%    The function expand the dynamic range of the input
%    signal x by using a technique opposite to the compressor.
%    The threshold th is the signal level at which the gain
%    is increased and ratio is the amount of amplification
%    that takes place past the threshold.
%
%  <See Also>
%    compressor --> Used to compress the dynamic range of a signal.

%  <Revision>
%    Peter S.K. Hansen, IMM, Technical University of Denmark
%
%    Last revised: February 7, 2000
%-----------------------------------------------------------------------

% Check the required input arguments.
if (nargin < 3)
  error('Not enough input arguments.')
end

% First, apply normal unity gain.
y = x;

% Find sequence values larger than the threshold level.
idx = find(abs(x)>th);

% Apply the ratio-to-1 expansion for levels above the threshold.
y(idx) = ((abs(x(idx))-th)*ratio + th).*sign(x(idx));

%-----------------------------------------------------------------------
% End of function expander
%-----------------------------------------------------------------------