function [Px,t] = sigpower(x,N)
%  sigpower --> Short-term power computation using a sliding window.
%
%  <Synopsis>
%    [Px,t] = sigpower(x,N)
%
%  <Description>
%    An initial short-term power estimate Px(1) based on the N-sample
%    vector x(1:N) is updated with the new sample x(N+1) and downdated
%    with the oldest sample x(1), resulting in Px(2). This update
%    procedure of the short-term power estimate continues until the
%    sliding window has reached the end of the signal x. The output
%    vector t is a time vector corresponding to center points of the
%    sliding window.

%  <Revision>
%    Peter S.K. Hansen, IMM, Technical University of Denmark
%
%    Last revised: February 22, 2000
%-----------------------------------------------------------------------

% Check the required input arguments.
if (nargin < 1)
  error('Not enough input arguments.')
elseif (nargin == 1)
  N = length(x);
end

if (N > length(x))
  error('The window length N can not be longer than the sequence length.')
end

% Make sure that x is a column vector.
x = x(:);

% Number of power estimates to make.
K  = length(x)-N+1;
Px = zeros(K,1);

% First power estimate.
Px(1) = x(1:N)'*x(1:N)/N;

% Updating of the power estimate.
for (k=2:K)
  Px(k) = Px(k-1) + (x(k+N-1)^2 - x(k-1)^2)/N;
end

% Time vector corresponding to center of window.
t = fix(N/2)+(1:K)';

%-----------------------------------------------------------------------
% End of function sigpower
%-----------------------------------------------------------------------