function psd = CalcPSD(x, calc_from_diff)
% CalcPSD.m
% Author: Alexander Khaustov; alexander dot khaustov at gmail dot com
% Copyright (C) 2008 St.-Petersburg Institute of Cardiological Technics (Incart), www.incart.ru
% This software is released under the terms of the GNU General
% Public License (http://www.gnu.org/copyleft/gpl.html).
%
% Calculates the Power Spectral Density of TWA series for each point
% withing ST segment
psd = zeros(floor(size(x,1)/2)+1, size(x, 2));
global Param
for timept = 1:size(x, 2)
a = x(:, timept);
if ~calc_from_diff
a = detrend(a);
end;
len = length(a);
b = periodogram(a,hamming(len),len);
psd(:, timept) = b * sum(a .* a) / sum(b) / length(a); % normalization independent of periodogram estimation method
% psd(:, timept) = PSD(a); % an attempt of logical normalization
end;
return;