function Px = per(x,n1,n2)
%PER	Estimate the spectrum of a process using the periodogram
%-----------
%USAGE	Px = per(x,n1,n2) 
%
%	The spectrum of a process x is estimated using the periodogram.
%	
%	x   :  input sequence
%       n1  :  starting index, x(n1)
%	n2  :  ending index, x(n2)
%
%	The periodogram is returned in Px using a linear scale.
%
%       If n1 and n2 are not specified the periodogram of the entire
%       sequence is computed.
%
%  see also BART, MPER, WELCH, and SPER
%
%---------------------------------------------------------------
% copyright 1996, by M.H. Hayes.  For use with the book 
% "Statistical Digital Signal Processing and Modeling"
% (John Wiley & Sons, 1996).
%---------------------------------------------------------------

x   = x(:);
if nargin == 1
    n1 = 1;  n2 = length(x);  end;
    Px = abs(fft(x(n1:n2),1024)).^2/(n2-n1+1);
    Px(1)=Px(2);
end;