function pp=lpcra2pp(ra)
%LPCAR2PP LPC: Convert ar filter autocorrelation to power spectrum polynomial in cos(w) PP=(RA)
% pp is a polynomial such that |polyval(ra,e^jw)| = polyval(pp,cos(w))

%      Copyright (C) Mike Brookes 1998
%      Version: $Id: lpcra2pp.m 713 2011-10-16 14:45:43Z dmb $
%
%   VOICEBOX is a MATLAB toolbox for speech processing.
%   Home page: http://www.ee.ic.ac.uk/hp/staff/dmb/voicebox/voicebox.html
%
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persistent tp;
[nf,p1]=size(ra);
% we check here if p is the same as before and reuse the tp matrix
if size(tp,1)~=p1
   p=p1-1;
   % chebyshev polynomials up to order p
   tp=zeros(p1,p1);
   tp(1,p1)=2;
   tp(2,p)=2;
   for i=3:p1
      tp(i,p+2-i:p)=2*tp(i-1,p+3-i:p1)-tp(i-2,p+2-i:p);
      tp(i,p1)=-tp(i-2,p1);
   end
   tp(1,p1)=1;
end
pp=ra*tp;
