function R=spec2ac(A)
% SPEC2AC - convert power spectrum to autocorrelation sequence
%
% assumes that S is a half-spectrum.
% R=spec2ac(S)

if isvector(A), A=A(:); end

S=[A; A(end-1:-1:2,:)]; 
R=real(ifft(S));					% autocorrelation
R=R(1:(1+size(R,1)/2),:);		% extract  first half (rest is symmetric)

