function [Rn,Fn]=auto_correlation(xn)
%該函數是求各的短時自相關函數Rn和短時平均幅度差函數AMDF Fn
N=size(xn,1);
n=size(xn,2);
K=N-1;
for i=1:n
    for k=1:K
        Rn(k,i)=sum(xn(1:N-k,i).*xn(1+k:N,i));
        Fn(k,i)=sum(abs(xn(1:N-k,i)-xn(1+k:N,i)));
    end
end