function y=circonvt(x1,x2,N)
if length(x2)>N
error('N must be>the length of X2')
end
x1=[x1 zeros(1,N-length(x1))];
x2=[x2 zeros(1,N-length(x2))];
m=[0:1:N-1];
x2=x2(mod(-m,N)+1);
H=zeros(N,N);
for n=1:1:N
H(n,:)=cirshftt(x2,n-1,N);
end
y=x1*H';