function y=circonv(x1,x2,N)


if length(x1)>N
    error('N must not be less than length of x1');
end 
if length(x2)>N
    error('N must not be less than length of x2');
end 

X1k=fft(x1,N);
X2k=fft(x2,N);
Yk=X1k.*X2k;
y=ifft(Yk);
if (all(imag(x1)==0))&(all(imag(x2)==0))
    y=real(y);
end