h(t) convolution analog filter (j≌2) 4Y(g) multiplication 高频处 From analog filter to digital filter digitalization h(n) convolution digital filter multiplication 八八→人从从 高频处x(t) t y(t) t analog filter convolution t h(t) X(j)  H(j)  multiplication Y(j)  digital filter convolution multiplication y(n) n n h(n) digitalization X(ej )  2  H(ej )  2  Y(ej )  2  From analog filter to digital filter 高频处 高频处 x(n) n