说明：  butterworth低通滤波器的原程序，可用ＭＡＴＬＡＢ的ＦＩＬＴＥＲ函数测试．可得到低通滤波器数字系统函数的系数．
(Butterworth low-pass filter the original procedure, MATLAB can be used to test the FILTER function. Available to low-pass filter function of the coefficient of digital systems.)