说明：  自己编写的巴特沃斯直接数字滤波器设计IIR低通滤波器实例函数，文件已带使用说明//编 写 人：Chernic//学 校：广州大学//联系邮箱：iamchernic@gmail.com
(I have written directly Butterworth IIR digital filter design examples of low-pass filter function, file with instructions for use// Write the person: Chernic// School: Guangzhou University// Contact E-mail: iamchernic@gmail.com)