用matlab实现巴特沃斯低通滤波器的M文件。本例读取电脑自带的XP开机声音，加上高频噪声，然后滤波看效果。-Butterworth low-pass filter using matlab M-file. This case, read the computer comes with XP start-up sound, coupled with high-frequency noise, and then filter to see the effect.