说明：  MATLAB 信号处理 带通、高通、低通滤波 FFT变换 显示并返回原始波形的FFT结果、滤波后的波形及滤波后的FFT结果
(MATLAB signal processing band-pass, high pass, low pass filter FFT of the waveform display and return to the original results of the FFT, the filtered waveform and the filtered results of the FFT)