中文说明：
根据滤波器指标选择合适的窗函数，计算窗函数的长度N，调用MATLAB函数fir1设计一个FIR低通滤波器。并编写程序，调用MATLAB快速卷积函数fftfilt实现对xt的滤波。绘图显示滤波器的频响特性曲线、滤波器输出信号的幅频特性图和时域波形图。
English Description：
Select the appropriate window function according to the filter index, calculate the length n of the window function, and call matlab function fir1 to design an FIR low-pass filter. And write a program to call matlab fast convolution function fftfilt to realize the filtering of XT. The drawing shows the frequency response characteristic curve of the filter, the amplitude frequency characteristic diagram and time domain waveform diagram of the filter output signal