中文说明：
用窗函数法设计一个线性相位FIR数字低通滤波器，用理想低通滤波器作为逼近滤波器，通带截止频率为0.2 ，阻带截止频率为0.4 ，阻带衰减不小于-40dB。
English Description：
A linear phase FIR digital low-pass filter is designed by window function method. The ideal low-pass filter is used as approximation filter. The cut-off frequency of passband is 0.2, the cut-off frequency of stopband is 0.4, and the stopband attenuation is not less than - 40dB.