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。-Window function method to design a linear phase FIR digital low-pass filter, as an ideal low-pass filter for approximation filter passband cutoff frequency of 0.2 stopband cutoff frequency of 0.4, the stop-band attenuation of less than-40dB.