说明：  用窗函数设计FIR数字滤波器。参数设置为：阶数 N=84，通带截止频率Wc=0.4*Pi，通频带为低通。
(FIR digital filter design with window function. Parameter is set: the order of N = 84, passband, the passband of the low-pass cutoff frequency of Wc = 0.4* Pi,.)