h（n）的长度N没有给定，仅给定通带边缘截止频率wc和阻带临界频率ws，以及相应的衰减，用窗函数法设计线性相位FIR低通滤波器-h (n) of length N is not given, only given pass band edge cutoff frequency wc and stop band the critical frequency ws, and the corresponding attenuation window function design linear phase FIR low-pass filter