仿真IIR 数字滤波器的直接I 型、II 型、级联型、并联型结构的特点和表示方法；FIR数字滤波器的直接型、级联型、频率抽样型以及快速卷积的结构特点和表示方法-Features and representation I direct simulation type IIR digital filter, type II, cascade, parallel type structure　a direct, cascade, the frequency of sampling and structural characteristics of fast convolution type FIR digital filter and representation method