说明：  通带截止频率 =1000Hz，阻带截止频率 =1200Hz，采样频率fs=5000Hz，通带衰减系数 =0.3dB，阻带衰减系数 =40dB。
(The digital filter are the digital signal processing important content, the digital filter may divide into IIR and the FIR two main kinds. As for the IIR digital filter design, we need the help of analog prototype filter, and then transform analog filter into digital filter. )