说明：  设计一个低通数字滤波器，满足如下设计指标： 通带截止频率：0.2pi，通带波动：1dB 阻带截止频率：0.3pi，阻带衰减：15dB 要求：分别采用巴特沃斯、切比雪夫Ⅰ型、切比雪夫Ⅱ型和椭圆滤波器原型进行设 计，比较各种设计的阶数、实际阻带衰减，分析实验结果
(Design a low pass digital filters that meet the following design specifications: passband cutoff frequency: 0.2pi, pass-band fluctuations: 1dB stopband cutoff frequency: 0.3pi, stop-band attenuation: 15dB requirements: respectively Butterworth, Chebyshev Ⅰ, Ⅱ and Chebyshev prototype elliptic filter design, comparing the order of the design, the actual stop-band attenuation, analysis of experimental results)