设计一个数字滤波器，在频率低于 的范围内，低通幅度特性为常数，并且不低于0.75dB。在频率 和 之间，阻带衰减至少为20dB。

(1)试求满足这些条件的最低阶Butterworth滤波器。-Design a digital filter, in the range of frequencies below the low-pass amplitude characteristic is constant, and not less than 0.75dB. Between the frequency and stop-band attenuation of at least 20dB. (1) Determine the lowest-order Butterworth filter to meet these conditions.