来自：https://www.dsprelated.com/freebooks/filters/Pole_Zero_Analysis_I.html
Transfer Function可以写为：
The order of a this function is defined as the maximum of its numerator and denominator polynomial orders. Such a transformation clearly preserves the number of Poles(极点) and Zeros(零点), provided poles and zeros at and
are either both counted or both not counted.
这是一个two poles two zeros 的filter，其增益Gain 。