说明：  FIR滤波器设计中往往是线性相位，但不是最小相位，所以它的零点有部分在单位圆外。把单位圆外的零点映射到单位圆内，构成最小相位滤波器。
(FIR filter design is often the linear phase, but it is not minimum phase, so it has some of the zeros outside the unit circle. To zero outside the unit circle is mapped to the unit circle, the minimum-phase filter constituted.)