FIR 滤波器之多相抽取器（Polyphase Decimator）

多项抽取滤波器基本原理：

将一组N个原型滤波器系数映射到M个多相子滤波器中，每个子滤波器中的系数个数为N/M，映射关系如下所示：

M-to-1多相抽取滤波器如下图3-26所示

如上图，多相子滤波器从第M个开始逐个以输出采样x(n)作为输入，到第一个输入后完成一轮循环，即将M个采样分别送入M个多相子滤波器后，开始得到输出，输出为M个多相子滤波器输出之和，输出采样速率是输入数据流采样速率的1/M。

The polyphase segments are accessed by delivering the input samples x(n) to their inputs using an input commutator which starts at the segment index and decrements to index 0. After the commutator has executed one cycle and delivered M input samples to the filter, a single output is taken as the summation of the outputs from the polyphase segments. The output sample rate is where is the sample rate of the input data stream .