fir_N

代码说明：

  根据过渡带宽及阻带衰减要求，选择窗函数的类型并估计窗口长度N（或阶数M=N-1），窗函数类型可根据最小阻带衰减As独立选择 (According to the transition bandwidth and stopband attenuation requirements, select the type of window function and estimate the window length N (or the order of M = N-1), type of window function according to the minimum stop-band attenuation can be independently selected As)