实现一个40阶的fir低通滤波器，通带截止频率0.35，阻带截止频率0.4.采用Hamming窗获得滤波器系数。-The realization of the fir a 40-order low-pass filter, passband cutoff frequencies of 0.35, stopband cutoff frequency 0.4. Using Hamming window to obtain filter coefficients.