用各种窗函数设计fir数字滤波器。 分别用矩形窗和hamming窗设计线性相位fir低通滤波器。要求通带截止频率，单位脉冲响应h(n)的长度N=21，绘出h(n)及其幅频响应特性曲线。 -Window function with a variety of fir digital filter design. Were rectangular window and hamming window design of linear phase fir low-pass filter. Req