矩形窗设计线性相位低通滤波器，该滤波器的通带截止频率为，单位脉冲响应的长度为N=21，并绘制其幅度响应特性曲线。-Rectangular window design linear phase low-pass filter, the passband cutoff frequency of the filter, the length of the unit impulse response is N=21, and to draw its amplitude response characte