用矩形窗设计了一个FIR线性相位低通数字滤波器。截止频率0.5π，阶数为21阶。求出了系统函数并画出了幅频响应。-A rectangular window design with linear phase FIR low-pass digital filter. Cutoff frequency 0.5π, the order of 21 bands. Find the system function and plotted the amplitude-frequency response.