说明：  截断效应，演示对直流信号进行截断后，信号频谱的变化，在演示程序中，图形的计算是由已知的时域和频域表达式计算。
(Truncation effect, demonstration of the DC signals are cut off, the signal spectrum changes in the presentation program, graphics is calculated from the known time domain and frequency domain expression of the calculation.)