说明：  1、观察采样引起的混叠。 2、判别离散时间系统的时不变性。 3、利用卷积计算信号通过FIR滤波器的输出，并观察输出信号的input-on暂态、input-off暂态和稳态阶段。
(One to observe the sampling due to aliasing. 2, determine the time invariant discrete-time systems. 3, the use of convolution calculation of the signal through the FIR filter output, and observe the output signal of the input-on transient, input-off transient and steady-state phase.)