说明：  设某信号 , N=20, 其中， 为白噪声。 求: 编程实现信号的DFT及其反DFT变换，画出原始信号、幅频特性和反变换后信号的曲线。
(Let a signal, N = 20, which, for the white noise. Demand: programming signal DFT and its inverse DFT transform, draw the original signal, the amplitude-frequency characteristic curve and inverse transformed signal.)