说明：  分别用非参数化谱估计中的直接法，间接法和加窗法对离散随机过程的已零均值化的N个数据样本估计了离散信号的谱密度。
(Were non-parametric spectral estimation of the direct method, indirect method and windowing method, discrete stochastic process has zero mean the data of the N-sample estimates of the spectral density of discrete signals.)