说明：  直接法又称周期图法，它是把随机序列x(n)的N个观测数据视为一能量有限的序列，直接计算x(n)的离散傅立叶变换，得X(k)，然后再取其幅值的平方，并除以N，作为序列x(n)真实功率谱的估计。
(The direct method is also called cycle diagram method, it is the random sequence x (n) n observation data as a energy limited sequence, direct calculation x (n) of discrete Fourier transform, x (k), and then take the amplitude of the square, and divided by n, as a sequence x (n) real power spectrum estimation. )