说明：  一维向量的离散短时Fourier变换函数。函数名：stft，输入：要处理的一维数组，加窗：矩形窗，窗口宽度：20。
(One-dimensional vector of discrete short-time Fourier transform function. Function name: stft, enter: To deal with the one-dimensional array, plus a window: rectangular windows, window width: 20.)