说明：  数字信号处理中的短时傅里叶变换程序，窗函数长度20， 默认为矩形窗，截取时间间隔 <=Nw/2。适用于时频分析
(Short time Fourier transform program in digital signal processing, window function length 20, default to rectangle window, intercept time interval <=Nw/2. Time frequency analysis)