实现短时傅里叶变换的功能。分为两种方式：1、matlab自带函数；2、自行编写程序。自行编写的程序效果更加，绘制时频分布图更清晰准确。（附带几篇参考文章）-Short time Fourier transform. Divided into two ways: 1 matlab own function　own programming. Write your own program was more plotted frequency distribution more clearly and accurately. (With several reference articles)