说明：  利用STFT可以估计信号在每片短时窗内的频率得到信号的瞬时频率，该曲线由一组时间和频率相对应的点组成，反映了信号频率随时间的变化。
(Using STFT can estimate the frequency of the signal in every piece of short-time window get signal instantaneous frequency, the curve consists of a set of time and frequency of corresponding points, reflects the signal frequency changes over time. )