1, Fourier transform: It is a mathematical tool to convert time domain signals into frequency domain signals. The signal is decomposed into sine wave components with different frequencies, and the amplitude and phase information of each component are given. Through Fourier transform, the spectrum characteristics of the signal can be obtained, including frequency components, frequency distribution and so on.

2. Short-time Fourier transform: an analysis method of non-stationary signals. The signal is divided into several short periods, and each period is Fourier transformed. This method can provide the frequency characteristics of the signal in different time periods, thus revealing the time-varying characteristics of the signal.