Digital filters are corresponding to analog filters. We use digital filters in commonly used discrete systems. Its main function is to use the characteristics of the discrete time system, where time is a variable, and then process the externally input signal. The input signal here is generally a waveform model in a broad sense, and the signal can be voltage, current, power, etc. Of course, there are similar to the frequency and so on. In actual operation, we can also turn the input signal waveform into an output, that is, invert the input and output. So as to achieve our purpose of modifying the spectrum of the signal.