In fact, digital TV signals will eventually be converted into analog signals before they can be displayed. But this step is to convert the digital signal after it is transmitted to the TV and finally display it. Theoretically, the digital signal has only two different signal states, high and low, so it is difficult to be disturbed during transmission, and it can be corrected by technology. Under the condition of ensuring the signal strength, it can be said that the digital signal is absolutely "pure"; The analog signal itself is a continuous electrical signal, which is mixed with useless interference information. During transmission, it may change at any time due to the influence of external interference. Generally, after being amplified several times, it will cause a lot of distortion and introduce a lot of interference, and the signal-to-noise ratio will be seriously reduced. The farther the transmission process, the worse the signal. Ordinary tape is a typical example, and it is almost impossible to listen to it after two or three copies.