Sending a sound signal that the speaker can pronounce is called an audio signal, and sending an image signal that can be seen on a TV screen is called a video signal.

These signals (information) cannot be turned into radio waves for long-distance transmission. They must be modulated into high-frequency (carrier) signals before they can be transmitted. Therefore, high-frequency signals are a carrier tool used to transmit information. . When receiving, this information must be "detected" from the high-frequency signal. The video signal that was sent up was the video signal, and the audio signal that was sent up was the audio signal. Because high-frequency signals are difficult to process because they are too high, many receivers first lower the frequency of the high-frequency signal to become an intermediate frequency signal, which is then more convenient for detection. Therefore, the intermediate frequency signal is still a "carrier" tool.