The sequential sampling, transmission and reproduction of TV signals from point to surface are completed by scanning. Television scanning systems vary from country to country. In China, there are 25 frames per second with 625 lines per frame. Each line is scanned from left to right, and each frame is divided into odd lines and even lines from top to bottom according to interlacing, so as to reduce flicker.
Image information is transmitted during scanning. When the scanning electron beam returns to the line retrace line before the start of the next line from the end of the previous line, and the field retrace line after each field is scanned from top to bottom, it should be blanked. During the blanking period of the line field, the line field synchronization signal is transmitted to synchronize the received and transmitted scanning, thus accurately reproducing the original image.
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The TV camera focuses the light image of the scene on the photosensitive (or light guide) target surface of the camera tube, and the photoelectronic excitation of each point on the target surface or the change of light guide changes with the brightness of each point of the light image. When scanning the target surface with an electron beam, an electric signal whose amplitude is proportional to the brightness of each scene image is generated and transmitted to the TV receiver, so that the electron beam scanned on the CRT screen changes with the intensity of the input signal. When scanning synchronously with the sender, the original image will appear on the screen of the picture tube.
The process of TV signal transmission and distribution, taking the live broadcast in other cities as an example, is generally from cameras, TV centers or vans, to microwave relay lines and transmitting stations, and finally to the user's TV receiver. In addition, TV broadcasting satellite and cable TV are also effective means of TV transmission and distribution in national and urban areas respectively.