Television Scanning

the sequential resolution of an image into its constituent elements, which takes place within a television camera tube, in order to obtain a video signal having instantaneous values proportional to the brightness of the image element being transmitted at a given moment; also, the reverse conversion, or synthesis, of the transmitted image in the kinescope of a television receiver. Scanning was achieved in the first television systems by mechanical means, such as the rotating Nipkow disk (1884). B. L. Rozing (1907) was the first to propose the use of an electron beam for scanning. In all modern black-and-white and color television broadcasting systems, an electron beam or light beam of constant intensity is used to scan the image and resolve it into elements (seeFLYING-SPOT CAMERA), and the image is synthesized on the screen of a kinescope by an electron beam, the intensity of which is modulated.

Television scanning is usually linear. The resolution and synthesis of an image are accomplished line by line from left to right and by fields from top to bottom. One horizontal sequence of elements forms a scanning line, and all the lines together constitute the television raster. The scanning beam in transmitting and receiving tubes is extinguished during the return trace, and no information about the image is produced.

Various methods of television scanning are used, including progressive scanning (line by line), interlaced scanning (by alternate lines), and dot-sequential scanning. Interlaced scanning is used for both black-and-white and color television broadcasting; the picture area is scanned twice for each time the entire raster is covered, and the resolution and synthesizing processes are synchronized.