Scanning

Although we see the image as occupying the full face of the picture tube, it is actually painted across the screen in constantly moving horizontal brush strokes, called “scanning lines”. At the rear of the picture tube is an electron gun, which is basically a heated wire filament. The heat causes electrons to form a “cloud” around the filament. A high positive voltage accelerates the negatively charged electrons from the filament in a thin beam toward the face of the TV screen where they strike a phosphor coating. The phosphors glow where the electrons strike. The image is formed by scanning the electron beam in 525 horizontal lines (625 in Europe) across the screen, with scanning line 1 at the top of the screen, and scanning line 525 at the bottom. (In reality, some of the 525 lines are lost during the time when the electron beam moves from the bottom of the screen back up to the top to begin the next scanning sequence, so, the actual number of scanning lines that end up being shown on the screen is 483, but for the sake of consistency with television literature, we will continue the discussion as if there were 525 lines.) Where the scene is dark, the electron beam is weak, so the phosphors do not glow very much. Where the scene is bright, the electron beam is intense, so the phosphors glow brightly. The scanning of the beam across the face of the picture tube is coordinated with scanning of the original scene (people, landscape, etc.) by a sensor in the television camera, and as the scanning lines become bright, dark, and intensities in between, the image is formed. The “resolution” of a TV image is the ability to distinguish alternating light and dark lines that are close together. If the lines were too close together (beyond the resolution limits), the lines merge into gray. The “vertical resolution” of NTSC TV is the 525 HORIZONTAL scanning lines. That does not change. However, the “horizontal resolution” (number of VERTICAL lines) changes with the source. The horizontal resolution of VHS tape is about 240 lines, broadcast TV is 330 lines, laserdisc is 420 lines, and DVD is 480 lines.