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Overscan is the situation in which not all of a televised image is present on a viewing screen. It exists because television sets from the 1930s through the early-2000s were highly variable in how the video image was positioned within the borders of the cathode ray tube (CRT) screen. The solution was to have the monitor show less than the full image i.e. with the edges "outside" the viewing area of the tube. In this way the image seen never showed black borders caused by either improper centering or non-linearity in the scanning circuits or variations in power supply voltage, all of which could cause the image to "shrink" in size and reveal the edge of the picture. ==Origins== Early analogue televisions varied in the displayed image because of manufacturing tolerance problems. There were also effects from the early design limitations of power supplies, whose DC voltage was not regulated as well as in later power supplies. This could cause the image size to change with normal variations in the AC line voltage, as well as a process called blooming, where the image size increased slightly when a brighter overall picture was displayed due to the increased electron beam current causing the CRT anode voltage to drop. Because of this, TV producers could not be certain where the visible edges of the image would be. In order to compensate, they defined three areas: * Title safe: An area visible by all reasonably maintained sets, where text was certain not to be cut off. * Action safe: A larger area that represented where a "perfect" set (with high precision to allow less overscanning) would cut the image off. * Underscan: The full image area to the electronic edge of the signal. A significant number of people would still see some of the overscan area, so while nothing important in a scene would be placed there, it also had to be kept free of microphones, stage hands, and other distractions. Studio monitors and camera viewfinders were set to show this area, so that producers and directors could make certain it was clear. When used, this mode is called ''underscan''. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Overscan」の詳細全文を読む スポンサード リンク
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