Overscan

Overscan is the situation in which not all of a televised image is present on a viewing screen. It exists because cathode-ray tube (CRT) television sets from the 1930s through to the early 2000s were highly variable in how the video image was positioned within the borders of the screen. The solution was to have the monitor show less than the full image, that is to say with the edges outside of 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.

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:

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 of unwanted elements. When used, this mode is called underscan.

Today's TV sets are based on newer fixed-pixel technology like liquid crystal displays (LCDs). As overscan reduces picture quality, it is undesirable for 1080i and 1080p sets; therefore, 1:1 pixel mapping is preferred.

On LCDs driven from a digital signal, no adjustment is necessary because all pixels are in fixed positions. Thus all modern computers can safely assume that all pixels are visible to the viewer. Analog video signals such as VGA, however, are subject to timing variations and even when using an LCD panel do not have this exactness. When video or animation content is designed to be viewed on computers (for example, Flash movies), it is not necessary to keep critical content away from the edge.

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