Overscan is a behaviour in certain television sets in which part of the input picture is cut off by the visible bounds of the screen. It exists because cathode-ray tube (CRT) television sets from the 1930s to the early 2000s were highly variable in how the video image was positioned within the borders of the screen. It then became common practice to have video signals with black edges around the picture, which the television was meant to discard in this way.
Origins Early analog 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 with additional black borders which weren't part of the original image. Fullscan: The full image area to the electronic edge of the signal (with the black borders of the image if they exist). Observable fullscan: An overscan image area which dismisses only the additional black borders of the image (if they exist). 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. Despite the wide adoption of LCD TVs that do not require overscan since the size of their images remains the same irrespective of voltage variations, many LCD TVs still come with overscan enabled by default, but it can be disabled by the user using the TV's on-screen menus.
Modern video displays
Today's displays, being driven by digital signals (such as DVI, HDMI and DisplayPort), and based on newer fixed-pixel technology (such as liquid crystal displays), can safely assume that all pixels are visible to the viewer. On digital displays driven from a digital signal, therefore, no adjustment is necessary because all pixels in the signal are unequivocally mapped to physical pixels on the display. As overscan reduces picture quality, it is undesirable for digital flat panels; therefore, 1:1 pixel mapping is preferred. When driven by analog video signals such as VGA, however, displays are subject to timing variations and cannot achieve this level of precision. CRTs made for computer display are set to underscan with an adjustable border, usually colored black. Some 1980s home computers such as the Apple IIGS could even change the border color. The border will change size and shape if required to allow for the tolerance of low precision (although later models allow for precise calibration to minimise or eliminate the border). As such, computer CRTs use less physical screen area than TVs, to allow all information to be shown at all times. Computer CRT monitors usually have a black border (unless they are fine-tuned by a user to minimize it)—these can be seen in the video card timings, which have more lines than are used by the desktop. When a computer CRT is advertised as 17-inch (16-inch viewable), it will have a diagonal inch of the tube covered by the plastic cabinet; this black border will occupy this missing inch (or more) when its geometry calibrations are set to default (LCDs with analog input need to deliberately identify and ignore this part of the signal, from all four sides). Video game systems have been designed to keep important game action in the title safe area. Older systems did this with borders for example, the Super Nintendo Entertainment System windowboxed the image with a black border, visible on some NTSC television sets and all PAL television sets. Newer systems frame content much as live action does, with the overscan area filled with extraneous details. Within the wide diversity of home computers that arose during the 1980s and early 1990s, many machines such as the ZX Spectrum or Commodore 64 had borders around their screen, which worked as a frame for the display area. Some other computers such as the Amiga allowed the video signal timing to be changed to produce overscan. In the cases of the C64, Amstrad CPC, and Atari ST it has proved possible to remove apparently fixed borders with special coding tricks. This effect was called overscan or fullscreen within the 16-bit Atari demoscene and allowed the development of a CPU-saving scrolling technique called sync-scrolling a bit later.
Datacasting Analog TV overscan can also be used for datacasting. The simplest form of this is closed captioning and teletext, both sent in the vertical blanking interval (VBI). Electronic program guides, such as TV Guide On Screen, are also sent in this manner. Microsoft's HOS uses the horizontal overscan instead of the vertical to transmit low-speed program-associated data at 6.4 kbit/s, which is slow enough to be recorded on a VCR without data corruption. In the U.S., National Datacast used PBS network stations for overscan and other datacasting, but they migrated to digital TV due to the digital television transition in 2009.
Overscan amounts
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