Overscan
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Overscan is extra image area around the four edges of a video image that is not normally seen by the viewer. It exists because TV sets in the 1930's through 1970s were highly variable in how the video image was framed within the CRT.
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[edit] Origins of overscan
Early TVs varied in their displayable area due to manufacturing tolerance problems as well as a process called blooming, where the image size increased slightly when a brighter overall picture was displayed. Because of this, TV producers could not be certain where the visible edges of the image would be. In order to cope with this, they defined three areas:[citation needed]
- 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 would cut the image off.
- Overscan: 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 to a scene could be placed there, it also had to be kept free of microphones, stage hands, and other distractions. Studio monitors and camera viewfinders can be set to show this area, so that producers and directors can make certain it is clear. When activated, this mode is called underscan.[citation needed]
[edit] Modern sets
Today's TV sets can be based on newer fixed-pixel technologies like LCDs, and those with CRTs have much less image drift with crystal-based timing, and so can have perfect image placement. Nevertheless, these sets when used for TV must overscan the image so that older programming will be framed as intended to be viewed. Even HDTV sets overscan, although implementation of this is inconsistent. It is common to see HDTV sets crop out text and station logos on HDTV programming. In response to different picture ratios, some broadcasters crop, magnify, or stretch the original video, further distorting the image a HDTV set may receive.[citation needed]
[edit] Overscan amounts
Main article: Overscan amounts
There is no hard technical spec for overscan amounts. Some say 5%, some say 10%, and the figure can be doubled for title safe, which needs more margin compared to action safe.
[edit] Overscan in Computers
CRTs made for computer display are set to underscan with an adjustable black border. 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 every last pixel is visible to the viewer. Analogue video signals like 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 PCs, (for example, flash movies) it is not necessary to keep critical content away from the edge. This can cause composition problems if such content is later repurposed for TV. The DVD of Happy Tree Friends suffers from this.[citation needed]
All videogame systems have been designed to keep important game action in the title safe area. Older systems did this with borders, newer ones frame content much as live action does, with the overscan area filled with extraneous details.[citation needed]
Within the wide diversity of home computers that arose during the 1980s and early 1990s a lot of machines like the Sinclar ZX Spectrum or Commodore C64 had borders around their screen, which worked as a frame for the display area. Some other computers such as the Commodore Amiga allowed the video signal timing to be changed to produce overscan. In the cases of the C64 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. The inventor and coder of the first fullscreen on the Atari ST was Ilja of Level 16 who is also known under the name of Andreas Franz.[citation needed]
Computer CRT monitors usually have a black border (unless they are fine-tuned by a user to minimise 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 analogue input need to deliberately identify and ignore this part of the signal, from all four sides). [citation needed]
[edit] See also
- Action safe
- Title safe
- Missing inch
- Nominal analogue blanking
- Television
