Overscan
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Origins of overscan
As vacuum tubes age, their values shift. Tubes in a TV set control where the electron beam paints the image, so as the set got older pictures would shrink and sometimes shift to one side or the other. The extra image area provided by overscan would then prevent blank space from showing on the set. Another need for overscan: until the 1990s, all TV faceplate designs had to have rounded edges and some early sets were nearly circular. The result of all this was that TV sets varied widely in what was visible, and what went off screen.
Because TV producers could not be certain where the visible edges of the image would be, 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 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, 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.
Modern sets
Todays CRTs have much less image drift, and other technologies like LCDs 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.
Consumer televisions have occasionally advertised the "missing inch" as an anti-feature, because television video normally has something to see where the plastic cabinets cover the glass tube. This represents a requirement unique to television, where an image with reasonable quality is expected to exist where (most) customers won't see it. (ref?)
Overscan in Computers
CRTs made for computer display are set to underscan with an adjustable black border. On LCDs no adjustment is necessary, pixels are in fixed positions. Thus all modern computers can safely assume that every last pixel is visible to the viewer. 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.
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.
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. Computers like the Commodore Amiga were capable of removing these borders by switching into a special overscan mode, while others like the Atari ST and Apple IIGS couldn't. At least that's what the original specifications said. However, the removal of borders was possible with special coding tricks and has been successfully performed on the C-64 and on the Atari ST. 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.
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).
See also
External links
Profile of Andreas Franz [link]
Atari ST Fullscreen Programming [link]
Atari ST Sync Scrolling (German)[link]
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