Overscan
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.
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.
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.
[6] When driven by analog video signals such as VGA, however, displays are subject to timing variations and cannot achieve this level of precision.
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).
[7] 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.
In the cases of the C64, Amstrad CPC,[8] and Atari ST it has proved possible to remove apparently fixed borders with special coding tricks.
Most figures serve as recommendations or typical summaries, as the nature of overscan is to overcome a variable limitation in older technologies such as cathode-ray tubes.
The following is a summary: Microsoft's Xbox game developer guidelines recommend using 85 percent of the screen width and height,[7] or a title safe area of 7.5% per side.
This presents a requirement unique to television, where an image with reasonable quality is expected to exist where some customers won't see it.
Digital foundations to most storage and transmission systems since the early 1990s have meant that analogue NTSC has only been expected to have 480 lines of picture[citation needed] – see SDTV, EDTV, and DVD-Video.
(View at full size to see the effects portrayed accurately.)
