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'''Overscan''' is the term used to describe the situation when not all of a televised image is present on a viewing screen. It exists because television sets from the 1930s through the 1990s were highly variable in how the video image was positioned within the borders of the cathode ray tube (CRT) screen. The solution was to have the monitor show less than the full image i.e. with the edges "outside" 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. With the ending of CRT displays, this issue has largely (but not completely) disappeared.
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).
{{Citation needed|date=February 2007}}
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 emulators|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.<ref name="caminos">{{cite web|url= |title=GDC 2004: Cross-Platform User Interface Development |publisher=Gamasutra |year=2004 |accessdate=2012-02-09 }}</ref>
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.<ref>{{cite web|url= |title=Microsoft Intellectual Property and Licensing | |date=2011-10-27 |accessdate=2012-01-04}}</ref> In the United States, National Datacast used Public Broadcasting Service network stations for overscan and other datacasting, but they Digital television transition|migrated to digital TV due to the digital television transition in 2009.
==Overscan amounts==
[[File:overscan examples.svg|thumb|500px|Illustration of Action Safe and Title Safe areas for 4:3 and 16:9 aspect ratios according to the BBC.]]
There is no hard technical specification for overscan amounts for the low definition formats. Some say 5%, some say 10%, and the figure can be doubled for title safe, which needs more margin compared to action safe. The overscan amounts are specified for the high definition formats as specified above.
* 625-line analogue video contains 575 active video lines<ref>ITU-R BT.470-6</ref> (this includes two half lines). When the half lines are rounded up to whole lines for ease of digital representation, this gives 576 lines, which is also the nearest mod(16) value to 575. To maintain the same picture aspect ratio, the number of active pixels could be increased to 703.2, which can be rounded up to 704.
* 525-line analogue video contains 485 active video lines<ref name="SMPTE 170M"/> (this include two half lines, though typically only 483 picture lines are present due to [[Closed Captions]] data taking up the first "active picture" line on each field). The nearest mod(16) value is 480. To maintain the same picture aspect ratio, the number of active pixels could be decreased to 706.2, which can be rounded down to 704 for mod(16).
The "standard" pixel aspect ratio data found in video editors, certain ITU standards, MPEG etc. is usually based on an approximation of the above, fudged to allow either 704 or 720 pixels to equate to the full 4x3 or 16x9 picture at the whim of the author.<ref>{{cite web|url= |author=facialz |title=Table of PAL PARs - DVD conversion |page=2 |publisher=Doom9's Forum |date=2008-03-09 |accessdate=2012-05-01}}</ref>
====625 / 525 or 576 / 480====
In [[broadcasting]], analogue system descriptions include the lines ''not used'' for the visible picture, whereas the digital systems only 'number' and encode signals that contain something to see.
The 625 (''PAL'') and 525 (''NTSC'') frame areas therefore contain even more overscan, which can be seen when vertical hold is lost and the picture rolls.{{Citation needed|date=August 2009}}
A portion of this interval available in analogue, known as the [vertical blanking interval, can be used for older forms of analogue datacasting such as Teletext services (like Ceefax and subtitling in the UK). The equivalent service on Digital television does not use this method and instead often uses MHEG.
Horizontally, the difference between 702/704 and 720 pixels/line is referred to as [[nominal analogue blanking]].
====480 vs 486====

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