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Overscan is extra image area around the four edges of a video image that may not be seen reliably by the viewer. It exists because television sets in the 1930s through 1970s were highly variable in how the video image was framed within the cathode ray tube (CRT).

Origins of overscan

Early televisions varied in their displayable area because of manufacturing tolerance problems. There were also effects from the early design limitations of linear power supplies, whose DC voltage was not regulated as well as in later switching-type power supplies. This would cause the image to shrink when AC power 'browned out', 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:

• 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.
• 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.

Modern sets

Today's TV sets can be based on newer fixed-pixel technologies like liquid crystal displays (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 to hide the possible artifacts that may lie at the edges of the picture. Even high-definition television (HDTV) sets overscan, although implementation of this was inconsistent while CRT sets persisted. With the accuracy attainable with digital type displays (e.g. plasma, LCD etc.), the area presented to the viewer is precisely defined. It is common to see HDTV sets crop out text and station logos on HDTV programming. In response to different picture ratios, some sources crop, magnify, or stretch the original video, further distorting the image a HDTV set may receive.

For the high definition formats the degree of overscan is specified. When a 1920x1080 high definition video is displayed (1080i or 1080p), only the central 1877x1000 pixels should be presented to the viewer. For a 1280x720 video (720p) only the central 1200x675 pixels should be visible. The low definition formats are not rigidly specified due to their history in CRT based dispalys.

Overscan in computers

CRTs made for computer display are set to underscan with an adjustable border, usually colored black. 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.

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. Analog video signals such as 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 computers (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 shown on television.

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 Sinclair ZX Spectrum or Commodore 64 (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.

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).

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 uses PBS network stations for overscan and other datacasting, but is migrating to digital TV prior to digital television transition in 2009.

Overscan amounts

There is no hard technical specification for overscan amounts for the low definition formatts. 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.

Different video and broadcast television systems require differing amounts of overscan. 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.

However the European Broadcasting Union has safe area recommendations regarding Television Production for 16:9 Widescreen.

The official BBC suggestions actually say 3.5% / 5% per side (see p21, p19). The following is a summary:

Microsoft's Xbox game developer guidelines recommend using 85 percent of the screen width and height, or a title safe area of 7.5% per side.

Terminology

Title safe or safe title is an area which is far enough in from the edges to neatly show text without distortion. If you place text beyond the safe area it may not display on some CRT TV sets (in worst case).

Action safe or safe action is the area in which you can expect the customer to see action. However, the transmitted image may extend to the edges of the MPEG frame 720x576. This presents a requirement unique to television, where an image with reasonable quality is expected to exist where (some) customers won't see it. This is the same concept as used in widescreen cropping.

TV safe is a generic term for the above two, and could mean either one.

Analog to digital resolution issues

720 vs. 702 or 704

• PAL - 702 is the width of analogue, not digital; the definition of what is 4:3, and what is 16:9, derives from here (702 can be either).
• NTSC - 704 is the width of analogue, not digital; the definition of what is 4:3, and what is 16:9, derives from here (704 can be either).

625 / 525 or 576 / 480

In broadcasting, analogue systems count the lines not used for the visible picture, whereas the digital systems only bother to encode (and compress) content that may contain something to see.

The 625 (PAL) and 525 (NTSC) line areas therefore contain even more to overscan, which can be seen when vertical hold is lost and the picture rolls.

A large part of the vertical overscan available in analogue only, 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 employ overscan and instead often uses MHEG.

Horizontally, the difference between 702/704 and 720 is referred to as nominal analogue blanking.

480 vs 486

The 525-line system originally contained 486 lines of picture, not 480.

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 – see SDTV, EDTV, and DVD-Video.

How this affects the interpretation of "the 4:3 ratio" as equal to 704x480 or 704x486 is unclear, but the VGA standard of 640x480 has had a large impact.

See also

• Nominal analogue blanking

Notes

1. Biggs, Billy. "Overscan and broadcast television". Retrieved 2007-01-01.
2. "Underscan/Overscan and The Safe Area Indicator". Retrieved 2007-01-01.
3. "ATI and NVIDIA: Quick Look at HDTV Overscan Compensation". Retrieved 2007-01-01.
4. http://hd.engadget.com/2010/05/27/hd-101-overscan-and-why-all-tvs-do-it/
5. Some 1980s home computers could change the border color: for example, see Apple IIGS#Technical specifications.
6. ^ GDC 2004: Cross-Platform User Interface Development on Gamasutra. Retrieved on 2008-11-19.
7. http://www.microsoft.com/about/legal/intellectualproperty/search/details.mspx?ip_id=IDAEQMXD&techType=&ipCat=Any&feeStructure=Any&keywords=&ipVenture=true
8. European Broadcasting Union - R95-2000 Television production for 16:9 widescreen: safe areas (02/12/08)
9. BBC Technical Standards for Network Television Delivery

External links

• How Overscan affects the DVD-viewing experience

• Television technology