Overscan
Encyclopedia
Overscan is extra image area around the four edges of a video
Video
Video is the technology of electronically capturing, recording, processing, storing, transmitting, and reconstructing a sequence of still images representing scenes in motion.- History :...

 image that may not be seen reliably by the viewer. It exists because television
Television
Television is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...

 sets in the 1930s through 1970s were highly variable in how the video image was framed within the cathode ray tube
Cathode ray tube
The cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam onto the fluorescent screen to create the images. The image may represent electrical waveforms , pictures , radar targets and...

 (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 display
Liquid crystal display
A liquid crystal display is a flat panel display, electronic visual display, or video display that uses the light modulating properties of liquid crystals . LCs do not emit light directly....

s (LCDs). For 1080i/p overscan is undesirable, as it reduces picture quality and 1:1 pixel mapping
1:1 pixel mapping
1:1 pixel mapping is a video display technique used in some devices, such as LCD monitors. A monitor that has been set to 1:1 pixel mapping will try to display an input source without scaling it, such that each pixel received is mapped to a single native pixel on the monitor...

 is preferred.

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
Adobe Flash
Adobe Flash is a multimedia platform used to add animation, video, and interactivity to web pages. Flash is frequently used for advertisements, games and flash animations for broadcast...

 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
Video game console
A video game console is an interactive entertainment computer or customized computer system that produces a video display signal which can be used with a display device to display a video game...

 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
Super Nintendo Entertainment System
The Super Nintendo Entertainment System is a 16-bit video game console that was released by Nintendo in North America, Europe, Australasia , and South America between 1990 and 1993. In Japan and Southeast Asia, the system is called the , or SFC for short...

 windowboxed
Windowbox (film)
Windowboxing in the display of film or video occurs when the aspect ratio of the media is such that the letterbox effect and pillarbox effect occur simultaneously...

 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
ZX Spectrum
The ZX Spectrum is an 8-bit personal home computer released in the United Kingdom in 1982 by Sinclair Research Ltd...

 or Commodore 64
Commodore 64
The Commodore 64 is an 8-bit home computer introduced by Commodore International in January 1982.Volume production started in the spring of 1982, with machines being released on to the market in August at a price of US$595...

 (C64) had borders around their screen, which worked as a frame for the display area. Some other computers such as the Commodore
Commodore International
Commodore is the commonly used name for Commodore Business Machines , the U.S.-based home computer manufacturer and electronics manufacturer headquartered in West Chester, Pennsylvania, which also housed Commodore's corporate parent company, Commodore International Limited...

 Amiga
Amiga
The Amiga is a family of personal computers that was sold by Commodore in the 1980s and 1990s. The first model was launched in 1985 as a high-end home computer and became popular for its graphical, audio and multi-tasking abilities...

 allowed the video signal timing to be changed to produce overscan. In the cases of the C64 and Atari ST
Atari ST
The Atari ST is a home/personal computer that was released by Atari Corporation in 1985 and commercially available from that summer into the early 1990s. The "ST" officially stands for "Sixteen/Thirty-two", which referred to the Motorola 68000's 16-bit external bus and 32-bit internals...

 it has proved possible to remove apparently fixed borders with special coding tricks. This effect was called overscan or fullscreen within the 16-bit
16-bit
-16-bit architecture:The HP BPC, introduced in 1975, was the world's first 16-bit microprocessor. Prominent 16-bit processors include the PDP-11, Intel 8086, Intel 80286 and the WDC 65C816. The Intel 8088 was program-compatible with the Intel 8086, and was 16-bit in that its registers were 16...

 Atari demoscene
Demoscene
The demoscene is a computer art subculture that specializes in producing demos, which are non-interactive audio-visual presentations that run in real-time on a computer...

 and allowed the development of a CPU
Central processing unit
The central processing unit is the portion of a computer system that carries out the instructions of a computer program, to perform the basic arithmetical, logical, and input/output operations of the system. The CPU plays a role somewhat analogous to the brain in the computer. The term has been in...

-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
Datacasting
Datacasting is the broadcasting of data over a wide area via radio waves. It most often refers to supplemental information sent by television stations along with digital television, but may also be applied to digital signals on analog TV or radio...

. The simplest form of this is closed captioning
Closed captioning
Closed captioning is the process of displaying text on a television, video screen or other visual display to provide additional or interpretive information to individuals who wish to access it...

 and teletext
Teletext
Teletext is a television information retrieval service developed in the United Kingdom in the early 1970s. It offers a range of text-based information, typically including national, international and sporting news, weather and TV schedules...

, both sent in the vertical blanking interval
Vertical blanking interval
The vertical blanking interval , also known as the vertical interval or VBLANK, is the time difference between the last line of one frame or field of a raster display, and the beginning of the first line of the next frame. It is present in analog television, VGA, DVI and other signals. During the...

 (VBI). Electronic program guide
Electronic program guide
Electronic program guides and interactive program guides provide users of television, radio, and other media applications with continuously updated menus displaying broadcast programming or scheduling information for current and upcoming programming...

s, such as TV Guide On Screen, are also sent in this manner. Microsoft
Microsoft
Microsoft Corporation is an American public multinational corporation headquartered in Redmond, Washington, USA that develops, manufactures, licenses, and supports a wide range of products and services predominantly related to computing through its various product divisions...

's HOS uses the horizontal overscan instead of the vertical to transmit low-speed program-associated data
Program-associated data
Program Associated Data or Program Service Data is a concept related to broadcasting which consists of a number of different fields or streams which are displayed on many HD Radio & satellite radio receivers in order to describe the program being transmitted, including different information such...

 at 6.4 kbit/s, which is slow enough to be recorded on a VCR without data corruption
Data corruption
Data corruption refers to errors in computer data that occur during writing, reading, storage, transmission, or processing, which introduce unintended changes to the original data...

. In the U.S.
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

, National Datacast
National Datacast
National Datacast Incorporated is a pioneer in data broadcasting.It is a for-profit subsidiary of the Public Broadcasting Service which handles datacasting on PBS TV stations throughout the United States...

 uses PBS
Public Broadcasting Service
The Public Broadcasting Service is an American non-profit public broadcasting television network with 354 member TV stations in the United States which hold collective ownership. Its headquarters is in Arlington, Virginia....

 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 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.

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 tube
Cathode ray tube
The cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam onto the fluorescent screen to create the images. The image may represent electrical waveforms , pictures , radar targets and...

s.

However the European Broadcasting Union
European Broadcasting Union
The European Broadcasting Union is a confederation of 74 broadcasting organisations from 56 countries, and 49 associate broadcasters from a further 25...

 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:
|
! colspan="2" | Action safe
! colspan="2" | Title safe
|-
|
! Vertical
! Horizontal
! Vertical
! Horizontal
|-
! 4:3
| 3.5%
| 3.3%
| 5.0%
| 6.7%
|-
! 16:9
| 3.5%
| 3.5%
| 5.0%
| 10.0%
|-
! 14:9 (displayed on 16:9)
| 3.5%
| 10.0%
| 5.0%
| 15.0%
|-
! 4:3 (displayed on 16:9)
| 3.5%
| 15.0%
| 5.0%
| 17.5%
|}>

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 older 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.

720 vs. 702 or 704

The sampling (digitising) of standard definition video was defined in Rec. 601 in 1982. In this standard, the existing analogue video standards are sampled at 13.5 MHz. Thus the number of active video pixels per line is equal to the sample rate multiplied by the active line duration (the part of each analogue video line which contains active video, i.e. does not contain sync pulses, blanking etc).
  • For 625-line 50 Hz video (usually, though incorrectly, called "PAL"), the active line duration is 52 µs, giving 702 pixels per line.

  • For 525-line 60 Hz video (usually, though incorrectly, called "NTSC"), the active line duration is 52.856 µs, giving ~713.5 pixels per line.


In order to accommodate both formats within the same line length, and to avoid cutting off parts of the active picture if the timing of the analogue video was at or beyond the tolerances set in the relevant standards, a total digital line length of 720 pixels was chosen. Hence the picture will have thin black bars down each side.

704 is the nearest mod(16) value to the actual analogue line lengths, and avoids having black bars down each side.

The use of 704 can be further justified as follows:
  • 625-line analogue video contains 575 active video lines (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 (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
Pixel aspect ratio
Pixel aspect ratio is a mathematical ratio that describes how the width of a pixel in a digital image compares to the height of that pixel....

 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.

Although standards-compliant video processing software should never fill all 720 pixels with active picture (only the center 704 pixels must contain the actual image, and the remaining 8 pixels on the sides of the image should constitute vertical black bars), recent digitally generated content (e.g. DVDs of recent movies) often disregards this rule. This makes it difficult to tell whether these pixels represent wider than 4x3 or 16x9 (as they would do if following Rec.601), or represent exactly 4x3 or 16x9 (as they would do if created using one of the fudged 720-referenced pixel aspect ratios).

625 / 525 or 576 / 480

In broadcasting
Broadcasting
Broadcasting is the distribution of audio and video content to a dispersed audience via any audio visual medium. Receiving parties may include the general public or a relatively large subset of thereof...

, 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
Vertical blanking interval
The vertical blanking interval , also known as the vertical interval or VBLANK, is the time difference between the last line of one frame or field of a raster display, and the beginning of the first line of the next frame. It is present in analog television, VGA, DVI and other signals. During the...

, can be used for older forms of analogue datacasting
Datacasting
Datacasting is the broadcasting of data over a wide area via radio waves. It most often refers to supplemental information sent by television stations along with digital television, but may also be applied to digital signals on analog TV or radio...

 such as Teletext
Teletext
Teletext is a television information retrieval service developed in the United Kingdom in the early 1970s. It offers a range of text-based information, typically including national, international and sporting news, weather and TV schedules...

 services (like Ceefax
Ceefax
Ceefax is the BBC's teletext information service transmitted via the analogue signal, started in 1974 and will run until April 2012 for Pages from Ceefax, while the actual interactive service will run until 24 October 2012, in-line with the digital switchover.-History:During the late 60s, engineer...

 and subtitling in the UK). The equivalent service on Digital television does not employ overscan and instead often uses MHEG
MHEG-5
MHEG-5, or ISO/IEC 13522-5, is part of a set of international standards relating to the presentation of multimedia information, standardised by the Multimedia and Hypermedia Experts Group...

.

Horizontally, the difference between 702/704 and 720 is referred to as nominal analogue blanking
Nominal analogue blanking
Nominal analog blanking or nominal analogue blanking is the outermost part of the overscan of a standard definition digital television image...

.

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
EdTV
EDtv is a 1999 American comedy film directed by Ron Howard. An adaptation of the Quebec film Louis 19, le roi des ondes , it stars Matthew McConaughey, Jenna Elfman, Woody Harrelson, Ellen DeGeneres, Martin Landau, Rob Reiner, Sally Kirkland, Elizabeth Hurley, Clint Howard, and Dennis Hopper.The...

, and DVD-Video
DVD-Video
DVD-Video is a consumer video format used to store digital video on DVD discs, and is currently the dominant consumer video format in Asia, North America, Europe, and Australia. Discs using the DVD-Video specification require a DVD drive and a MPEG-2 decoder...

.

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

  • 1:1 pixel mapping
    1:1 pixel mapping
    1:1 pixel mapping is a video display technique used in some devices, such as LCD monitors. A monitor that has been set to 1:1 pixel mapping will try to display an input source without scaling it, such that each pixel received is mapped to a single native pixel on the monitor...

  • HD ready 1080p
  • Bleed (printing)
    Bleed (printing)
    Bleed is a printing term that refers to printing that goes beyond the edge of the sheet after trimming. The bleed is the part on the side of a document that gives the printer a small amount of space to account for movement of the paper, and design inconsistencies...

  • Nominal analogue blanking
    Nominal analogue blanking
    Nominal analog blanking or nominal analogue blanking is the outermost part of the overscan of a standard definition digital television image...


External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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