Overscan

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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. It then became common practice to have video signals with black edges around the picture, which the television was meant to discard in this way.

Contents

Origins

Early analog televisions varied in the displayed image because of manufacturing tolerance problems. There were also effects from the early design limitations of power supplies, whose DC voltage was not regulated as well as in later power supplies. 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. Because of this, TV producers could not be certain where the visible edges of the image would be. In order to compensate, they defined three areas: [1]

A significant number of people would still see some of the overscan area, so while nothing important in a scene would be placed there, it also had to be kept free of microphones, stage hands, and other distractions. 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. When used, this mode is called underscan. [2]

Despite the wide adoption of LCD TVs that do not require overscan since the size of their images remains the same irrespective of voltage variations, many LCD TVs still come with overscan enabled by default, but it can be disabled by the user using the TV's on-screen menus. [3] [4]

Modern video displays

The effects of overscan on fixed-pixel displays
(View at full size to see the effects portrayed accurately.) Effects of overscan on fixed-pixel displays.png
The effects of overscan on fixed-pixel displays
(View at full size to see the effects portrayed accurately.)

Today's displays, being driven by digital signals (such as DVI, HDMI and DisplayPort), and based on newer fixed-pixel digital flat panel technology (such as liquid crystal displays), can safely assume that all pixels are visible to the viewer. On digital displays driven from a digital signal, therefore, no adjustment is necessary because all pixels in the signal are unequivocally mapped to physical pixels on the display. As overscan reduces picture quality, it is undesirable for digital flat panels; [5] therefore, 1:1 pixel mapping is preferred. [6] When driven by analog video signals such as VGA, however, displays are subject to timing variations and cannot achieve this level of precision.

CRTs made for computer display are set to underscan with an adjustable border, usually colored black. Some 1980s home computers such as the Apple IIGS could even change the border color. 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.

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 ]

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. [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. Some other computers such as the Amiga allowed the video signal timing to be changed to produce overscan. 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. 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.

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[ clarification needed ] 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. [9] In the U.S., National Datacast used PBS network stations for overscan and other datacasting, but they migrated to digital TV due to the digital television transition in 2009.

Overscan amounts

Illustration of Action Safe and Title Safe areas for 4:3 and 16:9 aspect ratios according to the BBC Overscan examples.svg
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.

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. [10]

The official BBC suggestions [11] actually say 3.5% / 5% per side (see p21, p19). The following is a summary:

Action safe 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, [7] or a title safe area of 7.5% per side.

Terminology

Title safe or safe title is an area that is far enough in from the edges to neatly show text without distortion. If you place text beyond the safe area, it might 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.

Analog to digital resolution issues

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 signals 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 that contains active video, that is to say that it 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, [12] giving 702 pixels per line.
  • For 525-line 60 Hz video (usually, and correctly, called "NTSC"), the active line duration is 52.856 μs, [13] 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 [14] (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 [13] (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. [15]

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).

The difference between 702/704 and 720 pixels/line is referred to as nominal analogue blanking.

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 ]

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.[ citation needed ]

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[ citation needed ] – 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.[ citation needed ]

See also

Related Research Articles

<span class="mw-page-title-main">NTSC</span> Analog television system

NTSC is the first American standard for analog television, published in 1941. In 1961, it was assigned the designation System M. It is also known as EIA standard 170.

<span class="mw-page-title-main">Standard-definition television</span> Digital TV with similar definition to analog broadcasts

Standard-definition television is a television system that uses a resolution that is not considered to be either high or enhanced definition. Standard refers to offering a similar resolution to the analog broadcast systems used when it was introduced.

<span class="mw-page-title-main">Interlaced video</span> Technique for doubling the perceived frame rate of a video display

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<span class="mw-page-title-main">ATSC standards</span> Standards for digital television in the US

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<span class="mw-page-title-main">Vertical blanking interval</span> Time between drawing frames or fields

In a raster scan display, the vertical blanking interval (VBI), also known as the vertical interval or VBLANK, is the time between the end of the final visible line of a frame or field and the beginning of the first visible line of the next frame or field. It is present in analog television, VGA, DVI and other signals.

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<span class="mw-page-title-main">Display resolution</span> Width and height of a display in pixels

The display resolution or display modes of a digital television, computer monitor, or other display device is the number of distinct pixels in each dimension that can be displayed. It can be an ambiguous term especially as the displayed resolution is controlled by different factors in cathode ray tube (CRT) displays, flat-panel displays and projection displays using fixed picture-element (pixel) arrays.

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<span class="mw-page-title-main">Safe area (television)</span> Visible portion of picture on TV screen

Safe area is a term used in television production to describe the areas of the television picture that can be seen on television screens.

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Nominal analogue blanking is the outermost part of the overscan of a standard definition digital television image. It consists of a gap of black pixels at the left and right sides, which correspond to the end and start of the horizontal blanking interval: the front porch at the right side, and the back porch at the left side. Digital television ordinarily contains 720 pixels per line, but only 702 (PAL) to 704 (NTSC) of them contain picture content. The location is variable, since analogue equipment may shift the picture sideways in an unexpected amount or direction.

<span class="mw-page-title-main">Pixel aspect ratio</span> Proportion between the width and the height of a pixel

A Pixel aspect ratio is a mathematical ratio that describes how the width of a pixel in a digital image compared to the height of that pixel.

<span class="mw-page-title-main">Windowbox (filmmaking)</span> The often undesirable combination of letterboxing and pillarboxing

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. Sometimes, by accident or design, a standard ratio image is presented in the central portion of a letterbox picture, resulting in a black border all around. It is generally disliked because it wastes much screen space and reduces the resolution of the original image. It can occur when a 16:9 film is set to 4:3 (letterbox), but then shown on a 16:9 TV or other output device. It can also occur in the opposite direction and a few films have been released with this aspect ratio — one example is The Crocodile Hunter: Collision Course, which had numerous scenes with Steve & Terri Irwin using widescreen pillar boxing.

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The technology of television has evolved since its early days using a mechanical system invented by Paul Gottlieb Nipkow in 1884. Every television system works on the scanning principle first implemented in the rotating disk scanner of Nipkow. This turns a two-dimensional image into a time series of signals that represent the brightness and color of each resolvable element of the picture. By repeating a two-dimensional image quickly enough, the impression of motion can be transmitted as well. For the receiving apparatus to reconstruct the image, synchronization information is included in the signal to allow proper placement of each line within the image and to identify when a complete image has been transmitted and a new image is to follow.

Broadcast-safe video is a term used in the broadcast industry to define video and audio compliant with the technical or regulatory broadcast requirements of the target area or region the feed might be broadcasting to. In the United States, the Federal Communications Commission (FCC) is the regulatory authority; in most of Europe, standards are set by the European Broadcasting Union (EBU).

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References

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