Overscan
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Overscan is extra image area around the four edges of a video image that is not normally seen 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).
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[edit] Origins of overscan
Early televisions varied in their displayable area because of manufacturing tolerance problems 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: [1]
- 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 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. [2]
[edit] 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 so that older programming will be framed as intended to be viewed. Even high-definition television (HDTV) sets overscan, although implementation of this is inconsistent. It is common to see HDTV sets crop out text and station logos on HDTV programming. In response to different picture ratios, some broadcasters crop, magnify, or stretch the original video, further distorting the image an HDTV set may receive.[3]
[edit] Overscan in computers
CRTs made for computer display are set to underscan with an adjustable black border. 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, newer ones frame content much as live action does, with the overscan area filled with extraneous details.[citation needed]
Within the wide diversity of home computers that arose during the 1980s and early 1990s, many machines such as the Sinclar 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. The inventor and coder of the first fullscreen on the Atari ST was Ilja of Level 16 who is also known under the name of Andreas Franz.[citation needed]
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]
[edit] 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. [1] In the US, National Datacast uses PBS network stations for overscan and other datacasting, but is migrating to digital TV prior to analog switchoff in 2009.
[edit] Overscan amounts
There is no hard technical specification for overscan amounts. Some say 5%, some say 10%, and the figure can be doubled for title safe, which needs more margin compared to action safe.
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 [4].
The official BBC suggestions[5] 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% |
[edit] 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.
[edit] PC Video Capture
PC capture cards act as high definition, with no overscan.
An image captured at 720x576 (with 4:3 content) generally looks like cropping to 658x540 when you account for what a CRT TV would remove from the edges. However, remember to scale this to 720x540 to restore 4:3 ratio (thanks to non-square pixels)
This is the rule of thumb to simulate CRT TV viewing, when capturing SDTV or EDTV on a PC:
| 4:3 | 16:9 | |||
|---|---|---|---|---|
| crop to | then scale to | crop to | then scale to | |
| NTSC 720x480 | 660x450 | 600x450 | 660x450 | 800x450 |
| PAL 720x576 | 658x540 | 720x540 | 658x540 | 960x540 |
(In general, HDTV needs no cropping or scaling unless it is pillarboxed.)
A minimal amount of subsequent cropping may be necessary to remove small black areas introduced by 14:9 content and various unpredictable idiosyncrasies. In addition to cropping, other factors to consider include gamma correction and frame rate correction.
[edit] Important esoteric values
[edit] 720 or 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).
720 is used to record analogue to digital safely, in case it shifts sideways, which it will.
Both 704 and 720 are divisible by 16, which comes in handy for high-speed digital operations such as MPEG compression. Additionally, 720 is a highly composite number.
[edit] 544x576 and 480x576 etc
(PAL) 540x576 and 480x576 are derived from 720x576, and defined as the same width as 720. 540 vs 544 is undefined (but divides into 16 neatly).
(NTSC) 540x480 and 480x480 are derived from 720x480, and defined as the same width as 720. 540 vs 544 is undefined (but divides into 16 neatly).
352x576 in MPEG-2 and 352x288 in MPEG-1 are derived from 704. This makes sense in NTSC land (704x480) but because PAL is 702, the difference is undefined. SDTV is messy.
[edit] 625 / 525 or 576 / 480
In broadcasting, analogue systems count the lines not used for 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.
Horizontally, the difference between 702/704 and 720 is referred to as nominal analogue blanking.
[edit] 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.
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.
Nevertheless, expectations for wastage and bad quality of the overscan area were far lower before the era of digital subsystems.
[edit] Trivia
During broadcasting a phone-in quiz called "Upitnik" on Croatian television, the correct answers could be found below the visible area of broadcast (could be seen on computer TV capture cards).
[edit] See also
[edit] Notes
- ^ Biggs, Billy. Overscan and broadcast television. Retrieved on 2007-01-01.
- ^ Underscan/Overscan and The Safe Area Indicator. Retrieved on 2007-01-01.
- ^ ATI and NVIDIA: Quick Look at HDTV Overscan Compensation. Retrieved on 2007-01-01.
- ^ European Broadcasting Union - R95-2000 Television production for 16:9 widescreen: safe areas (02/12/08)
- ^ BBC Technical Standards for Network Television Delivery
