Overscan amounts
From Wikipedia, the free encyclopedia
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.
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[edit] Tech Specs
There is no hard technical spec 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.
The official BBC suggestions actually say 3.5% / 5% (see p21, p19). The following is a summary:
| Action Safe | Title Safe | |||
|---|---|---|---|---|
| vertical | horizontal | vertical | horizontal | |
| 4:3 SD | 3.5% | 3.3% | 5.0% | 6.7% |
| 16:9 SD | 3.5% | 3.5% | 5.0% | 10.0% |
| 16:9 HD | 0.0% | 0.0% | 1.5% | 1.5% |
| 14:9 SD on 16:9 HD | 0.0% | 10.0% | 1.5% | 12.5% |
| 4:3 SD on 16:9 HD | 3.5% | 15.0% | 5.0% | 17.5% |
As you can see, "14:9" provides a lot less wastage for 16:9 viewers — that extra 5% real-estate looks a lot better for corner logos, watermarks and scrolling news tickers.
4:3 viewers get an automatic 4.1% added to both vertical figures thanks to 14:9's thin black bars — this is why 0% and 1.5% safe areas are acceptable.
[edit] Terminology
Title safe or safe title is an area which is far enough in from the edges to neatly show text without distortion (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 using PC video capture:
| 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 | |
| HDTV 1280x720 | does not exist | no need to crop/scale | |||
| HDTV 1920x1080 | does not exist | no need to crop/scale | |||
| note that a minimal amount of subsequent cropping may be necessary due to small black areas introduced by unpredictable idiosyncrasies | |||||
(Other factors to consider are gamma reproduction, and 50fps / 60fps motion reproduction).
[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.
It is also a highly composite number, useful for high-speed digital operations such as MPEG compression.
[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.
