Talk:Overscan
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Here is the big part I pulled, lets fix it here and restore it.
Contents |
[edit] old version
=== PC Video Capture ===
PC capture cards act as HD, 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 | ||
(Other factors to consider are gamma reproduction, and 50fps / 60fps motion reproduction).
[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] 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's also a highly composite number, useful for high-speed digital operations.
[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 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] 217.162.118.129's rewrite
Sorry about the revert, 217.162.118.129, but I feel that you went much too technical with the article to be useful. "Overscan" is one of the basics of video, so anyone who could understand things like timebase, geometry, and linearity would already know what overscan is. People looking up overscan in wikipedia are probably hobbyists who can't figure out why their TV keeps cutting off the sides of the graphics they did on their computer, so that is the audience we should be writing for. Algr 15:22, 28 July 2006 (UTC)
Yeah, actually I think you're wrong. The whole point of technical material on Wikipedia is that the technical parts are linked. Anyone who doesn't understand something can either skip over it if it's not important to them, or they can look into it just by clicking. For this reason I think your reversion was a very poor decision. What's more, I don't think there was even anything particularly technical in the new version.
(later) I've just looked at it again; there really isn't anything technical in the new version. If you don't know what geometry or linearity are you need to take some English lessons. What's more, the old version you restored is actually wrong!
- The latest version addresses my concern, so I'll go with it. Since you are new to wikipedia, here are some tips:
- Always sign your posts (on talk pages useing ~~~~ (four tildes). This will insert your name and timestamp. To insert just your name, type ~~~ (3 tildes). Otherwise it can get confusing where one comment stops and another starts.
- It's not useful to vaguely say something is wrong - you need to quote exactly what the problem is in the discussion.
- Read carefully. If the above refers to your edit note about CRTs, the article didn't site CRTs as the cause, it said vacuum tubes. This would include the tubes in the timing circuits. In the next section, CRT is shorthand for "TVs using CRTs". And even today they do drift - you can go into any store with TVs lined up and see variations.
- Be polite. The object of wikipedia is to get articles that everyone agrees on, so comments like "it sucks" aren't helpful for building consensus.
- The reader comes first. It is tempting to show off your technical or literary skills, but readers who get lost in the first few paragraphs won't skip further down or click on links - they'll go back to google and find something written for THEM. For articles with a wide range of possible readers, it is best to start simple and get into details further down in the article.
Algr 17:54, 29 July 2006 (UTC)
[edit] Spawned separate page
OK I've decided to 'be bold' and created a separate page at Overscan amounts. Feel free to delete the stuff off this page now, because everything on here was copied into that page, plus some tiny additions I made. But also feel free to reintegrate the two pages; I don't mind which way this goes, as long as the info is somewhere. Whophd 03:05, 18 September 2006 (UTC)
[edit] Overscan in computers
"Analogue video signals such as VGA, however, are subject to timing variations and even when using an LCD panel do not have this exactness."
Actually, for all practical purposes, "auto-setup" will make a one-to-one correspondnace between software pixels and the real LCD screen pixels. Howver, a one-to-one correspondance is impossible if the video card pixel resolution setting does not match the native LCD pixel resolution. Scetpfe 20:40, 24 February 2007 (UTC)
