What is the source of overscanning on TV?

Discussion in 'Archived Threads 2001-2004' started by John_Berger, Aug 7, 2002.

  1. John_Berger

    John_Berger Cinematographer

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    I just had a guest on my site ask me about why he loses some of the image on a TV as opposed to a PC. I explained that it's due to overscan and that the severity of overscan depends on the manufacturer. What I was not able to explain is why overscanning is used because, frankly, I don't know why.
    I did a search for it on HTF, but the threads that mentioned anything about it simply touched on the fact that overscanning exists and what percentage can be lost due to it. I didn't see anything that definiteively explained why overscanning exists, how it started, and why it continues to be factor. (That doesn't mean it's not there. It just means that I didn't see it. [​IMG] )
    Can anyone fill me in on this?
     
  2. Bill Slack

    Bill Slack Supporting Actor

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    Some amount of overscan is necessary because the very top of the picture has, shall we say, funny looking lines? Part of the picture information is used to transfer things like closed captioning, so you want to cut that portion off.

    Beyond that, it's more expensive to get the very edge of the picture, especially on a CRT to have correct geometry. So, minimal overscan is basically necessary, and the rest of overscan is largely for economic reasons.

    Most decent sets can handle 2.5%-5% overscan without much of a problem. Less than 2.5% (per side) is usually problematic. Also, TV shows are shot with padding to allow for 5%, or more, overscan.

    Some sets come with ridiculous amounts of overscan (sometimes even 10%!) which is hopefully (and usually) adjustable, but sometimes extreme overscan is masking just how crappy the set is.
     
  3. John_Berger

    John_Berger Cinematographer

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  4. Allan Jayne

    Allan Jayne Cinematographer

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    The source of overscan is deliberate design of the TV to have it.
    On many TV sets and computer monitors the picture expands slightly when the overall content is light and shrinks slightly when the overall content is dark. Using overscan this effect is less obtrusive. In extreme cases (crappy sets) there will be geometric distortion when the middle of the picture (all the way from left to right) is darker and shrunken compared with the top and bottom which is brigher and wider, or vice versa.
    As the TV ages, the picture tends to shrink, on average. With overscan, a longer time will pass before the picture gets smaller than the screen and only then the owner thinks something is wrong. The owner calls a serviceman who increases the overscan (hsize and vsize), the picture fills the screen again, and the serviceman submits a huge bill.
    To make sure the start button always shows up (lower left corner for windows 95) most people adjust their computer monitors to have some underscan rather than overscan.
    Video hints:
    http://members.aol.com/ajaynejr/video.htm
     
  5. Bill Slack

    Bill Slack Supporting Actor

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    Also, on computer monitors more of the actual tube is masked off the sides with the molding of the case, so you don't actually see the edges of the tube itself. As I understand it, TVs don't do this to the same degree.

    I've never seen a consumer priced CRT that has NO distortion on the edges when eliminating overscan, completely. Obviously, this kind of distortion and overscan would be an issue with computers, since there is no safe padded area in programs.
     

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