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Overscan: Correction Should Sharpen Picture?

Discussion in 'Archived Threads 2001-2004' started by James Edward, Apr 2, 2002.

  1. James Edward

    James Edward Supporting Actor

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    I was reading an article on video resolution and it touched on the subject of overscan. It brought to mind a question: If overscan is corrected, shouldn't the picture sharpness improve, since more of what you see is now using the full horizontal and vertical portion of the scanning area?

    I'm not sure if I'm phrasing this correctly, but I see a correlation between this and the 'anamorphic squeeze,' 'vertical compression' of 4:3 TV's improving resolution.

    Yes? No? What the heck am I talking about? And why am I obsessed with this hobby? I can GUARANTEE no one else in my office is remotely concerned with overscan.

    Thanks...
     
  2. Michael Reuben

    Michael Reuben Studio Mogul

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    No.
    Anamorphic mastering provides a more detailed (not necessarily a "sharper" picture) because scan lines that would otherwise be wasted on black bars are being used to contain additional picture information. This is part of the transfer process. The "vertical compression" applied on a 4:3 screen is just a way of seeing the additional resolution in its proper geometry.
    Reducing overscan doesn't increase resolution, doesn't redistribute scan lines, and only affects a specific playback device. Image that was hidden in the overscan now becomes visible at the edge of your screen. The image itself is unchanged; you just see more of it.
    M.
     
  3. RyanDinan

    RyanDinan Stunt Coordinator

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    Michael -



    I think you're not quite understanding what James was trying to get at -



    While it's not the same as an anamorphically transferred DVD, minimizing overscan will[/] tighten up the crispness of the image - which was the correlation James was trying to point out. Overscan occurs when portions of the video signal are pushed outside of the display's raster area, into the retrace/blanking intervals. This can happen on both the horizontal and vertical axis.



    Let's say 10% of the horizontal resolution is pushed into overscan. This means that only 90% of the total horizontal samples are being distributed along the same horizontal space. It's similar to taking a photo - say 5x7 - cropping off a half inch on each side, then blowing it back up to 5x7. Not only can this cause distortion, but it lowers resolution.

    The more you reduce overscan, the more of the samples get drawn in the same raster area, and the more of the image gets seen - i.e, more samples in the same amount of space - Higher resolution.



    -Ryan Dinan
     
  4. Allan Jayne

    Allan Jayne Cinematographer

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    Reducing overscan on a CRT based TV makes everything in the picture slightly smaller, but any given portion of subject matter is not made sharper. It may look sharper because the scan lines are closer together so gaps between them are less noticeable, and because the picture as a whole is smaller so defects are not as magnified. Then, it might look less sharp if the electron beam doesn't make a spot small enough to follow the finest details.
    In practice, when overscan is reduced (changed in any way) there is a good chance of increasing convergence errors which if not tweaked will make the overall picture less sharp.
    Video hints:
    http://members.aol.com/ajaynejr/video.htm
     
  5. Michael Reuben

    Michael Reuben Studio Mogul

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  6. RyanDinan

    RyanDinan Stunt Coordinator

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    While I agee with what Allan said - Having any amount of overscan is just like zooming in on the image, which is going to make things appear lower in resolution (however slight it may be)- Because of the fact that there are less samples being shown in the same viewable space than there could be. Convergence errors aside (which can normally be corrected easily with today's displays), minimizing overscan should improve the crispness of the image. That is unless of course, like Allen pointed out, the electron beam is not fine enough to resolve all the detail in that space. Which in case, would not necessarily lower the image crispness more so than what was displayed before overscan was corrected...The details may get blended together, but you'd see more of the image.

    Still today, it's common for consumer TV's to come with 15-20% overscan out of the box. Which is rediculous.

    On my set, for DVD, I have 5 "pixels" worth of overscan on the left and right (which equals out to about 1.4%), and 0 overscan on the top and bottom (due to the vertical squeeze).

    -Ryan Dinan
     
  7. RyanDinan

    RyanDinan Stunt Coordinator

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  8. Michael Reuben

    Michael Reuben Studio Mogul

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  9. James Edward

    James Edward Supporting Actor

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    At the risk of annoying a forum Administrator, my thinking was along the lines of what Ryan stated:

    The more you reduce overscan, the more of the samples get drawn in the same raster area, and the more of the image gets seen - i.e, more samples in the same amount of space - Higher resolution.

    This is why I was comparing it to raster 're-arrangement' on 4:3 TV's.

    But in reading the answers, the issue is much more complex than 'vertical compression.' And I see that 'sharpness' and 'resolution' cannot be used interchangeably...

    Thanks for the answers, they give me something to ponder.
     
  10. Michael Reuben

    Michael Reuben Studio Mogul

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    There's no risk of annoyance, James. You asked a perfectly reasonable question, which I tried to answer as best I could.
     

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