Letterbox Geometry (Avia)

Discussion in 'Archived Threads 2001-2004' started by Dave H, Dec 26, 2001.

  1. Dave H

    Dave H Producer

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    I was looking at the different aspect ratios on Avia under the "Letterbox Geometry" and would like to know what I should see ideally. As I stated in a previous post, I believe my centering is off just a bit.

    While looking at the 1:85:1 screen, here is what I see.

    This is kind of hard to explain so bare with me.

    On the extreme right side of the letter box, I see the upper right corner, middle side, and bottom left corner have lines on the grid pattern. However, I don't see these lines on the left hand side of the letterbox. Should these other lines be showing? I see the edge of the letterbox on both sides (left and right) but don't see the the lines I mentioned - they are only on the right side of screen - not on the left.

    I assume the the lines should be visible on the left part of the screen, as well - correct?

    Thanks
     
  2. Dave H

    Dave H Producer

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    Guy,
    What do you think? [​IMG]
     
  3. Allan Jayne

    Allan Jayne Cinematographer

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    First count the number of squares from right to left. If there are fewer than 16 (including the rightmost and leftmost which may be partially cut off) you have way too much overscan.
    If you are saying the faint safe action area L shaped lines 5% in from the grid corners are missing from the left, the overscan at the left may be far greater. Look for a "horizontal position" control to try to slide the entire picture to the right before adjusting the horizontal size control for overscan.
    I never heard of a TV whose gray scale was different at the left compared with the right but you might lower the contrast and raise the brightness for a moment to see if these L-shaped lines show up better. Use the 100 IRE version of the pattern.
    Are the grid lines thicker at the left, suggesting out of focus that may cause fainter lines to disappear?
    Video hints:
    http://members.aol.com/ajaynejr/video.htm
     
  4. Dave H

    Dave H Producer

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    Allan,

    The grid lines are exactly the same size everywhere. It is indeed, the L safe lines that are partially missing.

    The solution seems to be that if I could just very slightly

    slide my picture, the safe lines would then be exactly even on each side. It's really only the safe lines on the left that seem to be partially missing. The outer part of the actual grid is still shown.

    Does sliding the picture require a re-convergence? Or is this just with overscan?
     
  5. ThomasL

    ThomasL Supporting Actor

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    Dave, use the Avia overscan pattern to check to see what your overscan is. I can't remember if its the right or left but I read somewhere that it is common for one side to have a little more than the other (4 vs 5 or 5 vs 6). I think you can also use this pattern to check to see if your image is centered properly.

    cheers,

    --tom
     
  6. Allan Jayne

    Allan Jayne Cinematographer

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    I cannot guarantee that sliding the picture will not throw off the convergence or cause geometric distortion such as bowed lines. Sometimes you are lucky, sometimes you are not.

    When it comes to convergence, overscan, pincushion distortion, etc. technicians time and again fix something and something else goes wrong. Then they fix that something else and another something else goes wrong. Even convergence all by itself requires re-doing all four corners several times in rotation until it comes out reasonably right. So if the technician finds one happy medium early in the game, he will not want to try more different things on a "what if" basis. So you end up with the picture off center and the technician hopes you don't notice.
     

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