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Tech question. (theory)

Discussion in 'Projectors, Screens & Video Processors' started by cosaw, Sep 7, 2003.

  1. cosaw

    cosaw
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    Don't have a CRT projector yet and have been wondering if my thinking is correct:

    I beleive that tubes on CRT projectros don't have an inherently fixed resolution. Therefore I presume they don't have a shadow mask marking out pixels. It's my understanding that the picture is effectively drawn onto the face of each tube.

    So when performing an anamorphic squeeze the drawn lines get closer together. The main question: Are the lines also squeezed thinner themselves as well as being squeezed closer together? I would have thought this would be necessary in order that distortions in the picture are not introduced even at a point before the lines would start to overlap.

    cosaw
     
  2. EvilMudge

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    Unfortunately you can't squeeze the individual lines to be thinner, as the line height is determined by the electron beam in each tube, which produces what looks like a dot (although it's actually a diffuse blob, but that doesn't matter) if you were to look at the screen frozen in time.
     
  3. cosaw

    cosaw
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    Cheers Mark

    I expected this might be the case, what your saying then is that the dot size is fixed for any given tube? It's just hard for me to visualize how the dot size would relate to the size of an electron. After all if electrons are fired at one point on the face of a screen surely they can't all hit at exactly the same point, or do they? I would have thought there would be some kind of spread or variance. I then thought that if this is controlable in some way then the dot size/line height might be adjustable. Perhaps the dot size is just the minimum size achievable taking this sort of variance into account and any greater variance would be unacceptable i.e. too big a dot?

    cosaw
     
  4. EvilMudge

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    Actually the dot size depends on the accelerating voltage. As I said, it's more of a diffuse blob than a dot, because the electrons don't all arrive at precisely the same spot (Quantum mechanics forbids it).
    The more energy you give the electrons once they come off the cathode, the brighter the target phosphor will glow. But the greater the energy, the greater the uncertainty and the greater the theoretical dot size, and coincidentally the faster the tube wear rate.
    If you (and by this I mean a CRT designer) are willing to trade resolution and tube life for brightness, you can produce very bright images from a CRT projector. But the easiest thing to do is use more phosphor and more scan lines, so we wind up with monsters like the Barco Reality 12, which as Gordon Fraser once commented, looks like three portable TVs stuck together.
     
  5. Chris Frost

    Chris Frost
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    cosaw, whilst CRT's don't have a fixed resolution they do have a "sweet spot". That's the point between having gaps inbetween your lines and going too far when the lines overlap.

    Driving the source signal to create an image where the scan lines touch but don't overlap gives the best possible resolution from the projector. This is why variable rate scalers or scalers with Aspect Ratio control cost more but are better than fixed rate line multipliers like the iScan. They drive the projector at (or near its) sweet spot and handle the differences in resolution between Anamorphic and non-Anamorphic signals. You'll get to learn a lot more at Gordon's forthcoming Event II :)

    Regards
     
  6. Gordon @ Convergent AV

    Gordon @ Convergent AV
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    COSAW: You are also correct that the size of the dot is controllable. With a few Electro magnetic focusing PJ's it is possible to store different focus and astigmatism (shape of the dot) paramaters.

    The ability of a CRT PJ to draw each dot in exactly the same place everytime is something that most folk don't think about. It is, of course, important. I know of one chap in USA who spent ages looking in to this very parameter when re-engineering his CRT projectors. His modded one looked quite nice ;)

    Gordon
     
  7. cosaw

    cosaw
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    Thanks Mark, Chris, Gordon. All interesting information, I like to know the intricacies. Anyhow I might as well do the research while I wait until I can afford/find a place for a decent PJ.

    cosaw
     

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