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Picture refresh rate on projectors

Discussion in 'Projectors, Screens & Video Processors' started by Paul.M., Oct 31, 2004.

  1. Paul.M.

    Paul.M.
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    What hz are projectors refreshed at. I reckon 50hz is too low (whites flicker really badly at this level). Surely projectors have a higher refresh rate :lease:
     
  2. Kramer

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    Usually 60 Hz.
     
  3. Gary Lightfoot

    Gary Lightfoot
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    Projectors don't flicker as such, as the image is made up in a different way to a CRT, and the lamps output is constant.

    How the frame rate is delivered to the panel is another matter though, and differs between LCD and DLP as far s I know. DLP usualy runs at multiples of 50 or 60hz purely as a ranbow reduction effort. A single speed DLP could run at 50hz (my Davis DLS8 did), but most modern DLPs don't seem capable due to the colour wheel speeds.

    Anyone got anything definitive?

    Gary.
     
  4. Muf

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    Not definitive Gary but here is my understanding. The PJ Spec gives a range of frequencies the PJ will synchronise to, eg. 50 to 85Hz. So the refresh rate will depend on the signal being fed to the PJ. For DVD players and STB's the refresh rate will be a fixed 50Hz for PAL or 60Hz for NTSC. Feeding the PJ from a PC is another story, you can select a multitude of refresh rates including some which your PJ can't handle. I don't have experience of a wide range of PJ's, only the ones I have owned myself and when Kramer says "Usually 60Hz" I believe many PJ's are very limited and perhaps do some kind of conversion for 50Hz input.
    My current PJ is a Toshiba MT8 (InFocus7200) DLP. I have no trouble getting it to sync at 50Hz using Power Strip. BTW the colour wheel runs at a multiple of the refresh rate and you can hear it change speed when you switch from a 50Hz to 60Hz source or vice versa. Paul, whereas on a CRT 50Hz will have bad flicker, on a PJ it is beautiful.
    Here are two simple tests I use.
    1. Get out your camcorder (PAL I assume) Look at your screen CRT mon or PJ through the view finder. With a 50Hz refresh you will see a dark band scroll slowly up or down the screen. i.e. the refresh of the camcorder an pj are close to identical. Now change the pj to 60Hz and you will see a 10Hz flicker in the viewfinder (beat frequency).
    2. If you have a sat card in your PC tune in to CNBC Europe. Most of the time they have a double ticker tape at the bottom of the screen each scrolling at different rates. At 60Hz the slower tape looks a bit jittery but the fast one has a bad case of the jitters. At 50Hz both tapes are beautifully smooth. You could try 75Hz as well, many people use 75 as an alternative to 50 as 50 is not easily achievable. I find 75 is a very poor substitute for 50 and also my colour wheel stepper motor runs slower and quieter at 50Hz.

    Jim
     
  5. LV426

    LV426
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    Or.....to put it another way:

    A CRT screen is illuminated (energised) by a single focussed point beam of electrons that scans across the surface of the screen in a scan pattern. If you consider one point on the screen, it is momentarily energised, and then, immediately, the beam moves on and this one point begins to decay. 1/25 (or 1/30) of a second later, the beam returns to this point and energises it again at whatever brightness is needed for the next frame. So any (or every) point on the screen has a brightness pattern like a sawtooth. Quickly up to required brightness then falling away until the next visit by the beam. For a still image (for example) this sawtooth effect will still occur. Each "peak" on the sawtooth will be the same as the last. But in between, the brightness falls away.

    An LCD pixel is set to a given brightness level and stays there until it is reset to a new level for the next frame. So the illumination of a given point on the screen remains constant for a still image (assuming, of course, that the backlight itself doesn't flicker).

    In a single chip DLP, a DLP pixel is set to a given brightness for a colour (say, red) and stays there for a period of time that is a fraction of the frame rate, depending on the speed and number of segments in the colour wheel. Then, to coincide with movement in the colour wheel, its set to the required brightness for (say) blue. Then green, then back to red again. For a plain white image, the actual brightness of the pixel remains constant, but its colour is changed by the colour wheel.

    All of which shortens to:

    CRTs flicker by their nature.
    LCDs do not flicker by their nature.
    One-chip DLPs flicker, not in brightness, but in colour, by their nature.
     
  6. Gary Lightfoot

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    Muf, Nigel,

    Great info guys, thanks.

    I would have added in my original post (had I remembered) that 24fps is the usual frame rate for movie theaters, but due to the low light levels of reflected light specified for theaters (12ft lamberts with film in the gate), the flicker isn't noticable.

    Nigel - the colour flicker you mention with DLP - I assume that's the mirror flicker/dither used to achieve variations in colour intensity that you're refering to?

    Gary.
     
  7. LV426

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    I was referring to one-chip DLPs. A single DLP chip is only capable, itself, of modulating brightness levels; it can't produce colour. In other words, it is intrinsically a monochrome device. The same is true of an LCD chip.

    In an LCD projector, there are three chips each constantly modulating according to one of the three primary colours. There is one LCD panel dedicated to the red component, one for green and one for blue. The three images thus produced are colour filtered and optically combined into a single, full colour projected output. This output is constant (for an unchanging image).

    In a DLP projector, because (I guess) of the cost of DLP chips, there is only one chip. The image projected by a DLP machine is sequentially All Red, All Blue and All Green (maybe a different order).

    The one chip is sequenially energised with the red component, then green then blue. At this point, the image is still monochrome. To co-incide with this the colour wheel spins in front of the chip to colour the projected image. So, your projector throws, sequentually, the red component, then green, then blue. The theory is that this happens so quickly that your brain merges the three colour components together and you see full colour. It is the constant changing of each of the pixels on the chip, to generate the required intensity for red, then green, then blue that I was referring to.

    So, suppose for example we have an image that is meant to be yellow. This colour is achieved by mixing red and green, but no blue.

    On an LCD, the red chip and green chip are clear and the blue chip is opaque. This is constant. The projector throws yellow light (red and green mixed) constantly.

    On a DLP, the mirrors on the one chip will be reflective (when on the red part of the sequence, and the red filter on the wheel is in front of it); reflective (for green) and then non-reflective for blue; then back to red again. The image thrown is red, then green, then black, then red...... and so on.
     
  8. Paul.M.

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    When watching Prog scan on a projector is the flickering on whites as noticeable as when you see whites on a CRT tv? We just got a denon system/Panasonic 36 inch tv and prog scan is good, but very white scenes, i.e. 'Touching the Void' (brilliant film!) appear to be almost 'moving' as the white flickers so much. Would projectors, ie the ae700 or hs50 cause such bad flickering? Or is it just CRT tvs that have this problem?
     
  9. LV426

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    I think I already answered that question.

    CRTs flicker by their nature.
    LCDs do not flicker by their nature.
    One-chip DLPs flicker, not in brightness, but in colour, by their nature.
     
  10. Gary Lightfoot

    Gary Lightfoot
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    Nige,

    Yeah, I know how a DLP works, I was just making sure of what you meant by flicker - the DMD mirrors coming on and off for each respective colour section of the wheel (except in the case of 3chip DLP of course).

    Paul,

    If it's a digital tv, and you have the sharpness up too high, then sometimes solid colours appear to move, as the processing tries to differentiate each pixel. At least, that's what I've seen on most digital tvs with the processing on and sharpness up too high.

    I think tvs with digital processing tend to add more artefacts to the image than they cure - doing more harm than good.

    That may be what you're seeing, or is it more of a pulsing effect? Take a closer look and also compare it with the interlaced version and see how it compares.

    Gary.
     
  11. KraGorn

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    Why do some (most?) DLPs and LCDs that do accept signals at other than 60hz only do so via analogue inputs while their DVI connectors only allow 60hz? .. I seem to often see comments from people saying they're using component because they can set 48.9xxxxHz refresh but not on DVI .. or am I wrong and confused?
     
  12. Barcoing Mad

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    48fps used in cinemas, not 24 fps. Each fame repeated.
     
  13. Muf

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    KraGorn, I only have experience of my own PJ, a Toshiba MT8 and it has no problem accepting 50Hz through the DVI input. It is a clone of the InFocus 7200 so that is probably 2 to put on the list of 50Hz DVI capable PJs. I am sure their successors, the MT800 and 7205 (updated using HD2+ DMD) are also ok in this regard.

    Jim.
     
  14. KraGorn

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    Maybe I'm wrong then, I was sure my Z2 only allowed 60hz via DVI and my current Sharp seems to, or at least Windows thinks it seems to .. also, even though I select 50Hz using Powerstrip the projector's info display says its' getting 60hz.
     
  15. Muf

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    KraGorn, There is a problem with PowerStrip (even the latest version) whereby it will not give you two different refresh rates for the same resolution but there is a work around. If you look on the PowerStrip forum, I put in a question about this here:
    http://www.entechtaiwan.net/forums/viewtopic.php?t=1104&highlight=
    Notice he promises a better solution in version 4.

    Jim

    PS if you have version 3.42 or newer this is all you need
    "*Update: Beginning with PowerStrip 3.42, a right-click on the copy-to-clipboard speed button in Advanced timing options will give you the option of creating a shortcut on your desktop with the correct timing parameters, so you don't have to mess with the instructions above. (This is the small icon/button immediately to the right of the Apply button.)"
     

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