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DLP Rainbows and PJ resolution - a link?

Discussion in 'Projectors, Screens & Video Processors' started by nelliekins, Dec 16, 2004.

  1. nelliekins

    nelliekins
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    Hi all.

    I've been giving the matter of rainbows some thought recently (now demo'd a number of DLP PJs with a variety of colour wheels and resolutions), and I have a question for you wise people.

    Is there any connection between the native resolution, and the effect of the rainbows seen? It seems to make sense to me that the physical size of the rainbow has to be related to not only the color wheel, but the precision of the light being reflected off of the DMD as well.

    So, given

    1) an 854x480 panel and a 1280x720 panel
    2) the same type/speed of colour wheel
    3) the same size of output picture

    then I would expect significantly lower rainbow effect on the higher res PJ. This is because the total illuminated area for each pixel (given the same output picture size) is proportionally smaller, which should in turn reduce the area affected by rainbows.

    This seems to be backed up by a number of the threads reporting upgrades from X1/4800 to XGA panel PJs with 2x colour wheels (including Optoma 739, BenQ 6200, etc).

    Having said all that, I have not yet bought my first PJ, and I know that most of you reading this have a LOT more experience & knowledge about the matter than me.

    I would welcome your comments (even if they contradict me completely!).

    Neil.
     
  2. KraGorn

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    I don't see any connection :) .. a higher resolution will simply mean that more pixels are used to render any particular image component, the shiny edge of the sword in front of the dark background which is likely to cause the rainbow effect in someone susceptible to them. However, if there are enough pixels to render something then the image they produce will have the potential to cause a rainbow, the number of pixels making up that part of the image I don't see being relevant.

    Lumen output per unit area will be similar for similarly bright projectors, lower resolutions will have brighter pixels.

    The only issue I think where size would play a part is screen size .. a smaller image will have smaller highlights to cause rainbows and so they'll arguably be less noticeable, but for a given screen size I can't see that pixel packing density will affect things.
     
  3. Gary Lightfoot

    Gary Lightfoot
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    It's colour wheel related - you'll find modern 800 x 600 multi-speed pjs have less rainbows than the older 800 x 600 pjs that were single speed.

    Gary.
     
  4. LV426

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    I can't see how there'd be any connection. Regardless of how many mirrors (pixels) they are all (in effect) sequentially red, then green then blue (or whatever the sequence is). The effect is cause by the entire projected image being (say) red, then blue, then green; not by the behaviour of or quantity of mirrors.

    Or, to put it another way......

    Suppose you just shone the lamp through the colour wheel at a screen. No DMD device, just a lamp and colour wheel.

    Suppose you took a large black cloth with a straight vertical edge, and quickly revealed the screen by drawing the edge across it. And followed the edge with your eye. You'd see the rainbow.
     
  5. nelliekins

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    Hi KraGorn,

    So, DLP works by redrawing the entire image in red, then green, then blue - correct?

    And the reason that we only see rainbows along the edges is presumably because objects producing rainbows are > 1 pixel "thick", and therefore the colour for the other pixels are effectively overlapping where the colour deficiency occurs in interim pixels due to the rainbow effect?

    I think my understanding of DLP is fundamentally flawed... Let me draw a (really poor!) picture of what I _think_ is happening, using your sword edge as an example.

    Assume the "edge" consists of 10 white pixels (measuring in the direction of the motion).

    Source Line 1 of the sword edge is white. Therefore Output Line 1 of your sword edge _should_ be white, but the motion (which is the cause of the rainbow) means it produces the rainbow effect, meaning that what is output is:

    Output Line 1: RRRRRR
    Output Line 2: GGGGGG
    Output Line 3: BBBBBB

    However, this isnt _seen_, because motion is causing the same thing to happen to _source_ line 2, meaning that it effectively fills in the Red component on the _output_ line 2, giving R+G for output line 2, and G+B for output line 3. And of course, the 3rd _source_ line does exactly the same thing, i.e. filling in from _output_ line 3. Which gives

    Output Line 1: RRRRRR
    Output Line 2: YYYYYY (R+G=Y)
    Output Line 3: WWWWW (R+G+B=W)

    and of course you get the same sort of thing along the trailing edge as well, except it would be:

    Output Line 10: WWWWW
    Output Line 11: CCCCCC (G+B=C)
    Output Line 12: BBBBBB

    Is this right?

    If so, then the size of the rainbow _is_ directly proportional to the pixel density.

    In which case, the way I must be wrong is in assuming that the light spill between R, G, and B, is related to the pixels at all, when it is purely a function of colour wheel, contrast and lumens...

    ...Let me guess - I just answered my own question! :suicide:

    Of course if it _is_ just CW, contrast and lumens, how come nobody has devised the _formula_ for rainbow intensity? One for the moderators, methinks! :rotfl:

    Edit:

    Hi Nigel,

    I think I follow what you are saying - thanks!

    Gary,

    Surely more than just colour wheel is involved - otherwise how come everyone and their Granny keep asking "how good/bad are the rainbows" every time someone says they have a new model PJ? If it was _just_ the colour wheel, there'd be no point!

    Seasonal wishes to all,

    Neil.
     
  6. nelliekins

    nelliekins
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    Hi all,

    This is probably another _really_ silly question...

    ... But if a framebuffer stores the image information for the duration of the frame being drawn for RG+B colour components, how come you get rainbows at all? Are you saying the colour wheel is misaligned? Certainly the wheel spins fast enough to draw every frame at least twice per frame of input video signal (NTSC = 60Hz, PAL=50Hz, 2x colour wheel = 120Hz? 240Hz? Never can remember!) Given that, why is there even a need to see rainbows? If the framebuffer is used to "set" the DMD, its the same picture information for each colour...and I don't see how it can go wrong...

    Regards,

    Neil.
     
  7. Nick_UK

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    You are forgetting one fundamental fact - one rev of the colour wheel does not equal 50 or 60 Hz, because the colour wheel is showing 3 or more colours per revolution. Even if the colour wheel was spinning at 2 x 50Hz, you would still see the effect, because the refresh rate would be 100/3 or 33 Hz. The problem is not with the technology, the problem is down to persistance of vision, and this is why some people are affected more than others, because persistance of vision varies from person to person.
     
  8. LV426

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    Let's try this again.......

    The entire image projected onto the screen is completely red (for a fraction of a second). Then it's entirely green, then entirely blue, each for a fraction of a second.

    The projector never projects a full colour image, or more than one of the three primary colours at any one moment in time.

    The fraction of a second is refresh rate x actual wheel speed x number of segments.

    You percieve a full colour image because your brain merges the sequence of colours together.

    You see rainbows when something happens which allows your brain to perceive what's really on screen. Such as - scanning your eye across the screen (eg when following the trailing edge of a moving object). How well you percieve it depends on your own visual acuity - which fully explains why some people have no concept of how nausaeating DLP rainbows can be.

    It's nothing to do with the projector adjustment - it's a fundamental artefact of the sequential display of colours. There's absolutely nothing the can be done to a single-chip device to prevent it - it can only be sped up to a point where few people will ever notice it.
     
  9. Gary Lightfoot

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    Hi Neil,

    It is more or less just colour wheel - some people see rainbows on single speed colour wheel models (3 segements - R, G and B), but not on models with double segment (RGBRGB) and faster rotation (4 x RGBRGB). The Davis DLS8 was single speed 800 x 600 and the new Optoma H30 has a faster colour wheel - (4 x RGBRGB) and less or no rainbows for many of those that could see them before.

    Not everybody sees rainbows, and those that do will ask about them - if they're after a new pj and want to buy a DLP but are concerned about rainbows, they simply ask people if rainbows are visible. I never see them so cannot give them an answer, but you will find those that used to see them on older models no longer see them on some newer ones.

    If it was resolution based, there would be no rainbows on 1280 x 720 pjs, but some still see them.

    3 chip DLP no longer have rainbows because they don't have a colour wheel. Like Nigel said, you don't need anything other than a piece of card and a rotating wheel with a lamp behind it to see the rainbows. The same trick won't work with a 3 chip DLP though.

    edit - a framebuffer can't be used to store an image because the way the image is made still relies on the rotating wheel. You can't store mirror activity and RGB images unless you had a phosphore screen maybe. :)

    A tv makes up an image in a similar fashion, but because the tv has phosphor on the inside of the screen, the image remains on the screen for a short while while the image is being built up, so instead of seeing a line scanning across the tube, you see just the image. That, combined with your eyes persistance of vision is why you see the complete image and not an array of fast moving lines.

    Gary.
     
  10. nelliekins

    nelliekins
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    Hi everyone,

    Thanks for all your responses - it's really boosted my understanding of DLP. Makes you wonder why the manufacturers of PJs dont _all_ just use 4/6x color-wheels, doesn't it! I suppose its down to meeting a price point on the DMD (which has to change mirror state fast enough to keep up with the color wheel), and associated components.

    Sounds like you've just come up with the ultimate solution for the several million DLP owners with 1/2x colour wheels! Something that acts like phosphor that can be used to coat a screen. I've heard about rear-silvered mirror materials, with partial opacity on front surface, but never anything like using fluorescent coatings!!! :thumbsup:

    Neil.
     
  11. Gary Lightfoot

    Gary Lightfoot
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    This might help you visualise how it actualy works on a physical level:

    http://www.digitalhomemag.com/featu...d=28911&subsectionid=1300&subsubsectionid=935

    The pic only shows a single RGB wheel, but imagine two segments of each colour and that will represent the later models. Some have 7 or 8 segments which can reduce 'dithering' (mirror activity) in darker areas, but that's another story. :)

    The one drawback I think multi-segment colour wheels may have is increased possibility of colour banding - the less time the mirrors have to operate for each segment, the reduced range we may have in colour graduation. Some pjs with 8 segments seem more prone to banding than those with just 6 segments.

    Gary.
     
  12. Tempest

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    So you need to get 3 DLP chips and get rid of all the wheel mechanism.

    Makes you wonder dosen't it.

    Chips (etc) which DLP are generally not that expensive (In Real Terms) against physical moving/spinning things.

    3 colour filters that don't move, a few mirrors and 3 dlp chips.

    Why don't they stop ******ing about, get rid of the dam stupid wheels and just make them all this way?
     
  13. LV426

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    Hear hear. Well, actually SIM2 have. But they want GBP25k or so for it. I don't think so.....
     
  14. Tempest

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    Having read some thoughts on other forums on the very issue, it appears to be a common thought that the Single Chip with spinning wheel at a low consumer price and the triple chip at a astounding PRO price is mainly down to marketing and nothing to do with actual real manufacturing costs.

    They could (and still could) produce low cost 3 chip dlp projectors but don't want to as they want to have 2 seperate products for two different markets.

    If you make cheap ones the same you would destroy your high models.

    Just think how much a DLP chip does cost?

    $10 perhaps ?

    Who knows.

    So you could buy three whole single chip DLP machines, and be no where near the cost of one triple chip machine.

    I guess it's the trouble when things are controller too much by some people at the top.

    Ahhh, brings back memories of the old price fixing on RAM chips before it was blown apart. They held us all back for years on that one.
     
  15. mi_july

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    Ok, i have a question, if you had 3projectors, all the same make, same spec etc. and each one displayed a different colour projector 1- RED
    projector 2- GREEN
    projector 3- BLUE.
    would you still get rainbows, obviously you would also have to get them all lined up correctly but still would this eliminate the rainbow effect?
     
  16. Tempest

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    Well, I guess if you fed them all from the same source, then (perhaps) it could work, but you would be overlaying the colour on top of each other.

    In essence that's what a 3-chip DLP projector does.

    Although it's probably about the price of 6 single chip projectors.

    Go figure the logic behind that ! :mad:
     
  17. Gary Lightfoot

    Gary Lightfoot
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    Interesting idea.

    Off the top of my head without much thought:

    You could probably get them all to line up the image on the screen, but you'll have to get the colour wheels sinc'd so that each one is producing each colour at the correct time so all three colours appear at once. That would be the tricky (if not impossible) part.

    If you blacked out pj 1s red and green so it was showing just blue, pj 2 green and blue for red etc so they were each effectively just producing each colour, then I don't think you'd have to worry about them syncing.

    What might work better, would be to feed each pj with just one colour from the source, so they were all dedicated for each colour similar to the above. Not sure how the colour wheel syncing with the others would work though, i.e still see some form of rainbow. Anyone with 3 identical projectors wanna try it? :)

    Anyone else wanna debunk those ideas or come up with some better ones?

    Gary
     
  18. LV426

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    Lose the colour wheels. Fit each PJ with a filter; one red, one blue, one green. Drive each PJ with just one colour signal. QED. A 3-chip (3-lens!) DLP and no rainbows.
     
  19. MikeRJ

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    I don't, but I would expect it to cost significantly more than that, perhaps >$100. A 1280x720 has almost a million mirrors. It only takes a few of them is be faulty and the chip is scrap, so yields are almost certainly not very high (just as yields for high resolution LCD panels keeps the costs high).

    A colour wheel and the ascociated motor and synchronising circuitry MUST be significantly cheaper than two more DLP chips and the required optics to go with them or projectors using them simply wouldn't exist.

    Don't forget that DLP is still a relatively new and developing technology. Onec the production processes have been refined in order to improve yield and reduce production costs, expect to see 3 chip projectors being sold at affordable prices.
     
  20. Gary Lightfoot

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    Sounds promising!

    What about the stray light in each pj when the other colours aren't being generated though?

    Gary.
     
  21. GLADIATOR

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    Tempest

    You make some very valid points.

    I would like to add, if the magazines and people on the AV forums could be a little more honest about the rainbow effect, instead saying "only a few people suffer" and "some people can get used to them" then the DLP manufacturers might have a greater reason to produced reasonably priced 3 chippers. Lets hope the LCD mkt can continue to develop with machines liek the Sony HS50 and give the single chip DLP mkt a real run for its money, bringing the day when we can all afford 3 chippers a day closer.
     
  22. Gary Lightfoot

    Gary Lightfoot
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    I think people are being honest when saying that only as few people seem to suffer. Why should the general home cinema public on a forum lie? After all, they are consumers like you and me with no axe to grind or product to sell. I think all those on the US avs >$3500 forum are nearly all DLP owners since most LCDs fall below that price point. There are a hell of a lot of posters there too. A lot of the <$3500 are DLP owners as well, so it can't be that large a percentage.

    It is difficult to put an exact percentage to it though. In my case, I have never had any friends or family mention rainbows or headaches, even after being asked about them. I'm sure there are those who have had the opposite experience though.

    Gary.
     
  23. GLADIATOR

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    Gary

    I take your point but I didn't mean to imply that people lie. But I have seen numerous posts from people who say they are not bothered by the rainbows or they have trained themselves not to see them. I am sure not too many people would buy a DLP pj if they saw rainbows so comments from DLP owners who don't see them doesn't tell us much. I wonder how many people go to test a DLP pj, who see rainbows are a little put off then jack the whole pj idea in and go and buy a large LCD or plasma telly instead. We are not going to see their comments in the AV pj forums.

    Would be great for a Home Cinema mag to run a test, sample 100 people at random and find out how many suffer in any way.
     
  24. Gary Lightfoot

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    Numerous polls have been posted both here and on the US forum and the results can vary from 2% to somewhat greater. It seems to be inconclusive to an exact percentile, but lets say that tests may show that 25% of people see rainbows to one degree or another, but that still doesn't tell us if a specific individual will see them - you may be one of the 25% but you won't know until you demo one (or two). That's why buying blind is never a good idea IMHO.

    Gary.
     
  25. martian1

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    I have not got a problem with LCD its lovely...In my camera, pc monitor amd the boys game m/c [gameboy i think]. This is its place and a very good job it does, nice bright cartoon image.

    DLP for cinema......................If you cannot afford it then just winge about rainbows and buy a dust blob collector, with knackered pixels and dying panels.... :eek: :rotfl: Oh... just fu..ing with you guys.. :rotfl:
     
  26. Nick_UK

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    OK in theory. The reason why 3-chip DLP's are so expensive isn't really much to do with the price of the chips, it's more to do with the price of the optics. A single-chip DLP with a cheap-and-cheerful lens will give a picture which is (geometrically) pretty acceptable. But try to overlay three images over the top of each other, and you begin to see where the flaws in cheap optics are.
     
  27. Gary Lightfoot

    Gary Lightfoot
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    But isn't this how LCDs work?

    They use three panels in a similar fashion as well don't they?

    Gary.
     
  28. LV426

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    Exactly. I fail to see how the optics (and we're all agreed, that's the only thing missing from 3 x one-chip machines) can possibly add up to a retail price differential of - what - GBP10k or so?
     
  29. Mr.D

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    If you take 3 single chip dlp projectors and overcome the registration issues (which wouldn't be minor) and get them all outputting an individual colour channel (which would be quite easy I suspect) . You are still effectively going to have x3 residual light levels from x3 lamps .

    Upshot is you will end up with black levels which are probably no better than a fairly rubbish lcd.

    Now you could turn the lamps down a suitable amount for each colour record by adding resistors in place (if the lamp still functions with the drop in voltage , some likely just fail to energise when you drop the voltage below a certain point)
     

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