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Dispelling the Myth - Rainbow vs Screendoor

Discussion in 'Projectors, Screens & Video Processors' started by digisocialist, Oct 1, 2004.

  1. digisocialist

    digisocialist
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    It may be controversial and has been debated many times before but that does not negate the need to rear this again, especially for those like me who are relatively new to these forums. I have read many posts about the Pro’s & Con’s of both LCD & DLP and in general the arguments for either tend to have centered around the quality of darks on DLP vs the Lumens of LCD vs the effect of Rainbow on DLP vs the notorious Screen door of LCD. I am not here to weigh out the finer points of each, however it does occur to me that the notoriety of the Rainbow has put many off DLP as an option and I suspect there are many who don’t even demo DLP because of the notoriety alone. If my understanding is correct, the Rainbow is persistent; the separation of colour is by design and our susceptibility to it or not is purely based on how our brains maintain persistence of vision such that we are either fooled or not into seeing the separation at all. With the introduction of chip improvements, faster colour wheels and the concept of SCR, it appears that the separation of colour is becoming less noticeable simply due to speed i.e. our brains are more easily fooled and evolution has to adapt to get the upper hand (not that we would necessarily want an evolutionary step in this direction).

    What strikes me as important and here is my point, is just how susceptible are we to LCD screen door? The debate often seems bias to me as many suggest that they were put off DLP by Rainbow or steer away because ‘it may be an issue’, yet I have read hundreds of posts from people who complain of screen door. This suggests to me that there is no more of an ‘image’ issue with DLP as there is with LCD. The debate should be turned on its head…. There are people who don’t see Rainbow, and many who do see Screen door. Are we therefore not more susceptible to Screen Door than Rainbow, being as it is we may be less likely to be fooled by light dispersion through the pixels of LCD than more likely to be fooled by colour separation?

    I am not entirely in either camp, as I am not yet a PJ owner. I may however be bias myself as I am trying to convince myself to buy a DLP PJ; I won’t hide that. I may however be dissuaded from going down this route if a convincing argument can be put forward for LCD in the sub £1500 budget, where the general consensus may be that a particular PJ can beat the DLP in Darks and will not give me eye strain by trying to see beyond Screen Door and Dead Pixels.

    I look forward to your expert views.
     
  2. theritz

    theritz
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    I don't necessarily agree with everything in you post - for instance, I would not class Rainbows as being an "equal" artifact compared with screendoor - at the entry level, for instance, LCD projectors with 1280x720 panels do not present a screendoor problem when viewed from 1.5xscreen width (for most people at least), and current reports of the AE700 with smoothscreen generally declare screendoor to be a complete non-issue.

    Rainbows, on the other hand are a more personal issue - some are unaffected, some find the effect diminishes/becomes less a cause for concern over time, for some the effect is extreme, inducing fatigue/headaches/nausea. No amount of seating distance/de-focussing etc appears to remedy the effect in extreme cases. Reducing brightness (by using a Neutral Density filter, for instance, or not having the image concentrated on a small screen) has been reported to be beneficial, but for those most affected, the apearance of rainbows is a complete deal-breaker.

    I don't wish to de-construct your post for the sake of being contrary, but as you finished your post with the following...

    .... I don't believe that this is an issue on which you (or anyone else for that matter) should form a defined view based on a "general concensus" of argument/opinion in a thread here. All digital projector technologies are prone to their own foibles/artifacts to a greater or lesser extent. My recommendation is that you assemble a list of possible contenders in your budget, get on the phone and talk to reputable dealers with demo facilities and arrange to see them in action. Make up your own mind - it's your money, you'll end up watching the thing.

    Sean G.
     
  3. KraGorn

    KraGorn
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    Your description of the rainbow effect is correct, the ability (curse?) to see it is dependent on the individual. Screen-door OTOH is 100% guaranteed to be seen since it's 'built in' to the LCD panels, the ability to discern it descreases with viewing distance to the point it basically disappears, rainbows don't.

    You haven't mentioned a crucial difference though. Of those that see rainbows some actually get eye strain and in worst cases headaches .. whereas SD has no physiological effects, just visual ones.

    Given that both technologies are flawed the basic decision all owners have to make is which PQ imperfections they're prepared to accept, or in extreme cases which ones they can't possibly live with.

    Of course, the best solution is a CRT or 3-panel DLP .. and a bank balance to support one. :D
     
  4. Bristol Pete

    Bristol Pete
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    Isnt the ability to see rainbows something to do with how fast or good ones eye sight is?

    My eye sight has is pretty poor now, in fact I cannot read the LED dispay on my DVD deck from about eight feet unless I have my glasses on. (I am 33 mind you :( )

    Thus, when I have loaned and demoed a DLP machine I did not really suffer from the rainbow anomoly, but I did see it occasionally though not enough to discourage me from going down the DLP route.

    Not sure thought but thats my tuppence worth.

    Cap :)
     
  5. Spacecat

    Spacecat
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    i have worn glasses for years and i can see rainbows on DLP, its more to do with quickness of the eyes i think
     
  6. Paul D

    Paul D
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    Although this subject has been done to death, recent improvements in both camps means it is worth having a new discussion.

    Before going on, i will state i own a 1280x720 HD2+ DLP (5x/6x colour wheel).
    Also note that i have to generalise about both technologies, as there are both good and bad LCDs/DLPs etc.

    With LCDs, screendoor is mainly a resolution versus viewing distance(or screen size) issue. Sit far enough back, and all is well etc.
    Smoothscreen or slightly defocussing gives a worth while improvement if you really need a larger screen or need to sit closer etc.
    Higher resolution LCDs are a much better option, and the pixel gaps are getting slightly better.

    I feel vertical banding(VB) and fixed pattern noise(FPN) are bigger issues.
    Seeing this effect can destroy an image.
    It mainly shows in light, large expanses of the same colour like clouds or sky shots etc.
    It is part of how LCDs are driven, and i have yet to see an LCD that doesn't have it too some degree.
    A funny thing i found was that different signals exaggerated it differently.
    Lower quality signals(like composite/svideo) tended to be the worse, and RGBHV from an HTPC the best.
    Again higher resolutions help here by reducing the size of the bands.
    I have found Sony LCDs seem to have the least VB.
    Fixed pattern noise is harder to explain, but seems like your screen is dirty.
    I is mostly seen during pans and again where there are light expanses of the same colour.(sky/clouds etc)
    I'm not sure of the cause, but it may be a combination of screendoor/panel alignment/uniformity issues.

    Another problem is "native scan rate" of the panel. At least with the LCDs i have seen, only run at one true refresh rate.(56hz/60hz/etc etc)
    Any other refresh rate sent to the PJ is converted internally, leading to tearing and other artifacts.

    These issues need addressing no matter what advancements LCDs make with resolution/variable IRIS systems.

    The above issues aside, LCDs offer tremendous value.
    There is zero rainbow effect, and the images can be vibrant/bright.
    Using the variable IRIS means we can expect much higher on/off contrast in future, finally ridding LCD of the grey mist!
    1920x1080 + IRIS + Reduced pixel gap may just be LCDs saviour.
    LCD can be stunning if you realise it's limitations, and install accordingly.

    Single chip DLP:-
    DLPs still have screendoor but to a lesser degree, meaning there are still some screen size/viewing distance issues. Again the higher resolution models fair better, with the latest HD2+/DC3 being much improved.
    Vertical banding doesn't seem to be an issue at all.
    Fixed pattern noise doesn't seem to be a problem either.
    Rainbow is an issue still effecting some people, with the earlier DLPs being the worse offenders.
    I couldn't even watch a few minutes of video without feeling sick or dizzy.
    People who couldn't see them accused me of being mad!
    Again the latest high end models have made massive strides in minimising this effect.
    As stated i am very sensitive too them, even seeing a similar effect with CRT projectors.(yes even they have colour separation/persistence issues)
    Yet after demoing the Marantz S3, i was shocked to realise that i didn't see any rainbows.
    I have seen the odd rainbow since buying one almost a year ago, but at no point are they frequent or annoying. They seem to be best seen during dark backgrounds with something like a bright candle in the shot.

    Another issue with 1 chip DLP is temporal dithering. During pans, the bit depth has to be dropped due to way the shades of colour are controlled.(Pulse Width Modulation-PWM)
    This can give a fairly ugly artifact called clayface. It looks like peoples faces suddenly lose detail leading to graduations of colour, instead of smooth detailed skin.
    On the subject of dithering, some cheaper DLPs can struggle to give smooth graduations of colour and darker areas appear to be alive with restless dots.
    Higher end DLPs deal with this using a 7th segment on the colour wheel, and the new DC3 panel is supposed to prevent low IRE dithering all together.

    The 1 chip design doesn't have convergence or uniformity issues, and combined with zero vertical banding gives a pristine image.
    The picture is also razor sharp, and when connected digitally can be mind blowing.
    Contrast levels are already very good(ansi and on-off), and it will be interesting to see what variable IRIS adds to that. (almost salivating!)
    A plus point is also DLPs ability to adjust the colour wheel to suit the sources refresh rate, without any frame rate conversion.
    This allows 50hz/60hz etc to be driven without any tearing or other artifacts. Some models even allow 48hz(47.952hz) to perfectly match films native rate.
    I forgive the odd rainbow/clayface as they really are infrequent due to the rest of the image being so good.
    I never really settled with LCD (or CRT for that matter) due to some of the above mentioned issues.
    If rainbows are still an issue even with the latest models, then maybe a 3 chip DLP is your only option.(assuming your are feeling very rich)

    I feel it is time 3 chip DLP prices started falling, and the resolution raising(1920x1080).
    The reason?
    SXRD, or LCOS at the same price range (with higher resolution) etc
    But that's another thread!!! :smashin:
     
  7. hornydragon

    hornydragon
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    12 months ago a sub £1000 DLP was laughable....................... Oh how we learn forget the technology be it LCOS, CRT, DLP, LCD, UDP, ASS etc sort out budget and demo demo demo demo demo demo demo demo demo demo demo if it looks bloody incredible its probably the mchine for you!
     
  8. UrbanT

    UrbanT
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    I think KraGorn summarised this perfectly above. You have to accept that both technologies come with their problems, but only DLP can have a detrimental effect on the watcher.

    Personally, I do suffer rainbows badly, with a headache appearing normally within 10 minutes of the start of the film. Yes, I can see screendoor and VB, but fortunately the TX100 keeps these to an absolute minimum (IMO).

    Even if I couldn't see rainbows, I would be wary of buying DLP as my wife or daughter may suffer. Or my mates that come and watch movies etc.

    The £1000 LCD will keep me going nicely while new and better technology comes down in price :)
     

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