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Please explain the rainbow effect to me (DLP)

Discussion in 'TVs' started by natal, Sep 30, 2004.

  1. natal

    natal
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    Hi guys !

    Can someone explain to me what the so called "rainbow effect" is on DLP RPTV's .

    Im a bit intrested in the LG RE-44SZ21RD, I've read that some ppl have problems with the rainbow effect.

    Since I've have my share of problems with the Sony KP44PX3, I dont wanna exchange it for another problem.

    cheers

    /natal
     
  2. red16v

    red16v
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    In laymans terms. A normal tv picture is made up of red, green and blue (RGB). The pictures made up by these three signals are presented to your eyes, on a crt, at the same time. With DLP technology (in broad terms) the RGB pictures are presented to your eyes one after the other, ie NOT at the same time. However, to fool your eyes into thinking its seeing the RGB pictures all at the same time the individual R,G,B pictures are shown very very quickly in succession one after the other. This can lead to a strange effect called ' the rainbow effect' if an object moves very quickly across the screen - the problem is most prevalent when the object moves so fast that as the individual R,G,B pictures are presented to your eye the object appears in a different position on what would otherwise be identical R,G,B pictures. A practical example - visualise a man facing you standing in the centre of the picture. He is waving his hand (as though he was waving goodbye to you !) very quickly. Look at his hand - you MAY see a rainbow effect around his hand - this is because his hand is moving very quickly in relation to the R,G,B pictures being presented to your eye. NOT everyone will see this effect, some MAY see the effect - there is no consensus and it is entirely subjective. So, if you see a tv or PJ using DLP technology its absolutlely essential that you get all the members of the viewing family to see it first before making a firm decision - and, please don't castigate anyone who says they can see it if you insist you can't - it's subjective. regards, yt.
     
  3. ruhe

    ruhe
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    To try out your rainbow sensitivity, Gladiator Chapter 24 is very good. The rainbows should appear in the fires. When you go rainbow testing a DLP TV or a projector, the question is not only "Do I see rainbows?" but also "Am I disturbed by them?". I have seen posts from people who see rainbows, but aren't really bothered by them. Others get sick. Myself, I am sensitive, i.e. see them a lot. They also linger for me in the sense that disturbances remain in my field of vision, as if I had looked into a bright light. This is not acceptable.

    This is why I will go out to do some extensive rainbow testing once the Samsungs DLPs come out in the stores. Hopefully I will find a store where serious testing can take place, i.e. sitting for a long time watching, and in a fairly dark environment.
     
  4. whipitup

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    So the rainbow effect isnt the colour grading i see when the tv displys large area's of solid colour, for example: a painted wall in a house, with the shadow of a door passing over part of it, as the shadow gets deeper, you would expact to see a gradual change in colour, what I see are bands of ever deepening colour, or a sunset, rather than a gradual chage of colour moving away from the sun I see this banding effect..........................................................or is this a problem with the TV :rolleyes: :rolleyes:

    I must admit, Im not as happy with this TV as I hought I was going to be :mad:

    Comments please
     
  5. LV426

    LV426
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    whipitup: The effect I think you are describing is referred to as solarisation. (Why, I don't know). It arises when a given display is unable to reproduce all of the possible subtle variations in colour/shade. So a part of the picture that is colour A ends up being displayed exactly the same as the next part which is meant to be a very very slightly different colour B. Only when it gets to a more significantly different colour C does it actually change. As colour graduations in nature occur in all sorts of ways and shapes, the end result is curved shapes which are where A (or B) changes to C.

    The DLP rainbow effect is something completely different (as described).
     
  6. daddyo

    daddyo
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    After that explanation I think i've been seeing some solarisation rather than what I first though was a rainbow effect with my sagem axium.

    I notice it mainly with skin tones. For example, a close up of a face that moves across the screen can show some banding (i.e solarisation?) which disappears when the motion stops. Can't say i've noticed it on anything else - just skin!

    some channels don't have it. i.e. i've never seen it on DVD, don't notice it on feature films which I watch on freeview, I just notice it on regular TV soaps, dramas etc (again on freeview) where it is fairly obvious for most of the time!

    A bit strange really. It’s as if the sagem is having trouble with only certain types of signal.
     
  7. JohnMulcahy

    JohnMulcahy
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    Daddyo: There can be several reasons for this. DLP, in common with other fixed pixel display technologies like LCD and plasma, uses a combination of spatial and temporal dither patterns to extend the dynamic range of the basic technology being used. With moving objects the dithering that can be applied is reduced compared to stationary objects and the range of shades that can be generated is reduced correspondingly. Rapid motion is also problematic for the MPEG compression used in DTV and, depending on the bandwidth that has been allocated to the signal (which can vary at different times of day), some channels will suffer more than others. To add yet another variable, interlaced material can be more difficult to handle than material that was originally progressive (such as film sources) as the image content is changing twice as fast. On top of all that, the resolution (typically 8 bit vs 10 bit) of the underlying processing applied by the display engine can be a limiting factor.

    Regarding rainbows, the problem is not really with the motion of objects in the image. The red/green/blue images that make up what you see are all produced from the same frame of the material and are located in exactly the same place on the screen regardless of the speed of the object. The effect occurs when your eyeball is moving fast enough relative to a bright (typically white) part of the image that the red/green/blue components of the image fall on different parts of your retina. To see rainbows most easily, put up an image with very high contrast (e.g. a white grid on a black background), get very close to the set and look rapidly from one part of the screen to another. The further you are from the screen, the lower the contrast in the image and the less fast motion there is for your eyes to follow, the less likely you are to see this effect. There is also some physiological variation between individuals, and the worse your eyesight the less you are likely to notice as the images on your retina are more diffused. Personally I think the whole issue is vastly overstated and most people will rarely if ever notice this effect.
     
  8. LV426

    LV426
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    I'm sure you are right. However, speaking as one who is sensitive to them, the important point to make is that, unlike issues such as less than perfect geometry and screendoor effects, DLP rainbows can actually be physiologically uncomfortable to an affected viewer, even to the extent of nausea. Hence the need for any prospective purchaser to be pretty sure of themself (and of any others who will share their cinema with them) before adopting this technology. Roll on affordable 3-chip DLPs.
     
  9. daddyo

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    Thanks for great explanation on solarisation John especially the intermittent nature of the effect. This is exactly what i'm experiencing.

    I assume feature films, live TV etc are broadcast at a higher bandwidth when compared to the regular TV as I haven't detected solarisation with these programmes.
     
  10. Pavel

    Pavel
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    The best way of provoking rainbows that I know of is the aspect ratio test (white circle on black background) on DVDs with the THX setup routines, e.g. some Terminator and Star Wars DVDs). It's a white circle on black background. It's rainbow hell for me...if you don't see them with that you're probably safe.

    Luckily, even though I see them in normal viewing they don't bother me that much, and they always appear where there is white against a black background. They tend to be worse with high brightness and contrast settings and (strangely) when I'm tired.

    On John's point re. eyesight, be careful about this. I'm very short-sighted, but when wearing glasses I'm told by my optician that my eyesight is virtually perfect. It's the latter that seems to count. Without my glasses I can't even see the screen
    :D so rainbows are no problem!
     
  11. Mad Mikeyboy

    Mad Mikeyboy
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    I had never noticed any rainbow effects on my sagem till I played the set up screen on pirates of the carribean dvd. Like you described the white circle on black background. I only see it when I flick my eyes across the screen really quickly. For most, I think you only see them when your looking for them.

    Don't go looking!!
     

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