Answer to "What is the Colour Wheel/Rainbow artficact"! - AVS Forum
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post #1 of 10 Old 04-27-2001, 06:54 AM - Thread Starter
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Just thought I would post a response I had in another thread so any newbie can find it quickly:
EDIT--> There does exist two and three chip DLPS. Two chip still uses a colour wheel for 2 of the 3 colours, and hence must still exhibit some rainbow (or YM flashing http://www.avsforum.com/ubb/smile.gif) artifacts. 3 chip DLP does not have a colour wheel, so no rainbow, and better colours. However, they are big $$$, for the time being.

(From another thread, regarding a forum member who asked why he sees rainbows with his projector http://www.avsforum.com/ubb/smile.gif
Ok, before you get told to do a search, I will try to help you here:
The DLP you own is a single chip DLP. That means to produce an image, which LCD and DILA do using three panels (of different colour), the DLP must use one chip, and flash the colours on it at separate times, instead of making one image from the combination of three panels. To accomplish this, singlechip DLP uses a colour wheel, which flashes the colours on the chip one after another. The chip is sync'd with the wheel, so the mirrors are in the correct position when they get their dose of colour. Some colour wheels may use a clear section to boast the light output.

This is what the colour wheel does. The rainbow effect you see is what can simply be called an imprint on your eyes of the flashing of the colour wheel. That is, when you move your eyes quickly to a different location, you are getting glimpses of how the image is actually made up: of three separate, sequenced images. You normally won't see it if you don't move your eyes or head.

A new thing that is going on is to speed up the colour wheel, either by making it faster or adding more segments of colour, to the point where it is changing the colours faster than your eye differentiate and imprint.
Think of a strobe light. The faster it strobes, the less you can tell the difference between it being on and off. This is how film works, by flashing images faster than the eye can tell.

Hopefully, this was usefull, from all the knowledge I have aquired at my time here in the forum.



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[This message has been edited by David Mendicino (edited 04-28-2001).]

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post #2 of 10 Old 04-27-2001, 08:04 AM
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"the DLP must use one chip"

JUST so some newbies don't get the wrong idea. There ARE DLP technologies that use 2 or 3 DMD chips.

3-chip DLP projectors are very expensive, however, for a number of reasons. One of them is that they just don't make low-end 3-chip DLP projectors.

I personally don't like the name "rainbow." Rainbow usually means ROYGB?P. I like to call it RGB color-flashing for 1-chip and YM color-flashing for 2-chip.
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post #3 of 10 Old 04-27-2001, 08:28 PM
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Well, it's my understanding the 6 segment wheel is a reality and is scheduled to find its way into new products very shortly. Someone please correct me if I'm wrong!

Dan

My HT is an oldie but goodie!
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post #4 of 10 Old 04-27-2001, 09:15 PM
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Only a limited dataset here, but 'RGB color flashing' is not always a big deal. For 2 Fridays in a row, I have watched DVD at work with 40 or so co-workers and NOBODY has commented on or seen this effect (I asked).

Projector was an Infocus 425z driven by a PC, once with a G400 and once with a Radeon VE - both set to 800x600x32 at 72hz.

I've looked hard, and thankfully, seem to be immune.

[This message has been edited by JasG (edited 04-28-2001).]

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post #5 of 10 Old 04-27-2001, 09:31 PM
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I am a firm believer that you get nothing for nothing. I have always been wondering why the 6 segment wheel was not implemented in the newer HT models (Seleco and Dwin). Mmm. Let me think! There must be some kind of trade off. I could be wrong, but don't you think that increasing the rotation speed of the wheel has an impact on the actual DLP chip? I mean, the faster the wheel spin, the faster the mirror (mechanical) transitions, right? This could dramatically reduce the chip (and the projector) life. I am unfortunately very sensitive to the "rainbow" effect, but I can hardly see it with my new Seleco HT250, and would rather live with it...

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post #6 of 10 Old 04-28-2001, 08:22 PM
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Film itself gives me a hard time (under the best of conditions) when it comes to the 'willing suspension of disbelief'. So, to say that I can see the color wheel artifacts would be an understatement. Maybe that's because I prefer my reality straight up....

I cannot even move my eyes at all without seeing it. 1 chip DLP projectors ar (so far, in my experience) strictly out of the question for any type of motional video.

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post #7 of 10 Old 04-28-2001, 08:24 PM - Thread Starter
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KBK,
That is what I think for now, too. Hopefully, the new 6 segment colour wheel will eliminate this problem.



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post #8 of 10 Old 04-28-2001, 09:31 PM
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When i modified the 1 chip unit that I had, it helped noticably, but it was not eliminated. intrestingly, light ouput dropped enough to require a fully darkened room, and small screens. The it was nearly OK looking. At that point,a ECP4000 CRT projector looked better, for less money. The unit in question was rated originally at 700 lumens. It was the Nview 705z DLP projector. The ugliest unit I have ever seen.

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post #9 of 10 Old 04-28-2001, 09:55 PM - Thread Starter
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Ferret Lover:

The first qualification I said was that "the DLP you own is a single chip DLP" This entire post was in this context. Just clarifying. My bad to assume that all would know the difference. I will edit it...

Oh, don't you think rainbow sounds better than RGB flashing? We all know what it means..

JasG:

I believe it is the fact you are using HTPC, at 72hz, that is causing the reduction in rainbow. What is the regular frequency for NTSC, 60hz? Well, either way, the higher frequency of the HTPC causes the colour wheel to go farther.

Thanks


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post #10 of 10 Old 04-30-2001, 11:15 AM
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The Panasonic, Hitachi and Mitsubishi 1-chip DLP RPTVs have 6-segment colorwheels that spin at 2.5-3 times the VSync rate (150-180Hz) for an effective 5-6 times the VSync rate. For a quick test of the future, find one at a local dealer and see if the PWM artifact (rainbow) is still visible to you. http://www.avsforum.com/ubb/smile.gif

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