Understanding the rainbow effect, color wheel speeds, etc. - AVS Forum | Home Theater Discussions And Reviews
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post #1 of 13 Old 01-20-2016, 12:26 PM - Thread Starter
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Understanding the rainbow effect, color wheel speeds, etc.

I´ve recently discovered that I´m heavily affected by the rainbow effect 'RBE' produced by DLP projectors. I don´t notice it that much due to the motion in the video itself but when my eyes move rapidly across the image. This is quite annoying when watching movies with subtitles, as your eyes are constantly moving up and down and the subtitles have usually high contrast, which increases the chances of perceiving the RBE.

So while trying to figure out which DLP projectors, if any, could be better, and how to improve my experience, I have not been able to find an objective measure of how much RBE a projector may produce. That´s understandable, given that RBE is a subjective issue and many people can´t perceive. However, it seems that manufacturers advertise the projectors as having 4x or 6x speed color wheel, or having a RGBRGB wheel instead of RGB one, etc. But what does that exactly mean? Is a 6x speed wheel from two different manufacturers running at the same speed? How does DLP work with LED projectors, which don´t have colour wheel, because several people have reported to have perceived rainbow artifacts.

If my understanding is correct, the underlying reason for the RBE is the fact that with DLP technology each pixel´s R,G,B channels are not displayed simultaneously but sequentially. So the number of segments in the color wheel and the wheel speed directly affects the refresh rate of each colour channel per frame. So, in my opinion, what manufacturers should explain is how many times each channel is refreshed per frame.

In order to try to find out a little bit more I did a quick test, recording the test pattern of a BenQ W1110 with a 120fps camera. When moving the camera abruptly from side to side the same RBE that I see when I move my eyes across the image is clearly perceptible. Here is the video, there are a few good examples from around second 0:49 (playing the video at reduced speed helps):

Now, looking at the attached snapshot from the video, we can clearly see how the vertical white lines are decomposed in their RGB components when moving the camera from side to side. In the snapshot there are clearly two groups of RGB components per line in the pattern. Since the video is recorded at 120fps and each colour channel is refreshed twice per frame (from what we can see in the picture) we have a channel refresh rate of 240Hz. From the BenQ W1110 specs I got that with a 60Hz video mode (which is what is what set) the colour wheel runs at 4X. So if we consider that 4X speed means that the pixel colours are refreshed 4 times per frame I guess everything makes sense as 60Hz video and 4x pixel refresh per frame gives us the 240Hz pixel/colour channel refresh rate that I have estimated.

What do you think? Does any of the above make sense?

If so, I wonder why manufacturers won´t just tell use the RGB channels refresh rate. Now I know that I can´t deal with the 240Hz colour refresh frequency of the BenQ but there is now way to find out what are the figures for other manufacturers/models. For example I would like to know whats the colour channel refresh rate in a LED DLP projector such as the LG PF1500 but there is no way to find out (I asked the technical service, but from their answer I don´t think that whoever answered me even knows what RBE is).
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post #2 of 13 Old 01-20-2016, 02:00 PM
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The color separation occurs because a single chip exists in the DLP projector and the color wheel passes in front of the light source and the chip to get the colors.

I think the video you shot is excellent and a great representation of RBE breakup.

LED DLP doesn't have a color wheel, but flashes the LEDs one after the other to get RGB colors to the DLP chip. The effect to me seems like a good 6x color wheel. So, maybe an equivalent would be 7x or 8x, but it is still visible if you are looking for it.

Of course, single chip DLP can't show 3 channels at once with a single chip setup. 3-chip DLP and 3-chip LCD don't have RBE issues because they show all three colors at once instead of one color at a time like single chip DLP.

Why don't they tell us? MARKETING! There are a ton of 2x color wheel DLP projectors out there and they are inferior to 4x and 6x models. If you knew what you were getting, you might not buy it.

I've asked Texas Instruments this question directly. Why they don't require manufacturers to let consumers know what they are getting. TI basically said they discourage publication of specifications because it makes faster color wheels seem like they are better. Which, of course, they are.

2x color wheels are the imported GMO of projectors. They don't want to say what they are on the package and they don't want anyone better saying so either.
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post #3 of 13 Old 01-20-2016, 03:22 PM
 
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TI just needs to make 3 chip DLP affordable already. I would be interested. Single chip DLP is a non-starter for me due to RBE issues.
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post #4 of 13 Old 01-20-2016, 06:08 PM
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Quote:
Originally Posted by vvaldes View Post

In the snapshot there are clearly two groups of RGB components per line in the pattern. Since the video is recorded at 120fps and each colour channel is refreshed twice per frame (from what we can see in the picture) we have a channel refresh rate of 240Hz. From the BenQ W1110 specs I got that with a 60Hz video mode (which is what is what set) the colour wheel runs at 4X. So if we consider that 4X speed means that the pixel colours are refreshed 4 times per frame I guess everything makes sense as 60Hz video and 4x pixel refresh per frame gives us the 240Hz pixel/colour channel refresh rate that I have estimated.

....

I wonder why manufacturers won´t just tell use the RGB channels refresh rate.
Agreed: this should be a mandatory published spec since to people like myself and you, this matters.

As AV says, many manufacturers won't publish the figure because it's low and they'd rather not advertise it.


The other complexity is that the color wheel needs to complete a full revolution per input frame; so wheel speed varies with input refresh rate.

You'll see this on the spec sheet: with a video source of 60Hz, you get a 240Hz effective wheel spin rate (that's the 4x from the spec sheet - and it matches your findings).
If you run at 50Hz, you get 300Hz (that's the 6x from the spec sheet).

On the newer models (HT1075, HT1085, W1110/HT2050, HT3050), they don't list it, but 24Hz gives you 288Hz. So if you're sensitive and watching bluray (or any 24p) content, setting your video player to output at native 24Hz will give you a reduction in RBE, compared to 60Hz.



Cine4Home has a brilliant article on this if you're interested; it's here. German translated to English is here.

It's subjective of course, but they list the thresholds of RBE visibility as follows:

  • 0Hz to 150Hz: Visible and disturbing rainbow effect for almost any viewer.
  • 200Hz to 250Hz: Significantly reduced rainbow effect, little disturbing in the film operation, but sporadically perceptible in strong contrasts.
  • 300Hz: In video, barely perceptible rainbow effect, even in strong contrast scenes.


My findings are similar to theirs: 300Hz practically eradicates rainbows for me; so I run at 50Hz. On a newer model like yours you'll get similarly good performance from a straight 24Hz input refresh rate (since that's close to 300Hz; at 288Hz). 60Hz (which runs at 240Hz) is noticeable to me.
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post #5 of 13 Old 01-20-2016, 07:31 PM
 
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So how quickly does the Vivitek 1186 run? People say it's fairly rainbow free and I don't remember if I was running 1080/24p (but I most likely was) and watching it literally gave me a headache.
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post #6 of 13 Old 01-21-2016, 01:08 AM - Thread Starter
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Quote:
Originally Posted by kreeturez View Post
Agreed: this should be a mandatory published spec since to people like myself and you, this matters.

As AV says, many manufacturers won't publish the figure because it's low and they'd rather not advertise it.


The other complexity is that the color wheel needs to complete a full revolution per input frame; so wheel speed varies with input refresh rate.

You'll see this on the spec sheet: with a video source of 60Hz, you get a 240Hz effective wheel spin rate (that's the 4x from the spec sheet - and it matches your findings).
If you run at 50Hz, you get 300Hz (that's the 6x from the spec sheet).

On the newer models (HT1075, HT1085, W1110/HT2050, HT3050), they don't list it, but 24Hz gives you 288Hz. So if you're sensitive and watching bluray (or any 24p) content, setting your video player to output at native 24Hz will give you a reduction in RBE, compared to 60Hz.



Cine4Home has a brilliant article on this if you're interested; it's here. German translated to English is here.

It's subjective of course, but they list the thresholds of RBE visibility as follows:

  • 0Hz to 150Hz: Visible and disturbing rainbow effect for almost any viewer.
  • 200Hz to 250Hz: Significantly reduced rainbow effect, little disturbing in the film operation, but sporadically perceptible in strong contrasts.
  • 300Hz: In video, barely perceptible rainbow effect, even in strong contrast scenes.


My findings are similar to theirs: 300Hz practically eradicates rainbows for me; so I run at 50Hz. On a newer model like yours you'll get similarly good performance from a straight 24Hz input refresh rate (since that's close to 300Hz; at 288Hz). 60Hz (which runs at 240Hz) is noticeable to me.
Thanks a lot for the information! The article is great, I missed a few small details due to the translation but overall is excellent, the kind of meaningful information I was looking for. And also what they say agrees with what I have observed so I think I have a pretty decent understanding about what´s going on now. Unfortunately I have returned my BenQ so I can´t longer try other input signal frequencies and figure out if there is any improvement.

I´m still wondering what would be the effective colour refresh rate in projectors without a colour wheel, like the LG PF1500. I guess the limiting factors would be the maximum LED pulse frequency... And what about the DMD chip? How fast can the mirrors be repositioned to switch between colour channels?
I´m tempted to get the LGPF1500 and record another video, figure out the wheel refresh rate and see if my experience gets any better. Actually it would be awesome if somebody already owning the LG reading this could do that sort of test!
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post #7 of 13 Old 01-21-2016, 02:59 PM
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Quote:
Originally Posted by AMartin56 View Post
So how quickly does the Vivitek 1186 run? People say it's fairly rainbow free and I don't remember if I was running 1080/24p (but I most likely was) and watching it literally gave me a headache.
Unfortunately the spec sheet omits any info about this (probably not a great sign)... I mention 24Hz performing well on these new BenQ units; but unfortunately on my older BenQ W1070, the color wheel spins slowest at 24Hz (it runs at just 192Hz... which is why I mention sticking to 50Hz in my post above). Perhaps it's the same on this unit; I'm not sure... Their support team indicates a spin-rate of just 2x; though there's no way to tell if they're correct unless it's actually measured - like what the OP did here!


Quote:
Originally Posted by vvaldes View Post
I´m still wondering what would be the effective colour refresh rate in projectors without a colour wheel, like the LG PF1500. I guess the limiting factors would be the maximum LED pulse frequency... And what about the DMD chip? How fast can the mirrors be repositioned to switch between colour channels?
I´m tempted to get the LGPF1500 and record another video, figure out the wheel refresh rate and see if my experience gets any better. Actually it would be awesome if somebody already owning the LG reading this could do that sort of test!
The DMD probably won't be the limiting factor (those mirrors can move!); but there's still some RBE reported for those very sensitive. Less so then most other single-chip projectors, though. It's also less bright (which helps reduce RBE visibility); finding someone who's actually measured would be great.


One other side-note to this thread is why having an RGB/RGB color wheel (ie, a wheel without secondaries like yellow - or a white segment) provides such a significant advantage to reducing RBE: because a single revolution of the wheel produces the entire image twice. This means the wheel could spin physically 3 times per input frame, but still be 'effectively' a 6x wheel. Trying get 300Hz from a non-RGB/RGB color wheel requires the color wheel motor to spin at a whopping 18000rpm; whereas with an RGB/RGB wheel, just 9000rpm does effectively the same thing.

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post #8 of 13 Old 01-22-2016, 06:48 AM
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if i see rbe in that video when watching it on my Panasonic vt50 plasma could that indicate that i might be more susceptible to dlp rbe than if i didnt see it on my plasma ?
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post #9 of 13 Old 01-22-2016, 07:09 AM
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Originally Posted by ataqdroid View Post
if i see rbe in that video when watching it on my Panasonic vt50 plasma could that indicate that i might be more susceptible to dlp rbe than if i didnt see it on my plasma ?
I doubt there is any correlation. I see color separation in this video aswell but I havent ever noticed a single rainbow in my benq w10000.
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post #10 of 13 Old 01-22-2016, 07:10 AM - Thread Starter
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Originally Posted by ataqdroid View Post
if i see rbe in that video when watching it on my Panasonic vt50 plasma could that indicate that i might be more susceptible to dlp rbe than if i didnt see it on my plasma ?
The video that I recorded was shot at 120fps, an that's why it's able to capture the color separation. What you see in the video is pretty much what I see when I move my eyes rapidly around an image projected with a DLP projector. The effect captured in the video will be there with independence of what sort of display you use for watching it and its not really related to your susceptibility to RBE. Your eyes (or brain) may or may not be as 'sensitive' as the camera I used for recording the video.

As far as I know, the only way of figuring out if your are sensitive to RBE would be to try a real DLP projector. Moving your eyes rapidly around a high contrast image (like the one in the video) is the perfect trigger in my case to perceive the effect.
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post #11 of 13 Old 01-22-2016, 08:33 AM
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good to know i guess i will just have to buy a dlp and if i have issues i will return to amazon.
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post #12 of 13 Old 01-22-2016, 06:27 PM
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OMG I can see the RBE!

Does this mean I cant buy a benq because of this, or the demo was setup like this so everyone can see it.

I will have subtitles playing 50% of the time.

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post #13 of 13 Old 01-22-2016, 06:52 PM
 
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Originally Posted by mixo View Post
OMG I can see the RBE!

Does this mean I cant buy a benq because of this, or the demo was setup like this so everyone can see it.

I will have subtitles playing 50% of the time.

mixo
You would still need to test it for realsies..
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