Quote:
Originally Posted by
mdrew 
Could someone explain why the color wheel speed makes a difference? I hear it helps reduce RBE, but I don’t quite understand why. I also read that as the speed is increased, it does so at the expense of added wheel noise.
I will give a shot at a laymen's explanation. It is important to understand the technology to get a real grasp on it.
On a 3-chip DLP machine there is 1 chip that handles displaying the red image, one that handles the blue image, and one that handles the green image. When displaying a single frame each DLP chip projects its image at the same time, when all 3 are combined it results in the colorful image we see.
In contrast, a single chip DLP machine can only display a single color at any one time. So, first one color, say red, is displayed by the chip, then another, say the blue, is displayed, then the green. This is done sequentially, at no time is more than one single color displayed. This device relies on the human eye being able to combine these separate colors to fool our brains into seeing a single full color image. I think a lot of people don't actually realize this, if you recorded the displayed image from a single chip DLP in super slow motion and played it back frame by frame, you will never see a full color image, just images of the separate color components, i.e. there is never a time when there is a full RGB color image displayed.
But, the problem arises in that the human eye, and its ability to fool the brain, is not an exact science. Some people have different sensitivities. Some brains are not completely fooled and those are the people that see rainbows. The slower the wheel speed the longer each single color is displayed by itself before the next color is displayed. The quicker the wheel the more often each set of colors is displayed in succession per frame. So, on a 6x wheel the RGB frames are displayed 6 times for each frame in succession. If we stretch this out to give an example, say a video frame lasted for a complete second of time. With a 1x color wheel for the first 1/3 of a second you will see a red only image, then next 1/3 of a second you will see a blue only image, then the last 1/3 of a second you will see a green only image. On a 6x wheel speed you will see a red image for the first 1/18th of a second, then blue for 1/18th, then green for 1/18th, then red again for 1/18th, then blue again, etc... The faster the wheel spins the easier the brain is fooled into seeing a single full RGB image. It has been posted here before that the US Air Force has done tests and 100% of our brains are fooled at the equivalent of a 27x speed color wheel. So, even a 6x wheel will not mask the rainbows to some of us, nor will a 10x, or 15x. But, as the speed goes higher the percentage of us that see rainbows goes down.
As far as I know the technology to economically spin a color wheel at 27x speed is not in existence. The reason a color wheel is noisier at higher speeds becomes obvious when you realize how fast the wheel is actually spinning. A 1x speed wheel is spinning at 60 revolutions per second, assuming a 60 frames per second source video. This is 1,800 revolutions per minute (RPM). A 6x wheel runs actually at 5,400 RPM as they double up the color filters on the wheel, rather then having a single R, G, and B filter they put two of the filters in succession creating a wheel with RGBRGB allowing them to run double the "wheel speed" of a particular RPM. Thus, a 6x speed color wheel actually physically runs at a 3x speed while displaying 2 sets of R, G, and B frames per revolution of the wheel. A 27x speed wheel would have to run at 24,300 RPM assuming an RGBRGB filter section on it.
This is where LED lighting comes into play. LED lighting for single chip DLPs are promising because not only are they a more efficient light source, but they are able to switched on and off, so you can use an array of 3 LEDs, one R, one B, and one G, allowing them to replace a color wheel. LED's can be switched on and off at 27x speed and much higher. At 27x speed, this means the set of 3 RGB frames are displayed 27x per frame, at 60 frames per second this means the single DLP chip is displaying 4,860 frames per second, compare that to the lazy life of a 3-chip system where each chip displays just 60 frames per second. DLP mirrors can go from an on to off state in 15 microseconds, so the DLP chip theoretically can display over 66,000 fames per second if needed, well within the needs of the system.
