Reblogged from MaximumPC
Posted 05/18/10 at 06:03:33 PM
by Dr. Raymond Soneira
By now, you’ve surely seen ads for Sharp’s Quattron four-color technology. George “Mr. Sulu” Takei dons a lab coat, and fawns over Sharp’s introduction of a yellow primary-color sub-pixel to the traditional three-sub-pixel, RGB primary-color arrangement. According to Sharp, this results in “expanding the color gamut and faithfully rendering nearly all colors that can be discerned with the unaided human eye, especially golden yellow.”
If you have read this far, you already know that Quattron is just another shameful marketing gimmick. HDTV television and movie content is produced and color-balanced on three-color displays that are accurately calibrated to Rec.709. Sharp’s fourth primary color is yellow, and there isn’t anything for it to do because yellow is already being accurately reproduced with mixtures of the existing red and green primaries. More importantly, a Quattron display can’t show colors that aren’t in the original three-color source image. So what good is it? None, unless you like to see over-exaggerated yellows.
Well, if Shatner can debase himself as a TV pitchman, can we really begrudge Takei?
But could it be that existing consumer HDTVs are unable to reproduce the standard sRGB/Rec.709 color gamut, so Sharp’s fourth primary color actually has something useful to do? We decided to find out.
Colors and color gamuts can be accurately measured and then plotted in a chromaticity diagram to compare values to the standard. What follows is from a 2008 article where I used a spectroradiometer to measure the color gamut of HDTVs in the DisplayMate lab. To the right, are the results for a Sony consumer LCD HDTV. The black triangle is the Rec.709 standard and the red dots are the measured values for the red, green, and blue primary colors of the Sony display. Notice that the Sony measurements all fall exactly where they should on the triangle vertices. It’s perfect! In short, this Sony HDTV accurately shows exactly the same colors seen by, say, the director at a TV studio. Ipso facto, Sharp’s fourth color is absolutely superfluous and can only decrease picture quality and accuracy! Undoubtedly, part of the Quattron’s “Yellow Push” is being produced with simple video processing. Some people have been impressed watching the Sharp demos on the Quattron, but manufacturers demos are always fine-tuned to get a maximum wow response, so be careful before jumping to any conclusions about how it will perform displaying content at home.
See that narrow strip of gamut real estate between the top of the black triangle and the inside of the white line? That's where Quatron technology would have to live.
Note that in our figure, the outer white curve represents the limits of human vision. While the Rec.709 standard is much smaller, it’s important to note that the colors between the black triangle and white curve aren’t common in nature. Yes, a display can only reproduce the colors that lie inside of the polygon formed by its primary colors, but because yellow falls between the red and green primaries, Sharp’s yellow primary would need to lie somewhereoutside of the red and green leg of the color triangle. But there isn’t much room between the Rec.709 triangle and the human vision curve, is there? For this reason, it’s difficult to see why a yellow primary sub-pixel is needed unless Sharp isn’t able to put its red and green primaries where they belong.
Sharp shows its Quattron color gamut in some promotional material by using an old (x,y) distorted CIE Diagram from the year 1931 because it makes its extended color gamut look much larger than it really is. Our figure is the (u’v’) 1976 Uniform CIE Diagram and shows the color gamut accurately.
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Dr. Raymond Soneira is President of DisplayMate Technologies Corporation of Amherst, New Hampshire, which produces video calibration, evaluation, and diagnostic products for consumers, technicians, and manufacturers. A research scientist with a career that spans physics, computer science, and television system design, Soneira was a Long-Term Member of the Institute for Advanced Study in Princeton, and a Principal Investigator in the Computer Systems Research Laboratory at AT&T Bell Laboratories.
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