PAL Color Balance
Dear Gary: I have a query in which I hope you may throw some light on for me. I reside in Australia, the television system here uses PAL as opposed to your NTSC. I have always read that for the best picture on a set, the colour balance must be set to, or as close as possible to, 6500K. Is this colour setting also true for the PAL system or is there some other setting I should look at?
John Barsby
Video Technical Editor Greg Rogers Comments:
The one thing that all television systems around the world seem to agree on is a “white” reference color. This includes NTSC, PAL, SECAM and HDTV. All of these systems use the CIE (Commission Internationale de L’éclairage—International Commission on Lighting ) Illuminant D as the white reference standard. Illuminant D is also known as D65 (or D6500) and has a correlated color temperature of 6500K (degrees Kelvin). However, keep in mind that a correlated color temperature does not uniquely define a color. Colors (hue and saturation) can be defined by their position on the CIE Chromaticity (x,y) diagram. We publish a CIE x,y diagram with every display review. The standard white reference, D65, is always shown on that diagram and lies near the center at x = 0.3127, y = 0.3290. A color analyzer can be used to measure the x,y coordinates when calibrating a display. In order for a display to produce perfectly accurate colors, several conditions must be true. First, the display’s primary colors (red, green and blue) must match the primary colors specified by the television standard being used. Unfortunately, the primary colors specified for NTSC (now SMPTE C primaries), PAL and ATSC HDTV aren’t quite the same. But they are close enough that differences between them create fairly minor color errors. However, the primary colors found in consumer displays may deviate significantly from the standards, especially in front and rear projectors. Our CIE diagrams show the standard primary colors and the measured primary colors of the display being reviewed. The area inside a triangle formed by connecting the primary colors is called the color gamut. Significant deviations in any one primary can shift colors throughout the color gamut. In addition, a display can only produce colors that lie in its color gamut (the triangle). If the display’s color gamut is smaller than the standard color gamut, there will be valid colors that the display simply can’t produce at all. A second condition necessary to produce accurate colors is that the display maintain a constant D65 white reference over the entire brightness range from dark gray to peak white (this range is called the gray scale). If the gray scale value shifts toward red, green, or blue at a particular brightness level, the colors at that brightness level are also shifted accordingly. Another requirement is white field uniformity. If the gray scale color shifts over the screen area, other colors will shift in the same areas. Finally, any color decoders in the display or video sources must be accurate. Since the primary colors are red, green, and blue all displays must be internally driven by RGB signals. Any S-video (Y/C) or YPbPr signals must be accurately converted to RGB. You can use our CIE diagrams to get a good feel for the color accuracy of a display. Besides the primary colors, we also measure the complementary colors (magenta, cyan and yellow) produced by the display and plot those CIE (x,y) locations along with the standard complementary colors. This allows you to easily see what type of color errors the display may have. You can also use our CIE diagrams to see if the color decoding is accurate, independent of errors in the primary colors or the gray scale. The complementary colors should lie on a straight line drawn from a primary color through the display’s white reference point. This will always be the case (absent some unusual display problem) when using RGB inputs. If the color decoding is accurate it should also be true using any S-video or YPbPr input. But many times the complementary colors will be shifted well off the straight line. This indicates the color decoder has non-standard decoding angles. A too common problem is “red-push,” where the yellow and magenta colors are shifted along the edge of the color triangle toward red. This also pushes Caucasian flesh colors (that normally lie in the vicinity of x = 0.444, y = 0.400) toward red. It is unfortunate that some display manufacturers continue to set the default color temperature much higher than 6500K. This makes white colors more blue, which looks brighter. Apparently, they believe this sells more televisions. Since all colors are shifted toward blue, flesh colors would look very pale, which is why so many color decoders are still designed to add red-push.
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