S+V 101: HDTV Color Spaces Explained
Color is one of the most important aspects of a TV’s performance, right up there with contrast ratio, black level, and video processing. Yet not only does color get overlooked, it’s accepted when it’s displayed totally wrong. Welcome to the weird world of color.
The building blocks of television are the RGB primary colors: red, green, and blue. Where’s yellow, you ask? With paint, you’re using subtractive primary colors because the color of the paint you see is the only color not absorbed by the paint. So when mixing paint colors, you’re mixing what is being absorbed. This is where the red, yellow, and blue from art class come from. But because TVs are emissive (they create light), they use what’s called additive primary colors. When mixing light, you add new specific wavelengths to make new colors. In the TV world, green and red make yellow. Magazines are printed using CMYK subtractive primaries: cyan, magenta, yellow, and “K” for black. What you need to remember is this: When mixing wavelengths of light, red, green, and blue are all you need to create every color in the rainbow.
But what green do you use? Any kid with a Crayola box can tell you that there are eleventy zillion different shades of every color. To minimize the inevitable chaos this would have wreaked on color TV, standards were set up to specify exactly what red, green, and blue colors to use. As long as these standards were followed, the color going in would be the color going out. The colors chosen (referred to as color points) were limited by the technology of the day. So green isn’t that green, red is kind of a reddish-orange, and blue is . . . well, blue is actually fine.
The area inside the triangle created by the red, green, and blue color points is called the color space. The most common color space is Society of Motion Picture and Television Engineers “C,” or SMPTE C for short. These color points were chosen in the late 1960s and adopted as SMPTE RP 145 in the late 1980s. When HDTV rolled around, ITU-R Recommendation BT.709 (The ITU, or International Telecommunication Union, is actually a branch of the United Nations), also known as Rec. 709, superseded SMPTE C, although “C” is often still used in professional and consumer video equipment.
This is where it starts getting confusing for TV manufacturers. SMPTE C and Rec. 709 aren’t very wide color spaces. There are many colors that can’t be reproduced by either spec — for example, a good, deep purple. (BTW, a good Deep Purple hasn’t been around since, oh, 1973.) Modern technology — via filters on an LCD or phosphors on a plasma TV — can, and often do, create colors that go well beyond Rec. 709. Unfortunately, none of these fabricated colors exists in the source material. All HDTV/Blu-rays are, ideally, Rec. 709 (though often mastered on a SMPTE C display). So any colors created by the TV outside the 709 color-space triangle are technically not accurate.
But they are eye-catching. Catching an eye is what leads to TV sales; accordingly, TV manufacturers blow out the colors on their TVs so they appear flashy on the show floor. This may or may not look good at home. Many TVs have different color-space settings. Even just turning off the “Wide” color mode can result in more accurate colors. For displays with more elaborate color management system adjustments, fine-tuning color using just your eyes is extremely difficult.
Deep Color and xvYCC were created to help widen our color palettes. Deep Color adds extra digital bits to the color chain, giving more possible shades of color between each primary. With xvYCC, the color triangle is expanded, so more colors are available to use. You’ve likely seen these features mentioned in TV specs for years. The problem is, they’re bunk. In order to have either Deep Color or xvYCC, content has to be shot, mastered, stored, played back, and displayed all on equipment capable of Deep Color and/or xvYCC. So far, no commercially available content is. Some technologies, like Panasonic’s Adaptive High Precision 4:4:4, specifically upconvert color detail to take advantage of — and mimic — Deep Color. This can help somewhat, but like any upconverting technology, it’s not the same as the real thing. So, for the most part, these features lay dormant. Perhaps someday...