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| Executive Editor Rob Sabin prepares to adjust the color while performing basic calibration on a 70-inch JVC rear-projection HDTV. |
You've probably noticed the Test Bench box in S&V's HDTV reviews, which has a chart with before-and-after-calibration readings of the set's grayscale, measured from 30 to 90 IRE. Have you ever wondered what those readings mean or who performs this calibration? Or why 6,500 kelvin is so important? Sit back while I demystify the calibration process for you. I'll also tell you who can calibrate your TV and what you can expect them to do. (For more on why you should calibrate your set, see The Custom Installer: Your New TV Isn't Perfect.)
Setting the Standard
The Imaging Science Foundation (ISF) has done more than any other organization to raise public awareness of picture quality. It's even changed how some manufacturers calibrate TVs before they leave the factory and helped come up with the Certified Calibration Controls (CCC) program to make adjustment easier. But the ISF is best known for the more than 3,800 calibrators it's certified to adjust your TV for optimum performance. (To locate a certified technician, go to the ISF Web site at imagingscience.com/isf-trained.cfm or call 561-997-9073.) Having an ISF calibrator bring your set as close as possible to the analog (NTSC) and digital (ATSC) TV standards is the only way to be sure you're seeing DVDs and TV shows as they were meant to be seen.
First Things First
As with any service that requires training, not all calibrators are equally knowledgeable, so ask the calibrator if he's worked on your particular TV brand and model before. The ideal arrangement is when the dealer who sold you the set has a calibrator on staff, since he would be most familiar with that store's products.
If your set is brand-new, break it in before bringing in a calibrator. The ISF recommends roughly 200 hours of burn-in time for flat-panel TVs and around 100 hours for lamp-based front- or rear-projection displays. Have your TV on for at least 30 minutes before the calibrator arrives, since the black level and the grayscale on most sets will drift during warm-up.
Tools of the Trade
A calibrator has several tools that are critical to properly adjusting your TV. He might use some test DVDs to create the necessary test patterns, but a high-definition video signal generator, which enables him to precisely calibrate different resolutions, is even better.
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| A Sencore VideoPro multimedia video generator (left) creates the high-def test patterns, while a SpectraScan PR-650 colorimeter measures the screen’s color temperature. |
The first thing the ISF technician will adjust is your TV's basic controls: contrast, brightness, color, tint, and sharpness. (For tips on doing this yourself, see Step by Step: How to Calibrate Your HDTV.) Typically this is done in your set's service menu, where the calibrated profile can be recalled if the user settings are changed. To get to these menus, you usually need a code from the manufacturer that you enter using the remote and the TV's front-panel controls. But be warned! Since service-menu adjustments can ruin your TV, only a pro should do them.
A Whiter Shade of Pale
Like any color, white comes in more than one shade (see Grayscale Matters). In video, white is measured in kelvins (degrees Celsius above abolute zero), ranging from 2,800 K (the reddish-orange color of a 60-watt bulb) to 10,000 K (the purplish-blue you see in some headlights). The National Television Standards Committee (NTSC) that framed our system for analog color TV decided the correct shade of white is 6,500 kelvins — the color of sunlight at noon on a clear day — and the Advanced Television Standards Committee (ATSC) responsible for our digital TV system carried over the same color temperature as a reference point. More accurately, it specified point D65 along the "black body" curve of the CIE Chromaticity Diagram, and the correct color of white is expressed as points along the X and Y axis of the color spectrum (x = 0.3127, y = 0.3290).
Since a video signal is mostly black and white (or luminance), any deviation from D65 — whether toward the red or blue end of the spectrum — will bias the picture toward that color. The monitors used for color-correcting TV shows and DVD masters are also calibrated to D65, so your TV needs to be set to that color temperature if you want the images you see to match their sources. This setting isn't user-adjustable (except possibly by means of rather crude presets), so this is where the ISF calibrator really earns his keep.
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| A colorimeter is used to do grayscale calibration. The colorimeter’s readout shows 6,572 K — very close to the 6,500 K standard for white. |
Shades of Gray
The calibrator will bring up a window pattern with a gray rectangle centered on a black background. Most TVs only have two adjustments for color temperature: one for the low-IRE window (similar to a "brightness," or black-level, control) and one for the high-IRE window (similar to a "contrast," or white-level, control). While taking readings from his test equipment, the calibrator will change the red, green, and blue levels. He'll go back and forth between low and high windows until both measure as close to D65 as the set allows.
A calibrator working with an optical comparator can use only the high and low windows to make adjustments. Although someone who's good at using a comparator can do a decent job this way, his decisions are still subjective. Using a color analyzer, on the other hand, allows for more flexible adjustments and provides an exact reading. Analyzers are also better than comparators because they let the calibrator take readings all along the grayscale, not just the low and high windows.
Most analyzers are attached to the TV screen using a suction cup. They provide color-temperature readings in kelvins (K) and light-output readings in foot-lamberts (ftL) and also give the temperature's precise points on the CIE chart.
The calibrator next makes sure the color, tint, brightness, and contrast are okay — and that's it. Smile — you've just been ISF'd!
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| Point D, at coordinates x = 0.3127, y = 0.3290, on the CIE Chromaticity Diagram, is the NTSC/ATSC standard for white (6,500 K). (Click image for larger view.) |
Your Report Card
When he's finished, the calibrator will give you a report on your set's pre- and post-calibration results. Pay special attention to the graph for colorimetry calibration, which shows the color temperature before and after adjustment. "Before" is often a chaotic zigzag, with temperatures ranging from 5,000 K to 10,000 K or more. After calibration, you should have a line that holds steady at 6,500 K at all points from 20 to 100 IRE. But don't panic if your TV can't create a perfect line. Most sets will show some grayscale fluctuation, with variations of 200 to 300 K being common. Plasma, LCD, and DLP sets often have a large variance (500 K or more) at the low end of the scale. But this doesn't matter, since you can't see these deviations when you're watching your TV.
Less Bright, More Natural
The first thing you're likely to notice about your newly tweaked TV is its lower overall light output. A calibrated set is darker, because it's been adjusted to offer the best possible performance in a properly lit room — not to compete with banks of lights and other TVs. The picture will look more film-like, with natural colors and without artificially enhanced edges. (TVs that have the ISF's Certified Calibration Controls can have separate day and night modes. This allows the picture to be a bit brighter for daytime viewing but then be changed to deliver optimum performance when you're ready to roll the movie with the lights down.)
Since the color temperature has been properly adjusted away from its typical out-of-the-box blue bias (cool), video will look lusher and warmer. To see the improvement, have the calibrator go back and forth between the calibrated temperature and the TV's preset cool temperature. The difference will surprise you.
It's never too late to consult a professional. Whether you just bought a TV or are still using an old reliable one, every set can benefit from a visit from the ISF guy. Ready, set, calibrate!
Grayscale Matters
Adjusting the grayscale is probably the best-known part of calibration. But reviewing a bit of TV history will help you understand why it's so important.
We're deep into the transition from analog to digital TV. (All analog over-the-air TV transmissions are supposed to end on February 17, 2009.) When you consider that a top priority for the committee that established the NTSC standards in 1953 was to make sure that black-and-white sets would still be able to receive signals once color was introduced, the system has served us well. The solution was to lay the color information on top of the black-and-white picture information (a format called "composite video") — leaving the TV to separate the signals in order to make sense of them.
Even in component-video format (the way HD signals are broadcast), black-and-white signals are separate from the color ones. The green-colored connector on a component-video cable doesn't really carry the color green to your TV, but instead sends the luminance, or grayscale, information. The blue connector carries the blue-minus-luminance (B-Y) color-difference component, the red the red-minus-luminance (R-Y) component. The TV's color decoder adds and subtracts these three signal components to extract the three color primaries, red, green, and blue (RGB), in the correct proportions.
In video, every color and shade is produced by combinations of red, green, and blue light at different intensity levels. Full black is the complete absence of light, and an equal mix of colors at maximum intensity produces peak white. The grayscale is the range of steps between full black and peak white. Ideally, the only thing that changes from one shade of gray to the next is the intensity of the light, not the color mix. This is important because you can't have an accurately reproduced color picture if your gray isn't set correctly.
The signal's intensity is measured in IRE (named after the Institute of Radio Engineers). Full black is 0 IRE for HDTV (the NTSC standard was 7.5 IRE), and full white is 100. The grayscale makes up the transitions between 0 and 100 IRE.
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