Would you buy a sports car where the manufacturer tuned it to sound great in the showroom, at the cost of reduced real-world performance? Sounds improbable, but this is analogous to how most TVs are sold.
Manufacturers have determined that naive buyers are attracted to bright images. So, HDTVs are set by their creators to appear brighter in the showroom. Unfortunately, this cannot be accomplished while maintaining correct colors and calibration of the video image. In other words, TVs are intentionally mis-calibrated at the factory. This gives an unnatural blue tint to portions of the image that should be shades of gray
Most manufacturers have added accurate or cinema color settings to improve the picture after the sale, but often these too are far from correct. This leads to yet another intentionally introduced error: improperly brightening a TV picture not only makes gray appear blue, but it in fact corrupts all colors. Because viewers are very sensitive to inaccurate flesh tones, manufacturers add red push to the image to correct flesh-tone coloration. Some manufacturers also add green push to try to improve green colors. Red push changes the chroma gain and hue of the red channel. Green push does the same for the chroma green channel. While, red push does make flesh tones appear closer to their correct color, every other color is wrong. The net effect of this is that colors appear washed out, except for blue, which is over saturated.
So is this simply poetic license on the part of the manufacturers? That is, arent they just giving the customer what they want? In a word, no.
Video standards precisely define the correct colors that manufacturers are supposed to achieve, and people exposed to calibrated images overwhelmingly choose the properly calibrated image over the showroom version. Indeed, the availability of accurate or cinema settings gives evidence that the manufacturers understand that what they are doing is incorrect.
The good news is that consumers do want the best image quality when they purchase an expensive HDTV, and they look to the sales personnel to educate them on how to achieve this goal. Demonstrations that I have given to inexperienced viewers showing the differences before and after calibration also have resulted in an overwhelming preference for calibrated images. While not a scientific study, this demonstration can be given, and this thesis tested, by anyone willing to have a HDTV properly calibrated for before and after comparisons.
In effect, manufacturers have provided a great opportunity to add value by showing the consumer how proper video calibration will dramatically improve the quality of their viewing experience. Calibration benefits the consumer by offering improved picture quality, while providing a new revenue stream for the dealer.
While hands-on training provides the best way to learn the intricacies of video calibration, it is possible to discuss the basics in print. Some of these intricacies include input and output geometry (overscan and resolution), intensity (black and white levels), color (color-of-gray, color, and hue), and errors in the video, such as luma-to-chroma timing error (Y/C-delay). In addition, ambient conditions must be taken into account, such as if viewing is to be done in a darkened room, a bright room, or both.
Test patterns provide the only reasonable way to determine if the image calibration is correct, and to find out what is needed to eliminate errors. In addition, test equipment is necessary for some measurements. Many inexperienced sales personnel believe they can make video adjustments by eye, while watching video programming. However, the only thing that is certain in this assumption is that the calibration will be wrong. It is a small effort to purchase one of the available calibration discs and watch it to learn the basics of video calibration. These discs do not replace formal training, but they do lay a foundation for some of the steps used to calibrate HDTV displays. As a bonus they also describe the basics of audio setup.
The geometry referred to above involves finding out how your video image aligns with the screen. Manufacturers want to make sure that there is never a visible black-border to the video on the screen, so they often increase the image size well beyond the visible screen. While eliminating the possibility of a black-border, it also causes the edges of the image to disappear. This literally throws away resolution that could be used to improve image quality. Many TVs do not allow correction for this excessive overscan, so unless a video processor is available it may not be possible to correct image geometry.
Another aspect of image geometry involves obtaining a pixel perfect representation of the image. Unfortunately, many HDTVs will still apply scaling to an image even if it matches its native resolution. For example, a 720p display may still scale a 720p source to achieve the over-scanning mentioned above. This is a very poor decision made by manufacturers. Given the budget constraints placed on HDTV manufacturers and despite claims to the contrary, internal scalers do not do an optimal job of video processing. HDTVs that cannot be driven in pixel-perfect mode should therefore be avoided, because this limits the quality that can be achieved with a source at the displays native resolution, and when an external video processor is available.
Correct black and white levels also are critical for a quality image. If the black level is set too low, the dark detail will disappear. This is called black-crush. If the black level is set too high, then the image will appear washed-out. Similar problems occur with white. If the white level is set too high, then the bright detail will be lost. This is called white-crush. Fortunately all HDTVs have both black-level (brightness), and white-level (contrast), controls.
After initial adjustments of black and white levels, the grayscale should be calibrated. Grayscale is also referred to as color-of-gray or white balance. For grayscale, calibration test patterns and test equipment must be used. Using a window pattern and a colorimeter, the red, green, and blue levels at various gray input levels can be set properly. The correct color-of-gray, for color images, is referred to as D65. The D65 definition comes from the CIE Chromaticity diagram, and has a color temperature of 6500 degrees Kelvin. Note that the opposite is not always true. That is, a color-temperature of 6500 degrees Kelvin is not necessarily at the D65 point. The D65 definition comes from the CIE Chromaticity diagram.
The results of this calibration are often seen in magazines as a graph showing the color-temperature before and after calibration. Some hands-on training for grayscale calibration is required, but it remains a critical step in proper image calibration. Note that correcting the grayscale can change the black and white levels. So, it is important to repeat these two steps until black and white levels are correct and the grayscale is also correct. It is often best to have two or three grayscale calibrations available. These might be set for casual TV viewing, color film, and black and white film.
The color-gain (color) and color-phase (hue) should be adjusted next. This process is performed for all three primary colors (red, green and blue), but few HDTVs allow it. Most HDTVs have basic color and hue controls, but very few also have the necessary color and hue offsets for red and green. All three colors require adjustment to undo the errors created by red-push and green-push, and to account for manufacturing variance from display to display. If only the standard color and hue are adjusted after grayscale calibration, the color errors will only have been moved. That is, the color gray will be correct but all other colors will be wrong, including flesh tones.
Another overlooked calibration is called Y/C-delay. This refers to the difference in the physical position on the screen of the luma channel (Y) and the chroma channels (C). This is a common error, but cannot typically be corrected within the display. I have seen errors of up to 1.5 pixels between luma and chroma from a standard-definition source. This error is very noticeable on edges between two saturated colors and should be corrected. Unfortunately, HDTV manufacturers generally fail to take this issue into account and an external video processor is needed to correct the problem.
Room light level plays an important role in video calibration, as well. A system calibrated in a darkened room will not perform well with the lights on. Black and white levels will be wrong, and color-of-gray may be incorrect due to the color of the light source in the room. The HDTV must be calibrated at each of the various light levels that will be used for viewing. This often entails two or three independent calibration setups. This in turn requires multiple setup memories to store the various light level parameters. Regrettably, HDTVs typically do not have multiple calibration setups available, which limits quality if viewing at different light levels is desired.
While many HDTVs lack the majority of these required calibration features, some advanced video processors have all of these and more. Another advantage of a video processor is that it places the video switching near the source components where it should be, reducing the connections to the HDTV to a single cable. Advanced video processing can also improve on the displays internal video deinterlacing, scaling, and artifact reduction. Look for a video processor that supports extensive video switching, including HD analog input transcoding to a digital output, in addition to these important calibration adjustments and quality enhancement features.
It is not possible to cover all aspects of video calibration in an article of this length and, in any case, hands-on training is the best way to learn necessary skills. The Imaging Science Foundation (www.imagingscience.com) has an excellent introductory class. In addition, advanced calibration training is becoming available. Following CEDIA this year there will be a two-day calibration class, called, logically enough, Advanced Display Calibration. For information on this course, go to www.progressivelabs.net.
Please note that the author is not affiliated with either of these courses. These training seminars help provide a strong foundation for those who wish to begin offering video calibration services.
Jim Peterson is the founder and president of Lumagen Inc. He has been designing video and related products for more than 25 years. For information on Lumagens video switching, processing and calibration products, contact them at [email protected] or on the web at www.lumagen.com.