Secrets Q & A
- Written by Christian Eberle
- Published on 24 November 2008
- Professional Display Calibration: What It Is and What It Means to Your Home Theater Experience
- Page 2: Anatomy of a Professional Display Calibration
- Page 3: What Will Professional Display Calibration Do For Me?
- Page 4: THX and the Imaging Science Foundation (ISF)
- Page 5: Sample Data from a Calibration
- Page 6: Conclusions About Professional Display Calibration
- All Pages
Anatomy of a Professional Display Calibration
Let’s start laying the groundwork with levels. Brightness levels are expressed as IRE units from 0 to 100. This is a relative measurement. 0 IRE is the lowest brightness level, minimum black, and 100 IRE is the highest level, maximum white. These adjustments are made with the brightness (black level) and contrast (white level) controls. A calibrator will set these values using PLUGE patterns (photo below). These patterns display several bars on a black or white field. The bars are usually a bit above and a bit below 0 IRE and 100 IRE. When the below black and above white bars disappear, you have achieved the correct levels. One important note here: TVs are capable of 256 levels of brightness. Video levels as used on DVDs and broadcast content are from brightness levels 16 to 235. PC levels used by computers are 0 to 255. Video levels leave a bit of head and toe room to account for differences in mastering. Some sources and displays will clip the information below 16 and above 235. This is not necessarily a bad thing, but it makes the PLUGE patterns harder to use. It is very rare that actual DVD or broadcast content displays information below black or above white.
Now we move on to color gamut and color decoding. Pictured below is a CIE chart. Created by the International Commission on Illumination in 1931, this chart shows the range of color visible to the human eye. At its extremes are the three primary colors, red, green, and blue. You can see as the colors converge near the center of the chart, where they become white. All video displays are based on these three primary colors.
In the photo below, I have overlaid the color gamut that corresponds to HDTV, known as Rec 709. This is the colorspace used to master high-definition content. Most modern displays will come close to this standard. Some HDTVs, mostly in the front-projection category, have a color management system that allows adjustments to the primary and secondary color points.
The next graphic (below) shows the x and y coordinates for the standard definition and high definition color gamuts. All displays have the means to control color saturation and phase or color decoding. These are the color and tint controls. The color control adjusts the overall brightness of the three primaries. The tint control will move the cyan and magenta secondary colors so red phases correctly to blue and blue phases correctly to green. You may have heard of the blue filter method of adjusting the color and tint settings. This is usually inaccurate, as the color of the filter and the TVs color primaries must be correct for it to work. The easiest way to adjust color and tint is to shut off the red and green primaries, leaving only the blue on.
A split color bar pattern is displayed and the color and tint can be adjusted easily. Alternately, a color meter is used to measure the luminance of the primaries and the position of the secondaries can then be calculated.
The next major area is grayscale, or, the color of white. Since video is based on three color primaries, they must combine in a correct ratio to achieve a true white, so that white does not look slightly red, blue, or green. The SMPTE standard for color temperature or white balance is 6500 Kelvin or D65. Most modern HDTVs have at least one set of RGB controls to set the white balance. Ideally, there are two sets, one for the higher brightness levels and one for the lower. Again, a color meter is used with window patterns (photo below) of varying brightness to adjust the grayscale. It is important for the grayscale to track as closely to D65 as possible through the display’s full range of brightness. This is the area where consumer displays are the least accurate at their factory default settings. Whites will appear brighter if a bit of blue is added. Even the warmest or lowest color temperature setting will usually measure too blue. Getting the grayscale tracking correct has a huge impact on image quality. And it can only be done with instrumentation. There is no proper way to adjust white balance with the eye alone.
Now that we’ve covered the basics of levels, color, and white balance, let me highlight the other areas addressed by professional calibration. All displays have controls for edge enhancement (sharpness), options for video processing, and various aspect ratio choices. All these adjustments affect image quality. A calibration instrument has many test patterns that will determine the best values for all these controls. Some patterns include motion which helps determine the best settings for processing different signal sources. Since all incoming signals must be scaled and/or de-interlaced to match the HDTV's native resolution, these video processing options are very important. A calibrator can determine which component in the signal chain has the best video processing and make the appropriate settings.
Aspect ratios are also a very crucial area. The best image quality is usually found with an HDTV's 1:1 pixel mode. This means every pixel sent by the source device is mapped to the screen, there is no overscan. Overscan is only needed when there is unwanted material at the edges of the image. Some cable and satellite feeds will show this behavior. Since overscan means another video processing step, it should be used only when necessary.