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In a previous blog I talked about the cathode ray tube (CRT) and the fact that the CRT has been and continues to be the standard for visual confirmation of color accuracy in the professional production and mastering world. The standards for colorimetry, gray scale, gamma, and color space are all written for CRT displays. These standards dictate every recorded frame of video that is produced for home viewing. Is LCD catching up with the standard-barer CRT? Yes and no. For a high price, roughly between $25,000 and $40,000, a select few LCD monitors are available that claim to match a CRT’s image. Using the latest technology, they can produce as wide a range of colors and gray scale as represented by the best CRTs. The lack of true black is the bane of LCD technology, due to backlight leakage, which is evident in black scenes. As well, shadows and wide gray scales are difficult for LCDs to produce. To achieve a blacker black, some LCDs use a neutral density filter in front of the screen to darken the blacks, but this also limits the contrast ratio. RGB LEDs are used to adjust the amount of light to expand the color range. These display also use light sensors to measure the amount of light from each colored LED, in order to maintain the color balance through temperature changes, as well as aging of the LEDs. And by placing the LEDs across the entire background of the LCD, the light level can be adjusted dynamically, thereby, expanding the gray scale to produce a truer black. Other tricks include inserting black frames or turning off the backlight between image frames to emulate a CRT’s image fade and lag, which produces overall image smoothness. Another method to give an impression of a CRT scanned image, and thus image smoothness, is to back out a rolling section of the backlight. Image lag or latency is yet another issue and as LCD displays become larger, latency is an even bigger problem, requiring added audio delay to match the video, to avoid lip-sync issues. LCDs have limited viewing angles, with the very best displaying not more than 80 degrees off-center, or 160 degrees total. All this is not to say that LCDs will not one day rival the CRT in performance, at a price affordable by main street Americans. But since performance evaluation has not been taught to most consumers, there is limited appreciation of what it means for a color monitor to display an accurate representation of the original image. The appeal has been “flatness” and “brightness.” And while the LCD has found its way into almost all video studios and control rooms, mastering and broadcast professionals remain critical as to how closely LCD monitors represent the original or intended image displayed on professional CRTs. Thus, LCD (as well as plasma, LCoS, D-ILA and DLP) displays must, to be accurate, be able to re-create the same gray scale and colors that comprise the original mastered video. For professional CRT monitors, SMPTE C phosphors are used to create realistic colors. Thus, it is important is determine whether the new alternative display technologies can accurately reproduce SMPTE C colors and the new standards for high-definition digitally mastered video. Such performance evaluation is critical to knowing how accurate the image you see conforms to the image produced in mastering. Gary Reber Editor-In-Chief & Publisher Widescreen Review
Tags:
- editor's couch -
- CRT -
- LCD -
- RGB -
- black -
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