10.17.2008 05:19 PM
Replacing the CRT II

Today’s newest display technologies strive to replicate the contrast, colorimetry and viewing angle of the best cathode ray tube (CRT) displays. While most consumers do not think about the color monitor they view and if it is an accurate representation of the original image, those in broadcasting must pay attention to how closely the monitors represent the original or intended image. The liquid crystal display (LCD) has found its way into almost all TV studios and control rooms, but just how accurate is the LCD when it comes to colorimetry and linear gray scale reproduction? Only at trade shows do we sometimes get to compare different monitor technologies side by side to judge image quality.

LCD

LCD technology was introduced in the late 1960s and was used in wrist watches. Then in 1987, the first 3in LCD television was built. LCDs work on the principle of changing the opacity of a liquid via an electric charge. By using a backlight, the LCD can either block or pass the backlight through to the screen. By adding red, green and blue filters in front of individual pixels, almost any color can be shown.

In the first LCDs, all the LCD pixels were controlled directly by applying an electric charge to the LCD matrix, and this led to voltage buildup across other pixels. But these charges activated other, undesired, pixels causing them to be partially turned on, creating a washed out or low-contrast picture. The cure for this was the thin film transistor (TFT) LCD; this design incorporates a tiny transistor next to each LCD pixel to control it. TFT eliminated the leaking voltage that turned on undesired pixels and allowed for a higher-contrast picture.

Backlight and color gamut

The backlight for LCDs has traditionally been a cold cathode fluorescent (CCF) lamp, which gives off a light that does not cover the NTSC triangle of red, green and blue — meaning that the light emitted would never match that of a CRT. Another problem with LCDs is a lack of true black — a little of the backlight always leaks through the LCD on black scenes. When LCDs are placed next to CRT monitors, the difference in the blacks is quite evident. Shadows and wide gray scales are difficult for LCDs to produce, and it is common for whites to be crushed. Contrast ratios are hyped by manufacturers many times over, but the level of black achievable by the display is much more important, as is a proper gamma curve.

Image persistence

Other issues with the LCD include its lack of image fade and lag. When an image is presented on the LCD screen, it appears as a single picture fully formed, not scanned from top to bottom as in a CRT. The problem here is image persistence of the human eye; as LCD refresh rates have increased (the number of pictures per second on the screen), these fully formed pictures cause the eye to see the last frame and the current frame together. There’s a jerkiness to rapid motion, especially diagonal motion. In a CRT, the first frame would have faded before the new frame was scanned, but that is not so with LCDs.

Latency

Another issue is that of image lag or latency — in other words, the time between a video signal is input and its display onscreen. All LCDs all have built-in latency because of frame buffers and deinterlacing that must be done before displaying video. As LCDs grow in size, image lag becomes more of a problem. This latency can vary from as little as 12ms-15ms all the way up to 100ms. Some displays have added audio delay to match the video to avoid lip-sync issues. Faster processors are needed to reduce this number to at least a couple of frames.

Viewing angle

All LCDs have limitations on viewing angle. Unlike a CRT, the color and brightness on an LCD will shift as the angle of the viewer changes. Manufactures have been working on this, and the best that is available is about 80 degrees off-center, or a total of 160 degrees. Most professional monitors use what is called in-plane switching mode in the alignment of the liquid crystals, which allows for a wider viewing angle than that of older LCDs.

A “true” replacement

A few LCD manufactures are producing what they call a replacement for the CRT. 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. The issue of color range has been addressed by the use of RGB LEDs; by adjusting the amount of light from each of the LEDs, the color range has been expanded. And by using light sensors to measure the amount of light from each colored LED, the color balance can be maintained through temperature changes as well as aging of the LEDs. By placing the LEDs across the entire background of the LCD, the light level can be adjusted dynamically, thereby expanding the gray scale and produce a truer black.

To deal with the image fade issue, some LCDs insert black frames between image frames, thus erasing the image from the screen and from the eye. By increasing the display rate to double, or more, of the video signal, the LCD can be sent black frames — another way is to turn off the backlight in between the frames of video. An alternative method is to back out a rolling section of the backlight to give an impression of a scanned image (like the CRT).

Of course, matching a CRT’s image comes at a cost — the very few LCDs that claim to do this run anywhere from $25,000 to $40,000.

Conclusion

LCDs hold the greatest promise to replace the CRT to date. Using the latest technology, they can produce as wide a range of colors and gray scale as the best CRTs, but at a high price tag. The good news is that as these techniques are applied to the high-end monitors, they become available to a wider range of LCDs (i.e. more affordable).

The next "Transition to Digital" will explore other display technologies that rival, or claim to rival, the image quality produced by the CRT.

Read the first part of this tutorial?



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