What Will Replace The CRT for Professional Video Monitors?

The cathode ray tube (CRT), once the mainstay of the television display industry, has all but disappeared from the marketplace, and new display technologies are rushing in to take its place. A session at the upcoming NAB Broadcast Engineering Conference (BEC, April 18-23, 2009, Las Vegas, NV - see below for additional information) titled "Quality Control for Television" includes a paper, excerpted here, discussing the suitability of new display technologies, in particular LCD and plasma, as replacements for the venerable CRT in evaluation-grade monitor applications.

THE CHANGE - it's been apparent for some time that CRT technology is doomed to extinction, not just for professional monitor applications, but also for consumer television sets. While CRTs have done an excellent job for several decades, several developments in the 1990s and the first part of this century have clearly provided the epitaph. First, we are moving from a world of low-resolution standard-definition television to high-definition acquisition, production, and viewing. This means the link between resolution and brightness-long the weakness of CRTs-must be broken. Second, our picture shapes are changing, with a 1.77:1 (16:9) aspect ratio becoming the preferred display format over the older 1.33:1 (4:3) aspect ratio we've come to know and love for 60 years. Further, TV and monitor manufacturing has evolved into a high-volume business with highly competitive pricing. This makes it difficult for manufacturers to devote substantial resources to the development and manufacture of professional-grade monitors. Rather, any new monitors are usually derivatives of existing consumer television technologies.

THE CHALLENGERS - LIQUID CRYSTAL DISPLAYS (LCDs) - today, LCD imaging is a mature technology and is found in both direct-view and projection applications. Direct-view LCD monitors and TVs have been made with screen sizes measuring as much as 108 inches diagonally, although commercially-available sizes are limited to about 65 diagonal inches. Because LCDs are transmissive imaging devices, they are unable to completely block light when shut off. As a result, LCDs have a difficult time displaying "black" and very low shades of gray. This, of course, limits their dynamic range. LC transit (alignment) speeds are also hard-pressed to keep up with fast motion, resulting in blurred images. The most common illumination source for LCDs is the cold cathode fluorescent lamp, or CCFL. CCFLs are very bright, but have uneven spectral radiation characteristics and are also difficult to calibrate to a consistent white point. A new generation of LCD displays is now coming to market, equipped with light-emitting diode (LED) backlights. LEDs promise more accurate color control and improved contrast. A technique known as local area dimming, when combined with pulse-modulated LEDs, results in very high contrast and deeper black levels.

THE CHALLENGERS - PLASMA DISPLAY PANELS (PDPs) - Plasma display panels were first developed in the 1960s and were commercialized for consumer and professional use in the early 1990s. They are similar in function to CRTs, as both are classified as emissive devices. Plasma panels are made up of thousands of red, green, and blue pixels. Each pixel contains a mix of xenon and neon gases. A priming charge of several hundred volts is applied to the pixel electrodes, and when the voltage reaches a high enough level, there is a brief, but intense electrical discharge through the pixel (see figure). This causes the gas mixture to ionize and emit a broad spectrum of energy, including ultraviolet light. It is this UV light that causes the color phosphors within each pixel to glow brightly. A sustaining charge, roughly one-half the priming charge, maintains pixel activity, until a clearing charge resets the pixel to its off state. Plasma panels, unlike LCDs, are switching displays. They have only two states-"on" and "off." Therefore, PDPs must employ pulse-width modulation techniques to digitally create different luminance values. This is accomplished with switching speeds as fast as 600 Hz.

COLOR GAMUTS - Another area of concern is color gamut performance. An evaluation monitor can't cover just the SMPTE-C or ITU BT.709 color standard color spaces. It must instead display a wider range of color shades that are needed for encoding video and film content into extended color spaces, such as xvYCC or the DCI P3 color space. All professional LCD and plasma monitors I have tested to date are capable of showing 100% of the SMPTE-C and BT.709 standard color gamuts. However, only emissive displays and LED-equipped LCD monitors have shown the potential to cover xvYCC and P3 color shades. Shown in the figure are plotted color gamuts (white triangles) for commercial LCD/CCFL (left) and plasma monitors (right). While both cover 100% of BT.709 (represented by the black triangle in the figure), the plasma monitor is capable of a much wider gamut, particularly in the green channel.

This paper is authored by Peter H. Putman, ROAM Consulting L.L.C. It will be presented on Monday, April 20, 2009 starting at 2:00 p.m. in room S226 of the Las Vegas Convention Center. It will also be included in its entirety in the 2009 NAB Broadcast Engineering Conference Proceedings, on sale at the 2009 NAB Show Store and available on-line from the NAB Store (www.nabstore.com) after the convention.For a complete list and summaries of each paper that will be presented at the 63rd NAB Broadcast Engineering Conference, April 18 – 23 in Las Vegas click here. For additional conference, housing and registration information visit the NAB Show Web page at www.nabshow.com.

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