Can anything replace the old, reliable CRT? Yes!
We sometimes forget that the television medium is inherently analog. That is true for both picture and sound. Broadcasters are deeply caught up in the technology that has transitioned to digital over the last two decades, with transmission being the most recent advance.
High-power RF transmission is, however, an analog medium, with analog representations of digits modulated onto an essentially analog medium and transmitted so they can be decoded at the point of reception. Picture and sound also start and end as analog signals (light and sound pressure waves), with today's digital technology most often doing the processing and carrying the content.
Cameras must have lenses to capture the analog phenomenon of light reflected off of scenes. After television is processed in acquisition, emission and transmission, broadcasters provide consumers with a signal that they can turn back into light representing the original scene, with artistic intent included for rendering the desired images.
Factoring in color
For a long time, display technology consisted of electron beams scanning cathode ray picture tubes. During the development of the first high-quality monochrome television, then color and most recently HDTV, the picture tube remained remarkably unchanged in physics and manufacturing, other than the introduction of color picture tubes after World War II.
Color put many new constraints on display designs. In the early days of color production, professional displays seldom matched, and any comparison between consumer and professional CRTs made it clear there was much room for improvement. In time, improvements were made, and today it is practical to render a scene and have it show up at home remarkably close to the intent of the producer. This is facilitated by closely matched standards worldwide, principally promulgated by SMPTE and the EBU. Broadcasters are fortunate to have SMPTE-C Phosphors and great methods for calibrating all aspects of a display. Modern digital measurement technology allows contrast, brightness and white set point to be calibrated accurately, and all parameters are stable in modern CRT displays.
The problem with CRTs
Broadcasters face a growing dilemma. CRTs are built using rare earth elements in the phosphors, which are deemed toxic. Millions of CRTs from computer displays, consumer televisions and professional displays end up in landfills every year. As a result, there is a lot of regulatory pressure to eliminate CRTs from the marketplace. In 2003, California passed a law imposing a fee on the purchase of all CRTs to pay for disposal. Predictably, much of the recovered material that is restricted in U.S. landfills ends up overseas. But simply moving the problem to a location with less stringent legal constraints doesn't eliminate pollution.
Another reason why manufacturers are keen on eliminating CRTs is because of the movement to larger displays with wide aspect ratio screens. CRTs are too heavy to be practical in large sizes useful for HDTV. This presents no insurmountable technical problem, except for the fact that most other new display technologies do not produce the high quality and consistent results needed for professional monitoring applications.
This is because of several factors affecting the physics of the display in ways that were overcome with emissive CRT technology. For example, LCDs are transmissive devices and must contain a backlight to be useful for monitoring pictures. They would be a good solution provided the backlight provides a smooth response across a broad slice of the visual spectrum, and the color filters in the LCD closely match the color primaries standardized for television in ITU-R.BT709.
LCDs also do not have sufficiently deep blacks because they must cut off all light from the backlight source to achieve black. Lastly, LCDs take time to react to new data applied, creating significant lag in the response time to new content. This appears as smear on fast motion.
There are ways to minimize all of these effects. For example, LEDs can be used for the backlight and shut off when black is needed. Modern LCD materials can also largely eliminate the lag. However, for professional use, more engineering must be done to overcome resistance to implementation of LCD in QC applications.
Alternative display technologies
Plasma displays are a widely available type of new emissive display technology and can be extremely bright. They generate light in essentially the same physical process of stimulation as a phosphor with high-speed electrons used by CRTs. This allows them to be built with excellent color fidelity. Plasma displays larger than 50in can be more cost-effective than LCDs. This is not much of an advantage in a control room environment where more modest monitors are often preferred for camera matching and QC applications. The size of control rooms often precludes using single, large displays for each source, though monitoring processors can deliver multiple images to a single display in geometry more efficient than single monitors permit.
Other new emerging display technologies offer promise as well. Front projection using Texas Instruments' Digital Micromirror Device (DMD), also called Digital Light Processing (DLP), reflective technology works well when the light levels in the room are constrained. Because the light source can be full spectrum, colorimetry can be excellent, and contrast ratio is limited mostly by the power of the light source.
Another new technology called surface-conduction electron-emitter display (SED) offers the characteristics of a CRT, but in a panel only a few centimeters thick. (See Figure 1.) An electron beam produces the light, so the possibility of maintaining the colorimetry, dynamic range and temporal response capabilities of a CRT appears good. Power consumption should be low, making professional use possible.
In any event, until a display offers the characteristics of a CRT without significant drawbacks, professional applications will be challenged when trying to adopt technologies developed for consumer applications.
John Luff is a broadcast technology consultant.
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