Russell Brown /
11.02.2008
Originally featured on BroadcastEngineering.com
Replacing the CRT III

Today’s newest display technology strives to replicate the contrast, colorimetry and viewing angle of the best cathode ray tube (CRT) displays. The following technologies are either currently available or are in the pipeline. Broadcasters must pay attention to how closely the monitors they use represent the original or intended image. Rarely do engineers get to compare the actually image to the one on their screen; therefore, they must depend on the accuracy of the monitor to judge image quality.

Plasma

Plasma displays are based on changing xenon or neon gasses into plasma. This is sandwiched between two plates of glass with ribs that make small cells or subpixels, which are coated inside with phosphors of red, green or blue. Thin wires are run in front and behind the cells to form a matrix, and a sheet of dielectric material insulates the wires from the glass. When an electrical charge is applied across a cell, this charge transforms the gas into plasma and, in so doing, excites the phosphors that then emit colored light.

By using the same phosphors, plasma displays can create the same colors as a CRT. Plasma displays achieve high contrast ratios and wide viewing angles just like CRTs.
But unlike a CRT that varies its brightness by changing the amount of electrons in the beam sent to the screen, plasma displays vary the amount of light produced by pulse width modulation, which varies the on time of the pixels — more off time equals darker pixels and more on time equals brighter pixels.

Like CRTs, plasmas can have burn-in issues; although, all modern plasmas have circuitry to prevent this. Plasmas also require a precharge where a cell must be primed to decrease its turn-on time before being activated. This leads to the cell emitting a small amount of light, but, again, this has been addressed by manufacturers. With bright ambient light, plasma displays can appear to be washed out because of the light reflecting off the cells and making the screen look gray, but under normal control room lighting, they have blacker blacks than LCDs.

Although plasma displays have many of the characteristics of CRTs, their size prevents them from being used in racks, because they come in sizes from 37in to larger than 60in.

DLP

Digital light processing (DLP) technology uses hundreds of thousands of tiny mirrors to create an image by reflecting a light source onto a screen. The mirrors can move independently and can direct the light of individual mirrors (pixels) either on or off the screen. By rapidly moving the mirror, varying amounts of light can be displayed, thus changing the brightness of the picture. The device that accomplishes this is called a Digital Micromirror Device (DMD) and was invented in 1987 at Texas Instruments.

Color images are achieved in one of two ways. The first is by using a single DMD and a spinning color wheel. As the light source passes through either the red, green or blue section of the wheel, the mirrors are directed to reflect only that color’s image to the screen; as the wheel turns to the next color, the mirrors are directed to reflect that color’s image to the screen. In this way, all three colors are projected onto the screen in sequence, but this method can cause a rainbow effect where the separate scans of colors can be perceived with a person’s peripheral vision — such as when you change focus from one part of the screen to another.

The second way to display color images is with three separate DMDs and a prism to split the light source into the required red, green and blue light and direct them at the separate DMDs. This way, all three colors are directed at the screen simultaneously, thus eliminating the rainbow effect. Depending on the light source, the color gamut can match that of CRTs; they can even provide an wider range of colors by using six different color segments in the color wheel.

DLPs have been used in rear-screen displays but find their widest use in video projectors. The size of rear-screen displays precludes their use in racks, so, like plasmas, they will find limited use in broadcast facilities.

SED

Surface-conduction electron emitter display (SED) technology promises to be one of the best displays available when it gets out of R&D.

SEDs work just like a CRT but with one electron emitter per subpixel. They work by having two electrodes spaced very closely (a few nanometers) then placing a charge across them of about 10V. This causes electrons to be emitted across the gap, and another electrode is placed next to the glass screen with the RGB phosphors on it. An extra 10kV is applied to this electrode, which causes the emitted electrons to accelerate across the space between the emitters and the glass and strike the phosphors, thus emitting light.

The advantages of SED technology are brighter pictures with more contrast than any other display and a color range equal to that of a CRT. It is also suggested that these displays will use less power than any current display technologies. The downside is they are not available now, and there is no timetable for their introduction.

OLED

Organic light-emitting diode (OLED) technology is, as its name suggests, an LED display made with organic compounds, but in this case, the LEDs are the display. A group of RGB LEDs make up each pixel, and because these are LEDs, they can be made very thin. In fact, the first OLED is already on sale: an 11in screen 3cm thick that sells for more than $2000. The thinness of the screen is one of the amazing features of OLED technology — the display can literally be printed onto almost any surface, even a flexible one. OLED can match the NTSC color space and achieve a high contrast ratio (1:1000000), but right now, they are very new and very expensive.

FED

Field emission display (FED) technology is basically a cold cathode CRT without the scanning. The technology uses cone-shaped electrodes called “Spindts” (named after inventor Charles A. Spindt) to emit electrons (called field emissions) to excite the phosphor. To even out the difference in emissions between Spindts, bundles of nano-Spindts are used for each pixel, keeping differences in brightness within 2 percent across the screen. Only 9kV is required for these field emissions, but the Spindts must be kept in a vacuum.

FED displays promise to provide a 1:20000 contrast ratio, a color range that matches CRTs (using the same phosphors), a very wide viewing range and a delay of just several lines.

Difficulties in manufacturing have slowed FED rollout, but they should be on the market at the end of 2009, with the first likely applications in the broadcasting and medical industries.

Conclusion

Currently, several of these technologies are still in the laboratory but hold great promise. This leaves the LCD as the most likely display to replace the CRT now, with the help of supplemental technology to assist it in producing the color gamut, black level and white balance to match or surpass the CRT. The top of the line Grade 1 monitors will have a steep price tag, but so did the best CRT monitors. And as technology improves these displays, they will inevitability find their way into consumer and prosumer gear, reducing the cost. Until then, issues such as display latency, motion artifacts and square verses non-square pixels have yet to be overcome.

Next time

The next “Transition to Digital” will cover what it takes to convert an analog transmitter to digital.

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