Anyone who has been paying attention at all in recent years has noticed that both the types and sizes of television and video displays that are available have been increasing. Since before the beginning of commercial television in the late 1940s, the display devices used have overwhelmingly been cathode ray tubes (CRTs), although the flat screen television has been "just around the corner" since the 1950s. CRTs have offered very good performance as displays for television pictures: bright images, good contrast ratio, good color rendition, and a linear grayscale to name some of their more important characteristics. They do have their disadvantages, however: high weight, high vacuum, and the requirement for significant depth behind the screen, to name a few. The emergence of HDTV and the consequent demand for larger screens, along with the rapid development of alternative technologies, has resulted in the venerable CRT television display being strongly challenged by other types of display devices. What are the most prevalent TV display devices available, and how do they work?
CATHODE RAY TUBE
The cathode ray tube is a vacuum tube, and unless you are a tube audio enthusiast, it is probably the only vacuum tube to be found in your house. Operationally, it is a triode, employing one or more heated cathodes that emit electrons, each cathode having a control grid that modulates the voltage of the electron beam emitted from it. The bell-shaped sidewall of the cathode ray tube is coated with a conductive material that is maintained at a high positive potential. This is the triode's plate, which serves to attract and accelerate the modulated electron beam emitted by the cathode. The beam is electromagnetically focused and steered so that it scans across a matrix of phosphor dots on the front of the tube. When the electrons strike a phosphor dot, the dot emits light of a characteristic color. If the CRT is a color tube, there are three cathodes, each of which emits a beam modulated with the red, green, or blue signal respectively, or alternatively, a single cathode is driven with a multiplexed red, green and blue signal. Each modulated electron beam bombards a set of phosphor dots that fluoresce proportionally to the instantaneous amplitude of the beam. In this way, red, green and blue light analogs of the R, G, and B electron beams are generated, and the human visual system integrates these three light signals into color pictures.
The direct-view CRT is still the most prevalent type of television display, although there are indications that this will not always be the case. They have a significant portfolio of advantages: high light output, good contrast ratio and grayscale, capability for high resolution if a small dot pitch is used, wide-viewing angle, economies of scale in manufacture, and, in the professional area, the availability of standardized phosphor sets that produce well-controlled standardized colorimetries.
HDTV's detailed images are most fully appreciated on larger displays, and the CRT suffers from the requirement for a geometric increase in glass weight as screen size increases linearly. The practical limit on screen size for a cathode ray tube is about 40 inches, and a tube this size causes a TV set to weigh several hundred pounds. The trend toward larger screen sizes for HDTV gives advantage to alternative types of displays.
One method of achieving screen size larger than 40 inches is to project the images onto a large flat screen. Projection displays in both SD and HD resolutions have gained market share in recent years. TV projectors may employ either front or rear projection.
A front projection device is located some distance in front of an opaque flat screen, from which location it throws modulated red, green and blue light beams onto the screen. Front projection requires the projecting device to be located in the room with the viewers, and for optimal image quality, its location is critical. Chief among front projection's advantages is that it is possible to generate very large images if the projector is located sufficiently far away from the screen. In digital cinema, for example, screen sizes can range to 50 feet wide, or even wider. In the home, the front projection device must be either suspended from the ceiling or located in the prime seating area for viewing. In rear projection, the projection device is located behind a translucent screen. It solves the problem of locating the projector in the viewing space, but as screen size increases, the projector must be located increasingly far behind the screen, making the practical limit on rear projection screen size smaller than that possible with front projection. The devices that generate the projected light beams may use one of several technologies. The television projectors that have been used longest in the home use cathode ray tubes to generate their light beams. Such projectors typically use three CRTs, one each for red, green and blue signals respectively. The red electrical signal, for example, drives a CRT that draws a small version of the "red" image on its small screen. The light emitted from the CRT is passed through a red filter and lenses that focus it on the display screen. The green and blue signals are treated similarly, driving their own CRTs and focusing lenses. One disadvantage of CRT projectors is that they can generate only a certain amount of light. This shortcoming may be overcome to some degree by stacking two or more projectors on top of one another, aiming all the beams so that they properly converge on the display screen. The use of three discrete light beams creates the requirement for critically converging the beams: In order for the eye to perceive an integrated color picture, the separate images must fall on precisely the same screen location. The convergence requirement is increased in complexity when more than one projector is used, as the number of beams is multiplied.
DLP PROJECTORS
Many newer displays, both direct-view and projection, employ technologies other than CRTs. One of the new projection technologies that is gaining wide acceptance is the micromirror or DLP projector, a device which is fundamentally binary or "digital" in character. The DLP does not generate its own light, rather it requires an external light source to operate, usually a xenon lamp. The DLP device is a semiconductor device that is effectively a grid of tiny mirrors, each of which may be manipulated using a binary control signal so that at a given moment it either reflects light from the light source through a filter and focusing lens array out to a flat display screen, or does not reflect light out at all. The amount of light output from a given micromirror pixel cannot therefore be controlled using amplitude modulation, as it is in the CRT. It is controlled by modulating the on-time or duty cycle of the pixel. The micromirrors are controlled by a clocked digital signal typically in the 30 kHz range, and the intensity of light output from a given mirror over a period of time depends on the ratio of its "on" cycles to its "off" cycles.
DLP projectors may be of the one-chip or the three-chip variety. Three-chip units use a separate micromirror chip for each color, R, G and B; one-chip units use a single chip driven by a time-multiplexed signal containing all three color components. A color wheel with red, green and blue filter elements spins synchronously in front of the micromirror array, projecting the correct color of light when each signal is driving the mirrors.
An advantage of DLP projectors is that they can project very bright images, the image brightness dependent on the amount of light that is made available to the mirrors. This has led to their use in digital cinema.
Because the DLP display is strobed at a frequency in the tens of kilohertz, it offers another interesting property when projecting 24 frame-per-second material, as is done in movie theaters. The high strobe rate causes the perceived light flash rate to be far above the threshold of perceptual flicker. 24 fps material may therefore be projected at its native frame rate, instead of double-shuttering each frame as is done when 24 fps film is projected. This will of course eliminate the 2/2 pulldown judder that results from double-shuttering, and consequently, slightly change the perceived movement of the images. Whether this is a problem or a feature is a debate that will be left to others.
We have looked at two television and video display technologies; the oldest and the newest technologies that are in common use today. One of these technologies, the CRT, could be characterized as purely analog in nature, while the other, DLP, could be characterized as digital or binary in nature. Next time we will look at two rather more exotic display technologies that are based on chemical/physical phenomena: liquid crystal and plasma. Although this will not take us into additional dimensions or string theory, it will take us into additional states of matter beyond the three we learned about in science class.
