How To

It used to be that there were direct-view televisions and rear-projection models, all of them using the same old, venerable cathode-ray tube (CRT) technology. No one except home theater geeks like us even thought about the technology behind the screen. Today, the choices are more confusing. Here we answer the biggest question on many buyers' minds as they shop for a new video display: How do I cut through the babble of acronyms, initials, and conflicting claims about the various technologies used in today's televisions and video projectors, and what are their relative strengths and weaknesses?

There's one term you need to be familiar with up front to know what's going on in video today: fixed-pixel display. It applies to all of the display technologies described below except CRTs, and refers to the fact that the picture on such a display is made up of a fixed array of separately driven pixels (picture elements). Thus, a fixed-pixel display can properly be called a digital display. To activate all of the pixels and thus fill the screen, an incoming signal must be converted to the same horizontal and vertical resolutions as the display—the display's so-called native resolution.

Strictly speaking, a CRT is not a fixed-pixel device. The imaging elements—colored phosphor dots—are energized by a continuous electron beam. Because the speed of the beam and the number and spacing of the scan lines it paints on the face of the tube can be varied, a CRT has no native resolution. Its resolution, within limits, is determined by the signal that drive the beam. A CRT is, by nature, an analog device.

While there is no such thing as a digital CRT, a CRT can display digital sources. But these sources must first be converted to analog signals to drive the electron beam(s). A digital display, on the other hand, can be driven directly from a digital source (such as the DVI or HDMI digital output provided on some DVD players and HDTV set-top boxes), but can also function with an analog input (such as the same DVD player's component output). In the latter case, circuits in the display convert the analog input into a digital form that can be used to address the digital display's pixels.

As used here, the terms front-projection or front projector refer to any setup employing a separate projector and screen. This usage is common and provides for easy reference, even though a "front" projector can be used, with special screen material, in a custom-designed rear-projection arrangement. As used here, rear-projection television, RPTV, and rear projector refer to the familiar one-piece, big-screen sets that use projection technology, all of whose pieces are enclosed in a single box.

Not all one-box video displays come with built-in tuners (either standard- or high-definition) or speakers (particularly plasmas), though an increasing number do. If you need these features, check to be sure that the set you're considering has them. Front projectors typically do not have tuners or speaker systems.

With all fixed-pixel designs, there is the possibility of one or more pixels being inoperative out of the box or failing in use. In our experience this is rare, but check with the manufacturer before purchase to see what their warranty is in this regard. A bad pixel in the middle of the screen will be far more of a nuisance than one on the edge of the picture.

Cathode Ray Tube (CRT)
The CRT—the classic "picture tube"—is the technology that dominated video displays until just a few years ago. Electrons emitted from a cathode at the tube's narrow end, or neck, travel through a vacuum, strike colored phosphors that coat the inside of the wide, flat end of the tube, and cause the phosphors to glow. This beam of electrons is swept in horizontal lines across the face of the tube; hundreds of these lines, painted rapidly and sequentially on the tube, make up each frame of the video image.

In a so-called "direct-view" set, the flat face of the tube is itself the screen. The phosphors are arranged in patterns of red, green, and blue dots. When excited, these dots make up a full-color image—or, in the proper mixture, a black-and-white image.

In a CRT video projector, three separate tubes generate red, green, and blue images, each with an individual projection lens. The three separate images are overlaid, or converged, on a projection screen to generate a full-color image. Such projectors produce a larger (though less bright) picture than a direct-view set. Both front and rear projection are possible, but the most common form of CRT projector is the one-piece rear-projection set, or RPTV.

CRT Advantages
• An established, reliable, robust technology. Widely available and supported around the world by experienced service people. And CRTs last a long time. Manufacturers are claiming long lives for their new DLP, plasma, LCD, and LCoS displays; while we have no evidence to the contrary, we already know what to expect from a good CRT.

• Direct-view sets retain their brightness and overall image quality when viewed off-axis.

• Economical. Still the least expensive of the current technologies, except in front projection, where the few new models remaining are generally more expensive than competing fixed-pixel designs.

• The best examples offer excellent color quality, fine definition, and respectable though not overwhelming brightness (particularly with front projectors).

• Black level and shadow detail. Not all CRTs are superior in this regard—it still depends on the quality of the specific model. But no currently available fixed-pixel technology can equal the deep, rich black levels, shadow detail, and contrast ratios of the best CRTs.

CRT Disadvantages
• Big and heavy. For the large screen sizes sought by most home theater fans, whether direct-view or rear-projection, you need two or three chunky friends from the gym to carry it into the house, and lots of room to install it. Dealer delivery and installation is the only way to go. If you can arrange for in-home repair service as well before you buy, you'll be glad you did if anything goes wrong. (This isn't a bad idea with any but the smallest displays and projectors, but is doubly worthwhile with a big CRT.)

> Limited resolution. Only the largest projectors with 9-inch CRTs can fully resolve a 1920x1080 high-definition signal. While very few current fixed-pixel displays can do this either, more that do are expected in the next year or two. In the screen sizes found in most home theaters, however, this isn't necessarily a big deal, and some experts argue that the CRT's lack of visible pixel structure makes for a smoother, more filmlike picture. (The scan lines generated by a CRT display are usually invisible in a high-definition image, or in a standard-definition image scaled up to 720p or 1080i.)

• Off-axis limitations. This is not an issue with direct-view CRTs, or CRT projectors used with low-gain screens, but the high-gain screens used in all commercial rear-projection CRTs result in a significant loss of brightness when the image is viewed from above or below the center axis or, to a lesser degree, to the left or right of the center axis.

• Burn-in. Leave a stationary image on the screen, particularly when the Contrast control is turned up high—which it invariably will be as delivered from the factory—and you could burn a permanent shadow of the image into the screen. If you avoid excessively high contrast settings,and viewing of stationary or near-stationary material (for example, some video games), and alternate viewing material that covers part of the screen with material that fills it, burn-in should not be a worry.

• Convergence. On CRT front or rear projectors (but not direct-view sets), the red, green, and blue images from the three tubes must be converged—that is, precisely overlaid atop one another. Once converged, the three images can gradually drift out of alignment and require periodic adjustment. This isn't difficult on most sets (some projection sets offer more or less effective automatic convergence), but some buyers may find it a nuisance—or ignore it and watch degraded images. Sloppy adjustment of convergence is probably the single most significant failing that gave CRT projection sets an undeserved reputation for poor image quality.