November 2003

Television Technology: The Basics

A trip to your local audio/video retailer to investigate televisions will probably result in information overload as you take in the vast array of options. So before you go shopping, there are several basic technologies currently used in consumer televisions that you need to know about. While you don’t have to know exactly how every different type of TV works, it’s helpful to have a basic understanding of the technologies. Not one of them is inherently superior to the others; each has advantages and disadvantages.

Direct-view TVs

Except for the transition from black and white to color, standard televisions have not changed much in the past 60 years. We call such models direct-view CRTs (for cathode-ray tubes) because we look directly at the tube -- the device that creates the image -- rather than at an image projected onto another surface. A CRT is essentially a vacuum tube that produces images by projecting an electron beam onto the front of the tube, which is covered with phosphors that glow when excited by the beam of electrons.

Direct-view CRTs can provide a good, bright picture and are very inexpensive -- a good 27" set (measured diagonally) can be purchased for only a few hundred dollars. They’re easy to set up, and work well in a variety of locations and lighting conditions, including relatively high ambient light during the day. Their biggest drawback is that the size of the screen is limited to a maximum of about 40", due to the difficulty and expense of manufacturing the large glass tubes.

Projection TVs

Projection TVs can be divided into two types: front projection and rear projection. Rear-projection TVs, or RPTVs, are the large models typically seen at retailers, standing about five feet high and two to three feet deep. The projection unit is usually in the base of the TV; the image is projected off a mirror mounted on the inside back of the cabinet, then onto the back of the screen. Front projectors, on the other hand, work much like film projectors: a separate projection unit throws an image onto a wall or opaque screen.

RPTVs provide relatively large screen sizes of, usually, from 40 to 60 inches, and are easy to set up. However, they are large and cumbersome, and require much more space than a typical direct-view CRT. And the fact that they’re limited in how bright a picture they can produce means that their images suffer in environments with high ambient light. However, an RPTV does provide a big picture at a relatively low price in a simple, single box.

Front projectors are usually quite compact, yet can provide a very large picture of 100 inches or more. They are more difficult to install, as they require proper mounting and calibration, as well as a separate screen. Front projectors typically require a very dark room to be viewed in, and although they can be less expensive than RPTVs, the most expensive and highest-quality consumer video is usually provided by high-end front projectors.

Of the technologies used to generate images for projection TVs, CRTs have been around the longest and are still widely used in RPTVs. However, instead of having just one tube, as in a direct-view set, projection TVs have three CRTs -- one each for red, green, and blue light -- whose images are overlaid as they are projected onto the screen to produce different hues.

CRT-based projection TVs can produce some of the brightest, most natural-looking pictures of any projection technology, but they’re large and bulky, and the tubes lose brightness over time. Although many RPTVs are still CRT-based, most front projectors now use tiny computer-controlled digital displays to generate images.

Digital projection technology

Just a few years ago, CRTs were about the only viable technology for consumer projection televisions. Since then, rapid advances in digital microprocessor technology have resulted in new methods of generating the images required for projection TVs. These digital technologies all basically consist of a microchip with thousands of tiny pixels. These are used to create the video image, which is then magnified and projected onto a screen. These microchips generate images in black and white (called grayscale in this instance). Red, green, and blue light is used to project three different colored versions of the image, which are then combined to create all of the necessary shades of color.

Digital Light Processing (DLP): DLP, a technology developed by Texas Instruments, is now found in many low-cost front projectors. It is also used in some very expensive units, and is now beginning to be implemented in some RPTVs. The DLP chip is made up of many tiny mirrors that can be turned on and off several thousand times per second. When an individual mirror is switched off, it reflects no light and produces a dark pixel; switched on, it produces a light pixel. By leaving a mirror switched on longer than off, a light-gray pixel is projected; a dark-gray pixel is generated by leaving a mirror switched off longer.

Most DLP projectors use only one imaging chip, and a device called a color wheel that successively shines red, green, and blue light on the DLP chip. The switching of the mirrors is synchronized with the color wheel to reflect just enough light for each of the three primary colors so that the exact hue of color is reproduced when the three colors of light are combined. The video that you see from a DLP projector is actually a rapid series of red, green, and blue images that the eye-brain system combines into one picture of the correct color.

DLP projectors require only one imaging chip and can be made very compact and relatively inexpensive -- some DLP projectors sell for as little as $1000 USD. One drawback to single-chip DLP projectors is something called the "rainbow effect." Some people report seeing flickering and flashes of color with one-chip DLP projectors, presumably because it generates hues by alternating red, green, and blue images. This phenomenon varies with the individual; most people don’t find it a problem.

Liquid Crystal Display (LCD): LCD technology has been used in data-grade projectors for many years, but is now being used more frequently in consumer video projectors for home theaters. The LCD panel works similarly to a DLP chip except that, instead of reflecting light, the LCD panel blocks or lets light pass through; this makes it a transmissive rather than a reflective process, like DLP. LCD projectors always use three LCD panels, one each for red, green, and blue light, instead of a color wheel and the single chip used in DLP projectors.

Although there are some inexpensive LCD front projectors, they typically cost more than DLP projectors due to their more complex three-panel design. LCD projectors tend to have more saturated colors than DLPs, but they also appear to have more space between pixels, which can sometimes be noticeable as a blocky picture (called the screen-door effect).

Liquid Crystal on Silicon (LCoS): LCoS can be thought of as a hybrid of LCD and DLP technology. An LCoS panel reflects light, but instead of having tiny movable mirrors like a DLP chip, liquid crystals on a reflective substratum either block the light or allow it to pass through to the reflective surface. Like LCD projectors, LCoS projectors use three panels, but typically have higher resolutions than LCD- or DLP-based products.

LCoS technology is used mainly in high-performance professional displays, but is now finding its way into more consumer products. LCoS projectors are usually quite expensive, but are of very high resolution and provide what many believe is the best combination of performance characteristics currently available from the various digital projection technologies. Many manufacturers now offer LCoS projectors with their own variations. JVC calls their version D-ILA, for Direct-drive Image Light Amplifier; Sony has dubbed their implementation SXRD, for Silicon Crystal Reflective Display.

Flat-panel displays

This latest category of television displays is receiving the most attention among consumers because they take up so little room. These TVs are direct-view units, but instead of having a bulky CRT, they’re flat panels only a few inches thick. This makes them suitable for mounting on walls, or wherever space is scarce. These panels can be 60 inches or more, and are made up of small, individually controlled pixels.

Plasma: In a plasma display, each pixel contains a tiny bubble of gas -- essentially, a miniature CRT filled with a gas-like substance (plasma) and coated with phosphor. Each pixel is actually made up of three subpixels: one each for red, green, and blue. As an electrical current is applied to each subpixel, the plasma reacts with the phosphors inside the bubble and glows red, green, or blue. Plasma displays typically measure 40 to 60 inches and so are comparable in screen size to RPTVs, but without the bulk and weight of a large cabinet. Although their prices have come down considerably in the last few years, plasma TVs are still relatively expensive.

LCD: Flat-panel LCD TVs are similar to the displays built into laptop computers and the flat-screen LCD monitors used with desktop computers. They use the same core technology as LCD projectors, but in a much larger format. LCD flat screens are similar to plasma TVs, but unlike the tiny gas bubbles of a plasma display, which glow and emit their own light, liquid crystals generate no light; LCD displays must be backlit. The liquid crystals, placed between the light source and a color filter, control the amount of light that passes through the filter. The filter consists of red, green, and blue elements precisely aligned with the liquid-crystal subpixels so that different hues can be generated. Flat LCD screens look very much like plasma TVs, but are available in smaller sizes; the largest models are in the 30-40-inch range.

Which display technology is right for you?

No matter what your budget restrictions or performance requirements, there is probably a TV out there that is right for you. Deciding which one is the hard part.

It’s hard to beat the convenience and price of direct-view CRTs for everyday use and for home theaters of small to medium size. Inexpensive DLP and LCD front projectors are an excellent choice for those on a budget who want that big-screen home-theater experience. And for those who require a relatively large picture but don’t want to fuss with a projector and separate screen, RPTVs make a lot of sense.

For high-performance home theaters, an expensive DLP or LCoS projector is a good choice, but even then, there are other factors to consider. The higher-resolution LCoS projectors provide a smoother, more natural-looking picture, but lack the high contrast of DLP projectors. Also, some people actually prefer the sharper, more "digital-looking" images of DLP over the more "analog-like" picture of LCoS. Plasma and LCD flat panels have come a long way in terms of picture quality, and are coming down in price; there’s no denying the aesthetic appeal of these sleek units, which are ideal for lifestyle-oriented and custom-installation systems.

...Roger Kanno
roger@hometheatersound.com

• Go Back