Like acronyms? The home theater marketplace is full of 'em, like THD, AC3, RMS, VHS, NTSC, and DVD. If there's one product area that is especially prone to "alphabet overload", it's large-screen displays – video monitors and video projectors.

Don't believe me? Next time you're at a party, ask your fellow enthusiasts this question: "How many ways can I project video onto a large screen?" You'll probably get one or more of these responses:

"Buy a CRT Projector!"

"That analog stuff is old hat. You gotta go with DLP!"

"Nah, get an LCD projector."

"Forget the projector - check out one of those new PDP screens!"

Time for a follow-up question, like this: "Hey, what do all those initials stand for, anyway? Anyone actually know how those systems work?"

Chances are, all you'll get is a blank stare.

Well, there's no reason why you can't be the one person in your group to answer that question in a knowledgeable way, with the help of a quick "soft-cover seminar" on these varied and oft-times confusing display technologies.

CONNECT THE DOTS

There are two ways an electronic image can be formed on a projection screen or television monitor. The first is to scan an electron beam across the imaging surface at a very fast rate, thereby creating lines of picture detail (this is the system we've been using for 60 years worth of television broadcasts). The second method is to activate (modulate) a series of tiny square picture elements to pass or reflect light, creating dots of picture detail. The first system described is called a raster imaging system, while the second is known as a pixel imaging system.

Within each system there are several different approaches for displaying pictures. Our NTSC (National Television Standards Committee) system uses 525 interlaced scan lines to build each frame of video we see and typically employs a cathode-ray tube (CRT) to do the job. These lines are traced by a small dot, created by shooting electrons from the CRT onto a phosphor-covered surface. Hit a phosphor with electrical energy and it will glow. Build a picture tube with red, green, and blue phosphors; tickle them all with electron beams and voila - you've got color television.

CRT video projectors work the same way. Red, green and blue electron beams generated by picture tubes retrace the horizontal and vertical dimensions of the input video signal. Precisely align all three of these projected images on a screen or wall, and you have a full-color video image – it's that simple.

Of course there are a few catches. Large, conventional tube TV's are bulky and very heavy, due both to the power supply required and the weight of the picture tube and supporting chassis. Three gun CRT projectors also need large power supplies, they have limited light output, calibration is a constant problem, and they generate a lot of heat, (think fan noise!). You'll need to align and converge a CRT rear or front projector to get useable pictures out of it, (or pay someone to do it for you). Projection tubes tend to change as they age, so this may be required several times.

FIXED PIXEL DEVICES

Hmmm. Maybe that combination of weight and operating complexity doesn't appeal to you. If so, a flat-panel, pixel-based imaging system might be the way to go. There are several implementations of flat-panel technology, and they're all gathering lots of attention in the trade and consumer press. You'll find them in front and rear projectors, and one of them - plasma - has the potential to unseat the traditional direct-view CRT television.

Let's look at Liquid-crystal displays (LCD's) first. Liquid crystals were first discovered in the 1880s as naturally occurring compounds. Not much was done with this knowledge until after World War II, when RCA began experimenting with LCD's as an alternative to tube-based imaging.

Their operation is pretty basic: A "sandwich" is made up of two glass panels, inside of which are thousands of tiny pixels. Each sealed pixel is filled with a liquid crystal compound, and layered with electrodes. Under a magnifying glass, the liquid crystals appear to be tiny particles which float around in a random pattern - that is, until a small voltage is applied to both metal conductors on either side of the sandwich.

All of a sudden, these tiny particles align in rows like little soldiers and perform a neat trick with any light rays passing through the glass sandwich - they polarize them into horizontal and vertical components. Add a polarizing filter to the sandwich and you'll block 50% of this polarized light from passing through, thereby making the LC panel a form of light switch, or optical shutter. Remove the voltage and the LC molecules disperse back into their random movements.

Sound confusing? Think of a large building with thousands of windows in it. Each window has a light behind it, which can be infinitely varied in brightness. As you raise and lower the light level in each window, a shade of gray (remember those?) is created. Move back far enough and these windows appear as tiny pixels, or picture-forming elements. Set each window to a specific light level in precise patterns, and you've got an image.

By controlling the speed at which each of the liquid crystals align and disperse, we can form images with varying shades of gray and thereby create pictures. Add enough of these little marvels and you can show images with considerable detail. Attach some red, green and blue filters and presto! - full color video. Toss in a projection lamp, condenser and projection lens and you've got something that resembles a slide projector in both operation and simplicity. (Plus, it takes up a lot less space than an office building.)

Liquid crystal display panels are currently produced in two forms. The first is a single panel measuring from 6" to as large as 28" with built-in color filters . These amorphous silicon LCD panels are commonly used in notebook computers, and for a while were popular in front video projectors. Until 1994, all LCD projection panels and video projectors used amorphous LCD glass, making for some large but mechanically simple projector designs.

Smaller panels measuring as small as .7" are also manufactured. These polysilicon LCD panels are the panel of choice for both front LCD video projectors and LCD rear projection monitors. However, these panels are monochromatic and don't contain built-in color filters. An LCD projector must use three of these panels along with separate color filters to create the red, green, and blue parts of an image.

This makes the circuit more complex, but cuts down on weight and size. Still; LCD projectors have one big advantage over CRT projectors - LCD projectors use a single projection lens and don't require any external convergence or alignment. You just plug 'em in, turn them on, zoom, and focus. Sounds good, so what's the drawback?

LC glass is manufactured with a specific number of pixels, giving each panel a native resolution. Unlike the scanned lines from a CRT projector, these pixels are always present whether the projector is on or off. Some may even be defective as a result of normal factory tolerances for LC glass manufacturing, and these will show up as blue, red, green, black or white dots on the screen.

Because LCD panels were first manufactured for computer display applications, their pixel counts follow computer monitor standards such as VGA (640 x 480 pixels), SVGA (800 x 600), XGA (1204 x 768), and SXGA (1280 x 1024). Guess what? Unless your input signal matches the native pixel count exactly, you'll either miss some picture detail, or wind up with a lot of dark, unused pixels on the screen.

To get around this problem, some manufacturers use digital image manipulation to resize input signals. Video scalars make it possible to fill the available resolution on SVGA and XGA LCD projectors. But the quality varies considerably. The low end of the LCD market consists of "data grade" projectors, which are designed for static business presentations of pie-charts and graphs. (This means the manufacturers don't need to worry too much about color accuracy or signal processing.) When these projectors are used for movies and video, you'll often notice "dithered" areas of the picture, where video scan lines are straddling individual LCD pixels. Motion artifacts make the problem even worse.

Widescreen variations on traditional 4x3 panels have been introduced specifically for the home theater market. Sony's original VPL-VW10HT, and the current VPL-VW12HT are excellent front LCD projectors that use a special 1.35" LCD panel with 1366x768 pixels. These are true HDTV projectors with very accurate color temperature adjustments and high performance circuitry for video scaling.

THE MAGIC MIRROR

Another flat-screen imaging technology that has captured much media attention is Digital Light Processing (DLP) from Texas Instruments. Instead of using light shutters, the heart of the DLP system (called the Digital Micromirror Device, or DMD) employs thousands of tiny mirrors mounted on a dynamic RAM chip.

Electrical impulses received by each individual mirror cause it to tilt a maximum of twelve degrees towards (on) or away (off) from the projection lamp. By rapidly switching the mirrors between their 'off' and 'on' states, grayscale images are created. This technique is known as pulse-width modulation, and the grayscale values are determined by the ratio of 'on' to 'off' cycles in a given time interval.

The effect is not unlike that observed when hundreds of people in a football stadium hold up individual cards to form a great big picture or logo. DMD mirrors can cycle quite fast - quickly enough to show full-motion video. The red, green, and blue picture elements needed for life-like pictures are created by using a color wheel with a single DMD chip, or three separate color filters with three DMD chips.

Perhaps the most important aspect of DLP technology is that the signal communication system controlling the mirrors is 100% digital - not analog, as is the case in a CRT or LCD projector. This means that it will be possible in the future to directly modulate each of these tiny mirrors with an HDTV or other digitally-encoded signal, eliminating the possibility of analog chroma, moire and noise artifacts in the signal processing chain.

Disadvantages? Well, the most important is the fixed resolution of the DMD chip. Just as we saw with an LCD projector, the input signal source will look best on a DMD display if its resolution and the DMD chip size match up exactly. If not, the problem with unused pixels or unseen portions of the image will once again pop up.

At the present time, most DMD chips are designed for computer displays with either 848 by 600 DMD's or 1024 x 768 DMD'S, (although they've been widly used in front projectors and rear projection TV's). Recently however Texas Instrument's introduced a new chip specifically designed for high performance movie and video reproduction. This DMD has a widescreen aspect ratio of 16x9, and a display count of 1280x720 pixels. It will be available used in projectors made by Yamaha, InFocus, Runco and Sharp.

Even more impressive DLP images can be produced by projectors using a three-chip system (currently offered in the professional markets only). Separate red, green and blue color filters are used in conjunction with individual DMD chips, then combined in a prism before projection. This system combines the colorimetry of a CRT projector with the convenience of a single lens system, but is still limited by the DMD's native resolution.

For consumer use, a single DMD with a special color wheel is the imaging system of choice. The wheel is precisely synchronized to the DMD to image red, green, and blue light, plus a transparent band to boost brightness. The wheel moves at a very high speed - so fast that your eye shouldn't see any flicker as it strobes through the various color filters.

As with transmissive LCD, DLP front projectors and RPTV'S do not require convergence (you'd be nuts to try it anyway; it requires laboratory-grade equipment) and their maintenance merely consists of cleaning the air filter and changing the lamp as needed.

THE BEST OF BOTH WORLDS?

One hybrid flat-panel technology has been the "hot" ticket at professional and consumer trade shows. Plasma display panels (PDP's) offer what Buck Rogers dreamed of 60 years ago - a large television picture (37" - 63" diagonal) that can literally hang on the wall, or stand on a tabletop. Plasma displays employ an imaging system that combines the RGB phosphors and brightness of a CRT picture tube with the simplicity, low power consumption and construction of an LCD panel.

Like LCD'S and DMD'S, plasma panels have a fixed pixel structure whether they are "on" or "off". Individual red, green, and blue pixels are formed in crossing ribs between two glass plates, and a rare gas mixture is sealed in each pixel. When a charge is applied to any individual pixel, the rare gas is ionized, producing ultraviolet light.

This light then strikes a red, green or blue phosphor at the rear of the pixel, causing it to glow. Remove the charge and the gas de-ionizes. Extra electrodes are employed to charge and discharge the gas as fast as 85 times per second, making it possible to show full-motion video and still images with a technique similar to pulse-width modulation.

Despite the obvious appeal of a large, flat TV screen you can place on a tabletop, there are disadvantages to plasma technology. As we saw earlier with LCD and DLP displays, the fixed pixel structure in a plasma display gives it only one optimum display resolution - signals with higher and lower resolutions will be cropped or overscanned.

Sampling of grayscales in plasma panels needs to be improved. It has been demonstrated that 8-bit sampling does not provide a smooth-enough grayscale for viewing video. As a result, abrupt changes from one brightness level to another are observed, creating an artificial boundary or 'false contour' where there shouldn't be one.

Note that plasma display panels aren't tied to computer-industry display standards. Currently, they're being manufactured in several sizes - 37" and 40" 4:3 aspect ratio panels with 640 x 480 or 1024x768 pixels; 42" 16:9 panels with 852/853x480 or 1024x1024 pixels, 50" 16:9 panels with 1280x768 or 1365/1366x768 pixels, and 60"/61"/63" 16:9 panels with 1365/1366x768 pixels.

CLASS DISMISSED

Well, there you have it - a quick tour of the leading display technologies in use today. The four detailed - CRT, LCD, DLP, and PDP - each represent a working, practical large-screen display technology that is available now for consumer use. (No, there won't be a pop quiz on this, but at least you won't have to suffer from confusion in the future when discussing big-screen TV's and projectors!)

Copyright ©2002 Peter H. Putman. All rights reserved.