SEEING IS BELIEVING

A Primer on Large-Screen Technology

by Peter H. Putman, CTS, ISF

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 - direct-view monitors, video monitors and video projectors.

Don't believe me? Next time you're hanging out 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!"

"What about LCoS or D-ILA? They're hot!"

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. Who knows? After reading this article, your friends may even ask you along on their next trip to the dealer for a professional opinion!

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 (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. Align all three of these projected images precisely on a screen or wall, and you have a full-color video image - it's that simple.

Is CRT technology really obsolete, as many would claim? For starters, no picture information is wasted with a CRT display - every scan line is accounted for, no matter what the size of your monitor screen or projected image. In addition, your images will have very precise colorimetry since you are mixing equal amounts of red, green and blue light.

CRT projectors can achieve a wide contrast ratio - that is, the number of shades of gray in a given picture from the darkest shadows to brightest whites. For every shade of gray represented, a shade of color can also be reproduced. When you consider that humans can perceive millions of shades of color in real life, a projector or monitor must work pretty hard to produce images that appear "life-like"! As a result, CRT imaging technology still does the best job of accurately reproducing video images...so far.

Of course, there's a few catches involved. Both CRT-based monitors and projectors are bulky and heavy, due both to the power supply required and the weight of the picture tube(s) and supporting chassis. And while you usually don't have to align a direct-view TV set, you'll need to converge a CRT rear or front projector (or pay someone to do it for you) to get useable pictures out of it.

VENETIAN BLINDS

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 (LCDs) first. Liquid crystals were first discovered in the 1880s as naturally occurring compounds. However, have been around a long time, having first been discovered in the 1880s. Not much was done with this knowledge until after World War II, when RCA began experimenting with LCDs 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 - 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 to the consumer market. Sony's VPL-VW10HT (and soon-to-be-announced VPL-VW11HT) front LCD projectors use a special 1.35" LCD panel with 1366x768 pixels, and a version has also been incorporated into a Sanyo front projector for home use. Toshiba recently introduced the MT7, which uses an Epson-designed 1280x720 polysilicon LCD panel system.

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 may 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, DMD chips are available with either 848 by 600 DMDs or 1024 x 768 DMDs for front projectors and rear projection TVs. Texas Instruments has also introduced a variation of their SXGA DMD for consumer use. This DMD has an aspect ratio of 16x9, and displays 1280x720 pixels. It's been used in RPTVs made by Mitsubishi, Hitachi, and Panasonic, and has also been shown in front projectors made by PLUS and Sharp. (TI also announced recently a wide VGA (848x480 pixel) DMD to be used in lower-cost front projectors and RPTVs).

The most impressive DLP images seen by far are 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. Some folks (like me) do see the flicker, particularly with bright, white images.

YOUR BEST REFLECTION

LCD panels can also be manufactured as reflective devices. These panels fall into a fast-growing class of Liquid-crystal on Silicon (LCOS) displays, and there are plenty of companies jockeying for market position. LCoS panels are marketed and branded under a variety of names, such as JVC's Digital Image Light Amplifier (D-ILA) and Samsung's Ferroelectric LCD (F-LCD).

LCoS panels work almost exactly like transmissive LCDs, except that they reflect back polarized light. That's akin to two cars driving at opposite directions on a street that's only wide enough for one! If it were possible to have one of the cars drive sideways on the sidewalk, or on the side of a building, then both cars could pass. And that's a good analogy for LCoS - one light beam is polarized at 90 degrees to the other, allowing them to travel in both directions.

In an LCoS panel, the driving electronics are mounted directly to the backplane of the LCD. This provides a big advantage over transmissive LCD light efficiency. Typically, transmissive LCD panels throw away 50% of the light passing through them due to polarization losses, but reflective LCDs can do much better than that.

The down side is that the optical path for a three-panel LCoS projector or RPTV is much more complicated than that seen in a transmissive LCD projector. That's because the light is traveling in both directions at the same time, although polarized in two different planes. Using a color wheel with a single LCoS device is possible, provided the reflection angle was correctly designed.

Numerous manufacturers (most recently RCA) have announced plans to bring LCoS RPTVs to the home. To date, only one company - JVC - has been able to produce LCoS devices with acceptable manufacturing yields.. Their D-ILA product looks very much like a conventional polysilicon LCD panel, except it has a highly polished mirror surface and an opaque backing. It measures .9" diagonally and has a native resolution of 1365x768 pixels (SXGA).

JVC's early D-ILA front projectors were pretty large, but there have been size and weight reductions since then. Right now, JVC has re-introduced the D-Ahlia, a D-ILA RPTV ($13,999) that features a 61" 16x9 screen and uses special 1280x1028 non-square pixel D-ILA devices. It is also adequate for viewing of HDTV content.

As with transmissive LCD and DLP, LCoS front projectors and RPTVs do not require convergence (you'd be nuts to try it anyway; it requires laboratory-grade equipment) and their maintenance will consist of cleaning the air filter and changing the lamp as needed. One caveat - the lamp used in the D-Ahlia is a short-arc xenon, and it isn't cheap. Expect to get about 1000 hours from this lamp and pay about $700 for a replacement.

THE BEST OF BOTH WORLDS?

One hybrid flat-panel technology has been the "hot" ticket at professional and consumer trade shows. Plasma display panels (PDPs) 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 LCDs and DMDs, 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 five detailed - CRT, LCD, DLP, LCoS, and PDP - each represent a working, practical large-screen display technology that is either available now for consumer use, or will be at some time in the near future. (No, there won't be a pop quiz on this, but at least you won't have to suffer from "acronymity" in the future when discussing big-screen TVs and projectors!)

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