LCD and DLP Explained: What Goes on Here?

Flat-screen imaging technologies like LCD and DLP are slowly toppling the cathode-ray tube (CRT) from its pedestal. How much do you really understand about these new ways of watching TV?

Last fall, I was surfing one of the more popular home-theater forums on the Internet when I came across a heated discussion about an LCoS rear-projection TV that had just been announced. One of the participants was waxing rhapsodic about this product, saying that "it would blow away DLP" and "this is the display technology of the future." I posted some responses to find out if: a) he was secretly shilling for the company marketing the set or b) he was actually knowledgeable about LCoS (Liquid Crystal on Silicon) imaging.

After a few exchanges, I realized that this person was one of many who have been swept up by an effective press campaign. He didn't know how LCoS really works, or how difficult it is to manufacture in quantities. He was unaware that more than one company has been dragged into bankruptcy trying to deliver a similar product, and that even the world's leading maker of LCoS panels has problems with manufacturing yields.

The truth is, when it comes to the display industry, we are under a constant barrage of initials and acronyms.

New technologies for delivering and displaying audio and video are coming along at such a dizzying speed that it is virtually impossible to keep up with even a few of them, let alone all. Instead, we're seduced by these big screens with their thin profiles, wide viewing angles, and bright, high-resolution images.

Perhaps now is a good time to lean back in your recliner, switch off your home-theater system, and take a few minutes to bone up on what all the initials mean and how these highly touted technologies differ from one another. Are they really better than the plain-vanilla CRT? What are the disadvantages, if any, that you haven't heard about?

Is one better than another?

The Basics
For nearly a century, we depended on electron tubes to do a variety of things. While most of those functions have now been taken over by solid-state devices, the CRT still hangs in there, delivering video to the surface of a direct-view monitor, or tracing images on a front- or rear-projection screen.

By itself, a projection tube has no intrinsic resolution. It's a raster-based device; in other words, it needs to know the horizontal picture scan rate and the vertical picture refresh rate in order to draw anything from NTSC video to 1080i HDTV and XGA (1024x768) computer graphics. And tubes do a pretty good job of this—properly calibrated, they can hold a very wide gray scale and maintain consistent white balance along that scale. Depending on the size of the tube and the driving source, they can produce images of amazing detail and clarity.

But tubes are big and heavy. They use high voltages and require bulky chassis. They take up a lot of floor space. They drift out of alignment. These are all personality quirks we have come to know and accept in CRT-based imaging, and in many cases they don't cause us too much inconvenience.

However, tubes just don't work for personal electronics. Notebook computers, PDAs, cell phones, game controllers, and Watchman TVs would not have been possible if we hadn't come up with a smaller, lighter, lower-powered way to show video and graphics. It has taken the evolution of a new breed of electronic displays, based on fixed-pixel designs, to make possible these and many other products.

You might think that fixed-pixel imaging devices are a relatively new invention, but at least two of the systems currently in use, LCD and plasma, began to be developed some time ago. The light-polarizing behavior of liquid crystals, known as birefringence, was observed in the US in the latter part of the 19th century, and the pioneering work on LCD imaging was conducted by RCA in the 1950s and '60s. RCA abandoned its research in the latter part of the 1960s, only to see Sharp and Casio pick up the ball and run with it. Today, most large and small LCD panels are manufactured in Japan, China, and Korea.

Another technology invented here is plasma, first developed by professors at the University of Illinois–Champaign in 1964. Interested parties at the time included Owens-Illinois, IBM, and Fujitsu. Since then, like LCD, plasma manufacturing has moved offshore, where it is dominated by Asian manufacturers.

Unlike a CRT, a fixed-pixel device such as an LCD screen or DLP imaging chip has a specific, measurable pixel resolution, whether it's turned on or off. What's more, these devices do not actually trace images—they act as light shutters to modulate a light source and produce gray-scale pictures. The light source can be a super-thin backlight (used in large, amorphous-silicon LCD screens), or a high-intensity projection lamp (used in polysilicon LCD, DLP, and LCoS displays). The light can be reflected or transmitted and colored by the use of individual filters, colored mirrors, or even a high-speed, segmented color wheel.

One thing's for certain: the manufacture of picture tubes is a low-profit, high-volume business that more and more Asian companies want to abandon as soon as they can. The profit margins on projection CRTs aren't as bad, but almost no companies want to build front projectors using tubes any more. Mainstream manufacturers such as Mitsubishi and Hitachi have left the direct-view CRT business in recent years, while companies like Sony and Runco are drifting away from projection tubes in favor of fixed-pixel imaging.

Initials Explained
Let's take a closer look at each of the fixed-pixel systems you'll find in today's RPTVs and projectors.

LCD: The screens used for notebook computers, PDAs, cell phones, and a growing number of desktop computer monitors use variations of amorphous silicon liquid-crystal display (AM LCD) imaging. Depending on the voltage applied to each LCD pixel, the molecules inside align themselves in a specific fashion, and more or less light passes through the pixel to your eyes. The "amorphous" part describes the type of silicon used to form the transistors that switch each pixel.