Rear Projection TV Technology Primer By Shane Buettner   •   August, 2007 Big screen TVs ain't what they used to be- and that's a good thing. The CRT rear projection TVs of yesteryear were big. They still aren't flat screens, but digital technologies have shrunk RPTVs from front to back and dropped their weight in half. In addition, while CRT RPTVs die an irrevocable slow death from day one, digital RPTVs are lamp-based. When the lamp burns out, you buy a new one for a few hundred bucks and your TV is as good as new.

With flat panels being all the rage, RPTVs are not a growth category in the industry any more, but for the enthusiasts who will still consider one, our advice is to get 'em while you can.

First, there are many digital "microdisplay" RPTVs that outperform flat panels, especially in larger screen sizes, and for far less money in many cases—especially if we're talking about a premium flat panel. Also, while people love the flat panel form factor, very few people actually wall mount those sexy plasma and LCD screens. If you're table-mounting your flat panel then the in-room footprint isn't much different from one of these RPTVs.

Don't give up on the big screen TV just yet...

Two (And A Half) Basic Flavors
Two and half technologies currently dominate the rear (and front) projection market: LCD, Liquid Crystal on Silicon (LCoS), and DLP (Digital Light Projection). DLP is a technology developed and owned by Texas Instruments.

LCD and LCoS are related in that they both make use of liquid crystals (therefore the "half" in the "two and a half"). But they are also different in important ways. There are also proprietary variants of LCoS, such as JVC's D-ILA and Sony's SXRD.

Delineating all of the technical differences between these technologies would be enough to occupy several articles, so I want to focus here on practical differences. In very general terms, DLP is a reflective technology based on amazingly small micromirrors, while LCD is a "transmissive" technology, meaning that light is passed through rather than reflected off of the silicon substrate on the chip. LCoS can be thought of as something of a combination of the two, being a reflective liquid crystal technology- light is reflected off a mirrored surface beneath the liquid crystal substrate, passing through the liquid crystal layer twice.

The most obvious and important distinctions among these technologies for most consumers are in basic configuration and price.

LCD and LCoS projectors are exclusively three-chip designs, with separate chips handling red, green, and blue. As a result, LCD and LCoS projectors are always putting these three primary colors on screen in a continuous fashion.

DLP projectors, on the other hand, are predominantly single-chip designs that use a fast-spinning color wheel to emulate continuous color. This system works very well, but it's not perfect. Color separation "rainbow" artifacts can be seen occasionally in even the very best designs, and people vary in sensitivity to these artifacts.

Fortunately, looking at a single-chip DLP for just a few minutes will generally give you an idea of your sensitivity to the rainbows. If it's overwhelming, then you know to move on to a three-chip LCoS/LCD design.

Performance Characteristics
To some degree, rear projection DLP designs enjoyed the same considerable advantages over LCD and LCoS designs we see in front projection with respect to blacks and contrast ratio. But as with front projection, this has changed in the last few years, primarily due to the advent of the "dynamic" or auto iris, which is now being used in some DLPs as well as LCD/LCoS designs.

A dynamic iris is a complex system that opens and closes a physical iris depending on the light levels of the scene- it closes the iris down on darker scenes and opens it up on brighter scenes. In the best systems there are also complex electronic adjustments involved.

The first dynamic irises that appeared a few years ago were not free of artifacts- most obvious were brightness compression during bright scenes or "pumping," which means the iris' operation is too visible during scene transitions. These systems generally superior now, invisible in operation to even trained eyes with most program material.

As in front projection, 1080p resolution is becoming de rigeur among RPTVs. But single-chip DLPs for rear projection applications go about this differently. Rather than using native 1920x1080 chips, using a process once dubbed as "wobulation," a 960x1080 chip is used in conjunction with rapid shifting of the horizontal pixels to put a 1920x1080 pixel grid on-screen every 1/60th of a second. Someone in marketing decided that "wobulation" sounds more like something Foster Brooks used to do on TV, so now the process is called "smooth picture."

1080p LCD and LCoS RPTVs use native 1920x1080 imaging chips (and so do 1080p DLP front projectors). The early wobulators were a bit soft, but more recent designs have shown tack-sharp performance.