Four styles of HDTV

Rear-projection TVs

Thinking of going big? Rear-projection HDTVs still offer the biggest screen for your buck, although falling prices among big-screen plasmas--58 inches and up--are forcing RPTV makers cut prices on their own. It's pretty obvious, in fact, that eventually RPTVs will go the way of the dodo as flat-panel HDTVs take over completely, but we're still a couple years away from that. Rear-projection sets start at a sizeable 50 inches and go up from there.

A sea change has been happening in the rear-projection market as this TV type loses ground to flat panels: makers are dropping out. Heavy hitters Sony, Toshiba, and Hitachi no longer manufacture rear-projection sets, leaving only Mitsubishi and Samsung with significant lineups among major names. Both utilize DLP technology--former rear-projection stalwarts LCD and LCoS are no longer widely available.

Microdisplays
"Microdisplay" refers to any rear-projection HDTV that uses one or more microchips--again, DLP is the only one currently being manufactured--festooned with thousands or a just over a million pixels as part of a light engine. The light engine is driven by a lamp that bounces light off of chip, through an assembly that produces color, and onto the big screen. Below is a brief look at how microdisplays differ from standard televisions.

• The lamps inside these sets, which cost $200 and more, must be replaced every 3,000 to 6,000 hours, depending on conditions of use. You can replace most lamp assemblies yourself.
• The lamps take from 20 seconds to a minute to warm up and cool down. During the warm-up phase, the image is either dim or completely dark.
• The TVs are lighter and slimmer than older CRT-based rear-projectors, and you almost always need a stand to get them at eye level.
• Unlike older CRTs, all microdisplays can get quite bright without losing detail, so they're perfectly viewable in brightly lit rooms.

DLP

DLP was developed by Texas Instruments, and the company sells its chips, known as Digital Micromirror Devices (DMDs) to numerous manufacturers, including Samsung and Mitsubishi in the rear-projection arena. The DLP chip has thousands or just over a million tiny mirrors that flip back and forth to control light output.

Most DLP sets available today have 1080p resolution, and all use a technique "wobulation" to achieve their stated native resolutions. With wobulation, which Samsung calls SmoothPicture, the DLP chips have half as many mirrors, or physical pixels, as the stated native resolution (a 1080p chip has 960x1,080 pixels/mirrors). Another tiny mirror or lens moves rapidly back and forth to alternately create the first and second halves of the image and achieve 1,920x1,080 resolution. Wobulation can produce good results, and on some 1080p TVs, it can technically deliver all 1,920 lines of horizontal resolution; on other sets, it makes the image look softer.

One potential problem with DLP sets is known as the rainbow effect. Some people can see brief streaks of color on these TVs, especially in images with black fields and some bright features, such as a spaceship against a star field. This is caused by the fact that the single DLP chip uses a color wheel to create red, green, and blue, and hence all colors. The occurrence of these rainbows has been significantly reduced with the advent of newer, faster color wheels. Most people who watch a later-generation DLP never see rainbows at all, and the few who do usually only see them occasionally.

One potential problem with DLP sets is known as the rainbow effect. Some people can see brief streaks of color on these TVs, especially in images with black fields and some bright features (such as a spaceship). This is caused by the fact that the single DLP chip uses a color wheel to create red, green, and blue, and hence all colors. The occurrence of these rainbows has been significantly reduced with the advent of newer, faster color wheels, and most people who watch a DLP never see rainbows at all (and the few who do usually only see them occasionally). DLP HDTVs do introduce a bit more low-level video noise--which can look like tiny dancing pixels or motes in shadowy areas--than other microdisplay TVs.

New DLP technologies address a few of these issues. Samsung sells HDTVs, such as the HL61A750 pictured above, that use LEDs instead of the traditional bulb. In addition to lasting 20,000 hours, the LEDs eliminate the rainbow effect. Mitsubishi, for its part, has developed a high-end, laser-powered DLP HDTV that's available in limited quantities.