HDTVS--Today and Tomorrow

The TV You Want Today

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Refresh Rates, Continued

Panel manufacturers found that changing the cell structure and the formulations of liquid crystal material wasn't enough to overcome this problem: The trick was to refresh the image twice as often, doubling the refresh rate from 60 to 120 times a second. In addition, manufacturers improved their televisions' controlling circuitry so that it would look at the two original frames in the 60Hz image stream, and interpolate a new frame to provide an intermediate image.

This approach produced a marked improvement over traditional 60Hz sets, one that's well worth the extra investment in a set with a minimum of 120Hz. The price difference has narrowed, but you can still expect to pay approximately $100 to $200 more to step up to an HDTV set with this feature in the 40- to 42-inch range. It's a must-have feature if you plan to watch sports, but any content that includes panning scenes and fast action will benefit from this technology.

If 120Hz is good, then 240Hz must be twice as good, right? The answer is a lot murkier than that. The manufacturers that offer 240Hz refresh technology are divided into two camps, each with a different approach. Samsung and Sony use a true 240Hz technology, in which (as in 120Hz sets) the controller starts with a pair of frames from the 60Hz content stream--but then creates three additional intermediate frames, not one. This means that for each of the original frames, the set actually displays four frames. (The math changes for 24p signals, such as those piped out by Blu-ray Disc players, but the concept is similar.)

Adding these extra frames causes the liquid crystal material to move more quickly than it otherwise would, which in turn reduces the blur effects. The difference may be noticeable compared with 120Hz, but in our tests it wasn't as dramatic as the difference between 60Hz and 120Hz, even when we looked at the sets side by side. As such, 240Hz is probably not worth paying a lot more for over the cost of a 120Hz model. (Right now, the jump from 120Hz to 240Hz is about $300 to $600, a large premium compared with the step from 60Hz to 120Hz.)

LG 42LH50
LG approaches 240Hz by a different path: Its models with 240Hz performance generate one interpolated frame for each of the standard 60Hz frames, just as 120Hz models do, but they flash their backlights twice for every frame. Thus, 60 original frames plus 60 more interpolated frames make 120 frames, and then the backlight flashing twice for each frame yields 240 flashes per second. Like a strobe light in a disco dance hall, the flash of the backlight helps freeze the action on the screen and reduce motion blur. But this eye trick still presents only 120 frames per second, so asserting that its re­­fresh rate is faster than 120Hz rests on rather shaky science.

Panels with 120Hz (or faster) refresh rates have one additional benefit: Most television programming is recorded at 30 frames per second (fps), which is easy to double for the 60Hz refresh rate that most HDTVs have used. Movies, however, are filmed at 24 fps, which poses problems for technicians seeking to digitize them for DVD or broadcast formats. To fit the 30-fps timing, every four frames of movie film must be stretched to fit five frames of video. The process employed to achieve this, called "3:2 pulldown," uses two interlaced fields of the first film frame and then three interlaced frames of the next frame to produce the stretch.

This awkward conversion can create a motion artifact called "judder," a jerkiness or slight stutter visible in the finished image. But since 120Hz, unlike 60Hz, is an even multiple of 24, these panels can display 24-fps material without requiring any conversion; each frame just gets shown five times.

Note also that plasma does not have this problem. The individual plasma pixels can actually turn on and off much faster than an LCD pixel; in fact, Panasonic has taken to describing their panels as "600 Hz". Plasma needs this extra speed because a pixel can only be on or off, and so must be turned on and off rapidly during each frame of an image to create the correct shade of color. So in theory, motion blur is not a problem for plasma models. However, the PC World Labs' tests showed that, at least in the case of the two plasma models we tested from Panasonic--the TC-PS461 and the TC-P42X1--some plasma sets could learn a thing or two from LCDs.

Next: LED Backlighting

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