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LCD specs: not so swift

Different ways of reporting response times obscure the hard numbers

Faster LCD pixel response time is supposed to make scrolling, video and games move more smoothly. Unfortunately, the response time figures that vendors quote aren't particularly useful for comparison shoppers.

The lack of standard specifications for measuring pixel response time means that vendors can choose from different ways of reporting – or not reporting – response time. "For the consumer who's trying to make sense of this, it's really a case of buyer beware," said Rhoda Alexander, director of monitor research at the electronics-industry research firm ISuppli.

The LCD industry has traditionally defined response time as rise-and-fall: the time it takes a pixel to change from black to white (rise) and then back to black (fall). This is often depicted as a line graph with black at the bottom of the slope and white at the peak. But some vendors are now reporting response time as the time it takes to change from one shade of gray to another, termed gray-to-gray.

Even the companies selling LCD monitors get confused. For one of its monitors, Samsung advertised one response time and showed a different response time on the model's spec sheet, yet the company maintains that both figures were accurate. For about a month, an ad for Samsung's SyncMaster 915n LCD monitor touted the display's 4ms (millisecond) response time, while Samsung's own website indicated that the monitor's response time was 8ms.

"If you look at the printed ad, you'll see that the 4ms is black to white," says David Nichols, director of display marketing for Samsung's Information Technology Division. "The specs on the web quote the full cycle [of rise-and-fall--i.e., black to white to black]. That's where it gets to be tricky. The 4ms is absolutely correct, and the 8ms is as well."

And then there's the question of whether to measure response time using rise-and-fall or gray-to-gray. Rise-and-fall and gray-to-gray can both be useful measurements. Some vendors report one, some give the other, and some report both. Which response time measurement is more meaningful to users depends on how they intend to use their monitor. "Serious gamers usually consider themselves to be the most demanding users of displays," says Miller. "For them, the gray-to-gray, which is all about subtle shading, is far more important than the rise-and-fall."

VESA has a technical definition for rise-and-fall, but the gray-to-gray spec remains undefined. Industry experts agree that in LCDs, transitions between the fine distinctions of gray-to-gray are 3 to 4 times slower than those of rise-and-fall.
"The voltage required to turn something from black to white or white to black is greater, so it's faster. Controlling the voltage from gray to gray makes it slower," says Samsung's Nichols.

Other manufacturers question the usefulness of a gray-to-gray spec. "It [gray-to-gray] is totally meaningless because you don't know where the starting point is," says Ian Matthew, Sharp's product marketing manager for LCD monitors. In the absence of a standard, Sharp declines to publish a gray-to-gray spec at all.

VESA plans to introduce a spec encompassing both rise-and-fall and gray-to-gray response times. But that may not make its debut until mid-2006.

For now, it's better to trust your eyes. "Your average user running standard applications on an LCD monitor is not going to notice a difference between 12ms and 4ms, even assuming those are using the same criteria. Gamers and high-performance users will definitely notice," says ISuppli's Alexander. But "25ms to 30ms is fine for most business applications, unless you're in an application that requires a lot of scrolling."

Alexander cautions against losing perspective. "Look at overall front-of-screen performance instead of focusing on one spec," she says. In other words, don't miss the forest for the trees.


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