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," says 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 grey to another, termed grey-to-grey.
"There is an incredible degree of confusion in the general public about these specs. Terms are used almost interchangeably," says Ian Miller, chair of the board of directors of the Video Electronics Standards Association (VESA), a display-industry association. Miller is also director of technology at Samsung Information Systems America.
Which spec is it, anyway?
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 4-millisecond response time, while Samsung's own Web site 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." Nichols blames an internal misunderstanding for the use of two different response time measurements for the same monitor, and says that Samsung pulled the ad when the inconsistency surfaced.
And then there's the question of whether to measure response time using rise-and-fall or grey-to-grey. Rise-and-fall and grey-to-grey 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 grey-to-grey, which is all about subtle shading, is far more important than the rise-and-fall."
Hazy shades of grey
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 grey-to-grey 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 grey to grey makes it slower," says Samsung's Nichols.
A ViewSonic representative uses a car analogy to explain the same point. "Fully off is a black image, fully on is a white image. It's similar to a gas pedal on a car -- you're either off or on. But this spec doesn't cover all the speeds in between off and full throttle. Grey-to-grey is typically slower because it's in smaller differences that aren't normally overdriven," says Erik Willey, senior product manager for LCD at ViewSonic.
Willey says that ViewSonic shortens gray-to-gray response time on its VX724 and VX924 monitors by overdriving the panel's electronics using ViewSonic's Amplified Impulse technology. "Black to white to black is on/off, full-throttle or off. The difficulty is in how you get from one shade to another very well. With Amplified Impulse technology, we can accelerate this and keep accuracy" in both rise-and-fall and grey-to-grey measurements, Willey says.
Other manufacturers question the usefulness of a grey-to-grey spec. "It [grey-to-grey] 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 grey-to-grey spec at all.
Willey stands by ViewSonic's grey-to-grey spec. "We test from the darkest greys to the lightest greys. We conduct 256 test points across the grey-to-grey range, which are representative of all 64,000 transitions." He stresses that although ViewSonic considers grey-to-grey response time important, the company reports a rise-and-fall figure as well.
VESA plans to introduce a spec encompassing both rise-and-fall and grey-to-grey response times. But that spec may not make its debut until mid-2006. "Everybody would like it yesterday, but it's of negative value unless it's right," Miller points out. "It's going to take some months to get it right."
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.