LCD HDTV Motion Features: How Do They Work?
If you've been paying attention to LCD TV marketing pitches over the last few years, you may have noticed vendors touting faster refresh rates (120Hz, 240Hz, and even 480Hz) and various proprietary technologies that promise smooth motion. But what exactly is the problem here, and do any of the purported remedies help?
Several issues are involved. The first has to do with response time. LCDs depend on pixels that change their state to let varying amounts of light to pass through (or to go dark), depending on the image. If you're looking at images involving fast motion--sports and video games, for example--"the LCD can't turn off pixels fast enough," explains Ken Lowe, cofounder of flat-panel TV manufacturer Vizio. The result is what looks like a smear or a comet-like tail as the LCD tries to catch up.
To combat the problem, manufacturers began producing (and touting) displays capable of faster response times, typically measured in milliseconds. Most LCD TVs today claim response times of 2 to 8 milliseconds. But there are different ways of measuring response time--and no industry standard for doing so.
Consequently, manufacturers generally choose whatever methodology makes their panel look faster, which has made response time specs pretty useless in comparing sets from different vendors. This, by the way, is a long-standing issue that we've reported on in the past in stories such as 2005's "LCD Specs: Not So Swift."
However, today's panels are so fast that response time is no longer the sticking point. This is where refresh rates come into play. When LCDs first began replacing CRTs as computer monitors, they typically had a refresh rate of 60Hz, meaning that they redrew the screen image 60 times a second. That rate is fine for most content, but with some videos showing fast motion, the eye may perceive a blurry effect in moving objects.
LCDs with 120Hz refresh rates display twice as many images per second, but the content remains unchanged--and showing the same 60 images twice doesn't address the underlying problem with fast motion. That's why most LCD-TV vendors have introduced technologies that generate new frames of content based on interpolation of the source material. The programs use mathematical algorithms to analyze adjacent frames (or a group of adjacent frames) and figure out where moving objects might logically be in the fraction of a second that the source video doesn't contain.
Every vendor has its own way of making these calculations, and they appear under different names in marketing materials. Vizio, for example, calls its motion-smoothing technology MEMC (short for Motion Estimation, Motion Compensation)--but the ads simply refer to it as Smooth Motion. LG calls its motion interpolation technology TruMotion, Samsung has Auto Motion Plus, and so on.
How effective are these technologies? "Nobody's got it perfectly right yet," says Vizio's Lowe. That's because it's difficult to develop a mathematical algorithm that will work for all types of scenes and content.
Movies Muddle the Picture
Another cause of video artifacts is specific to movies--a problem that has been around since the days of analog CRT sets--and that is the mismatch between movie frame rates and television frame rates.
The NTSC sets used in the United States before the digital conversion had 525 lines of horizontal data and a refresh rate of 60Hz, but they were interlaced displays, meaning that each field consisted of every other line in the image: Odd numbered lines were drawn in one pass, and even number lines in the next. As a result, these sets effectively showed a complete frame 30 times a second.
Movies, however, are shot at 24 frames per second. So in order to transfer film to video for TV (a process called telecine), Hollywood had to come up with a way to add 6 frames a second. The movie industry settled on a technology known as 3-2 pulldown, which adds one frame to every four by painting half of every other frame twice.
It works like this: If you have four frames of film (A, B, C, D), your television would normally draw each in two passes, starting with the odd numbered lines of A followed by the even numbered lines of A and so on.
With telecine, the two A frames (odd and even) are followed by the two B frames, after which the odd-numbered B frame lines are repeated followed by the even-numbered C frame lines.
After that, the odd-numbered C frame lines are followed by the even numbered D-frame lines, and then the odd and even (again) D frame lines. The result? 10 fields or 5 frames, two of which are hybrids generated from two of the original four frames.
Telecine generally works well, but it can introduce artifacts called judder, a sort of jumpy look when the artificially generated frames don't make visual sense. Telecine judder is usually most apparent in slow panning shots.
The good news is that a 120Hz set doesn't need the artificially generated frames: It can simply show each of the 24 frames of filmed content five times. But in order for this to happen, you must have a 24-frame video source, such as a Blu-ray player with 1080p/24 support. Some DVD players and digital video recorders also have technology to detect and reverse telecine.
Even without telecine, you may detect jerkiness in film content. Some HDTVs come with technology designed to make 24p movies look smoother. Vizio has a Real Cinema control, which is separate from the Smooth Motion control. When turned on, the Real Cinema feature generates new frames of extrapolated content instead of simply repeating the original movie frame.
Interpolation Introduces Artifacts
Do smoothing technologies and higher refresh rates work? At PCWorld, we leave motion smoothing technologies on their default settings when we test, and we've noticed a general improvement in images involving motion. But occasionally we run into strange artifacts that these technologies have introduced.
For example, we recently tested a 40-inch Samsung LCD TV that looked terrific on most counts. But in a slow panning shot in the Blu-ray Disc version of The Dark Knight, we noticed that a window in an office building in the background appeared to switch floors--most probably because of an interpolation error in Samsung's motion-smoothing technology. (Samsung declined to make a representative available for interviewing for this story.)
Some display experts question whether any of these adjustment technologies help significantly. Ray Soneira, president of DisplayMate Technologies, says that consumers should take claims about refresh rates with a grain of salt.
Soneira points out that for an LCD to refresh at 120Hz, pixels must respond in 8 milliseconds; and to refresh at 240Hz, pixels must respond in 4 milliseconds. He is deeply skeptical about manufacturer claims regarding response times. "It's all marketing nonsense," he says.
Bill Schindler, a consulting engineer with Panasonic, is less critical of technologies for addressing motion artifacts, but he warns that consumers should be careful when fiddling with the controls for these technologies. "They're not going to have any idea of whether they're helping or hurting," he says.
"I generally would tell consumers to buy from a respected manufacturer and hope that they've set it [the TV] up with their default to be correct."
Ultimately, your appreciation of motion-smoothing technologies will depend on personal taste, especially where movies are concerned. Some people don't like the ultrasmooth look that these technologies can introduce (sometimes referred to as the soap-opera effect), preferring instead a slightly flickery look. If that's how you feel, your best bet might be to turn off any motion-smoothing controls.