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Motion video blurs picture for big panels at SID
By Ron Wilson, Executive Editor -- EDN, 6/14/2006
| More from SID: Small displays present unique problems Displays become more than dumb panels |
On one hand, the large-panel market is becoming more technologically stable. TFT LCDs dominate. Plasma panels are holding their own, despite a number of disadvantages. And Organic LEDs (OLEDs) are receiving more serious consideration as a future alternative.
But against this background of relative stability, there is turmoil in the trenches. The driving force behind the change is the problem of displaying rapidly moving images. For plasma and OLED technologies, both of which can be made with very rapid response times, this is not a serious issue. But LCDs often takes 30 msec or longer to respond to a change in electrical state, as their crystals retwist themselves and then slowly decay back to a ground state. Preventing blur for rapidly moving images is turning out to be a huge problem. And companies are marshalling resources all the way from solution chemistry to sophisticated digital image processing to solve that problem.
The most obvious way to attack the motion problem is to reduce the response time. Approaches include making the liquid-crystal solution less viscous so that the crystals can twist faster, or changing the whole mode in which the panel operates. OCB (optically corrected bend) mode, in which the crystals bend only a bit instead of twisting, is showing some promise here. Originally discussed a few years ago as a way of increasing viewing angle, OCB showed up on a number of prototypes this year as a potential answer to the motion problem. Samsung Electronics has achieve 6-msec response time in its version, according to Joseph Virginia, the company's vice president for LCDs.
Slashing the response time also permits Samsung to raise the refresh rate to 120 Hz. This, in turn, gives the impression of smoother motion. But it also requires motion interpolation between frames of video that are still arriving at 60 Hz. And that means some DSP capability and at least a couple of frames of local memory.
The approach also creates another problem. Even if you turn on the LCD pixel quickly, it stays on until you come back and change it again. Turns out this also plays a role in creating the apparent blurring of motion. A quick and dirty fix is to flash—or "sequence," as the industry says—the backlight, so that the user only sees the pixel just after the frame gets refreshed. Unfortunately, unless the refresh rate is very high, flashing can make motion jerky. It can also lead to visible flicker, especially for people with acute motion-sensitivity in their peripheral vision.
Moreover, reducing the duty cycle of the backlight raises other issues. A lower duty cycle means lower apparent brightness, and brightness has been one of the most stubborn demons to haunt LCD technology. Sequencing the backlight also translates to far more complicated control and power electronics for the backlight.
To try to make up for the loss in luminance, vendors are turning to both front- and back-side films that, essentially, waste less of the light falling on the back of the panel. in addition to increased luminance, films can also help with contrast ratios, which also helps with motion rendering. However, warned Sriram Peruvemba , senior manager of product marketing for Sharp Microelectronics, even a 600:1 contrast ratio is not ideal for moving images, and vendors are turning to image processing to increase the apparent contrast of rapidly moving images beyond what can be achieved with applied films.
Another change in the backlight is the growing competition between cold-cathode fluorescent lamps and LEDs. Fluorescents have the advantage of a lot of infrastructure and 50,000-hour life, according to Peruvemba. But they are unavoidably fragile and consume a great deal of power. "LEDs are challenging the fluorescents now," Peruvemba said. "Their lifetimes are getting better and their efficiency is getting better as well. But they generate heat, which is an issue, and it's not easy to diffuse the light from them."
White LEDs are the general choice in smaller displays, but in large panels designers are increasingly looking at combinations of red, green, and blue LEDs, which can deliver richer colors. Similarly, other new techniques are using cold-cathode fluorescents in green, blue, red, and crimson to achieve a richer color space.
Perhaps the most aggressive use of LED backlighting comes from an IP startup called BrightSide Technology, which is, not unlike many other vendors, using an array of white LEDs as a backlight source behind a diffuser. However, unlike other companies, BrightSide makes each of the LED squares independently controllable, so it can turn up, down, or off the light behind any particular area of the screen. An image-processing algorithm drives this capability on the fly by extracting average luminance data from the incoming video stream and, one suspects, smoothing out the boundaries between areas covered by the backlight squares. In order to keep up with HD data rates, the company implements the algorithm by reprogramming the shading engines in an nVidia graphics chip.
The approach allows BrightSide to expand the luminance range of the display far beyond the native capabilities of the LCD panel, simply by turning off the backlight behind dark areas of the image and turning up the light behind bright areas. The result is visually striking, and the technique provides an illustration of how image processing, backlight technology, power-supply design, and panel physics are all working together to improve the presentation of the moving image.
