Technical & Editorial

The Inner World of LCD TVs - Part II


Perennial Problems

LCD TVs have always suffered from several problems with picture quality, including low contrast, high black level, motion blur, poor uniformity, and narrow viewing angles. Each of these problems has been addressed in various ways, though some of the solutions also introduce problems of their own.

Contrast and black level are closely related. Virtually all LCD TVs can pump out lots of light at the bright end, so the lower the black level, the greater the contrast. However, achieving a low black level is very difficult, because the LCD material isn't 100% effective at blocking light from the backlight—some light always gets through.

One way to lower the black level is by using a so-called black-matrix design, in which the subpixels are separated by opaque black lines (see Fig. 8). Another technique is black-frame insertion, which, as the name implies, inserts black frames between the actual video frames. (This also helps reduce motion blur; more in a moment.) Both of these solutions also lower the maximum amount of light at the high end, which is called the peak white level, but most LCD TVs have plenty of light to spare, so this isn't really too much of a problem.

Fig.8: A black-matrix LCD panel has opaque black lines separating the subpixels.

Full-array backlighting with local dimming lowers the black level considerably and increases the overall contrast in actual images. However, the number of LEDs is always much less than the number of pixels, and they are controlled in groups or zones, not individually. As a result, local dimming has trouble with very small areas of brightness or darkness. For example, tiny bright spots, such as stars in space, often look like they have halos around them because the LEDs behind the stars also brighten the space around them.

Many LED-edgelit sets offer something akin to local dimming—for example, sets with edge illumination can turn off the LEDs adjacent to the black letterbox bars in widescreen movies to make these bars appear much darker than they otherwise would be. Many manufacturers call this "local dimming," but it isn't really the same, and it's not nearly as effective as true local dimming with full-array backlighting.

Another LCD bugaboo is motion blur—that is, objects in moderate to fast motion look blurry and can leave ghostly trails behind them. Of course, some blurriness is due to the fact that video consists of a series of still images taken at a certain frame rate, and all moving objects look blurry to one degree or another.

But LCDs exaggerate this effect mostly because liquid-crystal molecules take some time to shift their alignment from one frame to the next. Also, the LC molecules are still switching while the video frame is being displayed. This transition from one alignment to another is known as response time, one of the most widely recognized specifications of LCD TVs.

Unfortunately, TV and panel manufacturers rarely disclose exactly how response time is measured—from black to white to black or gray to gray—and the time to transition from one gray level to another changes depending on the start and end levels. Surprisingly, the time it takes to transition from one gray level to another is often longer than the transition from full black to full white and then back to full black.

Although overall response times have decreased over the last 10 years or so, motion blur remains a problem for LCD TVs. To minimize this effect, NEC developed a compensating circuit in which the voltage applied at the beginning of each frame is higher than what is ultimately needed. This higher voltage accelerates the movement of the liquid-crystal molecules to their new position. For those subpixels that do not change from one frame to the next—which is a majority of them most of the time—the voltage is held constant, reducing the time required to charge and discharge all the capacitors.

Another way to compensate for motion blur is black-frame insertion, in which a black frame is inserted between video frames. This technique lets the LC molecules settle into their new orientation before you can see the subsequent video frame. A related technique is called backlight scanning, which turns off the backlight in sequential horizontal segments from the top of the screen to the bottom during the transition from one video frame to the next.

Probably the most effective technique to combat motion blur is frame interpolation, which is available only in LCD TVs with a refresh rate of 120 Hz or higher. As shown in Fig. 9 below, the TV's video processor looks at two consecutive video frames, identifies objects in motion by the fact that they are in different locations from one frame to the next, and synthesizes (interpolates) one or more new frames to display between the incoming frames. Any objects in motion are placed in the new frames where they should be if those frames were actually present in the signal.

Fig.9: When frame interpolation is turned on, the TV's processor analyzes where moving objects are in each incoming frame and synthesizes new intervening frames with the moving objects placed where they would be if the scene had been captured at the higher frame rate.

Frame interpolation is quite effective at sharpening the detail of objects in motion. However, it also creates an artifact commonly called "the soap-opera effect," because it makes the moving image look like it was shot on live video at a higher frame rate, just like a soap opera. This is especially apparent in movies that were originally shot at 24 frames per second. Many viewers object to the soap-opera effect far more than they dislike motion blur, so they turn off the frame-interpolation function in the TV.

Most LED edgelit TVs suffer from uneven screen illumination, which is particularly evident in dark scenes, such as those depicting outer space. This usually manifests as lighter and darker areas on the screen, often in the corners, and is often known as "flashlighting."

Finally, we get to the bane of all LCD TVs, no matter what their illumination source is—limited viewing angles. As your viewing position moves off center with respect to the TV screen, black and dark gray levels on the screen appear to rise, brightness dips, and images consequently appear washed out. In addition, colors shift and lose saturation.

Basic TN LCD panels have very narrow viewing angles that are asymmetrical as well—the image degrades in one way when moving from side to side compared with moving up and down, which is why TN is not used in consumer TVs.

IPS and MVA technologies provide wider, more symmetrical viewing angles, but not nearly as wide as plasma or OLED (organic light-emitting diode) flat panels. In general, IPS offers wider viewing angles than MVA, but MVA typically provides lower black levels, at least when viewed on axis—that is, directly in front of the center of the screen.

Fig.10: TN LCD panels exhibit very poor, asymmetrical off-axis performance compared with advanced LCD technology such as IPS.

Things like black level and motion blur can be improved with technologies such as local dimming, black-frame insertion, and frame interpolation, but I know of no solution that completely eliminates viewing-angle limits. As a result, LCD TVs are generally not recommended if you routinely have lots of people watching the TV or you watch it while lying on the floor. Also, LCD TVs don't normally do well mounted high above the floor—say, over a fireplace—unless the mount can be tilted downward toward the viewing area. Go to Page 7: A Bright Future