Dynamic-Scanning Backlighting Makes LCD TV Come Alive

Dynamic scanning takes backlights a step beyond dimming for improved control of motion blur and more precise dynamic contrast control.

by Seyno Sluyterman

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ALTHOUGH liquid-crystal displays (LCDs) have been well accepted for computer-monitor applications, they still have two major drawbacks for TV application. The first drawback – severe motion artifacts – becomes apparent when viewing sporting events or action-oriented movies. The second drawback – insufficient contrast – is observed when viewing LCD TV in a room with reduced ambient illumination. Once the causes of these shortcomings became better understood, it was clear that both could be addressed by dynamic-scanning backlighting.

Improving the Illusion of Motion

Motion artifacts can have several causes, one of them being the slow response time of the panel. However, even after designers reduced panel response time to the point where they expected the motion artifacts to disappear, they were still very visible. Why? Because the smearing of the image actually occurs in the human eye.

The eye perceives moving objects as being sharp only when the eye can track the objects, and that is possible only for movements that are linear or nearly linear. This is where an LCD panel with a conventional, continuously lit backlight fails. Because LCDs sample incoming video data at the frame rate and hold the image over the entire frame period, objects on the screen exhibit a staircase-like movement instead of a continuous movement (Fig. 1).

Imagine an object that moves over the screen from left to right in 2 sec, which is not uncommon at all. Further assume a frame rate of 60 Hz for an LCD panel with 1440 pixels per line. Then, the image smearing resulting from the hold effect would be 12 pixels over one frame period! This is unacceptable even for standard definition television and is grossly inadequate for HDTV.

 

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Fig. 1: A plot of the location of a horizontally moving point shows a linear relationship between location and time (left). As exhibited on an LCD with a continuous backlight, this motion becomes a staircase, and the point now has a substantial apparent width (right).

 

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Fig. 2: The dynamic sampling and very short duty cycle of a CRT has proved its effectiveness in rendering moving images, as indicated here by its location-vs.-time diagram for a horizontally moving point (left). This successful approach is closely approximated by an LCD with a dynamic backlight (right).

 

Interestingly, the absence of an image does not contribute to image smearing, but an image at the wrong location does. This phenomenon is vigorously exploited in the CRT, where each part of the image is only present for less than a millisecond. By adopting a dynamic-scanning backlight, which illuminates each part of the panel for only a short moment, this principle can also be applied to LCDs.

Conceptually, a dynamic-scanning backlight is not the simplest approach to reducing image smearing, and other approaches have certainly been tried, both in the laboratory and in production.

Reducing Motion Smearing

One way of reducing the hold time of a display would be to increase the frame rate. This is, however, a complex method that requires even faster LCD responses, and even a doubling of the frame rate reduces the hold time only by a factor of 2. Furthermore, the required motion estimation and compensation techniques are not trivial.

Alternatively, a black frame can be inserted between each two-image frames, which eliminates the need for motion-compensated interpolation techniques. Several companies that manufacture LCD-TV modules are pursuing this approach today. But with this black-data insertion, both luminance and contrast suffer – unless the approach is combined with a scanning backlight! When black-data insertion is omitted in the bright areas of the screen, a technology also known as Grey Field Insertion, the loss in luminance is avoided, but there is no motion-fidelity improvement for these brighter areas of the screen.

Yet another approach is the use of a blinking backlight, defined here as a backlight that illuminates every point on the screen at the same time. In monitors and TVs with so-called side-lit backlight systems, all points on the screen are always illuminated at the same time. But such a backlight cannot provide optimum timing everywhere on the screen, as will be explained later in this article. As a result, optimum sharpness of moving objects can only be obtained locally, not everywhere on the screen. The reason for this is that the addressing of the entire panel takes time; in fact, it takes one frame period. We could consider drastically decreasing the addressing time of the panel, but that is a complex issue and we want the application of the backlight system to be simple, without having a major impact on the driving schemes of the panel.

Thus, for good motion portrayal, it is best to have a backlight with a small illumination duty cycle. But a small duty cycle is not in itself enough to solve the problem because the backlight illumination phase, i.e., the illumi-nation timing in relation to the writing sequence, is also important. To obtain the optimum representation of motion, the following timing has to be obeyed for each line on the panel:

(1) the image must be written during addressing,
(2) the pixels need time to respond,
(3) the panel must be illuminated for a short time.

 

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Fig. 3: By arranging for the required illumination period to end just before the new image is written into the panel, the longest possible panel response time can be achieved.

 

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Fig. 4: In a dynamic backlight, each of the lamps is activated for only a short period of time and motion portrayal is improved.

 

Ideally, the pixel response time is less than two thirds of the frame time when the illumination period is one third of the frame time. Arranging for the required illumination period to end just before the new image is written into the panel provides the most relaxed requirement for panel response time; i.e., the response time can be longer (Fig. 3).

Because the timing has to be chosen so that for each line of the display the light is switched off just before the new image is written to the panel, a free-running backlight system with a small duty cycle will not work. A synchronized system is required.

An image is usually written to the panel line by line, from top to bottom, during almost the entire frame time. So, from a timing point of view, the LCD illumination must also scroll down from the top of the screen to the bottom, in phase with the addressing of the panel (Fig. 4).

Improving Contrast

When using LCD TV in an environment with reduced illumination, it is quite apparent that LCD panels have poor contrast in dark scenes because even in the dark state there is always some light leakage from the backlight. This is particularly noticeable when the panel is viewed at an angle.

In dark scenes, the contrast of the displayed image can be improved by reducing the luminance of the backlight unit and, at the same time, increasing the panel transmission. Dimming can be accomplished by a reduction of the illumination duty cycle.

Combining dimming with a scanning backlight has additional advantages. With non-scanning dimmable backlights, there is no clear demarcation in illumination between frames, so that a dimming action on one frame may spill over into neighboring frames. With a scanning backlight, however, because of the accurate in-phase timing of the illumination with the addressing of the panel, it is possible to address each complete frame independently of the previous or subsequent frame. This allows much faster response to changes in illumination than is possible with non-scanning dimmable backlights, a feature that is particularly advantageous with transitions from dark to bright scenes.

 

Figure_5_tif Philips Lighting

Fig. 5: This see-through view of the Aptura backlight system for a 32-in. LCD shows the eight hot-cathode fluorescent lamps, each with a diameter of 16 mm.

 

Figure_6_right_tif Figure_6_left_tif Philips Lighting

Fig. 6: This image of a musical scale was moved from left to right at a speed of 10 pixels per frame on an LCD with a continuous backlight (left) and on a similar LCD with a scanning backlight (right). The reduction in motion smearing is evident.

 

Practical Embodiment

At Philips Lighting, we have developed Aptura Lighting Technology, and with that technology we have designed a backlight system that facilitates both scanning and dimming simultaneously.

To improve motion portrayal, the LCD illu-mination must have a small duty cycle. On the other hand, luminance should not be sacrificed nor should costs be allowed to escalate. With a duty cycle of 35%, a light source is required that can provide three times as much peak luminance as is required in a non-scanning application if the original luminance is to be maintained. We therefore decided to use eight fluorescent lamps with a diameter of 16 mm because of their exceptionally high light output and established high reliability. For a 32-in. backlight system, eight of these light sources are sufficient to reach a light output of 550 cd/m2 at a 35% duty cycle (Fig. 5).

Our light sources contain heat-controlled electrodes in order to guarantee a lifetime of more than 50,000 hours despite the dynamic lamp load caused by dimming and the higher peak currents required for scanning (these are hot-cathode fluorescent lamps, in contrast to the cold-cathode fluorescent lamps that are used in the vast majority of large-area LCD backlights used today). The backlight system produces significant improvements in the sharpness of moving images (Fig. 6) and in dark-scene contrast (Fig. 7).

Conclusions

We have designed scanning LCD backlight systems for LCD-TV sets with diagonals of 32 in. and larger by applying Aptura Lighting Technology. These backlights facilitate the representation of moving objects in a relatively simple way by using 35%-duty-cycle backlight operation at a 550-cd/m2 full-plane front-of-screen luminance. The technology also allows dynamic contrast improvement by employing backlight dimming up to a factor of 8 under scanning conditions. This intelligent backlight will be available for the next generation of LCD TVs in 2006. •

 

Fig__7_tif Philips Lighting

Fig. 7: In addition to reducing motion smearing, dynamic backlights can be used to improve the contrast in dark scenes on an LCD TV. Here, the improvement is shown at a rather large viewing angle.

 


Seyno Sluyterman is LCD Backlighting System Architect at Philips Lighting, Building HBX-p, P.O. Box 80020, 5600 JM, Eindhoven, The Netherlands; telephone +31-40-27-24928, fax +31-40-27-56503, e-mail: seyno.sluyterman@philips.com.