High-Contrast Low-MPRT OCB-LCD with Dynamic-Backlight-Control Technology
Slow response time and low contrast ratio still plague the performance of many TFT-LCDs. Solving one issue does not necessarily address the other. This article puts forth a potential solution: an OCB-mode LCD with dynamic-backlight-control technology.
by Shigesumi Araki, Kenji Nakao, Seiji Kawaguchi, Yuuki Nishimoto, Kazuhiro Nishiyama, Ken Shiiba, Akio Takimoto, Ryosuke Nonaka, Masahiro Baba, and Go Ito
WITH ALL THE PROGRESS MADE in the performance of TFT-LCDs, there are two important performance measurables that still need improvement: slow response time and low contrast ratio. Many improvements have been proposed to address these subjects. Among these, high frame rates, overdrive, and optically compensated bend (OCB) mode1 are used to improve slow response time. Of these techniques, dynamic-backlight-control technology dramatically improves contrast ratio.2 This article proposes a new dynamic-backlight-control technology for the high-speed OCB-mode.
Conventional LCDs
Dynamic backlight technology controls the backlight luminance and signal gamma data according to input images. For example, the backlight luminance is dimmed for dark scenes and the white luminance is optimized by adjusting the signal gamma data; therefore improving the contrast ratio.
Figure 1 shows a simple concept of a conventional LCD with dynamic-backlight-control technology illustrated with examples of a sequence of input images: the first frame is a dark image with a gray ball, the second frame is a bright large ball that suddenly appears on screen, and the third frame holds the previous image. Each frame's images are analyzed, and then the gamma function and backlight brightness are adjusted.
Fig. 1: Driving scheme of conventional dynamic-backlight-control technology.
In method #1, in which the backlight brightness is changed at the beginning of the second frame, gamma functions are changed at the beginning of the second frame. And the dark small-ball image is rewritten with the bright large-ball image. The scanning is performed from the top to the bottom of the screen. The mixed images appear in the second frame because the first frame image remains until it is rewritten by the second frame image. The optical response of the small-ball image shows that the brightness of the second frame is higher than that of the first frame. Therefore, the bright small ball appears for a moment, which creates a flashing problem.
Gamma functions are changed at the beginning of the second frame in method #2, in which the backlight brightness is changed at the end of the second frame. Since the driving mode is the same as that of method #1, the mixed images appear in the second fame. However, the flashing problem caused by the small-ball image does not occur in this method because the brightness of the small-ball image remains low during the second frame. However, it is also necessary to pay attention to the brightness of the large-ball image; during the second frame, it keeps its dark state and reaches its real brightness level in the third frame. The optical response of the large-ball image requires two steps, which makes the response time slower than that for a conventional LCD without dynamic-backlight-control technology.
Essentially, both methods weaken the picture quality compared to that of a conventional LCD without dynamic-backlight-control technology. Method #1 causes flashing problems, so this is not a suitable option. On the other hand, method #2 causes slow response time. Despite this and the fact that this method requires a very complex algorithm to prevent the flashing problem, this method has been applied to conventional LCDs.
Since the flashing problem and slow response problem are caused by the mixed images in the second frame, we came up with the concept of separating each sequence of frame images such as seen on a movie projector.
Conventional OCB-LCDs
The first version of our OCB-LCD, which we call "OCB-I," has adopted pseudo-impulse driving (also referred to as black insertion driving).3-6 With this driving, a black zone of constant width is running on the LC panel to separate optical outputs of consecutive frames and reduce motion blurring. The typical value for the moving picture response time (MPRT) is 8.2 msec. However, OCB-I had a low contrast ratio due to leakage of non-modulated light during the black-insertion period.
What You Should Know about the OCB Mode
As Szu-Fen F. Chen et al. explained in the November 2006 issue of Information Displaymagazine, the term "optically compensated bend (OCB) mode" was coined by Tatsuo Uchida in 1993. By employing biaxial compensation film, the required driving voltage becomes much lower than that for the original "Π-cell" – the use of an electrically controllable half-wave plate, as proposed by Phil J. Bos et al. earlier. The Π-cell was designed to enhance the LC response time by reducing the backward flow of the fluid by using surfaces that were treated so that their orientation was in the same pretilt direction. As illustrated here, the OCB mode, or Π-cell, transforms the splay state into the bend state when the applied voltage is higher than the critical voltage (Vcr). The white and dark states can then be switched in the bend state while increasing/decreasing the voltage.
In an OCB-LCD driving scheme, the LC phase transforms from the splay state into the bend state when a voltage level higher than the critical voltage is applied (Vcr is about 2 V). The voltage continues to increase in order to control the white and dark images in the bend state.
The LC molecules in the bend state are oriented 180° (Π) and vertically rotate between parallel rubbing directions within the OCB cell. The optical retardation of the upper and lower LC molecules compensates each other in the OCB cell. Therefore, the same optical retardations at different viewing angles can be seen. This intrinsic characteristic enables the OCB mode's wide-viewing-angle property. The bend-state orientation also enhances the LC switching speed to less than 4 msec between the ON and OFF states.
To overcome the trade-off between moving picture quality and contrast ratio, we developed a second version (OCB-II) using a CCFL blinking backlight.7 This driving method enables the reduction of the black image's luminance – without lowering the luminance of the white image – by turning off the backlight in sync with the black zone, thus improving contrast ratio. This improvement has already been incorporated into the mass-production of OCB-LCDs.
Furthermore, we have proposed a third version of the new driving method using an LED backlight (OCB-III).8,9 OCB-III includes a technology that can simultaneously improve moving picture quality (as fast as 2.0 msec) and contrast ratio.
Design of OCB-III with Dynamic-Backlight-Control Technology
Among conventional OCB driving methods, OCB-III driving is most suitable for dynamic-backlight-control technology because it separates each sequence of frame images. Figure 2 shows the driving scheme of OCB-III with dynamic-backlight-control technology. In this driving method, one frame is divided into three periods: (1) reset period, (2) addressing period, and (3) hold period. In the reset period, whole data are reset; in the addressing period, new data are written in each pixel by scanning in a similar manner. After whole data are written, the display data are held while the LED backlight turns on. Additionally, a dynamic controller has been added to the OCB-III driving. The dynamic controller first calculates gamma level and backlight level from the brightness level of an input image before addressing. Secondly, it controls the gamma level in the addressing period. Finally, it controls the backlight brightness by dimming in the hold period.
Theoretically, the driving method of OCB-III with dynamic-backlight-control technology causes no flashing problems. A diagram of the image operation is shown in Fig. 3. If the inputimages suddenly change between two frames – for example, from a dark image to a bright image – OCB-III with dynamic-backlight-control technology enables the display of the bright image by completing the image opera-tion before turning on the LED backlight; thus, the mixed image does not appear during the second frame. Therefore, this driving method does not create the flashing problem and keeps its fast response time. In addition, this driving method was realized by a simple algorithm.
The response-time property of a conven-tional LCD and the prototype OCB-III were measured with an oscilloscope and a luminancemeter with a photomultiplier tube. Input images are switched from full black to full white.
Fig. 2: Design of OCB-III LCD with dynamic-backlight-control technology.
Fig. 3: Driving scheme of OCB-III LCD with dynamic-backlight-control technology.
The measurement results are shown in Fig. 4. The response time of the conventional LCD with dynamic backlight control became slower than that of the LCD without dynamic backlight control. On the other hand, the OCB-III with dynamic backlight control shows the same response time as that of the OCB-LCD without dynamic backlight control.
Conclusion
The prototype 32-in. (1366 x 768) OCB-III LCD with dynamic-backlight-control technology features a maximum contrast ratio of 1,000,000:1 and an ultra-fast MPRT of 2.0 msec. High contrast ratio is consistent with high response time. Additionally, this LCD has a wide viewing angle of over 160° with a CR>50 and Δu¢v¢<0.02. Four additional prototype models have been developed: an 8-in. WVGA, 9-in. WVGA, 12.3-in. double-VGA (1280 x 480), and 12.1-in. SVGA. This shows that OCB-III with dynamic-backlight-control technology is also suitable for other OCB-LCD products.
OCB-III with dynamic-backlight-control technology achieves both high contrast ratio and fast response time. It shows high performance that exceeds that of CRTs in motion picture quality and almost the same performance in terms of contrast ratio. The combination of OCB-mode and dynamic-backlight-control technology provides the best solution for high-performance displays in various applications such as television, professional use (including medical and broadcasting equipment), and automotive.
References
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Fig. 4: Measurement result of response time.