Improving the Motion-Image Quality of LCD TVs

By using a scanning backlight and inserting a black frame between data frames in an LCD TV, the motion-blur artifact and the contrast ratio can be significantly improved.

Szu-Fen F. Chen, Chien-Lin Pan, Shen-Jiang Jeng, Chih-Liang Wu, and Peter Hsu

IN ORDER FOR thin-film-transistor liquid-crystal displays (TFT-LCDs) to capture an even larger share of the TV market, manu-factures must continue to improve image quality. The motion-blur artifact seen on TFT-LCD TVs has recently received a great deal of attention. The optically-compensated-bend (OCB) mode has been found to be effective in reducing this artifact. The OCB-LCD has the advantages of fast response time (less than 4 msec), wide viewing angle, and no gray-scale inversion. However, the contrast ratio of the OCB-LCD is limited to 600:1. To address both the motion blur and low contrast ratio, techniques such as scanning backlight with black insertion, dynamic backlight luminance control, non-parallel rubbing, and gamma adjusting have been investigated. As a result of our efforts, the motion-image quality of OCB-LCD TVs is now comparable to that of CRT TVs, and its contrast ratio has been increased by 60%.

OCB Mode

The term "optically compensated bend (OCB) mode" was coined by Tatsuo Uchida in 1993. By employing biaxial compensation film, the required driving voltage become 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 in Fig. 1, 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.


Szu-Fen F. Chen, Chien-Lin Pan, Shen-Jiang Jeng, Chih-Liang Wu, and Peter Hsu are with the TV Design General Division, R&D Center, Chunghwa Picture Tubes, Ltd., No. 1 Huaying Rd., Lungtan Shiang, Taoyuan, Taiwan 325, R.O.C.; telephone +886-3-480-5678 x7587, fax -0510, e-mail: fenny@mail.cptt.com.tw. Szu-Fen F. Chen is also with the Department of Photonics and the Display Institute, National Chiao Tung University, Hsinchu, Taiwan, R.O.C.

Fig. 1: 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. As a result, the OCB cell, possessing both the advantages of wide viewing angle and fast response time, could be the best candidate for motion-image-quality enhancement.

Scanning Backlight with Black Insertion

Because of the fast response time of an OCB-LCD, we have suggested the use of black-insertion (BI) technology, which alternates between a black data frame and a display data frame. The motion-image quality of LCD TVs is evaluated by using the well-known figure of merit — VESA's Moving Picture Response Time (MPRT). By using BI technology, the MPRT of OCB-LCD TVs can approach 8 msec. However, consumers wanted the motion-image quality of LCD TVs to approach the level of CRTs (~4 msec), and to accomplish this, we applied Scanning Backlight with Black Insertion (SBBI) technology to the LCD TV. It not only improves the motion-image quality but also makes the dark state of the OCB-LCD darker; i.e., the contrast ratio is also improved.

The control block diagram is shown in Fig. 2. According to the synchronizing signals of the double-frame unit and the timing controller (Tcon), the system can generate a pulse-width modulation (PWM) signal to control the backlight lamps. However, in order to achieve this, the scan timing of the backlight needs to be changed. When the input image is a black data frame, the system will turn off a few of the lamps at the black data positions. In other words, the turn-off timing of the lamps is synchronized with the black data insertion. The timing chart of SBBI technology is shown in Fig. 3.

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Fig. 2: Block diagram of Scanning Backlight with Black Insertion technology.

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Fig. 3: The timing chart shows how pulse-width modulation (PWM) controls the waveforms of the backlight.

 

As shown in the Fig 3, the vertical scanning lines are represented by m and the number of lamps by n. Each display section that includes m/n vertical scanning lines is displayed by one lamp. When the Tcon outputs the start pulse signal to the gate driver, the PWM generator synchronously starts the counter. While the black frame data is displayed, each m/n gate clock (GCLK) turns off the lamps in succession. The operation is illustrated in Fig. 4.

Due to the use of SBBI technology, the driving of the OCB-LCD TV is more similar to the impulse type than it is when using BI technology; the motion-image quality of OCB-LCD TVs can be further improved and made comparable to that of CRTs. The MPRT of a 32-in. OCB-LCD TV can be improved to 5.9 msec for average gray-to-gray levels, which is closer to the MPRT of CRT TVs (~ 4 msec) than any other LCD mode. A comparison of OCB- and MVA-LCD TVs is shown in Fig. 5. Clearly, the motion-image quality of the OCB mode is superior to that of the MVA mode by a factor of 3.

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Fig. 4: The schematic diagrams of a scanning backlight that matches the timing of black data insertion is shown. When the black data frame is displayed, as shown in (a), the lamps of the backlight are turned off at the corresponding position. When the image data frame is displayed, as shown in (b), the lamps of the backlight are turned on at the corresponding position.

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Fig. 5: The MPRT of conventional MVA-LCD TVs is approximated to be 18 msec (right). The performance of OCB-LCD TVs after the application of SBBI technology is 5.9 msec (left).

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Fig. 6: The improved image (a) by using Dynamic Backlight Luminance Control (DBLC) is compared with the original image (b).

Contrast-Ratio Improvement

Non-Parallel Rubbing Direction. Most LC molecules are vertically aligned in the dark state, but they are still not vertical next to the boundaries of an LC cell. Furthermore, the optical axis of the LC molecules diverges by 45° from the absorption axes of the polarizers. Therefore, an OCB-LCD can result in residual optical retardation, which in turn results in increased light leakage in the dark state.

In order to address the problem of light leakage, it is necessary to adjust the rubbing direction of LC molecules near the LC-cell boundary so that this direction is slightly closer to the absorption axis of the polarizer, which in turn will decrease the level of optical retardation of the LC cell in the dark state. This method will reduce the amount of light leakage and increase the contrast ratio. The angle between the top and bottom rubbing directions is defined by a1 for three conditions: 0, 6, and 12°. From the results shown in Table 1, it can be seen that when a1 is 6°, a better result is obtained. Moreover, the angular field of view of the cell caused by rubbing changed. Compared with parallel rubbing of the cell, the angular field of view is acceptable.

Dynamic Backlight Luminance Control. A dark image can be made darker by controlling the backlight luminance to match the gray-level histogram of the images. The enhanced image resulting from Dynamic Backlight Luminance Control (DBLC) and the original image are shown in Figs. 6(a) and 6(b), respectively. The level of darkness in the image can be improved from 0.9 to 0.45 cd/m2 when the maximum dimming of the backlight reaches 50%. As a result, the contrast ratio of an OCB-LCD is up to more than 1000:1. The block diagram of DBLC is shown in Fig. 7.

First, the system analyzes the input image and then determines the maximum data from the histogram. By using the proposed algorithm, the system can determine the improvement curve for the timing generation and the luminance of the backlight. Five improvement curves were generated and are shown in Fig. 8. If the maximum gray level is low, the system chooses curve 1; conversely, if the maximum gray level is high, the system chooses curve 5.

Independent Gamma Adjustment. Because of the high Δn LC that is used in an OCB-LCD, the retardation (Δnd) of different wavelengths is quite different (ΔnR<ΔnG<ΔnB) and larger than those of the other LCD modes. Hence, the transmittance of R, G, and B pixels is different when we drive them with the same voltage. In summary, Fig. 9 represents the transmittance curves of R, G, and B pixels when the driving voltages are different. If we choose the same voltage for R, G, and B, we could not obtain the optimum brightness level and contrast ratio.

An adjustable gamma circuit instead of a fixed one needs to be used to set a voltage for reference. We can set various voltages for the gray levels of the R, G, and B pixels. In other words, the signals will be separated from the three gamma curves of the R, G, and B pixels. In this way, the darkest level of the black image can be identified. Finally, in our experiment, the contrast ratio can be increased approximately by 5%.

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Fig. 7: The block diagram shows the control process of Dynamic Backlight Luminance Control (DBLC). After the image data is input into the system and the gray-level histogram is analyzed, the proposed algorithm can be used to generate contrast-enhanced output image data and a backlight inverter control signal.
Conclusion

The Scanning Backlight with Black Insertion (SBBI) technology that enhances the motion-image quality of OCB-LCD TVs has been described. By applying this technique, the MPRT of a 32-in.OCB-LCD TV can reach 5.9 msec for average gray-to-gray levels, which is comparable to that of CRT TVs (~4 msec) and better than that of other LCD modes. The contrast ratio of OCB-LCD TVs can be improved by over 60% by changing the structure of the LC cell and adjusting the gamma curves with respect to R, G, and B individually. By using dynamic controlled backlight luminance, the contrast ratio of OCB-LCD TVs can even be greater than 1000:1 at 50% dimming setting. Thus, the OCB-LCD is the best candidate for achieving high-video-image-quality TV. •

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Fig. 8: The improvement curves chosen for contrast enhancement.

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Fig. 9: The plot of the V–T curve of OCB-LCD TVs.