Tutorial on 3-D Technologies for Home LCD TVs

Technical advances are continuously being made in the area of shutter-glasses 3-D systems. Knowledge of several key elements – cross-talk, fast liquid crystals, high-speed driving techniques, and LED backlights – is essential in understanding this technology, as well as what comes next.

by Seongki Kim

3-D technology has been successfully deployed in the LCD industry since early 2010. Implementation of the stereoscopic 3-D technology currently in general use would not be possible without the development of the 240-Hz LCD system. This 3-D LCD system was the result of many advances in technical areas such as liquid crystals (LCs), driving circuits, and backlight controls. The fast response time of LCs in 3-D eyewear was particularly important to good 3-D performance. This article reviews technical areas related to shutter-glasses-type 3-D systems, including a recent advance, the polarization-switching 3-D technique, and concludes with a discussion of 3-D performance in the future.

An Ideal Stereoscopic System

The high-resolution 3-D system as seen in cinemas today is currently considered the best stereoscopic 3-D solution. The left- and right-eye frames actively switch on the screen and work in cooperation with the eyewear. The same principles can be adopted for an LCD-based 3-D home system. An additional panel on the TV would be necessary to convert the optical signal from the LCD panel and match it to the optical-filter characteristics of the eyewear.

Such an idea has become closer to realization as a result of frame-rate innovations in the LCD industry. Due to technical limitations, it has until recently been much harder to raise LCD frame rates to 240 Hz and beyond compared to impulsive-type emissive displays such as plasma displays.

The operational principle for shutter-glasses-type (SG-type) 3-D is to sequentially display left- and right-eye images so that the eyewear captures the left-eye image on the left lens while the left lens is open and the right-eye image on the right lens while the left is closed. The panel transmits a reference signal to the eyewear for synchronization between the two. Therefore, in order to achieve a flicker-free image for both eyes, the SG-type 3-D implementation requires at least twice the frame rate in 3-D mode than in normal 2-D mode.1–3

120 vs. 240 Hz

Theoretically, it would be possible to implement this type of stereoscopic 3-D system with a 120-Hz double-speed frame rate known as the "120-Hz driving scheme." However, there is a trade-off between brightness and cross-talk in 3-D mode that needs to be understood. For moderate 3-D brightness, for instance, designers have had to allow an unsatisfactory level of 3-D cross-talk. In contrast, acceptable cross-talk can be achieved but at the expense of suitable brightness. The problem is related to several aspects of the LCD architecture, including response time.

It is not simple to separate two consecutive frames without overlapping images in a progressive-scanning hold-type device. Theoretically speaking, a full image with no overlap would be available only during vertical blanking intervals, and elsewhere a current image would be written over a previous image. In most cases, LC response time is not fast enough.

Thus, the need for even higher frame rates and the advent of the 240-Hz LCD system contributed to the birth of the 3-D LCD TV system because the 240-Hz system generates four frames in order to process a frame each of 60-Hz 3-D input.

With the extra frames in the 240-Hz system, however, we can enhance the 3-D brightness. Backlighting techniques are used to allow time for the switching of the lenses of shutter glasses between the left (L) and right (R) eye images. Experience tells us that the duty ratio would be optimal at 40–50% for a reasonable amount of 3-D cross-talk. For black-frame insertion (BFI) such as LBRB, the shuttering of the 3-D eyewear can be managed relatively easily.

Two frames in the 240-Hz system can be assigned to display left and right stereoscopic images. The other two can be used to minimize 3-D cross-talk (read more about cross-talk below) in two different ways. Each extra frame can copy its previous frame, making LLRR (two identical left frames and two identical right frames) in order. Otherwise, two extra frames can be replaced with black frames, making LBRB (black frames between left and right frames) (see Fig. 1). The foregoing two methods can work with appropriate LED backlight techniques, which are also discussed later on.

 

Fig. 1: A 240-Hz 3-D timing diagram shows eight-block LED backlight scanning under an LBRB scheme.

 

The foregoing two methods have advantages and disadvantages from a product perspective. In slow LCs, the BFI technique is suitable. In general, 3-D brightness and cross-talk will vary on the rise (Tr) and fall (Tf) time of liquid crystals. In many cases, the slow rise and fall time of LCs can be overcome via electronic driving techniques and backlight controls.

We will now examine stereoscopic 3-D technology in terms of the following categories: cross-talk, LCs, drive circuitry, and backlight units.

3-D Cross-talk

Cross-talk is the undesired optical mixing of left- and right-eye images resulting from imperfections in the 3-D system. It is essential to understand the nature of 3-D cross-talk before discussing 3-D techniques in LCDs. Many formulas have been published, but there are no certified standard metrics yet.4–6 The most widely used formula for 3-D cross-talk is

 

eq1

 

Equation (1) evaluates the luminance difference between the left and right lenses after transitioning from the white level to the black level. An optimal result would be no difference between the transitions, so that cross-talk is zero. Note that LB is the black image at the left image, LW is the white image at the left image, RB is the black image at the right image, and RW is the white image at the right image.

An LCD is a progressive-scanned hold-type device. This characteristic by definition has a negative impact on 3-D cross-talk performance. As seen earlier, the desire is to measure the time taken in switching the 0–100% and 100–0% levels; i.e., the rise and fall time of LCs from the white image to the black images, and vice versa.

In 2-D mode, we are interested in the rise and fall time from 10 to 90% or vice versa. However, 3-D cross-talk has brought us to change the conventional definition in the 3-D mode.

In 2-D mode, the foregoing problem is shown as motion blur at edges of fast-moving objects, as in Fig. 2(a). In the 3-D mode, it appears as 3-D cross-talk between L and R stereoscopic images.

 

(a) fig2a           fig2b (b)

Fig. 2: Perceptions of artifacts under conditions of slow LC response differ between 2-D and 3-D: (a) motion blur is perceived on the edges of a moving object in 2-D mode and (b) as cross-talk in 3-D depth for a still object in 3-D mode.

 

High-Speed Liquid Crystals

LC response is sensitive to panel surface temperature. Figure 3 shows the relationship between LC response time and panel temperature. The rise time contributes to the brightness in the 3-D mode. So, fast-rising LCs will help enable brighter 3-D images. In the 3-D mode, the LC fall time is dominant in 3-D cross-talk. Hence, LC modes with fast fall times such as the VA mode7–9 will be beneficial to low cross-talk in the SG-type 3-D system. From a 3-D perspective, it is ideal if the rise and fall time can be smaller than the frame time.

 

fig3

Fig. 3: Panel-surface temperatures vs. LC response results are enhanced by small-cell-gap novel LCs.

 

In general, shutter glasses for the SG-type 3-D use the TN-mode. After one lens is closed, the other lens will open. Ideally, the sum of the rise and fall time of the LC in the panel is expected to be smaller than that of the eyewear, reducing the level of 3-D cross-talk.

Ad Hoc High-Speed Driving Technique

High-speed LCD panels are facing issues from excessive electrical rise time (distributed resistance and capacitance in the drive lines, sometimes referred to as "RC") delay and nearly optimized LC response time. The charging time can be doubled-up at a frame rate twice as high. The technique has been known as the half-G2D (hG2D) technique.10

The world's first 480-Hz full-HD LCD TV could help eliminate skepticism with regard to LCD operation speed. Many people thought that the LCD-TV panels, particularly large-sized LCD panels, could not run faster than 240 Hz due to the limitations on RC delay and LC response time. This might be true from a conventional design viewpoint, which has a single line for the gate and another single for the data (aka 1G1D).

However, in the new design architecture (aka hG2D), two scan lines share their charging time. So, the charging time for the 240-Hz remains same for 480 Hz. For instance, there are 1080 progressive scan lines, and each line will turn on and off sequentially in order in the 1G1D. The corresponding line-charging time in the 240-Hz FHD will be about 3.7 μsec. Likewise, the line-charging time for 480 Hz is about 1.8 μsec at the 1G1D. However, we can operate two scan lines at the same time by using hG2D, i.e., line 1 and 2, and line 3 and 4, up to line 1079 and 1080. Then, the 480-Hz line charging time can be increased to 3.7 μsec at the expense of twice as many data lines, which causes a small sacrifice in aperture ratio.

LED Backlights

LED backlights play an important role in the left frame – left frame - right frame – right frame (LLRR-type) 3-D implementation in separating consecutive frames; i.e., the left and the right. There are two types of LED implementations, which are direct and edge LED structures. The direct LED implementation is beneficial for warming up the panel-surface temperature and for better thermal-energy distribution. The increase of panel- surface temperature, as seen in Fig. 3, will cause the LC mode to have a fast rise and fall time.

Meanwhile, edge-type LED bars can be located at the top/bottom sides or left/right sides according to their design and cost target. In general, the top/bottom edge types are used for a global blinking effect, but the left/right edge types are good for a scanning effect. In the scanning mode, high-directional light-guide plates (LGPs) are advantageous in lowering 3-D cross-talk by reducing the leakage of light to adjacent blocks.

Figure 1 displays the timing diagram of the 240-Hz 3-D LCD system with an eight-block scanning LED module. The black solid lines denote driving signal inputs of the LBRB block, the red solid lines show LC response curves, the blue solid lines represent the shutter-glasses timing, and the white areas indicate the period of LED-on. The black shaded areas represent the period of LED-off. The left (L) frame input is a full white image and the right (R) frame input is a full black image, which is considered one of the patterns most conducive to 3-D cross-talk. One can observe that the undesired red tail in the black shaded area can be effectively hidden by turning off the light source.

Active-Shutter 3-D System

As shown in Fig. 4, the AS 3-D system consists of a TFT-LCD image panel for image display and an active-shutter panel that is a large single-pixel LC cell that causes the image polarization to be switched between left and right circular polarization. This shutter panel is synchronized with the left- and right-eye images being displayed on the image panel and works in tandem with passive glasses worn by the observer. This system is analogous to the RealD system used in movie theatres, and the choice of circular polarization for image selection was made to reduce the sensitivity to the observer's head tilt.

 

fig4

Fig. 4: This active-shutter 3-D system uses a TFT-LCD image panel, an active-shutter panel, and a quarter-wave plate.

 

The AS panel should have a faster rise and fall time than the image-display panel. The LC in the active-shutter panel would not degrade the image quality of the image panel. For instance, the OCB mode (aka pi-cell) could be a good candidate11,12 because it inherently switches faster than the TN or VA modes.

The operational principle of the AS 3-D system is illustrated in Fig. 4. The shuttering AS panel is running in synchronization with the 3-D images on the TFT-LCD image panel. It is actively alternating or switching from one state to the other between the two-phase states of 0 and λ/2, where λ indicates wavelengths of red, green, and blue colors. The QWP symmetrically compensates the foregoing two polarization states to –λ/4 and λ/4. The lenses of the passive eyewear are characterized to –λ/4 and λ/4, whose slow axes are orthogonal to each other. Thus, the state of –λ/4 on the AS panel is captured only by the lens of –λ/4, and the state of λ/4 only by the other lens of λ/4. The AS panel, including the QWP and the eyewear, is expected to have the same optical characteristics for the three primary colors of red, green, and blue as the image panel.

A 46-in. active-shutter 3-D (AS-3-D) panel was demonstrated by Samsung (a joint project between RealD and Samsung) at CES and at Display Week 2011. Previously known as active-retarder 3-D or polarization-switching 3-D, AS 3-D requires fast on/off switching times in order to maximize brightness and minimize cross-talk.

The Driving Techniques

The driving scheme of the AS 3-D panel differs according to the operation speed of the TFT-LCD image panel. TV mainly uses 240-Hz LCD panels, but 120-Hz panels are mainly used in monitors and notebooks. Theoretically speaking, there would be no need for backlight scanning control in the 240-Hz LBRB 3-D applications. The AS panel is fast enough to complete its state transition from one state to another during the period of a black frame. However, the 120-Hz 3-D system uses no black frames for the state transition. The backlight scanning control must be implemented in the 120-Hz 3-D system, synchronizing with the seg-ment of the scanning lines from the left-eye frame to the right-eye frame, or vice versa (Fig. 5).

 

(a) fig5a           fig5b (b)

Fig. 5: The operational principle of the active-shutter 3-D system includes (a) the off-state and (b) the on-state.

 

Driving Optimization

As mentioned in the previous section, the purpose of the BFI in the 240-Hz 3-D system is to reduce 3-D cross-talk that is caused by insufficient LC response time. In the author's experiment, the AS panel with a pi-cell mode was fast enough to complete its state transitions in one black-frame period. The AS panel in the 120-Hz image panel had no room for BFI, so it had to be divided into many segments. The AS panel was also fast enough to follow the state transitions according to the speed of the progressive-scanning lines on the image panel.

The Future

This article reviewed some basic technical concerns related to the shutter-glass-type 3-D system and the active-shutter 3-D system. They both belong to the active-shutter 3-D category since the left- and right-eye stereoscopic images switch on the LCD panel. The active-shutter 3-D has brought the shutter function onto the panel. Regardless of the shutter position, displaying the stereoscopic images remains the same.

As we prepare the autostereoscopic 3-D technology in the future, it may be necessary to attach an additional panel for the function of an active lens. This additional panel can be free from some critical requirements for the image LCD panel, such as the TFT structure and color filter. In addition, advances in plastic materials will lower the cost. It is important to keep in mind that as we prepare the next generation of 3-D technology, we should not degrade image resolution for cost competitiveness.

References

1D. Minoli, 3-D Television (3-DTV) Technology, Systems, and Deployment (CRC Press, 2011).

2D-S. Kim, S.-M. Park et al., "New 240-Hz Driving Method for Full HD and High-Quality 3-D LCD," SID Symposium Digest 41, 762–765 (2010).

3T. C. P. Hsu, J. C. L. Cheng et al., "High video image quality technology: dynamic scanning backlight with black insertion (DSBBI) implemented in a 32-in. OCB-LCD TV," SID Symposium Digest 38, 353–355 (2007).

4L. Meesters et al., "A Survey of Perceptual Evaluations and Requirements of Three-Dimensional TV," IEEE Trans. on Circuits and Systems for Video Technology 14 (2004).

5M. Siegel, "Perceptions of Cross-talk and the Possibility of a Zoneless Autostereoscopic Display," Proc. SPIE, Stereoscopic Displays and Virtual Reality Systems (2001).

6A. J. Woods, "Understanding Cross-talk in Stereoscopic Displays," 3-DSA (Three-Dimensional Systems and Applications) Conference, Tokyo, Japan, 19–21 May (2010).

7M. F. Schiekel and K. Fahrenschon, Appl. Phys. Lett. 19, 391 (1997).

8K. H. Kim et al., Proc. Asia Display '98, 383 (1998).

9S. S. Kim, "Super PVA Sets New State-of-the-Art for LCD TV," SID Symposium Digest 35, 760–763 (2004).

10S. S. Kim et al., "World's First 240-Hz TFT-LCD Technology for Full-HD LCD-TV and Its application to 3-D," SID Symposium Digest 40, 424–427 (2009).

11P. J. Bos and K. R. Koehler, Mol. Cryst. Liq. Cryst. 113, 329 (1984).

12M. Noguchi and H. Nakamura, SID Symposium Digest 28, 739–741 (1997). •

 


Seonki Kim is with Samsung Electronics Co., Ltd., LCD Business. He can be reached at seon.k.kim@samsung.com.