JP3974436B2 - Liquid crystal display - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a light and high-contrast display device by realizing an efficient driving method by detecting voltage between electrodes across a liquid crystal layer. <P>SOLUTION: Inverse transition preventive voltage is adjusted according to the transmittance (white display) of an image. The maximum transmittance (brightness) of the display image is detected and the inverse transition preventive voltage is adjusted accordingly. A means to accumulate the display image for at least one screen and a means to detect the maximum transmittance (brightness) of the image are provided and the inverse transition preventive voltage are adjusted according to the maximum transmittance. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an OCB type liquid crystal display device.
[0002]
[Prior art]
At present, liquid crystal display devices are often used for screen display of personal computers, car navigation systems, monitors, TVs, and the like. As the alignment mode of the liquid crystal used for these liquid crystal displays, a TN alignment mode and an STN alignment mode using nematic liquid crystal are often used, but there are drawbacks such as a slow response and a narrow viewing angle. In recent years, a liquid crystal display device in an IPS (horizontal plane drive) display mode driven by a horizontal electric field has been put to practical use as a display mode having an excellent viewing angle, but there are difficulties in response speed and aperture ratio. In addition, there are display modes such as ferroelectric liquid crystal with a fast response speed and a wide viewing angle, but there are difficulties in shock resistance and temperature characteristics.
[0003]
Recently, an optical compensation (OCB) type orientation mode has attracted attention as a display mode with excellent high-speed response and a wide viewing angle, and has been actively developed. In this mode, as shown in FIG. 4A, liquid crystal molecules (liquid crystal layer) 106 are aligned in parallel on the inner surfaces of upper and lower substrates 54 and 56 for a liquid crystal display panel to form a splay alignment liquid crystal layer. Next, a voltage is applied between the upper and lower substrates 54 and 56 to transfer the liquid crystal molecules 106 from the splay alignment to the bend alignment as shown in FIG. 4B, and further to FIG. 4C. It has a bend orientation state. (Here, various methods other than voltage application have been devised as a method of transition from the splay alignment to the bend alignment, but the details are omitted.)
FIG. 4B shows a state where the transmittance (luminance) is high when a low voltage is applied (commonly referred to as white display), and FIG. 4C shows a state where the transmittance (luminance) is low when a high voltage is applied (commonly referred to as black). Display), and this white display, black display, and black-and-white intermediate display display video.
[0004]
FIG. 5 shows a cross-sectional view of a liquid crystal display panel in which a retardation plate 53 and a polarizing plate 51 for optically compensating the liquid crystal layer 106 for widening the viewing angle and a backlight 55 for providing display luminance are arranged. In FIG. 5, a plurality of TFTs (the outline of driving of TFTs will be described later in the description of FIG. 7) arranged in a matrix on one of the substrates 54 and 56 (for example, the lower substrate) 54 is shown. In a video screen commonly called XGA, at least 768 vertical x 1024 horizontal) are formed.
[0005]
A video signal is supplied to the source electrode of the TFT, and a scanning signal is supplied to the gate electrode of the TFT. A video signal is input to the source electrode in synchronization with the ON state timing of the gate electrode, and a voltage is supplied between the drain electrode of the TFT and the counter electrode on the other substrate 56 via the liquid crystal layer 106 to display a liquid crystal display. To do. A backlight 55 controls the transmittance of the backlight light by the operation of the liquid crystal panel.
[0006]
FIG. 6 shows the voltage dependence on the transmittance of the liquid crystal display panel. In the relationship between the transmittance (brightness) of the pixel in FIG. 6 and the applied voltage, the voltage value on the horizontal axis indicates that the video signal is input to the source electrode of the TFT described in the description of the liquid crystal display panel in FIG. Therefore, when the voltage applied to the counter electrode is constant, the horizontal axis voltage is detected by detecting the video signal input to the source electrode. Can be detected. 4, 5, and 6 correspond to FIGS. 4B and 5B when approximately in the range of Vw, and when in the vicinity of Vb sufficiently exceeding the voltage value Vw, FIG. c) corresponds to FIG. 5 (c), respectively. Here, Vw is a voltage value called white display voltage, and Vb is a voltage value called black display voltage.
[0007]
This OCB type liquid crystal mode is a liquid crystal display device characterized by a high speed, a wide viewing angle, and a bright display performance. Furthermore, practical application of a liquid crystal TV using this is under study.
[0008]
As described above, the OCB mode liquid crystal display is a mode in which an image is displayed with an applied voltage corresponding to white display, black display, and black and white intermediate state. When the white display voltage Vw is approximately 0 V, FIG. There was a problem that the reverse transition from the bend orientation as shown in b) to the splay orientation shown in FIG. This reverse transition is an irreversible change, and in order to use the OCB type liquid crystal display device as video equipment, means for always maintaining bend alignment is required. Since the transition from FIG. 4B to FIG. 4C and FIG. 4C to FIG. 4B takes several milliseconds, it is suitable for a moving image such as a video device that requires a high-speed response. is there.
[0009]
Therefore, when the white display voltage Vw is approximately 0 V, a pulse voltage for a certain period (duty) is applied during one frame of the screen as means for constantly maintaining the bend orientation in the previous period. Therefore, a driving method peculiar to the OCB type liquid crystal for preventing the reverse transition from FIG. 4A to FIG. 4A is required. Such a driving method (reverse transition prevention technology) peculiar to the OCB mode is a unique driving method in which a pulse voltage needs to be applied for a certain period of time to prevent reverse transition during one frame period. The ratio (duty) of inputting the video signal is reduced, and the luminance is also reduced.
[0010]
Therefore, a method for increasing the ratio (duty) of inputting a video signal by applying a reverse transition prevention voltage efficiently has been studied. A block diagram (FIG. 7) of a liquid crystal display device described in Japanese Patent Application No. 11-369838 and an outline of the method are shown as a prior example of the prevention of reverse transition.
[0011]
7, TFTs 107 are formed in a matrix on the lower substrate 54, the gate electrodes 18 of the TFTs 107 are connected to gate lines X1, X2,..., Xn, and the source electrodes of the TFTs 107 are source lines Y1, Y2. ,..., Ym, and the drain electrode 17 of each TFT is connected to the pixel electrode constituting the pixel 1071. This pixel electrode is connected to the counter electrode 1051 through the liquid crystal layer 106 to constitute a liquid crystal display panel. The counter electrode is driven by a voltage supplied by the counter driver 105. Reference numeral 109 denotes an IC (hereinafter referred to as a gate driver) for applying a voltage for turning on and off a TFT to the gate lines X1, X2, and Xn. The gate driver 109 sequentially turns each TFT connected to the gate lines X1, X2,..., Xn in synchronization with the supply of data (video signal) 101 to the source lines Y1, Y2,. Turn on. Reference numeral 108 denotes an IC (hereinafter referred to as a source driver) that inputs the video signal 101 to the source lines Y1, Y2,. The potential difference between the voltage supplied to the counter electrode 1051 and the video input (signal) voltage supplied to each TFT connected to the source lines Y1, Y2,..., Ym is the both ends of the liquid crystal 106 in the pixel 1071. This voltage value determines the transmittance of the pixel 1071.
[0012]
FIG. 8 shows timing voltage waveforms applied to the gate electrodes 18 through the gate lines X1, X2,... Xn. In the line sequential drive, a video signal waveform (voltage) 101 is input to the source electrodes Y1, Y2,..., Ym during a timing period applied to the gate electrodes 18 of the m TFTs 107 connected to the gate line X1. . This is a method of displaying all the pixels (m pixels) in one line arranged in the lateral electrode X1 based on the interelectrode potential difference between the video signal waveform (voltage) 101 and the voltage applied to the counter electrode 1051. The voltage waveform in FIG. 8 is a principle schematic diagram for explanation. At this time, if the potential difference between the electrodes is kept long for a period of about 0 V as shown in FIG. 6, the reverse transition to the splay alignment has been made.
[0013]
[Problems to be solved by the invention]
In the preceding example as a device for preventing reverse transition,
The present invention is designed to always apply a uniform reverse transition prevention voltage in one frame period. In FIG. 6, a constant reverse transition prevention voltage is always applied to pixel display of white display and intermediate display or higher. Therefore, the problem is that the rate (efficiency) of inputting the video signal is low, and the reverse transition prevention voltage equal to that of white display (and intermediate display) is applied to the screen display of black display. The problem is that the black transmittance increases and the contrast decreases.
[0014]
[Means for Solving the Problems]
According to the present invention, the reverse transition prevention voltage and the voltage insertion ratio (hereinafter, the ratio of applying the reverse transition prevention voltage in one frame is defined as the black insertion ratio in accordance with the transmittance (luminance) level of the screen. ) Is the idea to change.
[0015]
The details of the present invention will be described below based on our experimental results.
[0016]
In FIG. 9, the critical black insertion rate is a black insertion rate indicating a critical point at which a reverse transition from bend alignment to splay alignment occurs at a black insertion rate lower than that. In FIG. 9, Vw means white display (high luminance) voltage, and the parameter Vib means reverse transition prevention voltage. FIG. 9 shows the relationship between the white display voltage (Vw) and the critical black insertion rate in one frame, with the reverse transition prevention voltage (Vib) as a parameter (Vib: 4.5 v, 5.0 v, 5.5 v) ( (Measured at room temperature). FIG. 9 shows that the critical black insertion rate is lower as the reverse transition prevention voltage (Vib) is larger than the same white display voltage (Vw). It can also be seen that the critical black insertion rate is lower as the white display voltage is higher for the same reverse transition prevention voltage. For example, when the white display voltage (Vw) is approximately 0 V, the critical black insertion rate is 10%, 12%, and 14% when Vib is 5.5V, 5.0V, and 4.5V, respectively. It can be seen that the higher the voltage (Vw), the lower the critical black insertion rate. That is, as the reverse transition prevention voltage Vib is higher and the white display voltage Vw is higher, reverse transition does not occur even at a lower black insertion rate. As a result, higher efficiency can be realized.
[0017]
From the relationship between the results of FIG. 9 and the transmittance (luminance) with respect to the vicinity of the white display voltage Vw and the black display voltage shown in FIG. The reverse transition prevention voltage and the black insertion rate can be determined. The present invention is a device for determining several efficient reverse transition prevention voltages and black insertion ratios based on the above experiments.
[0018]
From the above experimental results and discussion, as the first idea of the present invention, when externally inputting a video signal to the OCB type liquid crystal display device and driving the display video screen, the external video signal is detected and the external video signal is supported. Means for adjusting and applying the reverse transition prevention voltage and the black insertion rate so as to maintain the bend orientation of the liquid crystal molecules, and as a second idea, it corresponds to the maximum luminance level of the display image screen of the OCB type liquid crystal display device. Means for detecting a video signal and adjusting and applying a reverse transition prevention voltage and a black insertion rate so as to maintain a bend orientation according to the maximum luminance level;
As a third idea,
Means for accumulating at least one screen (one frame) of the display video screen displayed by line-sequential driving; means for detecting a video signal corresponding to a maximum luminance level and a minimum luminance level of the display video screen; and the maximum luminance level And by means of adjusting and applying the reverse transition prevention voltage and the black insertion rate so as to maintain the bend orientation according to the minimum luminance level,
It is intended to solve the problem.
[0019]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[0020]
(Embodiment 1) FIG. 1 is a diagram showing a typical mode of a video display screen. 1A shows a white display (high brightness) for the entire video screen (pixel), and FIG. 1B shows a black display (low brightness) for the entire video screen (pixel). ) Is a display in which the entire screen is divided into white display and black display areas along the gate line. FIG. 1D shows the center pixel of the screen in white display and the pixels in other areas in black display. Represents each. FIG. 2 shows specific values of the pulse video signal and the reverse transition prevention voltage of the liquid crystal display device implemented in the present invention. In the present invention, the white display voltage (screen of FIG. 1a) ranges from 0V to about 1.5V, the black display voltage (screen of FIG. 1b) is about 4.5V, and FIGS. 1c and 1d are the same as FIG. And the display voltage of FIG. Therefore, it is only necessary to drive and adjust the reverse transition prevention voltage and the critical black insertion rate as described above for each of the display modes.
[0021]
Hereinafter, each video screen will be described in detail.
[0022]
The video screen of FIG. 1a is driven by selecting an optimum reverse transition prevention voltage and insertion rate that are efficient from FIG. 9 according to the white display voltage Vw. Here, the higher the reverse transition prevention voltage, the better. This is because the higher the reverse transition prevention voltage, the lower the critical black insertion rate and the higher the efficiency. For example, when the white display voltage is displayed as 0 V in our experimental results (FIG. 9), the critical black insertion rate exceeds 14% when the reverse transition prevention voltage Vib is 4.5 V, whereas Vib = At 5.5 V, reverse transition does not occur even at a black insertion rate of about 10%, and high efficiency can be maintained.
[0023]
On the other hand, unlike the case of the white display in FIG. 1a, the video image in FIG. 1b is displayed with the reverse transition prevention voltage Vib equal to the black display voltage Vb.
[0024]
In this case, when the display is driven with different reverse transition prevention voltage Vib and black display voltage Vb, for example, when the reverse transition prevention voltage is 5 V and the black display voltage is 4.5 V, the black transmittance increases and the contrast decreases. It became a video, and gradation display was reversed. This cause will be described with reference to FIG. An OCB type liquid crystal display device has a minimum value of transmittance at a certain voltage (black display voltage, for example, 4.5 V), and the transmittance (luminance) is inverted when a voltage higher than that (for example, 5 V) is applied to the liquid crystal. (In FIG. 6, it corresponds to the transmittance pair in the vicinity of 4.5 to 5.0 V and the video input voltage). This is because the OCB type liquid crystal display device performs display in the birefringence mode and performs black display in a state where the phase difference of the liquid crystal layer is not 0 (a state in which the phase difference plate and the bend alignment are compensated). Further, even when the reverse transition prevention voltage Vib was made lower than the black display voltage Vb (here, 4.5 V), the black transmittance similarly increased and the contrast decreased. This is because the transmittance naturally increases when a voltage lower than the black display voltage is applied.
[0025]
Therefore, in the case of black display as shown in FIG. 1b, the reverse transition prevention voltage Vib is made equal to the black display voltage Vb. This is clearly different from the case of FIG. 1a described above, which is preferable as the reverse transition prevention voltage is higher.
[0026]
In addition, the video screen of FIG. 1c is driven by selecting an optimum voltage and an insertion rate that are efficient and provide high contrast according to the display voltage of each pixel. In FIG. 1c, when displaying the vicinity of the center portion of the screen (that is, white display), the reverse transition prevention voltage is set as high as possible to obtain higher efficiency. Further, when displaying the other display portion (that is, black display), the reverse transition prevention is made equal to the black display voltage so that the black transmittance does not increase. As described above, by optimizing the reverse transition prevention voltage according to the display voltage of each pixel, a higher contrast display is realized on the entire optimum screen.
[0027]
In addition, for the video screen of FIG. 1d, the reverse transition prevention voltage and the voltage insertion rate are made uniform on the screen, thereby realizing optimization of the image quality (efficiency / contrast) of the entire screen. The idea is as follows. First, since the display other than the center of the screen is black, the reverse transition prevention voltage is set equal to the black display voltage so that the black transmittance does not increase. In addition, the black insertion rate and the reverse transition prevention voltage were determined so that reverse transition does not occur even in white display when displaying the central portion of the screen (that is, the portion where white display and black display are mixed along the gate line). Here, if the reverse transition prevention voltage is equal to or higher than the black display voltage, the black insertion rate can be reduced and the efficiency of the white display portion can be further increased, while the transmittance of the black display portion is increased. This resulted in a level difference in brightness with the black display portion other than the central portion of the screen, resulting in a display with a sense of discomfort over the entire screen. Therefore, the reverse transition prevention voltage is made equal to the black display voltage in the display at the center of the screen. As a result, for the screen of FIG. 1d, making the reverse transition prevention voltage and its voltage insertion rate uniform on the screen was effective in optimizing the display on the entire screen.
[0028]
As described above, it is possible to display images with higher efficiency and higher contrast by adjusting the reverse transition prevention voltage and the critical black insertion rate according to each screen.
[0029]
Here, four examples of screen display are shown in FIGS. 1a to 1d, but the present invention is not limited to this. In short, by detecting the maximum luminance (transmittance) level and minimum luminance (transmittance) level in the video signal included in the screen, and setting the black insertion rate and reverse transition prevention voltage accordingly, Efficient and higher contrast display can be realized.
[0030]
(Example 2)
The first embodiment is an invention in which a video signal is detected for each pixel or for each gate line selected corresponding to the video signal, and a black insertion rate and a reverse transition prevention voltage are set accordingly. This is particularly effective when the video signal can be predicted in advance (for example, display of video stored in a recorded broadcast / recording medium). On the other hand, when a video signal cannot be anticipated (for example, live broadcasting on a TV or the like), since a very high transfer rate is required for transferring luminance level data, it has been very difficult to implement at the present time.
[0031]
The second embodiment is a device that accumulates a display image at least every frame, detects a luminance level, determines a black insertion rate and a reverse transition prevention voltage corresponding thereto, and optimizes image quality (efficiency and contrast). is there. This is because the luminance level data is stored and transferred in one frame, so that the data transfer is easy and effective even when the video signal is displayed in an unexpected manner.
[0032]
A specific method of this embodiment will be described with reference to the drawings.
[0033]
FIG. 3 is a block diagram of a liquid crystal display device showing a second embodiment of the present invention. This device has a storage unit 1033 for storing one screen (one frame) of a display image and a detection unit 1032 for detecting a voltage corresponding to the maximum luminance in the stored one screen (one frame). It is a device for preventing the reverse transition by adjusting and applying the reverse transition prevention voltage and the critical black insertion rate so as to maintain the bend orientation according to the voltage value corresponding to the luminance. At the same time, the detection unit 1032 has a function of detecting the minimum luminance (minimum luminance detection unit). When the voltage value corresponding to the minimum luminance is a substantially black display voltage, the reverse transition prevention voltage is equal to the black display voltage. By doing so, it is devised to achieve high contrast.
[0034]
Specifically, through the following processes (1) to (5), the reverse transition prevention voltage and the critical black insertion rate in one frame are determined, and the display is performed under the optimum conditions.
[0035]
Process (1): First, data in one frame is stored in the storage unit. The maximum luminance level and the minimum luminance level are detected from the data by the detection unit.
[0036]
Process (2): When the minimum luminance level in the one frame is substantially black display, the reverse transition prevention voltage is made equal to the black display voltage. This is to suppress a decrease in contrast, and details are as shown in the first embodiment.
[0037]
Process {circle around (3)}: When the minimum luminance level is sufficiently brighter than black display (specifically, within approximately 4.0V), the reverse transition prevention voltage is applied as high as possible (for example, 5.5V).
[0038]
By the processes (2) and (3), the reverse transition prevention voltage is optimized so as to realize a high contrast. Next,
Process {circle over (4)}: A black insertion rate is determined such that the maximum efficiency is obtained from the maximum luminance level in one frame and the optimized reverse transition prevention voltage Vib and no reverse transition occurs. Specifically, it is determined using the reverse transition prevention voltage Vib and the critical black insertion rate shown in FIG. At this time, the data shown in FIG. 9 was introduced into the display system in the form of a ROM (Read Only Memory) IC. For example, when the voltage corresponding to the maximum luminance is about 1.0 V, the reverse transition is less likely to occur compared to the case of about 0 V. Can be increased.
[0039]
Process {circle over (5)}: The black insertion rate and reverse transition prevention voltage determined in steps {circle around (1)} to {circle around (4)} are transferred to the gate driver and source driver through the drive control unit (FIG. 3) and actually displayed.
[0040]
The above is the process for optimizing the reverse rotation prevention voltage and the black insertion rate in one frame. This is done for each frame, and maximum efficiency and maximum contrast are displayed according to each screen.
[0041]
Note that this invention is a block diagram of a driving method including both an accumulating unit and a luminance detecting unit in the video signal of one screen. Of course, a block diagram of a driving method including only the detecting unit is also included. .
[0042]
【The invention's effect】
As described in detail above, the present invention eliminates the need to apply a reverse transition prevention voltage for a certain period that is not always necessary for each frame, and increases the ratio (duty) at which a video signal can be applied during one frame period. Can do. In addition, by detecting the voltage corresponding to the maximum luminance / minimum luminance and adjusting and applying the reverse transition prevention voltage and the black insertion rate according to the maximum luminance / minimum luminance, the ratio that the video signal can be applied is more It can be set appropriately to maintain high-efficiency and higher-contrast display. Then, the reverse transition prevention voltage and the black insertion rate are adjusted so that the bend orientation is maintained according to the voltage corresponding to the maximum luminance and the minimum luminance in the transmittance accumulated for one frame, and the contrast is not lowered. This provides an efficient reverse transition prevention drive method that can increase the rate at which the video signal can be applied to all the video signals for one screen, and is bright and high-contrast video from an industrial standpoint. The present invention is effective for manufacturing an OCB type liquid crystal display device with a screen.
[Brief description of the drawings]
FIG. 1 is a diagram showing typical modes of a video display screen. FIG. 2 is a diagram showing specific numerical values of a video signal and a voltage for preventing reverse transition implemented in the present invention. 3 is a block diagram of a driving method according to a third embodiment of the present invention. FIG. 4 is a diagram showing splay alignment and bend alignment of the OCB type liquid crystal according to the present invention. FIG. 5 is an OCB type liquid crystal. FIG. 6 is a diagram showing a graph showing transmittance versus voltage characteristics of the liquid crystal display device of the present invention. FIG. 7 is a block diagram of a conventional driving method for an OCB type liquid crystal display device. FIG. 8 is a timing diagram of drive waveforms. FIG. 9 is a diagram showing the relationship between the critical black insertion rate and the reverse transition prevention voltage Vib.
101: Video signal 102: Synchronization signal 103: Video signal detector (input signal level detector)
105: Counter driving unit 1051 Counter electrode 106: Liquid crystal (liquid crystal molecule)
107: TFT
1071: Pixel 1032: Maximum and minimum transmittance (luminance) detection unit 1033: 1 frame storage unit 51: Polarizing plate 53: Phase difference plate 54: Substrate 55: Backlight 56: Substrate 17: Drain electrode

Claims (3)

When an external video signal is inputted to the OCB type liquid crystal display device and the display video screen is driven, the external video signal is detected and reverse transition is performed so as to maintain the bend orientation of the liquid crystal molecules corresponding to the external video signal. Liquid crystal display device that adjusts and applies prevention voltage and black insertion rate. A video signal corresponding to the maximum luminance level of the display video screen of the OCB type liquid crystal display device is detected and applied by adjusting the reverse transition prevention voltage and the black insertion rate so as to maintain the bend orientation according to the maximum luminance level. The liquid crystal display device according to claim 1. Means for accumulating at least one screen (one frame) of the display video screen displayed by line-sequential driving; means for detecting a video signal corresponding to a maximum luminance level and a minimum luminance level of the display video screen; and the maximum luminance level 3. The liquid crystal display device according to claim 1, further comprising means for adjusting and applying a reverse transition prevention voltage and a black insertion rate so as to maintain a bend alignment in accordance with the minimum luminance level.

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