JPH0568221A - Driving method for liquid crystal display device - Google Patents

Abstract

PURPOSE:To provide a driving method for a liquid crystal display device by which an inversion phenomenon at halftone display is prevented and a wide view field angle characteristic is realized. CONSTITUTION:High and low relation between a video input level and the half tone of 50% transmittivity is detected by a detector 1, and either a black level or a white level is selected by a switch 6 in accordance with this output. The video input is also level-converted by a time division conversion circuit 4 in conformity to a prescribed conversion table. By switching a switch circuit 7 by a signal obtained by a frequency doubler circuit 5, the write-in is executed to a liquid crystal display panel two times in one field period by the binary signal of the white or the black level obtained from the switch circuit 6 and the signal obtained from the conversion circuit 4.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of driving a liquid crystal display device.

[0002]

2. Description of the Related Art The electro-optical characteristics of a liquid crystal display cell can be roughly divided and calculated from two parts.
The first is the calculation of the displacement amount of the liquid crystal molecules, and the second is the calculation of the optical characteristics when light passes through the liquid crystal molecules. The displacement amount of the first liquid crystal molecule is represented by the Frank continuum theory, and the elastic free energy gd of the liquid crystal is given by the following equation.

[0003]

[Equation 1] The elastic displacement is also caused by the electric field applied between the electrodes, and the energy ge due to the electric field is expressed by the following equation.

[0004]

[Equation 2] Therefore, the entire arrangement is determined so that the change amount g (= gd-ge) of the displacement amount is minimized in the entire liquid crystal cell.

Regarding the second optical characteristic, this is basically calculated according to Barrman's 4 × 4 matrix method (DW Derreman: “Liquid-crystal twist cell dynami”).
cs with backflow ”, J.Appl.phys., 46, 9, 3746 (19
75))).

It becomes clear that an optical simulation of the liquid crystal cell is performed by numerical calculation based on the above process.
The viewing angle characteristics of the display cell greatly change depending on the pretilt angle and birefringence Δn generated when controlling the alignment of the liquid crystal molecules, the thickness d of the liquid crystal layer, and the like. Therefore, it has been studied to improve the viewing angle characteristics of the liquid crystal display device by optimizing the parameters of the liquid crystal cell (see Okano, Kobayashi: "Liquid Crystal (application)", Baifukan, p152 (1985)).

In addition, focusing on the fact that the viewing angle characteristics of a liquid crystal display device depend not only on the characteristics of the liquid crystal cell but also on the characteristics of the polarizing plate, an example of expanding the viewing angle by newly adding a phase film is also reported. (Yamagishi et al .: “Wide viewing angle LCD using phase film”) TV Technical Report 14, IDY90-
47, p35 (1990)).

However, by just optimizing the physical property parameters as described above, sufficient performance cannot be obtained yet, and the method of adding the phase film improves the viewing angle characteristics of the polarizing plate, but it is a fundamental liquid crystal cell. The viewing angle characteristics of are not improved.

11 to 14 show viewing angle characteristics of a liquid crystal panel whose parameters are simply optimized by a conventional method. FIG. 11 shows changes in transmittance in the vertical direction, FIG. 12 shows changes in contrast, FIG. 13 shows changes in transmittance in the horizontal direction, and FIG. 14 shows changes in contrast.

The normal viewing angle is calculated by the contrast C = from the transmittance TW for white display and the transmittance TB for black display.
It is defined as the angle when TW / TB drops to a certain value (usually 10/1 to 20/1). However, in actual images, this definition is often not sufficient. For example, a liquid crystal television has a black and white reversal phenomenon. This is a phenomenon in which a bright part appears dark and a dark part appears bright when viewed from a certain angle or more. When this phenomenon occurs, the image becomes extremely unnatural. This reversal phenomenon is likely to occur particularly at the halftone level, and it is necessary to define the viewing angle including this phenomenon.

From FIGS. 11 to 14, the angles when the contrast drops to 10/1 in the normal definition of the viewing angle are 34 ° and 16 ° in the vertical direction and 54 ° and 50 ° in the horizontal direction. However, if the point at which the reversal phenomenon occurs is defined as the viewing angle, the angle drops to 28 ° in the upward direction.

[0012]

As described above, the conventional liquid crystal display device has a problem that the viewing angle is narrow and the viewing angle is narrow, regardless of the definition of the viewing angle. For this reason, the contrast deteriorates or the color tone changes depending on the viewing angle, and the image quality deteriorates. In particular, when the screen becomes large or the number of viewers increases, there is a problem in that the image quality remarkably differs depending on the position of the screen and the viewing position.

The present invention has been made in consideration of such circumstances, and an object thereof is to provide a driving method of a liquid crystal display device which realizes a wide viewing angle characteristic by using the same cell structure as the conventional one.

[0014]

According to the present invention, when a liquid crystal display device in which a plurality of display pixels are arranged in a matrix is line-sequentially scanned, a signal is written to one pixel at least twice or more in one field period. Characterize.

[0015]

Operation: The state in which the liquid crystal has the widest viewing angle is shown in FIGS.
As is clear from FIG. 4, this is the case of white level or black level display. Therefore, if the writing is performed twice or more in one field period and at least one of the writing is performed at the white or black level, it is possible to compensate for the viewing angle characteristic deteriorated at the halftone level.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The details of the present invention will be described below with reference to the drawings.

FIG. 1 is a transmittance-voltage characteristic of a liquid crystal cell for explaining the principle of the present invention. As can be understood from FIGS. 11 and 13, the viewing angle characteristics of white display and black display change in substantially opposite directions. For example, in the downward direction, the transmittance is higher when viewed from below when displaying black, and is lower when viewed from below when displaying white. Therefore, in the present invention, in the case of performing a gray scale display, instead of obtaining the transmittance of 50% at the voltage VM shown in FIG.
The combination of the high transmittance state due to V and the low transmittance state due to the voltage VB for displaying black, that is, the combination of binary display of white and black is used. As a result, the viewing angle characteristics for white display and the viewing angle characteristics for black display mutually compensate each other, and as a result, a wide viewing angle characteristic that cannot be obtained with the viewing angle characteristics of halftone display by applying an intermediate voltage once. The corner is obtained.

2 and 3 show the method of the present invention.
The vertical and horizontal viewing angle characteristics in the case of performing a halftone display that obtains a transmittance of 50% by the binary average of the white level and the black level are shown in the graphs of FIGS. 11 and 13, respectively. is there. From these, the viewing angle of the halftone display represented by the black and white binary average is larger than that of the normal halftone display.
It is clear that it is extremely wide in both the vertical and horizontal directions.

The explanation so far has been about only one point of the transmittance of 50% even though it is a halftone. For other levels, for example, a combination of a monochrome binary level and a halftone level is used. Halftone display with a transmittance of 50% or more is a combination of a white level for maximum transmittance and a halftone level with a transmittance of 50% or less, and halftone display with a transmittance of 50% or less is
It is realized by a combination of a black level that obtains the minimum transmittance and a halftone level that has a transmittance of 50% or more. This is based on the following idea. As is clear from the vertical viewing angle characteristics, it can be said that the difference in the viewing angle characteristics due to the transmittance is such that, when the viewing angle is the phase, the phase advances as the transmittance increases. Therefore, assuming that the ideal viewing angle characteristic has a constant transmittance with respect to the viewing angle,
By adding the transmittances having different phases or having a phase difference of close to 180 °, it is possible to approximate the ideal state. Based on such an idea, an ideal addition method for expressing one transmittance from two transmittances is expressed by the following equation. T (Vin) = {T (VW) + T (V1)} / 2> 50 [%] = {T (VW) + T (VB)} / 2 = 50 [%] = {T (VB) + T (V2) } / 2 <50 [%]

Here, T (VW) is the transmittance of white display by the voltage VW, T (VB) is the transmittance of black display by the voltage VB, and T (V1) is 50% obtained by the voltage V1.
The following transmittance, T (V2), is obtained by the voltage V2 5
The transmittance is 0% or more.

That is, in order to obtain a transmittance higher than 50%, one of the two uses the maximum transmittance, and in order to obtain a transmittance lower than 50%, one of the two uses the minimum transmittance. By using, the maximum phase difference can be obtained. The above is a case where one transmittance is obtained from two transmittances, but this method can be extended to a case where one transmittance is obtained from three or more transmittances.

Next, a concrete method for creating one transmittance from two or more transmittances will be described. This includes
There are roughly three possible methods. The first is a method of applying modulation in the time axis direction, the second is a method of applying spatial modulation, and the third is a combination thereof.

The first time base modulation is 120H, which is normally driven at 60 Hz when the resolution is not desired to be lowered.
This is a method of driving by increasing the frequency at z and 180 Hz. That is, while the signal writing is normally performed once in one field period, the writing is performed twice or more. Since this also depends on the response speed of the liquid crystal, it is desired to increase the response speed.

The second spatial modulation is a method in which instead of increasing the driving frequency, the actual number of pixels is increased and adjacent pixels are respectively driven by the above-mentioned signals showing the two transmittance characteristics. In the future, it is not necessary to increase the response speed of the liquid crystal,
Since the number of pixels increases, improvement in manufacturing technology is desired. The third method is an intermediate method between the first and second methods, and can be a highly practical method by selecting the optimum combination.

FIG. 4 is a specific circuit configuration example of an embodiment for realizing the driving method of the present invention described above. The liquid crystal display panel is, for example, a so-called active matrix type normal display panel in which display pixels arranged in a matrix are composed of liquid crystal cells and switching elements. Therefore, the liquid crystal display panel has a signal line driving circuit and a scanning line driving circuit for arranging the signal lines and the scanning lines between the display pixels in each row and each column of the matrix and performing line-sequential scanning.

The video input signal level detector 1 of FIG.
This is a comparator for detecting whether the video input signal level has a transmittance of 50% or more or a transmittance of less than 50%. The reference input terminal of detector 1
The white level signal and the black level signal are added by the adder 3, and the value averaged by the averaging circuit 2 (that is, the signal level for obtaining the transmittance of 50%) is given. The detector 1 drives the switch circuit 6 according to the detection result, and when the video input signal obtains the transmittance of 50% or more, the white level signal is changed to the transmittance of 5 by the switch circuit 6.
The black level signal is selected when 0% or less is obtained.

On the other hand, the video input signal enters the time division conversion circuit 4. The time-division conversion circuit 4 is composed of, for example, a ROM having a conversion table as shown in FIG.
The meaning of this conversion table is as follows. When the video input signal level is the white display level, the output also becomes the white display level, and when the video input signal level is the black display level, the output also becomes the black display level. Looking at the vicinity of the halftone display level (the level at the position of the broken line in FIG. 5) where the video input signal has a transmittance of 50%, when the video input signal is slightly on the white display side, it is converted to a level close to the black display level. An output is obtained, and conversely, when it is slightly on the black display side, an output converted to a level close to the white display level is obtained.

In this way, the two signals obtained from the time-division conversion circuit 4 and the switch circuit 6 satisfy the preferable relationship for expanding the above-mentioned viewing angle. The outputs of the time-division conversion circuit 4 and the switch circuit 6 are switched by the switch circuit 7 by the control signal obtained from the field signal frequency doubling circuit 5, that is, in a cycle of half of one field period, and the liquid crystal display panel is displayed. Is sent to the drive signal.

As described above, by writing twice to one pixel with a predetermined combination in one field period, the viewing angle in the halftone display can be changed as described above with reference to FIGS. 2 and 3. Wider viewing angle characteristics than before can be obtained.

Although a linear conversion table is shown in FIG. 5, it is a non-linear conversion table when the response speed of the liquid crystal is slow, or when the speed is different depending on the level between rising and falling.

FIG. 6 shows a drive signal generation circuit of another embodiment. In this embodiment, a time division conversion circuit 11 is used in which all the functions of the comparison detector 1 and the switch circuits 6 and 7 of the previous embodiment are tabulated.

FIG. 7 shows a conversion table of the conversion circuit 11. Two kinds of conversion outputs a and b are obtained in the range where the input level is 50% or less of the transmittance, and two kinds of conversion c and d are obtained in the range where the input level is 50% or more of the transmittance. It has a conversion table so that the output can be obtained. Then, for a certain input level, two types of converted outputs are sent as drive signals to the liquid crystal display panel in one field period in a combination of a and b or c and d. Also in this embodiment, the viewing angle characteristics can be improved as in the previous embodiments.

In the above embodiment, the number of times of writing in one pixel is set to two in one field period. At this time,
There are three levels that can be driven only with binary values of white and black: white, black, and 50% gray. Therefore, in general, when driving n times in one field period, n + 1 levels are driven only by binary values of black and white, and the other levels are driven by using a combination of gray level and white or black, so that the binary values are binary. The drive can be optimized.

In the present invention, it is preferable that at least one of the signals written in one field period has a white level with a transmittance of 86.6% or more or a black level with a transmittance of 3.3% or less. This is for the following reasons. Normally, the contrast of a liquid crystal display device is represented by the ratio of the luminance when displaying black and the luminance when displaying white, and the optimum contrast is 30.
/ 1 to 50/1 or more (see, for example, Color Science Handbook). From this, the transmittance of black must be 3.3% or less.

Further, in the normal image pickup apparatus, the signal level is set so that an image with no white spots can be reproduced even when the optical input is 300%. That is, after γ output with 100% light input,
The signal level is set to about 223 levels (IRE100) by 8 bits (Nagoya et al .: "Signal processing LSI for all digital color cameras", TV Technical Report 8, TEBS 96-6, p.
27). Therefore, when 255 is the maximum value (100% transmittance), the black level is 16, and the white level is 86.6%. In the case of liquid crystal, it is necessary to set the white level larger than this in order to obtain a large dynamic range. By setting the white level and the black level as described above, a high quality liquid crystal display device can be realized.

FIG. 8 shows drive waveforms of an embodiment in which time axis modulation and spatial modulation are combined. The solid and broken lines in the figure are drive waveforms of pixels adjacent to each other. That is, a certain pixel is driven with a positive voltage of V2 at a certain time, while an adjacent pixel is driven with a negative voltage of -V1. This is alternately performed in the field cycle Tf.

In the system of this embodiment, when one pixel is viewed, flicker occurs at the frame cycle (30 Hz).
Flicker is eliminated by compensating each other in units of two adjacent pixels. Therefore, according to this method, it is possible to realize wide viewing angle driving without increasing the driving frequency and without causing flicker.

If a normal still image is used, a direct current component remains. Therefore, it is necessary to change the polarity inversion phase in units of one pixel or several pixels for each field. In the case of a moving image, it is considered that there is almost no DC component left after being averaged, and not only is it always positive,
Even if it is not balanced, it is driven with a negative polarity, so there is almost no problem with deterioration of image quality.

In the embodiments described above, it is not mentioned whether the display device is a monochrome display or a color display, but the present invention is not limited to monochrome display, but the color filter as shown in FIGS. 9 (a) and 9 (b) is used. It can also be applied to a color display device with.

In the case of a color display device, when the system of the embodiment of FIG. 6 is applied, the structure of the drive circuit section becomes as shown in FIG. The basic configuration is the same as for black and white display, but R,
Since it is necessary to process the G and B signals separately, as shown in the figure, the time division conversion circuit 11R is provided for each of the R, G and B signals.
, 11G, 11B are provided.

The time division conversion circuits 11R, 11G,
All of 11B basically have the same structure, but can be different if necessary. When the liquid crystal display panel is constructed with the same cell gap d, the optical characteristics are Δnd / λ (Δ
n: anisotropy of refractive index, λ: input wavelength)
Strictly speaking, different signal processing is required for each of R, G, and B.

[0042]

As described above, according to the present invention,
By driving the liquid crystal display panel by combining the white / black binary drive and the halftone drive, the viewing angle characteristics of the liquid crystal display panel are improved, and in particular, the inversion phenomenon in the halftone display is prevented and the effective viewing angle is improved. The corners can be widened.

[Brief description of drawings]

FIG. 1 is a diagram showing a transmittance-voltage characteristic of a liquid crystal display panel.

FIG. 2 is a diagram showing vertical viewing angle characteristics according to the method of the present invention.

FIG. 3 is a diagram similarly showing a viewing angle characteristic in the left-right direction.

FIG. 4 is a diagram showing a circuit configuration of an embodiment of the present invention.

FIG. 5 is a diagram showing a conversion table used in the embodiment.

FIG. 6 is a diagram showing a circuit configuration of another embodiment of the present invention.

FIG. 7 is a diagram showing a conversion table used in the embodiment.

FIG. 8 is a diagram showing drive waveforms of another embodiment of the present invention.

FIG. 9 is a view showing an arrangement example of color filters when the present invention is applied to color display.

FIG. 10 is a diagram showing an example of a circuit configuration when it is similarly applied to color display.

FIG. 11 is a diagram showing changes in transmittance in the vertical direction of a conventional liquid crystal display panel.

FIG. 12 is a diagram showing a contrast change in the vertical direction.

FIG. 13 is a diagram showing a change in transmissivity in the left-right direction of a conventional liquid crystal display panel.

FIG. 14 is a diagram similarly showing a contrast change in the left-right direction.

[Explanation of symbols]

 1 ... Video input signal level detector, 2 ... Averaging circuit, 3 ... Adder, 4 ... Time division conversion circuit, 5 ... Frequency doubling circuit, 6, 7 ... Switch circuit, 11 ... Time division conversion circuit.

Claims (2)

[Claims] 1. A driving method of a liquid crystal display device, wherein when a liquid crystal display device having a plurality of display pixels arranged in a matrix is line-sequentially scanned, a signal is written to one pixel at least twice in one field period. Method. 2. When n is the number of times signals are written to one pixel in one field period, n + 1 levels are driven by black and white binary values only, and the other levels use a combination of gray level and white or black. The driving method for a liquid crystal display device according to claim 1, wherein the driving method is a driving method.