JP2003195830A - Active matrix liquid crystal display device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To actualize uniform display by suppressing intensity change above and below a window pattern which is generated when an active matrix liquid crystal display device displays the window pattern on the background of a specified display pattern. <P>SOLUTION: In a selection period of a scanning signal, the polarity of an image signal is inverted about an image signal center Vsc as shown in Fig. 6 (a). When the effective area S7 of a screen upper part and the effective area S8 of a screen lower part are equalized in value as shown in Fig. 6 (b) and Fig. 6 (c), no crosstalk is generated. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an active matrix liquid crystal display device for improving display quality by improving a driving method.

[0002]

2. Description of the Related Art An active matrix liquid crystal display device displays a single screen by turning on a TFT (thin film transistor) for each line of scanning signal wiring to give an image signal and sequentially turning on all scanning signal wiring. There is. One screen is
It is rewritten in a cycle that does not cause discomfort so that flicker cannot be seen and the movement of the image is smooth. Usually, the switching cycle is often 16.6 ms, and the switching frequency at this time is 60 Hz. However, since the characteristics of the liquid crystal deteriorate unless it is driven by an alternating electric field, two screens are used as a pair and a positive electric field and a negative electric field are repeatedly applied to the liquid crystal. In this case, the alternating electric field of 30 Hz is applied to the liquid crystal, and the flicker is easily visible at this frequency.

In order to make such a flicker inconspicuous, conventionally, there is a driving method for inverting the polarity of the voltage applied to the liquid crystal for each one or two scanning signal wirings.
This driving method has the effect of canceling out the flicker when the liquid crystal panel is viewed from a distance. For the same reason,
There is also a driving method in which the voltage applied to each image signal wiring is inverted. However, in the active matrix liquid crystal display device, the parasitic capacitance Csd exists between the image signal wiring and the pixel electrode, and when the image signal changes, the potential of the pixel electrodes other than the pixel electrode also changes due to the influence of the parasitic capacitance. To do.

Therefore, when the polarity of the image signal is inverted for every one or two scanning signal wirings, the potential of the pixel electrode is changed and the voltage applied to the liquid crystal is changed in accordance with the polarity inversion. become. Normally, when the entire screen is displayed uniformly, changes in the potentials of the pixel electrodes alternately appear with the same amount of positive and negative, so they do not cancel each other out and no difference in brightness occurs. However, when a certain specific pattern is displayed, the change in the potential of the pixel electrode differs depending on whether it is positive or negative, which may cause a difference in brightness.

As a specific example, when black pixels in a halftone are arranged for each scanning signal line and a black or white window pattern is displayed in the background arranged for every three image signal wirings, A difference in brightness from the surroundings occurs above and below the window pattern, and an image with a vertical band is displayed. This phenomenon is called vertical crosstalk here. FIG. 5 shows an example of a display state in which vertical crosstalk has occurred. The vertical crosstalk greatly depends on the parasitic capacitance Csd and the image signal amplitude Vspp, and the crosstalk can be reduced by designing them to be small. However, other display performance, such as brightness,
Since the contrast, the viewing angle, and the like are deteriorated, it is difficult to sufficiently improve the vertical crosstalk, and the crosstalk cannot be completely eliminated only by reducing the parasitic capacitance Csd and the image signal amplitude Vspp.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems of the prior art, and has vertical crosstalk regardless of various display patterns, parasitic capacitance Csd, and image signal amplitude Vspp. It is an object of the present invention to realize an active matrix liquid crystal display device capable of suppressing the occurrence of the above and obtaining uniform display.

[0007]

The invention of claim 1 of the present application is an active matrix liquid crystal display device in which the polarity of the image signal is inverted with respect to the center of the image signal during the selection period of the scanning signal.

According to a second aspect of the present invention, in the active matrix liquid crystal display device according to the first aspect, the image signals input to the adjacent image signal wirings have the same polarity when viewed from the center of the image signal. It was done.

The invention of claim 3 of the present application is the same as claim 1 or 2.
In the active matrix liquid crystal display device, the image signal whose polarity is inverted within the scanning signal selection period has the same voltage difference with respect to the image signal center before and after the polarity inversion.

The invention of claim 4 of the present application is the same as claim 1 or 2.
In this active matrix liquid crystal display device, the polarity of the image signal is inverted at the timing of 1/2 of the scanning signal selection period.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An active matrix liquid crystal display device according to an embodiment of the present invention will be described with reference to the drawings. In the active matrix liquid crystal display device, image signal wirings, scanning signal wirings, pixel electrodes, TFTs and the like are formed in a matrix. FIG. 1 shows an equivalent circuit diagram for one dot.

When the two-dimensional coordinates of the liquid crystal display device are (x, y), a plurality of scanning signal wirings 1, 5 along the x-axis direction are provided.
Are formed at a constant pitch in the y-axis direction. A plurality of image signal wirings 2 and 8 along the y-axis direction are formed at a constant pitch in the x-axis direction. As shown in FIG. 1, the circuit for one dot includes a set of scanning signal wiring 1 and image signal wiring 2, and a TFT in which a source and a gate are connected to this wiring.
3 and a pixel electrode 4 connected to the drain of the TFT 3.

Since the image signal wiring 2 and the pixel electrode 4 are arranged close to each other or partially overlapped with each other, a parasitic capacitance Csd is generated. The parasitic capacitance Csd exists on the left and right sides of the pixel electrode 4, and the parasitic capacitance on the left side is Csd.
If d1 and the parasitic capacitance on the right side are Csd2, then Csd
= Csd1 + Csd2. The parasitic capacitance between the pixel electrode 4 and the scanning signal line 1 is Cgd, the parasitic capacitance between the pixel electrode 4 and the scanning signal line 5 for capacitive coupling is Cst, and the equivalent capacitance of the liquid crystal to the pixel electrode 4 and the common electrode 6 is Clc
And

Assuming that the total capacity of one pixel is Ctot, the value is as follows. Ctot = Clc + Csd + Cgd + Cst When the image signal amplitude is Vspp, the potential variation ΔV of the pixel electrode 4 due to the parasitic capacitance Csd is given by the following equation. ΔV = Vspp * Csd / Ctot

Here, the image signal is sent to the image signal center (hereinafter referred to as the center voltage) Vs for each scanning signal wiring.
Consider a liquid crystal display device whose polarity is inverted with respect to c. In such a liquid crystal display device, in the odd-numbered and even-numbered scanning signal lines 1, the pixel electrodes 4 held for one frame period
The potential polarities of are opposite to each other. If the pixel potential fluctuation due to the parasitic capacitance Csd increases the voltage applied to the liquid crystal of the pixel on the odd scan line, the voltage applied to the liquid crystal of the pixel on the even scan line decreases. Therefore, the actual voltage applied to the liquid crystal differs for each scanning line.

When writing an image which is uniform over the entire surface, the voltage applied to the liquid crystal increases and decreases alternately for each scanning period, so that the effective value voltage applied to the liquid crystal is actually the same and no display difference appears. FIG. 2 is a waveform diagram showing a state in which the intermediate value is displayed on the entire screen. As shown in FIG. 2A, the image signal has a V voltage in a certain scanning period with respect to the center voltage Vsc.
It becomes higher than sc to reach the voltage V1 and becomes V during the next scanning period.
It decreases from sc and changes to voltage V2. FIG. 2B shows the pixel potential at the top of the screen, and FIG. 2C shows the pixel potential at the bottom of the screen. Here, the potential variation ΔVa of the pixel electrode at the upper and lower portions of the screen is determined by the two continuous frames F.
The following values are obtained for x and Fy. ΔVa = (V1−V2) * Csd / Ctot Further, as shown in the figure, the polarity of the pixel potential changes with one frame period as a cycle. Therefore, the effective voltage applied to the liquid crystal is
RMS voltage at the top of the screen (hereafter called the effective area)
Is S1 and the effective area at the bottom of the screen is S2, S1
= S2.

Next, consider the case where the display pattern is a black, halftone, and a repeating pattern of black and halftone for each scanning line. FIG. 3 is a waveform diagram showing the state in this case. Figure 3
As shown in (a), the image signal is asymmetric with respect to the center voltage Vsc. In a certain frame Fx, the voltage for halftone display is V1 and the voltage for black display is V4. In the next frame Fy, the voltage for halftone display is V2 and the voltage for black display is V3. FIG. 3B shows the pixel potential at the top of the screen, and FIG. 3C shows the pixel potential at the bottom of the screen.

The potential variation ΔVb of the pixel electrode in the frame Fx at the top of the screen has the following value. ΔVb = (V1−V4) * Csd / Ctot Potential fluctuation Δ of the pixel electrode in the frame Fy at the top of the screen
Vc has the following value. ΔVc = (V3-V2) * Csd / Ctot

In the frame Fx at the bottom of the screen, the potential variation ΔVd of the pixel electrode during the scanning period at the beginning has the following values. ΔVd = (V1−V2) * Csd / Ctot In the frame Fx at the bottom of the screen, the potential variation ΔVe of the pixel electrode in the subsequent scanning period has the following value. ΔVe = (V1−V4) * Csd / Ctot In the frame Fy at the bottom of the screen, the potential variation ΔVf of the pixel electrode during the scanning period at the beginning has the following value. ΔVf = (V1−V2) * Csd / Ctot In the frame Fy at the bottom of the screen, the potential variation ΔVg of the pixel electrode in the subsequent scanning period has the following value. ΔVg = (V3-V2) * Csd / Ctot

As described above, the polarity of the pixel potential changes in a cycle of one frame period, but even if the same halftone signal is applied, the potential of the pixel electrode is different from that in the case where halftone display is performed on the entire screen. Since the fluctuation ΔV changes for each frame and for the upper and lower portions of the screen, the effective value voltage applied to the liquid crystal also differs in the frame period.

This RMS voltage difference differs depending on the scanning line. Even if the same halftone signal is applied, the RMS voltage on the first scanning line is small and the RMS voltage on the last scanning line is large. Therefore, a luminance gradient is generated both at the top and bottom of the screen. As shown in FIG. 3, when the effective area in the upper part of the screen is S3 and the effective area in the lower part of the screen is S4, the following relationship is established. S3 <S4, S3 <S1, S4> S2

Further, even when there is a black or white window pattern in the repeated pattern of black, halftone, black, and halftone for each scanning line, the effective value voltage is different above and below the window pattern. This is shown in FIG. FIG. 4A shows a voltage change of the image signal. In the frame Fx, the image signal voltage changes to V1, V2, V4 for each scanning line with respect to the center voltage Vsc outside the window pattern. In the window pattern, the image signal voltage changes to V4 and V3 for each scanning line. In the frame Fy, the image signal voltage changes to V1, V2 and V3 for each scanning line outside the window pattern, and the image signal voltage changes to V4 and V3 for each scanning line within the window pattern. FIG. 4B shows the pixel potential at the top of the screen.
(C) shows the pixel potential at the bottom of the screen.

In the frame Fx in the upper part of the screen, the potential variation ΔVh of the pixel electrode outside the window pattern has the following value. ΔVh = (V1−V4) * Csd / Ctot In the frame Fx at the upper part of the screen, the potential variation ΔVi of the pixel electrode in the window pattern has the following value. ΔVi = (V3−V4) * Csd / Ctot In the frame Fy at the top of the screen, the potential variation ΔVj of the pixel electrode outside the window pattern has the following value. ΔVj = (V3−V2) * Csd / Ctot potential variation ΔV of the pixel electrode in the window pattern
k has the following value. ΔVk = (V3-V4) * Csd / Ctot

In the frame Fx at the bottom of the screen, the potential fluctuations ΔVl, Δ of the pixel electrodes outside the window pattern.
Vm has the following values. ΔVl = (V1-V2) * Csd / Ctot ΔVm = (V1-V4) * Csd / Ctot In the frame Fx at the bottom of the screen, the potential variation ΔVn of the pixel electrode in the window pattern has the following value. ΔVn = (V3−V4) * Csd / Ctot In the frame Fy at the upper part of the screen, the potential fluctuations ΔVo and ΔVp of the pixel electrode outside the window pattern have the following values. ΔVo = (V1-V2) * Csd / Ctot ΔVp = (V3-V2) * Csd / Ctot Potential fluctuation ΔV of the pixel electrode in the window pattern
q has the following value. ΔVq = (V3-V4) * Csd / Ctot

In FIG. 4, the effective area of each frame is determined in consideration of these potential fluctuations ΔV. When the effective area at the upper part of the screen is S5 and the effective area at the lower part of the screen is S6, the following relationship is established. S5> S3, S6 <S4

In the case of such a display pattern, even if the same halftone signal is given, as shown in FIG. 5, a difference in luminance occurs between the upper point A of the window pattern and its left and right sides A'A ", and the TN liquid crystal displays. A for normally white mode
The point becomes darker than the left and right A'A ". Also, at the lower point B and the left and right B'B" of the window pattern, the point B becomes brighter than the left and right B'B ". This phenomenon is vertical crosstalk. The display pattern in which vertical crosstalk occurs occurs when the image signal is asymmetric with respect to the center voltage Vsc within one frame period, which is a parasitic capacitance Csd.
This is because the value of the potential fluctuation ΔV that fluctuates due to the capacitive coupling changes according to the vertical position of the screen, as described with reference to FIGS. 3 and 4.

Therefore, according to the present invention, the image signal is inverted between the image signal center Vsc and the image signal center Vsc within the selection period of the scanning signal so that the image signal becomes symmetrical with respect to the center voltage Vsc within one frame period. Just drive. FIG. 6 shows an example of the voltage waveform of the image signal that realizes such a driving method.

FIG. 6A shows a voltage waveform of an image signal when a black window pattern is displayed at the center of the screen and a repeating pattern of black and halftone is displayed as a background on the other screens. When the halftone is written as Gr and the black is written as Bl, for example, in the frame Fx, Gr, Bl, Gr, Bl, Bl, Bl, Bl, Bl,
Bl, Bl, Bl, Gr, Bl, Gr ... Are displayed. Then, five pixels of the window pattern in the central portion viewed from the left and right of the screen are designated as Bl, Bl, Bl, Bl, Bl, Bl. Such a pattern is the same as that shown in FIG. 4 for comparison. Then, the voltages of Gr are V1 and V2,
The voltage of Bl is V3 and V4. Note that V1 and V3 are on the positive side with respect to the center voltage Vsc, and V2 and V4 are on the negative side with respect to the center voltage Vsc. Actually | V1
Let | = | V2 | and | V3 | = | V4 |.

The polarity of the image signal is inverted with respect to the center voltage Vsc within the scanning signal selection period. Then, the image signals input to the adjacent image signal wirings have the same polarity with respect to the center voltage. Further, the polarity of the image signal is inverted within the selection period of the scanning signal, and the same voltage difference is made to the center voltage before and after the polarity inversion. The polarity inversion of the image signal is performed at the timing of 1/2 of the scanning signal selection period.

Therefore, the following expression holds for Gr. | V1-Vsc | = | Vsc-V2 | Further, the following formula holds for Gr. | V3-Vsc | = | Vsc-V4 |

When the liquid crystal display device is driven under such conditions, the pixel potential at the upper part of the screen becomes as shown in FIG. 6 (b),
The pixel potential at the bottom of the screen is as shown in FIG. Assuming that the effective areas of the frames Fx and Fy in the upper part of the screen are S7 and the effective areas of the frames Fx and Fy in the lower part of the screen are S7, the following equation is established when the same analysis as in FIGS. 3 and 4 is performed. S
7 = S8

As a result, the effective area, that is, the effective value voltage becomes equal regardless of the display pattern and the position in the vertical direction of the screen, and the vertical crosstalk does not occur. On the other hand, when the image signal as shown in FIG. 7 is applied, that is, when the voltage applied to the liquid crystal for each scanning line is not inverted, the center voltage V
Since it is symmetrical with respect to sc within one frame period, vertical crosstalk does not occur.

However, in the driving method shown in FIG. 7, the effective area at the top of the screen is S9 and the effective area at the bottom of the screen is S10.
Then, S9 = S10 The above relationship holds, but flicker becomes more noticeable,
Not practical.

As described above, according to the present embodiment, the effective voltage at the top and bottom of the screen is the parasitic capacitance Csd and the image signal amplitude Vs.
Since it has nothing to do with the influence of pp, it is possible to obtain a liquid crystal display device that is less likely to be affected by performance restrictions such as reduction in brightness (aperture ratio) and contrast.

[0035]

EXAMPLES As Example 1 of the present invention, the evaluation results of a normally white TN liquid crystal mode and a 15-inch XGA TFT liquid crystal display device which is driven in reverse every scanning signal line will be described. As a background, halftones and black are alternately displayed for each scanning line, and a black window pattern having a display area of 1/2 is displayed in the center both vertically and horizontally. Before using the driving method of the present invention, the upper portion of the window pattern was dark and the lower portion was bright, as shown in FIG. The difference in brightness between when the window pattern was displayed and when it was not displayed was about 10%, which was a level at which vertical crosstalk could be clearly confirmed visually. Next, when the driving method of the present invention was used, almost no vertical crosstalk could be visually confirmed, and the luminance difference was 1% or less.

As a second embodiment of the present invention, an evaluation result of a normally white TN liquid crystal mode, a 15-inch XGA TFT liquid crystal display device driven in reverse (dot inversion drive) for each scanning signal line and each image signal line. I will describe. In the dot inversion drive, almost no vertical crosstalk can be confirmed even if a halftone and black are alternately displayed on the background for each scanning line and a window pattern is displayed. This is because the signal is inverted between the adjacent image signal wirings and the parasitic capacitance Cs
This is because the voltage fluctuations due to d cancel each other on the left and right of the pixel. Here, if the background display pattern is halftone every other pixel in the upper, lower, left and right, and black, vertical crosstalk occurs,
The brightness difference was about 3%. Next, when the driving method of the present invention was used, as in Example 1, almost no vertical crosstalk could be visually confirmed, and the luminance difference was 1% or less.

Although the normally white mode, TN liquid crystal display device is used in the above embodiments, the present invention is not limited to this.
The principle is the same in the PS mode and the VA mode, and the present invention is effective.

[0038]

As described above, according to the present invention, it is possible to suppress the occurrence of vertical crosstalk regardless of various display patterns, parasitic capacitance Csd, and image signal amplitude Vspp, and realize uniform display.

[Brief description of drawings]

FIG. 1 is a circuit diagram of one dot of an active matrix liquid crystal display device.

FIG. 2 is a waveform diagram showing an image signal and a pixel potential when halftone display is performed on the entire screen.

FIG. 3 is a waveform diagram showing an image signal and a pixel potential when displaying halftone and black for each scanning line.

FIG. 4 is a waveform diagram showing an image signal and a pixel potential when a halftone and black are displayed for each scanning line and a black window pattern is displayed at the center of the screen.

FIG. 5 is an explanatory diagram showing vertical crosstalk that occurs when a halftone and black are displayed for each scanning line and a black window pattern is displayed at the center of the screen.

FIG. 6 is a waveform diagram showing an image signal and a pixel potential when a halftone and black are displayed for each scanning line and a black window pattern is displayed at the center of the screen in the active matrix liquid crystal display device of the present invention.

FIG. 7 is a diagram showing an active matrix liquid crystal display device of the present invention, in which a halftone and black are displayed for each scanning line when the voltage applied to the liquid crystal is not inverted for each scanning line, and a black window is displayed at the center of the screen. Waveform diagram showing image signals and pixel potentials when a pattern is displayed

[Explanation of symbols]

1,5 Scan signal wiring 2,8 Image signal wiring 3 TFT 4 pixel electrodes 6 common electrode Csd1 parasitic capacitance (left) Csd2 parasitic capacitance (right) Clc Liquid crystal capacity Cst storage capacity Cgd parasitic capacitance

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) G09G 3/20 642 G09G 3/20 642A

Claims (4)

[Claims] 1. An active matrix liquid crystal display device in which a polarity of an image signal is inverted with respect to an image signal center during a scanning signal selection period. 2. The active matrix liquid crystal display device according to claim 1, wherein the image signals input to adjacent image signal wirings have the same polarity when viewed from the center of the image signal. 3. The active matrix liquid crystal display device according to claim 1, wherein the image signals whose polarities are inverted within the scanning signal selection period have the same voltage difference with respect to the image signal center before and after the polarities are inverted. 4. The image signal is 1 / th of the selection period of the scanning signal.
3. The active matrix liquid crystal display device according to claim 1, wherein the polarity is inverted at the timing of 2.