JPH1039837A - Liquid crystal display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To shorten the response time of liquid crystal and to reduce the deviation of gradation based on hysteresis by providing a frame memory means temporarily storing a successively inputted video signal and outputting the video signal of one frame before and outputting a liquid crystal driving signal while inputting the successively inputted video signal and the video signal of one frame before. SOLUTION: An A/D converter 3 converts a video signal 1 being an analog signal into a digital signal 2. A frame memory means 5 temporarily stores the successively inputted video signal 1 and outputs the video signal 4 one frame before. A video signal converter 7 inputs the video signal 2 of a present frame and the video signal 4 of one frame before and outputs a liquid crystal driving signal 6. A D/A converter 9 converts the liquid crystal driving signal 6 being the digital signal into an analog signal 8, which is transmitted to a liquid crystal driving circuit. Moreover, a memory look-up table is used in the video signal converter 7.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a liquid crystal display device in which an active element is arranged for each pixel electrode.

[0002]

2. Description of the Related Art Conventionally, a TN (twisted nematic) type liquid crystal has been used as a liquid crystal in a liquid crystal display device in which an active element is arranged for each pixel electrode. The TN type liquid crystal basically needs two polarizing plates because of its optical rotation. Since the polarizing plate absorbs about half of the light, the TN type liquid crystal has a problem that the light use efficiency is small. For this problem,
Attempts have been made to solve this problem by using a transmission-scattering type liquid crystal that does not require a polarizing plate. As one of the liquid crystals, a polymer dispersed liquid crystal (hereinafter abbreviated as PDLC) has been proposed.

[0003] The response time of the PDLC is as short as several milliseconds, for example, from black display to white display or from white display to black display. However, the response time from halftone display to halftone display is very slow, several hundred milliseconds. If the response time is slow, there is a problem that the afterimage is conspicuous in the case of a moving image having a fast movement.

[0006] PDLC has a hysteresis in the voltage-display gradation characteristic. If there is hysteresis,
When the display image changes, a gradation shift occurs. For this reason, depending on the display image, there is a problem that information of the previous screen remains (afterimage based on hysteresis). FIG. 3 shows the hysteresis of the voltage-display gradation characteristic of the PDLC and the deviation of the gradation due to the hysteresis. The display gray scale of the PDLC is different between the step of increasing the voltage and the step of decreasing the voltage. Therefore, the display gradation of the PDLC depends on the past display state. For example, 256
Assume that gradation display is performed and a voltage V0 is applied to a specific pixel of the PDLC. The gradation of the pixel is set to 0 before V0 is applied.
If so, the gradation of A1 is displayed. On the other hand, if the gradation of the pixel is 255 before applying V0, the gradation of A2 is displayed. Thus, even if the same voltage is applied,
There is a problem that the display gradation is different depending on the past history of the pixel.

As described above, the display characteristics of the liquid crystal display device are as follows.
It largely depends on the characteristics of the liquid crystal. For this reason, a driving method for suppressing the image lag due to the response time delay and the hysteresis characteristic of the liquid crystal is required. Heretofore, examples of conventional driving methods include JP-A-5-40252, JP-A-5-153530, and JP-A-7-20828.

Japanese Unexamined Patent Publication No. 5-40252 discloses a driving method in which a display gradation is changed between two values by repeatedly applying two voltages having different amplitudes. In this way,
The afterimage phenomenon can be reduced by reducing the difference between when the voltage is increased and when the voltage is decreased. However, depending on the liquid crystal material, there is a problem that flicker is noticeable by changing the display gradation between two values, and conversely, there is a problem that display quality is deteriorated. Although this method can improve the afterimage phenomenon based on hysteresis, the afterimage problem based on the response time has not been solved.

Japanese Patent Application Laid-Open No. 5-153530 discloses a method of multiplying gradation data by a correction coefficient using a motion detection circuit between frames. However, depending on the response characteristics of the actual polymer-dispersed liquid crystal, it sometimes takes several hundred ms depending on the gradation change pattern. Therefore, even if the correction coefficient is multiplied to improve the hysteresis based on this method, the liquid crystal cannot always respond within one frame. Therefore, this method cannot improve the hysteresis of a liquid crystal having a slow response time that takes more than one frame to respond.

Japanese Patent Application Laid-Open No. Hei 7-20828 discloses Japanese Patent Application Laid-Open No. Hei 5-15353.
This is a method in which response prediction means is used as the means disclosed in Japanese Patent Publication No. 0.
Although this method can solve both the response time and the hysteresis, it has a problem that the circuit scale becomes large.

[0009]

SUMMARY OF THE INVENTION An object of the present invention is to provide a liquid crystal display device which produces an afterimage due to a slow response time of the liquid crystal, and a liquid crystal display device which makes such an afterimage inconspicuous. Is to do.

Further, the present invention provides a liquid crystal display device in which the response time of the liquid crystal is reduced for any display pattern.

Further, the present invention not only eliminates the afterimage based on the hysteresis but also shortens the response time of the liquid crystal for a liquid crystal having a hysteresis in the voltage-display gradation such as PDLC. And
An object of the present invention is to provide a liquid crystal display device having a small circuit scale.

[0012]

A liquid crystal display device according to the present invention comprises a display panel having liquid crystal sandwiched between an active matrix substrate and a counter electrode substrate, a driving circuit for the display panel, and a video signal sequentially inputted. And a video signal converting means for receiving the sequentially input video signal and the video signal of one frame before and outputting a liquid crystal drive signal. You.

Further, in the liquid crystal display device of the present invention, the video signal conversion means is provided with a memory look-up table.

Further, in the liquid crystal display device of the present invention, the memory lookup table is provided with an interpolation operation.

Further, in the liquid crystal display device of the present invention, a polymer dispersed liquid crystal is used as the liquid crystal.

[0016]

FIG. 1 is a block diagram of a main part of an embodiment of the present invention. Video signal 1 that is an analog signal
A / D converter 3 for converting an image signal into a digital signal 2, frame memory means 5 for temporarily storing video signal 1 sequentially inputted and outputting video signal 4 one frame before, video signal 2 of the current frame and one frame A video signal converter 7 that inputs the previous video signal 4 and outputs a liquid crystal drive signal 6, and converts a liquid crystal drive signal 6 that is a digital signal into an analog video signal 8.
/ A converter 9. The video signal 8 is sent to the liquid crystal drive circuit 11 shown in FIG. The video signal converter 7 uses a memory look-up table as shown in FIG. The look-up table stores the voltage data to be applied to the liquid crystal, using the binary value of the gradation value of the video signal of the current frame and the gradation value of the video signal of the previous frame as an argument. For example, if the gradation value of the video signal of the current frame is a and the gradation value of the video signal of the previous frame is b, the voltage data to be applied to the liquid crystal is c. Note that the video signal 1 in FIG. 1 performs the same processing on the R, G, and B signals of the video signal, and therefore, only one of them is shown here.

FIG. 2 shows the configuration of the liquid crystal drive circuit 11 and the liquid crystal display panel 12. The liquid crystal drive circuit 11 includes a video signal control unit 13, a data line drive circuit 14, a scan line drive circuit 15,
Consists of In addition, the liquid crystal display panel 12 includes the pixel 1
Liquid crystal is sandwiched between an active matrix substrate 18 provided with an active element 17 for each 6 and a counter electrode substrate 20 provided with a counter electrode 19.

Next, how to determine the value of the lookup table will be described. FIG. 4A is a waveform example showing an applied voltage in a specific pixel of the present invention. FIG. 4 (b)
FIG. 4A schematically shows a gradation change of the liquid crystal due to the applied voltage in FIG. In FIG. 4, the temporal transition of the input gradation signal is a gradation value a at time t ', and a gradation value b from time t0 to time t3. That is, it is assumed that the gradation changes from b to a at the time t0. Here, the definition of the word is such that the gradation value of one frame before is set as the current gradation value, the current frame gradation value is set as the target gradation value, and the gradation value of the video signal converted by the video signal converter is applied. The gradation value is used. The applied gradation value corresponding to the current gradation value b and the target gradation value a is assumed to be c,
b is smaller than a. Further, the voltage values corresponding to the gradation values a, b, and c are defined as Va, Vb, and Vc. FIG.
(B) The waveform 1 is changed from the time t0 to the time t3 by the conventional driving method (method of applying a voltage corresponding to the target gradation value).
4 shows the optical response of the liquid crystal when Va is applied. The response time of the waveform 1 is (t3-t0) time. Waveform 3 in FIG. 4B is an optical response of the liquid crystal when Vc is applied from time t0 to time t3. In the waveform 3, the display gradation becomes exactly a at the time t1. Waveform 2 in FIG. 4B is obtained by applying Vc from time t0 to time t1 and applying time Vc from time t1 to time t1.
This is the optical response of the liquid crystal when Va is applied up to 3, that is, when a voltage is applied as shown in FIG. Therefore, by applying a voltage as shown in FIG.
The response of the liquid crystal can be completed within one frame.
Therefore, the applied voltage value c corresponding to all patterns of gradation change
Is obtained in advance, a lookup table can be created.

Hereinafter, embodiments of the present invention will be described for a liquid crystal display device using PDLC as a liquid crystal display element. Of course, the present invention can be applied to other liquid crystals such as a TN (twisted nematic) type.

The operation of the present invention will be described with reference to FIGS.
This will be described with reference to FIG. As a precondition, it is assumed that the gradation value of the video signal changes from gradation value b to gradation value a at time t0. FIG. 7A is a voltage waveform diagram applied to the actual liquid crystal in FIG. Va, Vb, and Vc in FIG. 7A are voltage values corresponding to gradations a, b, and c, respectively. One frame was set to 33 ms (frequency: 30 Hz). FIG. 7 (b)
FIG. 7A shows that the transmittance of the PDLC changes according to the waveform of the applied voltage in FIG.
Waveforms 1, 2, and 3 in FIG. 7B show changes in the display gradation of the PDLC when the applied voltage value from t0 to t1 in FIG. 7A is changed.

Waveform 1 is a case where Vc = Va, that is,
This is a conventional driving method (a method of applying a voltage corresponding to a target gradation value). The time required for the gradation to change from b to a (the time from 0% to 90% when the rate of change from b to a is 100%, which is called the response time) requires t3 hours. However, the waveform 1 cannot reach the target gradation value a. This is because the hysteresis phenomenon occurs in the voltage-gradation characteristics of the PDLC. Here, the target gradation value is an average value of the voltage increase process and the voltage decrease process of the applied voltage of the gradation characteristic.

FIG. 5 shows the hysteresis of the voltage-display gradation characteristic of the PDLC and the deviation of the gradation due to the hysteresis. As shown in FIG. 5, the display gray scale of the PDLC is different between the step of increasing the voltage and the step of decreasing the voltage. For this reason, PDLC
Display gradation depends on the past display state. For example, 25
It is assumed that six gradations are displayed and a voltage V0 is applied to a specific pixel of this PDLC. If the gradation of the pixel is 0 before applying V0, the gradation of A1 is displayed. On the other hand, if the gradation of the pixel is 255 before applying V0, the gradation of A2 is displayed. As described above, even when the same voltage is applied, there is a problem that the display gradation is different depending on the past history of the pixel. To solve this problem, FIG.
(B) For waveform 2, there is a method of applying a voltage that satisfies Vc> Vb and does not overshoot (a voltage application method similar to that of Japanese Patent Application Laid-Open No. 7-20828 although the configuration of the invention is completely different).
The response time is t4, which is faster than waveform 1. However, the waveform 2 looks as if it has reached the target gradation value a for a moment, but after the time t1, the display gradation of the liquid crystal is attenuated in a direction to return to the original state. This is because the response characteristics of the PDLC differ depending on the region. PDLC
As shown in FIG. 8, a liquid crystal in the form of droplets is dispersed and arranged in a polymer film, and electrodes are opposed to each other so as to sandwich the polymer film. Response characteristics are different between an interface region of the droplet-shaped liquid crystal, for example, with a polymer film, and a central region. That is, the response is slower at the interface, and faster at the central region. When a voltage is applied to such a liquid crystal, the central region of the droplet-shaped liquid crystal responds first, and the interface region responds slowly. Therefore, in the case of the waveform 2 in FIG. 7B, the liquid crystal that has responded to the voltage Vc in the central region returns to the alignment corresponding to the voltage Va, and the liquid crystal at the interface that has not been completely followed in one frame responds slowly. Continue. As a result, the display gradation slightly returns. The cause of the hysteresis phenomenon is, in particular, the liquid crystal alignment portion at the interface. Therefore, the hysteresis problem cannot be solved unless the liquid crystal alignment portion at the interface has reached display gray scale a.

In order to solve this problem, a device is required. For example, when it is desired to display a gradation from 0 gradation to A3 in FIG. 6, a gradation level of overshoot is applied to A4. On the other hand, when it is desired to display a gradation from 255 gradations to A3, an overshoot gradation level is applied up to A5. As a result of overshoot, V3
Is applied, a target gradation of A3 can be displayed. Therefore, hysteresis can be eliminated by applying a voltage that causes overshoot.

FIG. 7B shows a waveform 3 in which Vc> Vb and
This is a case where a voltage is applied so as to overshoot to the gradation d. Since the response time overshoots, it takes a little longer to reach t5 to settle within 90 to 100% of the change rate than the waveform B. However, it is possible to reach the target gradation a, and therefore, it is possible to eliminate the deviation of the gradation based on the hysteresis of the PDLC.

The present invention is not limited to the above-described embodiment, but can be implemented in various modifications without departing from the scope of the invention.

[0026]

According to the present invention, the driving method of a liquid crystal display device in which active elements are arranged for each pixel can be improved only by adding a video signal converter and a frame memory. Therefore, the response time of the liquid crystal is shortened, and at the same time, the shift of the gradation based on the hysteresis is reduced.
According to the present invention, the afterimage phenomenon is eliminated, and the display quality is dramatically improved.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a main configuration of the present invention.

FIG. 2 is an explanatory diagram illustrating a configuration of a liquid crystal display device.

FIG. 3 is an explanatory diagram of a memory lookup table.

FIG. 4 is an explanatory diagram of a waveform example showing an applied voltage in a specific pixel of the present invention and a gradation change of PDLC by the applied voltage.

FIG. 5 is an explanatory diagram showing hysteresis of a voltage-gradation characteristic of a PDLC.

FIG. 6 is an explanatory diagram showing that hysteresis is eliminated.

FIG. 7 is an explanatory diagram showing a waveform example showing an applied voltage in a specific pixel according to the present invention and a gradation change of PDLC by the applied voltage.

FIG. 8 is an explanatory diagram showing a configuration of a PDLC.

[Explanation of symbols]

1: input analog video signal, 2: digital video signal, 3
... A / D converter, 5 ... frame memory, 6 ... liquid crystal drive signal, 7 ... video signal converter, 8 ... video signal to be output to the liquid crystal drive circuit, 9 ... D / A converter.

Claims (5)

[Claims] 1. A liquid crystal display comprising a display panel in which liquid crystal is sandwiched between an active matrix substrate provided with an active element for each pixel electrode and a counter electrode substrate provided with a counter electrode, and a driving circuit therefor. In the apparatus, a frame memory means for temporarily storing a sequentially input video signal A and outputting a video signal B one frame before, and a video inputting the video signal A and the video signal B and outputting a liquid crystal drive signal A liquid crystal display device comprising: signal conversion means. 2. A liquid crystal display device according to claim 1, wherein a memory look-up table is used for said video signal conversion means. 3. The liquid crystal display device according to claim 2, wherein said liquid crystal drive signal is obtained by interpolation of said memory look-up table. 4. The video signal according to claim 2, wherein a change in optical response is equal to or greater than a value corresponding to said video signal A when displaying a video corresponding to said video signal A from said video signal B. A liquid crystal display device in which data of a memory lookup table is set as described above. 5. The liquid crystal display device according to claim 1, wherein the liquid crystal is a polymer dispersed liquid crystal.