JP3210385B2 - Liquid crystal display - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display device which performs gradation display by a frame thinning method.

[0002]

2. Description of the Related Art In recent years, liquid crystal display devices have been widely used as display devices for personal word processors and personal computers.

In order to use this liquid crystal display device as a display device, it is indispensable to cope with multi-stage display and high pixel count for realizing colorization and high-quality image display. In order to drive a display device in a matrix and display such a high-quality image, a driving unit that reduces crosstalk between adjacent pixels and obtains high contrast is required.

As the driving means, a driving circuit of a multiplex driving method based on a voltage averaging method is generally used.

FIG. 7 shows an example of an electrode configuration of a matrix type liquid crystal display device. FIG. 8 is a waveform diagram showing a driving voltage according to the multiplex driving method.

As shown in FIG. 7, the matrix liquid crystal display element has a plurality of signal electrodes Xi (i = 1, 2, 3,... M) and a plurality of scanning electrodes Yj (j = 1, 2, 3,...). .N) are arranged in a matrix.

Then, for example, the pixel (X1, Y1) on the same scanning electrode Y1 is turned on and the pixel (X2, Y1) is turned off.
In the case of the F state, as shown in FIG. 8 (b), the signal electrodes X1 and X2 are applied to the signal voltage V whose polarity is inverted in one frame cycle.
(X1) and V (X2) are applied.

A scanning voltage V (Y) as shown in FIG.
1), V (Y2)... V (YN) are applied.

Thus, the signal electrode Xi and the scanning electrode Yj
And the scanning voltage V (X1), V (X2).
When (Y1), V (Y2)... Are applied, the driving voltage V () shown in FIG. 8C is applied to the above-mentioned pixels (X1, Y1) and (X2, Y1) among these opposing electrodes. X1, Y1) and V
(X2, Y1) will be applied.

Since the peak value of the applied voltage in the ON state is (a + 1) V and the peak value of the applied voltage in the OFF state is (a-1) V, the difference between the driving voltages is 2 V. The ON state pixel and O corresponding to the difference of the effective value of the voltage
The contrast of the pixel in the FF state can be obtained.

By further pushing this method, the contrast ratio between the ON state pixel and the OFF state pixel is increased,
And as a driving method to reduce crosstalk,
There is an optimum bias driving method, which is generally used.

A liquid crystal display device adopting the multiplex driving method basically performs gradation display by a frame thinning method in which the frequency of on-frame and off-frame is changed, even when the display is basically performed in binary of on / off. be able to. (For example, JP-A-58-57192 and JP-A-60-205496) As an example, a case of performing four gradation display will be described.

To express a certain pixel as the first gradation among the four gradations, it is always turned on (all frames).

To express the second gradation, two frames out of three frames are turned on and one frame is turned off.

To express the third gradation, one of the three frames is turned on and the other two frames are turned off.

To express as the fourth gradation, it is always turned off (all frames).

However, if the on / off timing is the same for all the pixels in the portion displaying such an intermediate gradation, flicker (display flickering) occurs and the screen becomes very difficult to see.

Therefore, in order to suppress the flicker, adjacent pixels are driven by shifting the on / off timing.

The frame thinning method expresses an intermediate gradation by intermittently lighting pixels on a frame-by-frame basis.

[0020]

The flicker in the above-described frame thinning method has no problem since it is not perceived by human eyes if the response of the liquid crystal is sufficiently slow with respect to the frame period.

However, in the case of a liquid crystal excellent in high-speed response, the light transmittance repeatedly changes sharply between the ON state and the OFF state of the pixel within the time of several frames in which the thinned-out lighting is performed. There is a problem that this is perceived as flicker by human eyes, and the display screen becomes hard to see. The flicker becomes more remarkable as the response time of the liquid crystal display element becomes shorter with respect to the frame period.

Therefore, if the frame period is shortened, that is, if the frame frequency F is relatively increased with respect to the response speed, flicker can be reduced.

However, on the other hand, as the frame frequency F increases, the power consumption of the liquid crystal display device increases and crosstalk frequently occurs. Therefore, it is not preferable to set the frame frequency F higher than necessary. . The frame frequency F is generally about 60 to 70 [Hz].

The state of occurrence of such flicker and crosstalk depends not only on the relationship between the frame frequency F and the response time τ of the liquid crystal display element, but also on the number of gradations N. In particular, when a multi-gradation display is to be performed as in a recent liquid crystal display device, the occurrence of flicker and crosstalk becomes more remarkable depending on the number N of the gradations.

A liquid crystal display device using an STN (super twisted nematic) type liquid crystal is used in a liquid crystal display device capable of driving a multiplex and realizing an image display with a high contrast ratio and corresponding to a large number of pixels. The response time τ is longer than about 300 [ms]. This STN
Response time τ is 300, as in a liquid crystal display using liquid crystal display.
In the case of being longer than [ms], flicker did not cause any problem even when a display by a frame thinning method of about 16 gradations was executed at a generally used frame frequency of 60 to 70 [Hz].

However, when such a liquid crystal display device is used as, for example, a display device of a personal computer, the response time τ is long, so that it is impossible to follow a display such as movement of a mouse or scrolling. Further, when used as a television, it cannot follow changes in images. For this reason, it is necessary to increase the response speed of liquid crystal display devices, including those using STN type liquid crystal, that is, to shorten the response time τ, and research and development has been advanced, and it has become possible to shorten the response time τ. Liquid crystal display devices have also been realized.

However, in the liquid crystal display device having such a short response time τ, flicker becomes more remarkable as the response time τ becomes shorter with respect to the frame period, as described above.
There is a problem that a display image becomes hard to see.

In particular, when a multi-gradation display is to be performed as in a recent liquid crystal display device, not only the relationship between the frame frequency F and the response time τ of the liquid crystal display element, but also the gradation Even with the number N, there is a problem that the occurrence of flicker and crosstalk becomes more remarkable, and the displayed image becomes hard to see.

The present invention has been made in order to solve such a problem, and an object of the present invention is to provide a liquid crystal display device which displays an image having a gradation expression driven by multiplex driving by a frame thinning method. An object of the present invention is to realize high-quality image display by solving the problem of occurrence of flicker and crosstalk.

[0030]

According to a first aspect of the present invention, there is provided a liquid crystal display device having an XY matrix electrode, and a driving circuit for driving the liquid crystal display device. In a liquid crystal display device having
The liquid crystal display device further comprises a driving circuit in which a response time of the liquid crystal display element is set to 300 [ms] or less and a frame frequency for driving the liquid crystal display element by a frame thinning method is set to 80 [Hz] or more. And

The liquid crystal display device according to the second invention is
In a liquid crystal display device including a liquid crystal display element having an XY matrix electrode and a driving circuit for driving the liquid crystal display element, when the response time of the liquid crystal display element is τ [ms], the liquid crystal display element is When the frame frequency driven by the frame thinning method is F [Hz], when the frame frequency F is 200 [ms] ≦ τ ≦ 300 [ms] with respect to the response time τ, F = 80 [Hz]; When 150 [ms] ≦ τ ≦ 200 [ms], 80 [Hz] ≦ F ≦ 100 [Hz], When 100 [ms] ≦ τ ≦ 150 [ms], 100 [Hz] ≦ F ≦ 120 [Hz] ], 80 [ms] ≤ τ ≤ 100 [ms], 120 [Hz] ≤ F ≤ 140 [Hz], and τ ≤ 80 [ms], the drive set as 140 [Hz] ≤ F It is characterized by having a circuit.

Further, the liquid crystal display device according to the third invention is characterized in that:
In a liquid crystal display device including a liquid crystal display element having an XY matrix electrode and a driving circuit for driving the liquid crystal display element, when the response time of the liquid crystal display element is τ [ms], the liquid crystal display element is Assuming that the frame frequency driven by the frame thinning method is F [Hz] and the number of gray scales to be displayed on the liquid crystal display element is N [gray scale], the relationship between the response time τ, the frame frequency F, and the gray scale number N Is characterized by having a driving circuit set as F ≧ N / 2τ + 40, where F ≧ 80.

The frame frequency F may be set in advance in the drive circuit of the liquid crystal display device at the time of manufacture of the liquid crystal display device, or the total number of gradations N expressed by the device may be varied. The setting of the frame frequency in the drive circuit may be automatically changed in conjunction with the switching of the setting of the number of gradations N, or may be manually changed by the user.

The liquid crystal display devices according to the first and second aspects of the present invention have a frame frequency F for driving the liquid crystal display element.
Focusing on the response time τ as a determining factor, the number of gray scales N by the frame thinning method can be reduced from the commonly used 8 gray scales to 16 gray scales.
To fix the flicker to the response time τ and to realize a good image display with less crosstalk without taking into account the difference in the number of tones N, taking into account the difference in the number of tones N fixedly as the number of tones, This is a liquid crystal display device including a drive circuit in which an appropriate frame frequency F is set.

In the third invention, the relationship between the frame frequency F, the response time τ, and the number of gradations N is further taken into account in consideration of the number of gradations N than in the liquid crystal display devices according to the first and second inventions. This is a general formula. Further, since the frame frequency F can be defined more finely than the first and second inventions in accordance with the number of gradations N, it is possible to achieve high-quality display with flicker suppressed further.

[0036]

In a liquid crystal display device which performs gradation display by the frame thinning method, flicker can be reduced by increasing the frame frequency relatively in accordance with the response characteristics. Since the power consumption of the liquid crystal display device increases and crosstalk frequently occurs, it is not preferable to set the frame frequency higher than necessary. Therefore, when expressing an image on the liquid crystal display element, an appropriate frame frequency F [Hz] range in which flicker is reduced and crosstalk is reduced with respect to the response time τ [ms] inherent to the liquid crystal display element is determined in advance by experiments or the like. It should be noted that the frame frequency F of the driving circuit of the liquid crystal display device is set according to the response time τ of the liquid crystal display element. Specifically, the response time τ [ms] of the liquid crystal display element is set to 300 [ms] or less,
At this time, the frame frequency F [Hz] for driving the liquid crystal display element by the frame thinning method is set to 80 [Hz] or more.

Further, the response time τ inherent to the liquid crystal display element
If [ms] is divided into several types and the frame frequency F is set more finely for each response time τ, flicker and crosstalk can be minimized. That is, when 200 [ms] ≦ τ ≦ 300 [ms], F = 80 [Hz], and when 150 [ms] ≦ τ ≦ 200 [ms], 80 [Hz] ≦ F ≦ 100 [Hz], 100 When [ms] ≦ τ ≦ 150 [ms], 100 [Hz] ≦ F ≦ 120 [Hz], and when 80 [ms] ≦ τ ≦ 100 [ms], 120 [Hz] ≦ F ≦ 140 [Hz] , Τ ≦ 80 [ms], setting 140 [Hz] ≦ F makes it possible to realize a high-quality gradation display in which flicker and crosstalk are more effectively suppressed.

Further, in a liquid crystal display device that performs grayscale display by the frame thinning method, if attention is further paid to the number of grayscales N [grayscale] of an image to be displayed on the liquid crystal display element, the frame frequency F [ Hz], the flicker is not always minimized depending on the value of N, especially when the number of gradations N is large. May become noticeable.

Therefore, the first invention is further advanced, and a frame frequency F corresponding to the number of gradations N and the response time τ is previously determined so that an optimum image display can be performed in consideration of the number of gradations N. The frame frequency F of the liquid crystal display device is set to the optimum frame frequency according to the response time τ of the liquid crystal display element and the number of gradations N set in the drive circuit. Specifically, the relationship between the frame frequency F, the number of gradations N, and the response time τ is set such that F ≧ N / 2τ + 40, where F ≧ 80.

The liquid crystal display device having the driving circuit set as described above can realize high-quality gradation display in which flicker and crosstalk are sufficiently suppressed.

[0041]

Embodiments of the present invention will be described below in detail with reference to the drawings.

(Embodiment 1) FIG. 1 is a block diagram showing the structure of the liquid crystal display device of the first and second inventions, FIG. 2 is a cross-sectional view showing the structure of a liquid crystal display element used therein, and FIG. FIG. 10 is a diagram showing the results of an experiment performed to derive a relationship between a response time τ and a frame frequency F of a liquid crystal display element used in the liquid crystal display device of the invention of the present invention.

As shown in FIG. 1, the liquid crystal display device of the first invention comprises an LCD drive circuit 10 and the LCD drive circuit 1.
And a liquid crystal display element 20 which is multiplex-driven by 0 to perform frame thinning-out gradation display.

As shown in FIG. 2, the liquid crystal display element 20
The signal lines (X electrodes) 21 composed of 640 band-shaped transparent electrodes and the scanning lines (Y lines) composed of 200 band-shaped transparent electrodes are arranged so as to be orthogonal to the opposing surfaces of the two glass substrates 25 and 26.
An electrode 22 is provided to form an XY matrix, on which alignment films 23 and 24 are formed.

The periphery of the matrix is sealed and fixed with a sealant 28, and a liquid crystal composition 27 is sandwiched between the two glass substrates 25 and 26 and the sealant 28.

On the display surface side of the liquid crystal display element 20, an optical retardation film 29 and a polarizing plate 30 are attached, and on the rear surface side, a polarizing plate 31 and a backlight 32 are attached.

For the liquid crystal composition 27, ZLI-2293 manufactured by Merck was used. The rubbing directions of the alignment films 23 and 24 are set such that the twist angle of the liquid crystal is 240 degrees.

Further, in order to carry out an experiment for obtaining an optimum frame frequency F for a response time τ specific to each liquid crystal display element, a plurality of liquid crystals having different response times τ by changing the electrode spacing d of the liquid crystal cells respectively. A display element was prepared and used as the liquid crystal display element 20.

That is, (1) a liquid crystal display element having an electrode interval d = 2.9 μm; response time τ = 80 ms; (2) a liquid crystal display element having an electrode interval d = 3.2 μm; response time τ = 100 ms; d = 3.9 μm; Liquid crystal display element with response time τ = 150 ms (4) Electrode interval d = 4.5 μm; Liquid crystal display element with response time τ = 200 ms (5) Electrode interval d = 5.5 μm; Response time τ = 300 (6) Liquid crystal display element of (6) Liquid crystal display element having electrode spacing d = 7.1 μm; response time τ = 500 ms.

When display image data such as a data signal and a control signal is input, the LCD drive circuit 10 generates an multiplex drive voltage based on the display image data, and an LCD drive voltage generation circuit 14 A signal line driver IC 13 and a scanning line driver IC 12 are connected to the liquid crystal display element 20 and apply a driving voltage to the liquid crystal display element 20 by a frame thinning method.

The LCD driving circuit 10 makes the frame frequency F variable in order to conduct an experiment for finding an optimum frame frequency F for a response time τ unique to each liquid crystal display element.

The LCD driving circuit 10 configured as described above
Using the liquid crystal display element 20, a 16-gradation display is performed by the frame thinning method, and the above-described six liquid crystal display elements are sequentially replaced to obtain a response time τ specific to each liquid crystal display element. An experiment was conducted to find the optimum frame frequency F with less flicker.

As a result, a result was obtained in which there was a relationship as shown in FIG. 3 between the frame frequency F and the state of occurrence of flicker during driving.

From this experimental result, it is found that the frame frequency F is 80
At [Hz] or higher, almost no flicker was observed, and it was confirmed that generally good image display was achieved.

When the response time τ is equal to or longer than 300 [ms] as in the liquid crystal display device according to the prior art, it is impossible to follow the change in the image. In response to this, the response time τ may be set to 300 [ms] or less by adjusting the composition of the liquid crystal and the cell gap of the liquid crystal display element.

According to the experimental results shown in FIG. 3, the lower the frame frequency F and the shorter the response time of the liquid crystal, the more noticeable the occurrence of flicker.

However, it is known that the higher the frame frequency F, the larger the power consumption of the liquid crystal display device and the more pronounced the crosstalk.

Therefore, by repeating the above experiments, the occurrence of crosstalk is examined in addition to the occurrence of flicker, and the response time τ is set such that the occurrence of flicker is not remarkable and the occurrence of crosstalk is reduced. , The optimum frame frequency F was determined.

As a result, the frame frequency F is changed to the response time τ
It has been found that an image having a gradation expression in which flicker and crosstalk are more effectively suppressed can be obtained by setting as follows. That is, when 200 [ms] ≦ τ ≦ 300 [ms], F = 80 [Hz], and when 150 [ms] ≦ τ ≦ 200 [ms], 80 [Hz] ≦ F ≦ 100 [Hz], 100 When [ms] ≦ τ ≦ 150 [ms], 100 [Hz] ≦ F ≦ 120 [Hz], and when 80 [ms] ≦ τ ≦ 100 [ms], 120 [Hz] ≦ F ≦ 140 [Hz] , Τ ≦ 80 [ms], the frame frequency F is set as 140 [Hz] ≦ F.

The above-described response time τ (response characteristic of liquid crystal) will be briefly described.

The transmittance in the on state and the transmittance in the off state are set to 100 and 100, respectively, for normally black display.
0, 0 and normally white, respectively.
Set to 100.

The time required for the transmittance of the liquid crystal to change from 0 to 90 from the moment when the applied voltage is switched from the off state to the on state in the case of normally black display, or the transmittance in the case of normally white display. The time required to change from 100 to 10 is τon, and conversely, in the case of normally black display, from the moment when the applied voltage is switched from on to off, until the transmittance of the liquid crystal changes from 100 to 10 , Or the time required for the transmittance to change from 0 to 90 in the case of normally white display, is represented as τoff, and the response time is represented as τ = τon + τoff.

In this embodiment, the LCD driving circuit 10
Has a variable frame frequency F in order to conduct an experiment for finding an optimum frame frequency F for a response time τ specific to each liquid crystal display element. For example, a liquid crystal display device in which the frame frequency is set using the present invention is used. Is mass-produced, an optimum frame frequency F corresponding to a liquid crystal display element incorporated in the liquid crystal display device is set in advance in a drive circuit of the liquid crystal display device.

(Embodiment 2) Next, in the liquid crystal display device having substantially the same configuration as that of the first embodiment, the liquid crystal display element 20 is changed by changing the value of the electrode interval d of the liquid crystal cell. A plurality of types of liquid crystal display elements set so as to have different response times τ are prepared, and the setting of the number of gradations N and the frame frequency F of the drive circuit 10 are changed to several types of driving conditions, thereby changing the plurality of types of liquid crystal display elements. An experiment was conducted in which the liquid crystal display element was driven to determine when the relationship between the response time τ of the liquid crystal, the number of gradations N, and the frame frequency F causes flicker.

Specifically, the liquid crystal display element 20 includes: (1) a liquid crystal display element having an electrode interval d = 2.9 μm; a response time τ = 80 ms; and (2) an electrode interval d = 3.9 μm; a response time τ = 150 ms. (3) Liquid crystal display element with electrode spacing d = 5.5 μm; response time τ = 300 ms. Three types of liquid crystal display elements are used. The frame frequency F is set from about 40 Hz to 300 Hz in setting the drive circuit. And in each case the number of tones N
The test pattern was displayed with three different gradations, 16 gradations and 32 gradations, and the display quality was visually inspected.

The results are shown separately in FIGS. An area A shown in the figure shows an area under conditions where it was confirmed that flicker was inconspicuous and was sufficiently resistant to practical use, and an area B was an area where conditions were confirmed that flicker was noticeable and was unbearable in practical use. Is shown. Also, plots indicated by x in the figure indicate points where flicker occurred frequently and the screen became hard to see, and plots indicated by Δ indicate points where flicker was slightly observed and the screen became slightly hard to see, The plots shown show good display with almost no flicker.

Based on the above experimental results, the total number of gray scales N [gray scales] and the liquid crystal response time τ [ms] of the liquid crystal display device for displaying an image having a gray scale expression by the frame thinning method are considered. Thus, it was concluded that the frame frequency F [Hz] at which the optimum image display can be performed should be set as F ≧ N / 2τ + 40, where F ≧ 80. Here, the condition that F ≧ 80 is given is that the frame frequency F
Is less than 80 [Hz], it has been confirmed in the above-described first embodiment that flicker occurs significantly.

As described above, a liquid crystal display device in which the frame frequency F was set was manufactured, a test pattern was displayed, and the image quality of the displayed image was verified by visual observation. No flicker or crosstalk was found in the displayed image. It was confirmed that high-quality image display was realized.

[0069]

As described in detail above, the liquid crystal display device of the present invention is suitable for a liquid crystal display device that is driven by multiplexing and performs gradation display by a frame thinning method and a driving method thereof, and is suitable for a liquid crystal display device. By setting the frame frequency according to the response characteristics and the number of gradations, it is possible to provide a liquid crystal display device that realizes high-quality image display while suppressing flicker and crosstalk.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a configuration of a liquid crystal display device of the present invention.

FIG. 2 is a cross-sectional view illustrating a structure of a liquid crystal display element used in the liquid crystal display device of the present invention.

FIG. 3 is a diagram showing the results of an experiment performed to derive a relationship between a response time τ and a frame frequency F in the liquid crystal display devices of the first and second inventions.

FIG. 4 shows a liquid crystal display device according to a third aspect of the present invention, in which an experiment was conducted to derive a relationship between the total number of gradations N, the response time τ, and the frame frequency F, and the response time τ was set to 80 ms. The figure which shows the result at the time.

FIG. 5 shows a response time τ = 150 [ms] of an experiment performed to derive a relationship between the total number of gradations N, response time τ, and frame frequency F in the liquid crystal display device according to the third invention.
FIG.

FIG. 6 shows a response time τ = 300 [ms] in an experiment performed to derive a relationship between the total number of gradations N, response time τ, and frame frequency F in the liquid crystal display device of the second invention.
FIG.

FIG. 7 is a partially omitted plan view showing an electrode configuration of a matrix type liquid crystal display element used in a conventional liquid crystal display device.

FIG. 8 is a waveform diagram showing a driving voltage of a conventional liquid crystal display device by a multiplex driving method.

Claims (2)

(57) [Claims] 1. A liquid crystal display device comprising: a liquid crystal display element having an XY matrix electrode; and a drive circuit for driving the liquid crystal display element, wherein a response time of the liquid crystal display element is τ [ms].
Assuming that a frame frequency for driving the liquid crystal display element by the frame thinning method is F [Hz], when the frame frequency F is 200 [ms] ≦ τ ≦ 300 [ms] with respect to the response time τ, F = 80 [H
z], 150 [ms] ≦ τ ≦ 200 [ms], 80 [Hz] ≦
When F ≦ 100 [Hz], 100 [ms] ≦ τ ≦ 150 [ms], 100 [Hz]
When ≤ F ≤ 120 [Hz], 80 [ms] ≤ τ ≤ 100 [ms], 120 [Hz] ≤
The liquid crystal display device characterized by having a set driving circuit as F ≦ 140 [Hz]. 2. A liquid crystal display device comprising: a liquid crystal display element having an XY matrix electrode; and a drive circuit for driving the liquid crystal display element, wherein a response time of the liquid crystal display element is τ [ms].
Assuming that a frame frequency at which the liquid crystal display element is driven by the frame thinning method is F [Hz] and a number of gradations to be expressed by the liquid crystal display element is N [gradation], the response time τ, the frame frequency F, A liquid crystal display device comprising a drive circuit in which the relationship of the tone number N is set as follows: F ≧ N / 2τ + 40, where F ≧ 80.