JP2006018042A - Method for driving liquid crystal display device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for driving a liquid crystal display device capable of suppressing an influence on an gamma characteristic even if temperature changes, and suppressing blurring of an animation and occurrence of a pseudo contour. <P>SOLUTION: The driving method in which when letting a display means DS, where two or more pixels are arranged in a matrix form by enclosing a liquid crystal LC between an active matrix substrate 2 with two or more reflection type pixel electrodes 4 and a transparent counter electrode CE, perform displaying by applying digitized image signals Video thereto, each field of the image signals is constituted of two or more sub-fields, respectively, and each of the two or more sub-fields is selectively turned on or off to display a predetermined number of gradation levels, comprises a sub-field for displaying a product of a display period and a display level according to the image signals to be displayed within each field period by dividing it into two at least. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for driving a liquid crystal display device.

  Conventionally, in a method of driving an active matrix type liquid crystal display device, a driving voltage of liquid crystal is generally controlled by an analog signal (for example, Patent Documents 1 and 2). As the liquid crystal mode, VA (Vertical Aligned) or MTN (mixed-mode Twisted Nematic) is used, and VA is used particularly for improving the contrast ratio. In this type of liquid crystal display device, a liquid crystal is sealed between an active matrix substrate and a counter substrate to form a large number of pixels, an image signal is written to each pixel, and a capacitor (signal auxiliary) attached to each pixel. The liquid crystal is driven by accumulating in the capacity.

  One example will be described with reference to FIG. FIG. 19 is a schematic view showing an example of the configuration of an active matrix type liquid crystal display device. The display means DS of this liquid crystal display device has a plurality of column signal electrodes D1, D2, D3,... Di arranged in parallel on the active matrix substrate 2, and each of these column signal electrodes D1, D2, D3, ... a plurality of row scanning electrodes G1, G2, G3,... Gj are arranged in a direction orthogonal to Di. Hereinafter, the symbols D1 to Di may be collectively represented as a symbol D, and the symbols G1 to Gj may be collectively denoted as a symbol G. At the intersection of each column signal electrode D and row scan electrode G, as shown in FIG. 20, pixels Px including a pixel switching transistor Tr and a signal auxiliary capacitor Cs and a liquid crystal LC corresponding to each pixel are arranged in a matrix. Has been placed. The column signal electrode driving circuit 100 includes a horizontal shift register 101 and a switch group including a plurality of analog switches S1, S2, S3,. The input side of each analog switch S1, S2, S3,... Si is connected in common to the display signal supply wiring L to which the image signal Video is supplied, and the output side is respectively corresponding column signal electrodes D1, D2, D3,. ... connected to Di. Further, the output of the horizontal shift register 101 is connected to the control signals of the analog switches S1, S2, S3,.

  In the column signal electrode drive circuit 100 having such a configuration, the horizontal shift register 101 is driven by a horizontal start signal HST and a horizontal clock HCK supplied from a drive timing pulse generation circuit (not shown), and an output pulse from the horizontal shift register 101 is output. By sequentially turning on the analog switches S1, S2, S3,..., Si, the image signal Video for one horizontal period is sequentially sampled to the column signal electrodes D1, D2, D3,.

On the other hand, the row scanning electrode driving circuit 102 includes a vertical shift register having a number of stages corresponding to the total number of display rows. This vertical shift register is driven by a vertical start signal VST supplied from a drive timing pulse generating circuit (not shown) and a vertical shift clock VCK synchronized with a horizontal period, and is 1 for each row scanning electrode G1, G2, G3,. Scan pulses are sequentially output for each horizontal period (each row). Here, the vertical period of the image signal Video is synchronized with the vertical start signal VST.
As a result, the pixel switching transistors Tr connected to the row scanning electrodes G1, G2, G3,... Gj are sequentially turned on row by row, and the voltage of the image signal Video sampled to D1, D2, D3,. In this way, it is accumulated and held as charge information in the signal auxiliary capacitance Cs of the adjacent pixels.

As a result, the signal voltage accumulated in each signal auxiliary capacitor Cs is applied to the liquid crystal LC corresponding to each pixel Px via the reflective pixel electrode 4, and the light modulation degree of the liquid crystal LC changes accordingly. As a result, an image corresponding to the image signal Video is displayed. The liquid crystal LC is sealed between each reflective pixel electrode 4 on the active matrix substrate 2 and the transparent counter electrode CE.
In such a display method, the voltage applied to the liquid crystal LC is constant in time and expresses the gradation by changing according to the signal level. In addition to the disadvantages that noise is easily applied to the level and that it is easily affected by the pseudo signal, a direct current component is easily applied to the liquid crystal LC, resulting in problems with the remaining image and the panel life.

  On the other hand, there is a digital driving method in which an analog image signal is converted into a digital signal and a voltage is applied to the liquid crystal in a pulsed manner to drive the liquid crystal. For example, there is a system in which one field (1 TV field) is divided into a plurality of subfields, and lighting / non-lighting is controlled for each subfield. As this method, a method using a weighted subfield, an intra-field dispersion method, and a CLEAR driving method are known (for example, Patent Documents 2, 3, and 4).

JP-A-11-174410 JP 2004-69788 A JP 2001-343950 A JP 2003-295839 A

However, the above-described liquid crystal display device using the digital driving method has the following problems. That is, the responsiveness to the driving pulse of the liquid crystal changes due to the temperature change of the liquid crystal display device itself. As a result, there is a problem that the gamma characteristic indicating the relationship between the input signal and the output signal changes and the display characteristic deteriorates. In addition, blurring may occur when displaying a moving image, or pseudo contour may occur.
The present invention has been devised to pay attention to the above problems and to effectively solve them. An object of the present invention is to provide a driving method of a liquid crystal display device capable of suppressing the influence on the gamma characteristic even when the temperature changes, and suppressing the occurrence of moving image blur and pseudo contour.

The invention according to claim 1 is digitized into display means in which a liquid crystal is sealed between an active matrix substrate having a plurality of reflective pixel electrodes and a transparent counter electrode, and a plurality of pixels are arranged in a matrix. When the image signal is applied and displayed, each field of the image signal is composed of a plurality of subfields, and each of the plurality of subfields is selectively turned on or off to give a predetermined number of gradations. A driving method for displaying a level, comprising: a subfield for displaying a product of a display time and a display level corresponding to the image signal to be displayed in each field period divided into at least two parts. Is the method.
In other words, the digitized image signal is displayed on the display means in which liquid crystal is sealed between an active matrix substrate having a plurality of reflective pixel electrodes and a transparent counter electrode, and a plurality of pixels are arranged in a matrix. When applying and displaying, each field of the image signal is composed of a plurality of subfields, and each of the plurality of subfields is selectively turned on or off to display a predetermined number of gradation levels. In the driving method, the display is performed in the middle of the gradation level so as to have two or more convergence points so that the brightness of the display hardly changes even if the temperature during driving of the liquid crystal changes. This is a method for driving a liquid crystal display device.

In this case, for example, as defined in claim 2, in one field of the image signal, there is provided means for causing two or more portions above a certain peak of the modulation of the liquid crystal with respect to the image signal to appear.
For example, as defined in claim 3, in one field of the image signal, four or more highly modulated portions of liquid crystal appear in a low gradation level region, and in a middle gradation level to high gradation level region. Display is made so that two or more highly modulated portions of the liquid crystal appear.

According to the driving method of the liquid crystal display device of the present invention, the following excellent operational effects can be exhibited.
Provided is a driving method of a liquid crystal display device that can suppress the influence on the gamma characteristic even when the temperature changes, and can suppress the occurrence of moving image blur and pseudo contour.

Hereinafter, an embodiment of a method for driving a liquid crystal display device according to the present invention will be described in detail with reference to the accompanying drawings.
<Description of principle>
First, the principle of the present invention will be described.
Conventionally, the liquid crystal is pulse-driven. In the old days, when a substrate on which strip-shaped electrodes are arranged is arranged orthogonally to form a simple matrix liquid crystal element and a liquid crystal located at the intersection of the electrodes is driven, it has been a substantial pulse drive. In order to avoid crosstalk, a liquid crystal that changes sharply at a certain voltage, such as STN liquid crystal, is used, but the image quality remains poor. In addition, a liquid crystal display using a ferroelectric liquid crystal that is pulse-driven has been developed, but it has not been put into practical use because a reset pulse is required and gradation is not sufficient. On the other hand, the drive system proposed by S Vision is superior in that a predetermined pulse is driven to the nematic liquid crystal, but the state of the pulse response of the liquid crystal changes with temperature. There was a drawback that the characteristics changed.

  In this regard, the present inventors' knowledge is introduced. Consider a case in which there is no internal ion movement in the liquid crystal, and the voltage drop in one field period is negligible with a very high resistance. In this case, it does not matter whether the voltage is applied to the plus side or the minus side for the liquid crystal. That is, FIG. 1 shows the relationship between the voltage applied to the liquid crystal and the output light, and the change in the liquid crystal is the same in both the case of FIG. 1A and the case of FIG. FIG. 1A shows the time change of the driving pulse voltage to the liquid crystal and the response of the liquid crystal to the change. FIG. 1B shows that the same response is obtained when the polarity of the driving pulse to the liquid crystal is the same. In this case, it is assumed that linearly polarized light enters the liquid crystal cell, reflects off the electrode surface, and exits from the transparent counter electrode. Outside the liquid crystal cell, a light analyzing means such as a polarizing beam splitter is provided, and an optical system that transmits only polarized light orthogonal to incident polarized light and blocks the same light as incident polarized light is provided.

When the response of the liquid crystal is fast, it responds to the input voltage, as shown in FIG. 1C, but when the response speed of the liquid crystal is slow, it is as shown in FIGS. 1A and 1B. The response is delayed. FIG. 1C shows the response of a liquid crystal having a very fast response when the same drive pulse as in FIG. 1B is input. However, when pulse driving is performed on the liquid crystal, the relationship between the driving interval of the pulse and the response of the liquid crystal is very deep, and this point is solved in Patent Document 2 by the present applicant. .
Here, assuming a case where a short pulse is applied to the liquid crystal, the liquid crystal does not respond completely in one pulse driving time, and the modulation of the liquid crystal increases by overlapping the pulses many times. FIG. 2 is a diagram showing a voltage applied to the liquid crystal and a change state of output light at that time. As shown in FIG. 2A, when the pulse driving time is long and the pulse interval is long, the output light greatly fluctuates in a wave shape.

  Further, when the pulse interval is shortened with respect to the response speed of the liquid crystal, the liquid crystal is in a modulation state that increases in a certain time as in the case of the continuous pulse (FIG. 2B). The width of one pulse is t seconds (FIG. 2A). However, it should be noted that if the pulse is simply repeated, the substantial drive voltage behaves as an effective value, and the modulation factor of the liquid crystal cannot be taken up to 100%. In order to eliminate this, it is necessary to increase the pulse drive voltage to 1 / √2 and change the peak voltage. This is an effective relationship from a linear relationship to the frequency of pulse driving. For example, FIG. 3 is a graph showing changes in output light when the width and height of the pulse voltage are changed. FIG. 3 (a) shows the response (output light) of the liquid crystal when the pulse voltage is the same and the pulse is thinned and when a long pulse is applied. It can be seen that the modulation degree of the liquid crystal decreases. FIG. 3B shows that the same brightness (the degree of modulation of the liquid crystal) is obtained when the voltage is increased even at the same pulse interval. In this case, the voltage at which the liquid crystal has the maximum degree of fluctuation is 1 / √2 when the duty of the pulse is 50%.

  FIG. 4 is a graph showing the relationship between the arrangement of pulses and the degree of modulation of liquid crystal (output light) when the pulse voltage and the pulse width are the same. Thus, it can be seen that the output light differs depending on three different pulse arrangements even if the pulse voltage and the pulse width are the same. This is a proximity effect of the pulse. When one pulse and another pulse are close to each other and when they are separated, the response of the liquid crystal is different, and as a result, a phenomenon is observed in which the brightness is substantially observed as a result of temporal integration. is there. If the pulse interval is wide, an isolated waveform pulse is applied to the liquid crystal. When the pulse interval is narrow, the liquid crystal operates as if the response speed of the liquid crystal is substantially increased. In FIG. 5, one field is divided into 24 subfields, and the lighting sequence of 6 pulses with the same interval (1 subfield period) 300 μsec is connected with priority (blue) and every other lighting sequence. The brightness obtained is displayed for the ones that were turned on (purple) and the ones that were turned on every other two.

  Here, the lighting patterns are as shown in FIGS. 5 (a) to 5 (c). In FIG. 5, the shaded portion means that the pulse is lit. The horizontal axis indicates the subfield number, and the vertical axis indicates the gradation level. FIG. 5 (a) shows a pulse pattern of a type (one peak) that lights up in the order used in FIG. 4, and FIG. 5 (b) shows the drive pulses to the liquid crystal used in FIG. FIG. 5C shows a pulse pattern in which the driving pulses for the liquid crystal used in FIG. 4 are sequentially turned on every two subfields.

Even if the effective value of the voltage applied to the liquid crystal is the same, the brightness obtained varies depending on how the pulse is applied to the liquid crystal. Furthermore, when the physical properties of the liquid crystal change, the same liquid crystal cell is used, and even when the same temperature change occurs, the rate of change in brightness depends on how the pulse is applied to the liquid crystal (in this case, the liquid crystal is driven). This means that the length, number, and type of pulses are exactly the same, only the sequence is different.)
In the previous example, the case of FIG. 5C is the most stable with respect to temperature, FIG. 5B follows, and FIG. 5A is the worst. Therefore, it is possible to improve the temperature characteristics by optimizing the way of applying the pulse group to the liquid crystal.

Now consider the response waveform of the liquid crystal, not the pulse train applied to the liquid crystal. The response of the liquid crystal is not a condition for responding 1: 1 to the pulse, but is a condition having a response speed that does not respond completely with one pulse. When the pulses have the same wave height (in this case, equivalent to a voltage), and those with a certain pulse width gather, the liquid crystal has the same effect as driving with a large pulse, and the modulation degree of the liquid crystal is large. Become. Therefore, it becomes a problem how many blocks the light modulated by the liquid crystal becomes.
When a portion strongly modulated by the liquid crystal appears once in one field (1/60 sec), it is expressed as one mountain. When this appears twice in one field, it is called two peaks. In one drive pattern in one field (corresponding to the case of FIG. 5A), the output change when the temperature changes is large as described above, so the temperature stability is low and the flicker is conspicuous. It turns out that there are drawbacks.

  This flicker is not so conspicuous in a high gradation and bright part, but is particularly conspicuous in a gradation from the middle to the lower. In particular, it is more conspicuous in PAL (used in Europe and the like) in which one field is 1/50 msec than in the case of the NTSC system in which one field is 1/60 msec. In order to avoid this, at least two-climbing driving in which a mountain appears twice in one field is necessary. When the nematic liquid crystal is pulse-driven, a predetermined target luminance characteristic cannot be obtained unless the liquid crystal is driven in consideration of such points.

On the other hand, in the white state where all the pulses are turned on, an effective driving voltage is applied to the liquid crystal. In this state, even if the response of the liquid crystal changes depending on the temperature, there is little influence. Even in the black state where all the pulses are turned off, the liquid crystal is applied with an effective driving voltage, so that even if the response of the liquid crystal changes due to temperature, the influence is small. Therefore, the portion that is an intermediate gradation is particularly changed.
At this time, if a portion where the modulation of the liquid crystal is high and a low portion are formed, a portion where the brightness hardly changes even if the temperature at the time of driving the liquid crystal is changed in a region above the halftone. Such a phenomenon is called a convergence point here. For example, FIG. 7 is a graph showing the temperature dependence of the gamma characteristic, and it can be seen that the output ratio largely fluctuates depending on the temperature, but hardly changes at a certain part (convergence point). FIG. 6 is a graph showing the temporal response of the liquid crystal to the pulse voltage when the liquid crystal driving temperature is changed. Here, the temperature is changed in the range of 20 to 60 ° C. FIG. 7A shows an output change at a gradation portion exceeding the first convergence point, and FIG. 7B shows an output change at a gradation portion not exceeding the first convergence point. One field is 16.7 msec, and there are two peaks every 16.7 msec. That is, there are two large blocks of subfields that are lit in the subfield. In the gradation portion higher than the convergence point, as shown in FIG. 7A, even if the temperature of the portion where the modulation of the liquid crystal is high rises and the degree of modulation further increases, the output of that portion is It does n’t change much. However, the non-saturated part (the part where the drive pulse is actually off) follows because the response of the liquid crystal is fast, and the one-field average output is a region where the output slightly decreases even when the liquid crystal temperature rises. Become.

  On the other hand, in the region below the convergence point, as shown in FIG. 7B, when a pulse voltage is applied to the liquid crystal, the temperature rises, and the response is accelerated due to the influence of the viscosity of the liquid crystal and the like. In some cases, the response of the liquid crystal to the voltage pulse becomes faster, and as a result, the output tends to increase. As a result, a convergence point where the output hardly changes even when the temperature changes is generated due to the influence of both. This convergence point occurs in FIG.

<First embodiment>
Here, an example of a liquid crystal display device used in the method of the present invention will be described with reference to FIG. Since the basic configuration is the same as that of the apparatus shown in FIG. 19, the same components are denoted by the same reference numerals and the description thereof is omitted. This device configuration is the same as that disclosed in Patent Document 2. In the liquid crystal display device 6, the input image signal is converted into a digital signal through the AD converter 12, and each data is supplied to each column signal electrode D1 to Di. Each data is held in the SRAM circuit 15 of each pixel shown in FIG. The pulse width modulator 13 shown in FIG. 8 is connected to the buffer circuit 16 shown in FIG. 9, and sets the time during which the data held in the SRAM circuit 15 is applied to the liquid crystal by the pulse width modulator 13. Corresponding to the column signal electrodes D1 to Di, column signal electrodes * D1 to * Di for inverted signals for supplying these inverted signals are respectively provided. Each of the column signal electrodes D1 to Di is provided with a pair of switches S1 to Si that are turned on and off in synchronization with the respective analog switches S1 to Si. The inverted image signal Video is input to the inverted signal column signal electrodes * D1 to * Di via the inverter 14.

Here, the pulse driving circuit shown in FIG. 9 will be described. When the liquid crystal LC is driven by digital driving by this pulse width modulation, each electrode potential is fixed, and the structure is extremely difficult to be affected by the stray capacitance between the electrodes. As a result, there are few problems in image display compared to analog driving.
The pulse driving circuit includes an SRAM (static RAM) circuit 15 that holds input data and a buffer circuit 16 that transfers the data to the pixel electrode 4. The SRAM circuit 15 stores information in a flip-flop circuit composed of transistors Tr1 to Tr4. The switches S2 and S1 are respectively connected to the column signal electrode D and the other column signal electrode * D through which an inverted signal of this data flows.

  In synchronization with the image signal data flowing through the column signal electrodes D and * D, a pulse is input to the row scanning electrode G connected to the gate, and the data is temporarily held in the SRAM circuit 15. The buffer circuit 16 includes a switch (not shown). A signal for turning on the switch is input from the outside. When the buffer circuit 16 is turned on, data stored in the SRAM circuit 15 is added to the liquid crystal LC to drive liquid crystal molecules. Here, the liquid crystal LC uses a mode in which, for example, a nematic liquid crystal having a negative dielectric anisotropy is aligned substantially perpendicular to the active matrix substrate 2 when no voltage is applied.

Next, a light modulation optical system package using this liquid crystal display device will be described. Here, for the sake of simplicity, description will be made with reference to FIG. 10 using a case of a light modulation optical system package 60 in which a color quad proposed by Color Link Co., Ltd. is modified. Here, three liquid crystal display devices 6 are used as the spatial light modulators 6A, 6B, and 6C.
In FIG. 10, a light source 62 on the light incident side of the light modulation optical system package 60 and a first polarization unit that selectively transmits only predetermined linearly polarized light from indefinitely polarized white light that is emitted from the light source 62. (For example, a polarizing plate) 64, a second polarizing means (for example, a polarizing plate) 66 that selectively transmits only predetermined linearly polarized light to the light emission side of the light modulation optical system package 60, and a projection lens 68. And are arranged. A screen (not shown) is provided in front of the projection lens 68 to display a color image.

The light modulation optical system package 60 includes cubic or prismatic first, second, third, and fourth polarizing beam splitters 70, 72, 74, and 76 having polarization separating surfaces 70A, 72A, 74A, and 76A. They are arranged so as to intersect in a substantially X shape, and the whole is bonded and fixed by a base (not shown). A light shielding means 78 is provided at the center of the package 60.
Here, when the first polarizing beam splitter 70 is selected as the incident-side polarizing beam splitter, the fourth polarizing beam splitter 76 at the diagonal position is set as the output-side polarizing beam splitter. Further, the light modulation optical system package 60 has the first color light (for example, G light) on the light incident side light transmitting surface of the first polarizing beam splitter 70 and the light emitting side light transmitting surface of the fourth polarizing beam splitter 76. First wavelength-selective polarization conversion means (for example, a G phase plate) 80 for rotating the plane of polarization by 90 ° is provided, respectively, and the gap between the opposing portions of the first and third polarization beam splitters 72 and 74, and the third and Second gap-selective polarization conversion means (for example, R phase plate) 82 that rotates the polarization plane of the second color light (for example, R light) by 90 ° is provided in the gap between the opposed portions of the fourth polarization beam splitters 74 and 76. Each is equipped.

Spatial light modulation elements 6A, 6B, and 6C, which are reflective liquid crystal display devices, are disposed on the light transmitting surfaces of the second and third polarization beam splitters 72 and 74 of the light modulation optical system package 60, respectively. . Here, the spatial light modulators 6A, 6B, and 6C are used for G light, R light, and B light, respectively.
Phase plates 20A, 20B, and 20C are respectively inserted between the spatial light modulation elements 6A, 6B, and 6C and the polarization beam splitters 72, 74, and 74 positioned immediately before. As each of the phase plates 20A, 20B, and 20C, for example, a quarter wavelength plate is used, and the angle of the quarter wavelength plate can be changed around the optical axis so as to compensate for the birefringence of the polarizing beam splitter and the liquid crystal. It has a structure. In addition, each of the phase plates 20A, 20B, and 20C is desirably a quarter wavelength at the center wavelength of the irradiated light.
In such a light modulation optical system package 60, the image signal is modulated for each of G color, R color, and B color in each of the spatial light modulation elements 6A, 6B, and 6C, and these colors are synthesized by the projection lens 68. Then, it is projected onto the screen and an image is displayed.

Here, in the first embodiment of the present invention, the first convergence point described above is obtained by the following method.
The arrangement of the pulse train is devised during one field period, and a portion where the liquid crystal is driven is high and low during one field period. In this state, the liquid crystal parameters such as the liquid crystal viscosity and the liquid crystal cell thickness are set appropriately, and the response speed of the liquid crystal is set so as not to follow each pulse. It was confirmed that the overall stability is improved when the first convergence point is brought as low as possible. This can be realized by increasing the response of the liquid crystal so as not to follow the pulse.

The second convergence point is obtained by the following method.
Adjust the angle of the phase plates 20A, 20B, and 20C between the spatial light modulation elements 6A, 6B, and 6C and the polarizing beam splitters 72, 74, and 74 facing the spatial light modulators 6A, 6B, and 6C so that the most black appears with respect to the temperature change. To do. It is important to perform this adjustment at a temperature within the operating temperature range, and it is desirable to perform the adjustment at the middle of the operating temperature range or at a slightly higher temperature.
When the temperature of the liquid crystal rises, the following two effects occur simultaneously. One effect is a decrease in the viscosity of the liquid crystal. Another effect is a reduction in the birefringence of the liquid crystal. Thus, when the viscosity of the liquid crystal is lowered, the response to the pulse is improved, and the output is increased in the low gradation portion. On the other hand, when the birefringence of the liquid crystal decreases due to the temperature rise, the black position combined by the phase plates 20A, 20B, and 20C shifts from the optimum point. For this reason, the position where the voltage is applied to the liquid crystal is the position where black can be taken most. Since both the above effects work in the direction of canceling, a convergence point appears in the low gradation portion.

  In addition to the first convergence point and the second convergence point, for example, at the 0th gradation and the 255th gradation, since there are many pulses in time, an effective voltage when taken on the liquid crystal In this case, there is almost no temperature change. Therefore, the 0th gradation and the 255th gradation have no temperature change, and have two convergence points in the middle area of the gradation, so that even if the temperature changes, the influence on the gamma characteristic can be reduced. Display quality can be maintained high. For example, the specific conditions for generating the first and second convergence points are as follows. When the first convergence point generates 10 or less peaks in 1/60 seconds, the response speed of the liquid crystal ( This occurs when the response time (rise time + fall time) has a response speed of about 3 msec or more. If the response speed of the liquid crystal exceeds 20 msec, the moving image characteristics deteriorate, so it is desirable to have a response speed of 3 msec to 20 msec. The second convergence point is NB (normally black), and can be achieved by aligning the optimum point with a wave plate in the operating temperature range and by pulse driving.

  For example, FIG. 11 is a graph showing the principle that the second convergence point (dark part) is generated when the angle of the phase plate is adjusted with respect to the optical axis. The horizontal axis indicates the pulse voltage and the vertical axis indicates the output light (No. 1). Standardized with 1 being a bright spot). The temperature of the liquid crystal is changed within a range of 20 to 60 ° C. As is apparent from FIG. 11, the blackest part occurs in the temperature range of 40 to 60 ° C. FIG. 12 is a diagram showing a state in which the first convergence point and the second convergence point appear in the middle of the gradation level, and two portions surrounded by dotted lines are the first and second convergence points P1, P2. Here, the temperature of the liquid crystal is changed in a range of 20 to 60 ° C. As the spatial light modulation elements 6A, 6B, 6C, liquid crystal display devices as shown in FIGS. 19 and 20 may be used instead of the liquid crystal display devices shown in FIGS.

<Second embodiment>
Next, a second embodiment of the method of the present invention will be described.
In the second embodiment, display is performed so that two or more highly modulated portions of the liquid crystal LC appear in one field of the image signal.
In the case of a normal transmission type liquid crystal display device, it is said to be a holding type driving method. This is because the pixel has a pixel switching transistor and a storage capacitor. When a signal is written to the pixel, the corresponding pixel switching transistor is turned on, a voltage is applied to the pixel, and the voltage is stored in the storage capacitor for a certain period. Thus, the image is displayed. In this method, it is pointed out that moving image blur occurs when an image moves because the image is held for one field period.

  In particular, in a normal analog device that controls the signal auxiliary capacitance with a voltage value, in the case of white, the voltage can be driven high, but in the gradation portion (area) below the halftone, the driving voltage is low, and the voltage There is a relationship that the response speed of the liquid crystal is further reduced as the value of the value becomes lower, and there has been a problem of improving the response speed from the halftone to the lower gradation part (region). It has been found that this phenomenon can be improved by putting black in the image, but there was no specific method. This time, by using a method in which the liquid crystal is digitally driven and divided into several subfields, all subfields are lit at high gradations, and the liquid crystal is not driven in the gradations below the halftone. , It was found that the video characteristics can be improved. The difference from the conventional method is that the white display does not change, but the moving image performance is improved in halftone.

  From the viewpoint of liquid crystal driving, it becomes a problem how many blocks a pulse applied to the liquid crystal is divided into in one field. This point will be explained in detail. Here, in order to simplify the story, consider the case where one field is divided into 30 subfields and displayed. In order to simplify the story, the weights of the subfields are completely the same. FIG. 13 is a diagram showing the relationship between subfields and gradation levels. The horizontal axis indicates the number of subfields, and the vertical axis indicates the gradation level. In the figure, shaded portions indicate lighting. FIG. 13A shows a pattern in which each pulse is lit in order from the first subfield toward the 30th subfield. In this case, the liquid crystal is modulated in one subfield in the middle gradation. Since only one peak (degree of modulation) appears, it is classified as one peak drive. The number of gradations that can be expressed here is 31 types from 0 to 30 gradations.

  In the lighting pattern as shown in FIG. 13 (b), the first lighting is the first subfield without any change, but the next lighting is not the second subfield but the 16th subfield. The second subfield is lit third. With such an arrangement, a two-crest pattern in which two portions with strong modulation of liquid crystal (modulation crests) appear in one field is obtained. In the lighting pattern as shown in FIG. 13C, the first subfield is lit first, the eleventh subfield is lit next, and the twenty-first subfield is lit next. Go on in order. As a result, there are three portions where the liquid crystal is strongly modulated (the peak of modulation) in one field. In this way, the pulse pattern is determined with great care in order to affect the brightness and temperature characteristics in the manner in which pulses are applied to the liquid crystal.

  Here, in the above description, an eternal bit that does not turn off (turns off) when the lighting pattern is turned on once is used. However, if a pulse train is composed of only this eternal bit, the number of gradations obtained is reduced. . Therefore, in such a case, the number of gradations that can be expressed dramatically can be increased by adding binary bits, which are repeatedly turned on / off as shown in FIG. In the following example, 8 gradations are expressed by 3 bits. If each bit is weighted 1, 2, and 4 and lights up from 0 gradation to 7 gradations as shown in FIG. 14, 8 gradations can be expressed. If this binary bit and the eternal bit are combined and the weight of the eternal bit is 8, for example, in the case of FIGS. 13 (a) to 13 (c) described above, 31 × 8 = 248 gradation representations Is possible.

This is almost a gradation close to 256 gradations, but if further gradation is required, this binary bit is set to 4 bits, the weights are set to 1, 2, 4 and 8, respectively, and the weights of the eternal bits are set. If it is 16, 31 × 16 = 496 gradations, and a bit indicating the brightness of 256 gradations may be selected and used from among them.
There are an infinite number of combinations, but there are points to note. That is, if the modulation of the liquid crystal is one in one field, the temperature characteristic is deteriorated, and since flicker is conspicuous, there are two or more modulation peaks of the liquid crystal in one field. is necessary.
Also, from the viewpoint of blurring of moving images, the pulses are arranged so that the modulation of the liquid crystal is reduced at the field switching portion. That is, since all the pulses are lit in the portion close to white display, which is a high gradation portion, a pattern in which the last portion of the field is not lit in the portion from the middle gradation to the lower gradation is desirable. By doing so, the moving image blur phenomenon peculiar to the holding type liquid crystal display device is eliminated, and the moving image of the intermediate gradation can be clearly seen.

FIG. 15 is a diagram showing a display state and liquid crystal response when a blur is simulated when a vertical stripe moves in the horizontal direction at the speed of one pixel in one field in an analog (holding type) liquid crystal display device. FIG. 5 is a diagram showing a display state and liquid crystal response when a blur is simulated when a vertical stripe moves in the horizontal direction at a speed of one pixel in one field in a digital (pulse type) liquid crystal display device.
When the liquid crystal characteristics have the characteristics as shown in FIG. 15B, how much the edge is blurred when an image having vertical stripes every other line moves horizontally by one pixel per field. It is an example calculated by. The brightness of the bright portion of the vertical stripe is set at a brightness level of 30% with respect to the saturation brightness. In this case, since the voltage application to the liquid crystal decreases, the response speed of the liquid crystal becomes slow. In this case, it can be seen that there is considerable blurring.

On the other hand, FIG. 16A assumes the case where digital driving is performed with the same liquid crystal cell. Here, since a pulse voltage is applied, the drive voltage to the liquid crystal is high, and the same voltage is applied to the drive voltage even in the halftone portion. Further, the pattern of pulse driving to the liquid crystal is such that when the liquid crystal characteristics are driven in two ridges as shown in FIG. 16 (b), the portion is not driven by the switching of the field. The calculation shows that the stripes are clearly separated. The luminance level here is set to 30% of the saturated luminance (128 tone pulse train).
For example, a specific condition satisfying the second embodiment as described above occurs when the response time (falling) of the liquid crystal is within one frame period and a part of one frame is not lit. That is, it is observed when the light output fluctuates in one frame period. If the display with the highest gradation level is 128 gradations, which is half the gradation of 256 gradations that are white display, and there is an area that is 30% or less of the peak brightness within 10 frames or more in one frame, It was confirmed that there was an effect of improving the video display characteristics. In order to achieve this, the sequence of pulses to be lit may be selected in the frame.

<Third embodiment>
Next, a third embodiment of the method of the present invention will be described.
In the third embodiment, in one field of the image signal, four or more high-modulation portions of the liquid crystal appear in the low gradation level region, and the modulation of the liquid crystal occurs in the medium gradation level to the high gradation level region. Display so that two or more high parts appear.
When the liquid crystal display element is digitally driven, the pulse drive to the liquid crystal is a driving method that changes the period between low gradation and high gradation, and the period at low gradation is greater than the period at high gradation. The pulse train is arranged so that the period shifts naturally.

  This can be done as follows. As shown in FIG. 17, the order of driving to the liquid crystal is such that two peaks of the modulation peak of the liquid crystal occur in one field. In this state, the eternal bit that is lit first is sufficiently separated from the eternal bit that is lit next in time. In this way, when the operating temperature of the liquid crystal changes most, the low gradation portion, which is the portion where the change is the largest, substantially doubles to four peaks and responds to the response of the liquid crystal pulse. An effect of reducing the change in the modulation degree (the brightness when averaged over one field) is obtained.

  In FIG. 17, an area A1 shown in FIG. 17A is a pattern that lights up in the relationship between the subframe and the gradation level as shown in FIG. 5A, and an area B1 is as shown in FIG. It is a pattern that lights up in relation to subframes and gradation levels. In addition, the parts indicated by X1 and X2 in FIG. The region C1 shows an example of a lighting pattern in which four peaks appear at a low gradation. In this case, the number of peaks is not limited to four. However, if two or more bright portions appear at a low gradation and become brighter, a lighting pattern with two peaks in one frame may be taken. The region C2 shows an example in which two bright portions appear in one frame in high gradation.

  The result is shown in FIG. FIG. 18 shows temperature characteristics in the above two drive patterns. Curve A is a simple two-crest drive, and curve B is an array of pulse trains arranged so that the number of modulation degree peaks of the liquid crystal increases at a low gradation as described above. It can be seen that the temperature change rate is significantly reduced and the temperature characteristics are good.

It is a graph which shows the relationship between the voltage applied to a liquid crystal, and output light. It is a figure which shows the change state of the applied voltage to a liquid crystal, and the output light at that time. It is a graph which shows the change of output light when changing the width | variety and height of a pulse voltage. It is a graph which shows the relationship between the arrangement of a pulse and the modulation degree (output light) of a liquid crystal in case pulse voltage and pulse width are the same. It is a figure which shows the aspect of the lighting sequence of six pulses of the same space | interval (1 subfield period) when 1 field is divided | segmented into 24 subfields. It is a graph which shows the time response of the liquid crystal with respect to a pulse voltage when the drive temperature of a liquid crystal is changed. It is a graph which shows the time response of the liquid crystal with respect to a pulse voltage when the drive temperature of a liquid crystal is changed. It is a figure which shows an example of the liquid crystal display device used for this invention method. It is a block diagram which shows an SRAM circuit. It is a block diagram which shows the light modulation optical system package using a spatial light modulation element. It is a graph which shows the principle which a 2nd convergence point (dark part) generate | occur | produces when angle-adjusting a phase plate with respect to an optical axis. It is a figure which shows the state which the 1st convergence point and the 2nd convergence point appeared in the middle of the gradation level. It is a figure which shows the relationship between a subfield and a gradation level. It is a figure which shows a state when the binary bit which repeats lighting / astigmatism is added. It is a figure which shows the display state at the time of simulating the blurring when the vertical stripe moves to the horizontal direction at the speed of one pixel in one field in the analog (holding type) liquid crystal display device and the response of the liquid crystal. It is a figure which shows the display state at the time of simulating the blurring when the vertical stripe moves to the horizontal direction at the speed of one pixel in one field in the digital (pulse type) liquid crystal display device and the response of the liquid crystal. It is a figure which shows the order of the drive to a liquid crystal. It is a graph which shows the relationship between a gradation level and a ratio. It is the schematic which shows the example of a structure of an active matrix type liquid crystal display device. It is a figure which shows the circuit structure of a pixel.

Explanation of symbols

DESCRIPTION OF SYMBOLS 2 ... Active matrix substrate, 4 ... Pixel electrode, 6 ... Liquid crystal display device, 6A, 6B, 6C ... Liquid crystal display device (spatial light modulation element), 60 ... Light modulation optical system package, 68 ... Projection lens, 70, 72, 74, 76 ... Polarizing beam splitter, 80 ... Phase plate, CE ... Counter electrode, DS ... Display means, D1-Di ... Signal electrode, G1-Gj ... Row scanning electrode, Tr ... Pixel switching transistor, LC ... Liquid crystal, Px ... Pixel, Video ... Image signal.

Claims (3)

Liquid crystal is sealed between an active matrix substrate having a plurality of reflective pixel electrodes and a transparent counter electrode, and a digitized image signal is applied to a display means in which a plurality of pixels are arranged in a matrix for display. In the driving method, each field of the image signal is configured by a plurality of subfields, and each of the plurality of subfields is selectively turned on or off to display a predetermined number of gradation levels.
A driving method of a liquid crystal display device, comprising: a subfield for displaying a product of a display time and a display level corresponding to the image signal to be displayed in each field period at least in two.   2. The method of driving a liquid crystal display device according to claim 1, further comprising means for causing two or more portions of the liquid crystal modulation with respect to the image signal to exceed two or more peaks in one field of the image signal. In one field of the image signal, four or more portions with high liquid crystal modulation appear in a low gradation level region, and two or more portions with high liquid crystal modulation appear in a middle gradation level to high gradation level region. 3. The method of driving a liquid crystal display device according to claim 1, wherein the display is performed as follows.

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