JP2637515B2 - Liquid crystal device and driving method of liquid crystal element - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal device for gray scale display on a display panel, and more particularly, to a liquid crystal television using a liquid crystal material having bistability, in particular, a ferroelectric liquid crystal. And a liquid crystal device for gradation display in a display panel as described above.

(Prior art)

In a conventional liquid crystal television panel using an active matrix driving method, thin-film transistors (TFTs) are arranged in a matrix for each pixel, and a gate-on pulse is applied to the TFTs to make the source and the drain conductive. Is applied from a source and stored in a capacitor, and a liquid crystal (for example, Tw) is stored in accordance with the stored image signal.
isted Nematic-TN-liquid crystal) is driven, and at the same time, the voltage of the video signal is modulated to perform gradation display.

However, in such an active matrix drive type television panel using TN liquid crystal, since the TFT used has a complicated structure, the number of structural steps is large, and the high manufacturing cost is a net. There is a problem that it is difficult to form a thin film semiconductor (for example, polysilicon, amorphous silicon) constituting a TFT over a wide area.

On the other hand, a passive matrix drive type display panel using TN liquid crystal is known as one that can be manufactured at a low manufacturing cost. In this display panel, as the number of scanning lines (N) increases, one screen (one frame). 1 while scanning
The time during which an effective electric field is applied to one selected point (duty ratio) is reduced at a rate of 1 / N, which causes crosstalk and has the disadvantage that a high-contrast image is not obtained. In addition, when the duty ratio is low, it is difficult to control the gradation of each pixel by voltage modulation. For example, it is not suitable for a display panel having a high number of wirings, particularly a liquid crystal television panel.

By the way, the use of a liquid crystal device having bistability as an improved type of the conventional liquid crystal device has been disclosed by both Clark and Lagerwall in JP-A-56-107216.
And US Patent No. 4367924. As the bistable liquid crystal, a ferroelectric liquid crystal having a chiral smectic C phase (SmC ^* ) or an H phase (SmH ^* ) is generally used. In these states, the first liquid crystal responds to an applied electric field. Has the property of taking one of the optically stable state and the second optically stable state and maintaining the state when no electric field is applied, that is, it has bistability, and has a quick response to a change in the electric field. Thus, wide use in fields such as high-speed and storage type display devices is expected.

However, the above-described ferroelectric liquid crystal element has a problem that flicker occurs during multiplexing driving. In particular, in European Patent Publication No. 149899, an alternating voltage in which the phase of the scanning selection signal is reversed is applied to each writing frame, and white writing (arranged so that the cross Nicol is in a bright state) is performed in the previous frame. Then, in the following frame, black (arranged so that the cross Nicol is in a dark state)
A multiplexing driving method for performing selective writing of data is disclosed. Also, in addition to the driving method described above, U.S. Pat.
A driving method disclosed in, for example, US Pat. No. 6,655,561 or US Pat. No. 4,655,561 is known.

In this driving method, at the time of black selective writing after white selective writing, white pixels selectively written in the previous frame are half-selected, and an effective voltage smaller than the writing voltage is applied. Therefore, in the multiplexing driving method, at the time of selective writing of black, the half-selection voltage is uniformly applied to the white selected pixels which are the background of the black character by a half frame period (one screen scanning time corresponding to one frame scanning time). The optical characteristics of a white selected pixel to which a half-selection voltage has been applied and a half-selection voltage has been applied change every half-frame period. For this reason, in the case of a display in which black characters are written on a white background, the number of pixels that have selected white is overwhelmingly greater than the pixels that have selected black, and the white background appears to flicker. Also, in contrast to the above-described display in which black characters are written on a white background, the occurrence of flicker is also observed in the case of a black character and a white character display. When the normal frame frequency is set to 30 Hz, the above-mentioned half-selection voltage is applied at 15 Hz, which is a half frame frequency, so that it is perceived as flicker by an observer, and display quality is significantly impaired.

In particular, the ferroelectric liquid crystal requires a longer drive pulse (scan selection period) in low-temperature driving compared to, for example, high-temperature scanning driving at a 15 Hz frame frequency. Scan driving at a low frame frequency was required. Therefore, when driving at low temperatures,
Flicker caused by scanning drive at a low frame frequency has occurred.

[Summary of the Invention]

SUMMARY OF THE INVENTION An object of the present invention is to provide a liquid crystal device for gray scale display in which flicker caused by scanning drive at a low frame frequency is suppressed.

Another object of the present invention is to provide a liquid crystal device for gradation display in which occurrence of image deletion is prevented.

According to the present invention, first, a liquid crystal element having a matrix electrode formed of a plurality of scanning electrodes and a plurality of information electrodes and a chiral smectic liquid crystal, and a scanning electrode are supplied with a scanning selection signal within one vertical scanning period.
One vertical scan by the interlace application is repeated a plurality of times so that one screen scan for displaying one screen is completed by applying an interlace at every second or more, and the scan selection signal is changed to at least two consecutive ones. In the vertical scanning, the first means for performing one screen scan applied to non-adjacent scan electrodes and performing one gray scale screen scan by repeating the one screen scan a plurality of times, and the plurality of information electrodes A liquid crystal device having second means for applying an information signal in synchronization with a scanning selection signal has a first feature. Secondly, a liquid crystal device includes a matrix electrode formed by a plurality of scanning electrodes and a plurality of information electrodes. In a method for driving a liquid crystal element having a chiral smectic liquid crystal, one screen for displaying one screen by applying a scan selection signal to every one of the plurality of scanning electrodes by skipping at least three lines in one vertical scanning period. Scan is complete One vertical scan by the interlace application is repeated a plurality of times, and one screen scan in which the scan selection signal is applied to non-adjacent scan electrodes in at least two consecutive one vertical scans is performed. A second feature is provided in a method for driving a liquid crystal element that performs one-gradation screen scanning by repeating screen scanning a plurality of times and applies an information signal to the plurality of information electrodes in synchronization with a scanning selection signal. Third, in a method of driving a liquid crystal element having a matrix electrode having a plurality of intersections formed by a plurality of scanning electrodes and a plurality of information electrodes and a chiral smectic liquid crystal, the alignment of the chiral smectic liquid crystal at all the intersections One-gray-screen scanning is performed by performing one-screen scanning for selecting a state a plurality of times, and all one-screen scanning periods in the one-gray-screen scanning are set to be different from each other. In addition, the ratio of the period in which the chiral smectic liquid crystal of the pixel portion is aligned in one alignment state during the one-gray-screen scanning period is determined according to the gray-scale information, and the one-screen scanning is performed one vertical scanning a plurality of times. In each vertical scanning period, a scanning selection signal is interleaved and applied to the scanning electrodes at intervals of three or more lines, and the scanning selection signal is applied to at least two consecutive vertical scannings which are not adjacent to each other. A third feature of the liquid crystal element driving method is to apply an information signal to the electrodes and to apply the information signal to the plurality of information electrodes in synchronization with the scan selection signal.

(Detailed description of embodiments of the invention)

An embodiment of the present invention will be described using a ferroelectric liquid crystal (hereinafter, FLC).

FIG. 1 shows a first embodiment of the present invention (FIG. 2 shows A in FIG. 1).
-A 'is a sectional view), showing upper electrode groups 11A and 11B.
(Hereinafter referred to as an information electrode group) and a lower electrode group 12 (hereinafter referred to as a scan electrode group C) are formed so as to be in a matrix with each other, are formed on glass substrates 13 and 14, respectively, and have an FLC material 1 between them.
5 is sandwiched between. As shown, the scanning electrode group C is C ₀ , C ₁ , C ₂ ..., And the information electrode group is A (A ₁ , A ₂ , A
₃ ) and B (B ₁ , B ₂ , B ₃ , B ₄ …), and one pixel is a region E (electrode line width A> B) surrounded by a dotted line in the drawing, ie,
For example, a region E where the scanning electrode C ₂ and the data electrodes A ₂ and B ₂ are Obaratsupu. At this time, the electrode line width is A> B. Each scanning electrode group C and information electrode groups A and B are respectively
Connected to a power supply (not shown) via SW
The SW is also connected to a controller circuit (not shown) for controlling its ON / OFF. With this configuration, under the control of the controller circuit, for example, the gray scale expression at the pixel E is performed as follows. When the common electrode C ₂ is being scanned, white (W) signals are applied to A ₂ and B ₂ respectively, and gray 1 (Gray 1) is W and B to A _2.
When a signal that becomes black and (hereinafter B) is given to 2, a gray 2
When (hereinafter GRAY2) is imparted with signal W to A ₂ B, the B _2,
Black gives the third signal when A signal is applied to A ₂ and B ₂ respectively.
The figure shows the halftone expression of W, Gray1, Gray2, and B described above.

With such a simple configuration, it is possible to express a quaternary gray scale on a binary FLC.

In a preferred embodiment of the present invention, a plurality of intersections constituting one pixel E are formed with different intersection areas, respectively.
In particular, this different intersection area is 2 for the minimum intersection area 1:
4: 8: 16:...: 2 ⁿ (n = the number of intersections in one pixel E) is preferable.

In the present invention, the scanning electrode is divided into two, and when the electrode line width C = B, eight gradation levels are possible, and when C ≠ D, 16 gradation levels are possible.

In addition, when the division is performed only on the information electrode side, if the electrode line width is A = B and the color filters are provided so as to have a complementary color relationship with A and B, four colors can be displayed. For example, by arranging complementary color relations of [A = yellow; B = blue], [A = magenta; B = green] or [A = cyan; B = red], the colors of white, black, A and B Color 4
Color display becomes possible.

The polarizers 16A and 16B shown in FIG. 2 are arranged with their polarization axes crossing each other, and display black in a dark state and display white in a bright state.

Although the matrix electrode shown in FIG. 1 is driven by a driving example described below, the present invention can also be applied to a matrix electrode formed by a scanning electrode and an information electrode having the same electrode width.

FIG. 4 is a liquid crystal display drive control circuit diagram of the present embodiment.

In the figure, DSP is a liquid crystal display unit, A ₁₁ ,
A ₁₂ ,..., A ₄₄ indicate respective pixels. M1, M2 and M3 are frame memories each having a memory capacity of 4 × 4 = 16 bits. Data is sent to the memories M1, M2, and M3 from the data bus DB, and writing / reading and addresses are controlled by the control bus CB.

FC is the field switching signal, DC is its decoder, MPX
Is a multiplexer for selecting one of the outputs of the memories M1, M2, M3, MM is a monostable multivibrator, GT is a gate signal, FG is a clock oscillator, CK is a clock signal, AND is an AND gate, and F is a row scan clock. A signal, CNT is a counter, SR is a serial input parallel input shift register, DR1 to DR4 are column drive circuits, and DR5 to DR8 are row drive circuits.

Hereinafter, the operation of the circuit shown in FIG. 4 will be described with reference to FIGS. 5 to 7.

FIG. 5 shows gradation data of each pixel in one gradation screen scanning (a period for forming one screen of a gradation video image; referred to as "one frame"), and the most significant bit MSB of each gradation data is stored in the memory M3. , The middle bit is stored in memory M2 and the least significant bit LS
B is input to the memory M1 via the data bus.

Then at time t ₁ one screen scan ( "one subframe") when switching signal FC is generated decoder DC is excisional as to select the data of the multiplexer MPX from memory M1. At the same time, the FC is input to the monostable multivibrator MM to generate the gate signal GT, open the AND gate AND, and output the four clocks of the clock signal CK to the counter CNT as the row scanning signal F. The counter CNT turns on the driver DR5 at the first clock. At this time, the data of the first row of the memory M1 is input to the shift register SR, and only the driver DR4 is in the ON state. Therefore, the liquid crystal pixel A
₁₃ is set to the dark level, and the other liquid crystal pixels A ₁₁ , A ₁₂ , A
₁₄ is set to the light level. The row scanning signal F is input to a controller (not shown) as a memory row switching signal,
From the memory M1, the data of the next second row is stored in the shift register SR.
And the driver DR6 is turned on by the next row scanning signal F, and at the same time, the second digit data of M1 is input from the shift register SR to the drivers DR1 to DR4, respectively. At this time the driver
DR2, DR3, DR4 is turned on, the pixel A _22, A _23, A ₂₄ is set to the dark level, A ₂₁ is set to the bright level. The above operation is repeated for the third and fourth rows.

The fourth row scanning signal F for selecting the fourth row is a counter CN.
When input to T, the counter CNT outputs the memory switch request signal MC
Is output to a controller (not shown), the memory is switched to M2, and the process moves to the second subframe. At this time, each liquid crystal pixel set in the bright or dark state in the first sub-frame maintains the state because the ferroelectric liquid crystal has a memory function.

Similarly, in the second sub-frame, the multiplexer MPX selects the data from the memory M2 by the sub-frame switching signal FC, and the row scanning signal is input to the counter CNT and the shift register SR by the gate signal GT. Then, row scanning is performed at the same cycle as the first sub-frame, and each liquid crystal pixel is set to a dark state or a bright state. The same applies to the third subframe.

In this embodiment, the ratio of the first, second, and third sub-frame periods is set to 1: 2: 4, similarly to the weighting of each bit. Thus, for example, the gray scale data of the pixel A ₁₁ is a 2 as shown in FIG. 6, in this case becomes a dark level only the second sub-frame period, 2/7 one frame period becomes a dark state. The grayscale data of the pixel A ₂₄ is 5. In this case, the first and third sub-frame periods are at the dark level, the second sub-frame period is maintained at the light level, and 5/7 of one frame period. Becomes dark. The gradation data of the pixel A ₄₂ is 7, and at this time, the dark state is maintained during all the sub-frame periods. That is, in this embodiment, halftone expression of eight gradations is possible.

By controlling the ratio of the display time within one frame, that is, the display duty, it is possible to express an apparent halftone. When the third sub-frame ends and one frame ends, the data in the memories M1 to M3 is rewritten by the control bus CB and the data bus DB, and the data of the next frame is stored in the memory.

In this embodiment, one frame is divided into three sub-frames. However, if two or more sub-frames are divided, halftone display is possible. In addition, each subframe period is determined by the variable amount with the same weight as the data bit, but it is also possible to equalize the subframe period by equal division. However, in this case, it is necessary to decode the gradation data.

Figure 8 (A) is a scanning selection signal S _S used in the present invention, the scan non-selection signal S _N, a white information signal I _W and a black data signal I _B represents. FIG. 8 (B) shows a pixel on the scanning selection electrode to which the scanning selection signal is applied (intersection between the scanning electrode and the information electrode).
The voltage waveform applied to the selected pixel (the pixel to which the white information signal I _W is applied and the voltage (I _W −S _S ) is applied), the non-selected pixel (the black information signal) on the same scan selection electrode voltage (I _B -S _S) is a voltage waveform and a scan non-selection signal is applied to the applied) is applied to the two pixels on the applied scan non-selection electrodes in pixels I _B is applied The voltage waveform is shown. According to FIG. 8 and (A) (B), the voltage to the non-selected pixel on the scan selection electrode at a phase t ₁ is a voltage exceeding one threshold voltage of the ferroelectric liquid crystal - (V ₁ + V ₃ ) Is applied to generate one of the alignment states of the ferroelectric liquid crystal, thereby generating a dark state and writing black. At the phase t ₁ at this time, a voltage (−V ₁ + V ₃ ) which is a voltage lower than the threshold value of the ferroelectric liquid crystal is applied to the selected pixel on the scanning selection electrode, and the state changes to the alignment state of the ferroelectric liquid crystal. Does not occur. In phase t _2, the selected pixel on the scan selection electrode is a voltage exceeding the other threshold voltage of the ferroelectric liquid crystal voltage (V ₂ + V ₃₎ is applied,
When the ferroelectric liquid crystal is aligned in the other alignment state, a bright state is generated and written in white. Further, a voltage (V ₂ −V ₃ ) which is equal to or lower than the threshold value of the ferroelectric liquid crystal is applied to the non-selected pixels on the scanning selection electrode at the phase t ₂ , and the alignment state at the previous phase t ₁ Does not change. On the other hand, the pixels on the scanning non-selected electrodes in the phase t ₁ and t _2, the voltage ± V ₃ a threshold voltage below the voltage of the ferroelectric liquid crystal is applied. Therefore, in the present embodiment, the pixels on the scanning electrodes selected in the phase T ₁ is performed is white or black writing, followed by even in a state in which the scanning non-selection signal is applied, the time before writing Is maintained as it is.

Further, in this embodiment, the phase T _2, the voltage of opposite polarity is applied from the information electrode to the information signal in the writing phase T _1. Therefore, as shown in FIG. 8 (C), an AC voltage is applied to the pixels when scanning is not selected, and the threshold characteristics of the ferroelectric liquid crystal can be improved.

FIG. 8C shows a timing chart of the drive voltage in the sub-frame shown in FIG. In this example, the scan selection signal is skipped and applied to the scan electrodes every five lines, and the scan selection signal is applied to the non-adjacent scan electrodes in six consecutive fields. In this example, the scanning selection period (T ₁ + T ₂ ) is set to be long at low temperatures by selecting every five scanning electrodes and performing one sub-frame scanning (one screen scanning) with six field scans. As a result, even when the scanning drive is performed at a low frame frequency (for example, a frame frequency of 5 to 10 Hz), the occurrence of flicker caused by the scanning drive at a low frame frequency can be significantly suppressed. By applying a scan selection signal so as to select non-adjacent scan electrodes in field scan, image deletion could be effectively eliminated.

Figure 8 (D) is a diagram showing an driving waveform example at the intersection of the scanning electrode S ₁ and data electrode I ₁ and I ₂ in an frame and the first to third sub-frame. According to FIG. 8 (D), each sub-frame period a first sub-frame: second sub-frame: the third sub-frame = 1: 2: set to 4, the scanning electrode S ₁ and data electrode I ₁ the intersection, light in the first subframe, bright in the second sub-frame, gradation display in accordance with the state of the dark in the third sub-frame is obtained, and the intersection of the scanning electrode S ₁ and data electrodes I _2, A gradation display corresponding to a bright state in the first subframe, a dark state in the second subframe, and a dark state in the third subframe is obtained. Moreover, in this embodiment, as shown in Figure 3, the scanning electrode S ₁ and the crossing area scanning of the data electrodes I ₂ electrode S ₁
And is set to twice the intersection area between the data electrode I _1, gradation display corresponding to this intersection area ratio is made.

FIG. 4 (E) is an example using the drive waveform of FIG. 4 (A). In this example, every two scanning electrodes are skipped and selected within one sub-frame period, and two successive scanning electrodes are selected. A scan selection signal was applied to select non-adjacent scan electrodes in one field scan.

The present invention is not limited to the above-described example. In particular, the scan selection signal can be skipped and applied to every four or more scan electrodes, preferably every five to twenty scan electrodes. In the present invention, the peak values of the voltage signals V ₁ , −V and ± V ₃ are | V ₁ | = |
−V ₂ |> | ± V ₃ |, preferably | V ₁ | = | −V ₂ |> 2 | ± V ₃ | The pulse width of these voltages is generally set to 1 μsec to 1 msec, preferably 10 μsec to 100 μsec, and the pulse width at low temperatures is preferably set longer than the pulse width at high temperatures.

In the present invention, various types of ferroelectric liquid crystal elements can be used. Specifically, SSFLC disclosed by Clark et al. In U.S. Pat. A ferroelectric liquid crystal element in an alignment state disclosed in the specification No. 2159635 can be used. Further, in the present invention, a color filter is arranged in each pixel in, for example, a stripe shape or a mosaic shape, and a liquid crystal element having bistability is prepared. Can be displayed.

Therefore, the method of the present invention can be applied to a liquid crystal television for displaying a monochrome or color image having a gradation characteristic, especially a conventional CR.
The present invention can be applied to a liquid crystal pocket color television which is much smaller and lighter than a T color television.

〔The invention's effect〕

According to the present invention, it is possible to effectively suppress the occurrence of flicker caused by scan driving at a low frame frequency such as 2 Hz to 15 Hz. Does not occur, and a high-quality display gradation screen can be obtained over a substantially wide temperature range. Further, according to the present invention, image flow can be effectively prevented, and in this sense, a high-quality gradation display screen can be obtained.

[Brief description of the drawings]

FIG. 1 is a plan view of a matrix electrode used in the present invention. FIG. 2 is a graph showing A- of the ferroelectric liquid crystal element used in the present invention.
It is A 'sectional drawing. FIG. 3 is an explanatory diagram schematically showing a halftone. FIG. 4 is an explanatory diagram showing a drive control circuit of the present invention. 5 and 6 (a) to (d)
FIG. 5 is an explanatory diagram illustrating one example of grayscale data of a pixel. FIG. 7 is an explanatory diagram showing a time chart when used in the driving method of the present invention. 8 (A) to 8 (E) are waveform diagrams showing examples of driving waveforms used in the present invention.

 ──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-63-244021 (JP, A) JP-A-61-272724 (JP, A) JP-A-62-9322 (JP, A)

Claims (8)

(57) [Claims] 1. A liquid crystal device having a matrix electrode formed of a plurality of scanning electrodes and a plurality of information electrodes and a chiral smectic liquid crystal, and a scanning electrode, wherein at least three scanning selection signals are applied within one vertical scanning period. One vertical scan by the jump application is repeated a plurality of times so that one screen scan for displaying one screen is completed, and the scan selection signal is output in at least two consecutive one vertical scans. A one-screen scan applied to non-adjacent scan electrodes, and a one-tone screen scan is performed by repeating the one-screen scan a plurality of times.
And a second means for applying an information signal to the plurality of information electrodes in synchronization with a scanning selection signal. 2. The liquid crystal device according to claim 1, wherein the scanning selection signal is a signal having one polarity voltage and the other polarity voltage based on a voltage applied to a scanning electrode that is not selected. 3. The liquid crystal device according to claim 1, wherein the liquid crystal element includes a plurality of regions each having a different area from each other for gray scale display. 4. A method for driving a liquid crystal element having a matrix electrode formed of a plurality of scanning electrodes and a plurality of information electrodes and a chiral smectic liquid crystal, wherein the plurality of scanning electrodes are selectively scanned within one vertical scanning period. One vertical scan by the interlace application is repeated a plurality of times so that one screen scan for displaying one screen is completed by repeating the scan selection signal at least two times. In one continuous vertical scan, one screen scan applied to non-adjacent scan electrodes is performed, and one screen scan is repeated a plurality of times to perform one gradation screen scan, and a scan selection signal is applied to the plurality of information electrodes. A method for driving a liquid crystal element, wherein an information signal is applied in synchronization with a liquid crystal element. 5. The method of driving a liquid crystal element according to claim 4, wherein a signal having one polarity voltage and the other polarity voltage based on a voltage applied to an unselected scan electrode is applied as the scan selection signal. 6. The driving method for a liquid crystal element according to claim 4, wherein the liquid crystal element includes a plurality of regions each having a different area from each other for gradation display. 7. A method of driving a liquid crystal element having a matrix electrode having a plurality of intersections formed by a plurality of scanning electrodes and a plurality of information electrodes and a chiral smectic liquid crystal, wherein the chiral smectic liquid crystal at all the intersections is provided. By performing one screen scan for selecting the alignment state a plurality of times, one-gradation screen scanning is performed, and all one-screen scanning periods in the one-gradation screen scanning are set to be different from each other, and one-gradation screen scanning period is set. The ratio of the period during which the chiral smectic liquid crystal of the pixel portion is aligned in one alignment state is determined in accordance with the gradation information, and the one screen scan is performed by a plurality of one vertical scans. In the period, a scan selection signal is skipped and applied to the scan electrodes at intervals of three or more, and the scan selection signals are applied to at least two consecutive one vertical scans. A method for driving a liquid crystal element, comprising applying an information signal to a plurality of information electrodes in synchronization with the scan selection signal. 8. A method for driving a liquid crystal element according to claim 7, wherein one pixel is composed of a plurality of intersections having different areas from each other for gradation display, and said plurality of intersections are on the same scanning electrode. .