JPH10161613A - Image display unit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To perform the display of high speed high contrast ratio and low crosstalk by using a plural line simultaneous selection method, by largely setting the current supply capacity of a power source which supplies a part of voltage level of which the load by a display is high. SOLUTION: A scanning electrode is classified into plural subgroups by L lines (L is an integer of 2-8), and the specific voltage is applied to a scanning electrode belonging to one sub-group, for collectively selecting the sub-group. The current supply capacity of a power source which supplies a part of the voltage level of which the load by the display is high, is set to be larger than that of a power source which supplies the other voltage level. Each voltage level obtained by the division of the resistance, is output to a power source circuit as V0 -V4 through the operation amplifier OP0-PO4. For example, when the load is concentrated on V1 , V3 , the current supply capacity of the operation amplifiers OP1, OP3 connected with V1 , V3 are controlled to be larger than V0 , V2 , V4 .

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for driving a liquid crystal display device suitable for a liquid crystal which responds at high speed. In particular, the present invention relates to a method for simultaneously selecting a plurality of lines (Japanese Unexamined Patent Publication No.
907, U.S. Pat. No. 5,262,881) which performs multiplex driving.

[0002]

2. Description of the Related Art Hereinafter, in this specification, a scanning electrode is called a row electrode or simply a line, and a data electrode is also called a column electrode.

[0003] With the progress of the advanced information age, the need for information display media is increasing more and more. Liquid crystal displays have advantages such as thinness, light weight, and low power consumption, and are expected to become more and more popular with semiconductor technology. On the other hand, with the spread of the screen, a large screen and a high definition are required, and a search for a method of displaying a large capacity has begun. Among them, the STN (super twisted nematic) method has a simpler manufacturing process than the TFT (thin film transistor) method and can be produced at a low cost, so that it will be the mainstream of the liquid crystal display in the future.

[0004] In order to perform large-capacity display by the STN method, line-sequential multiplex driving has been conventionally performed. In this method, each row electrode is sequentially selected one by one, and a column electrode is selected corresponding to a pattern to be displayed.
The display of one screen is completed by selecting all the electrodes.

However, in the line sequential driving method, it is known that a problem called a frame response occurs as the display capacity increases. In the line sequential driving method, a relatively large voltage is applied to the pixel when selected and a relatively small voltage is applied when not selected. Generally, this voltage ratio increases as the number of row lines increases (as the duty becomes higher). Therefore, when the voltage ratio is small, the liquid crystal responding to the effective voltage value responds to the applied waveform. That is,
The frame response is a phenomenon in which the transmittance at the time of OFF increases because the amplitude of the selection pulse is large, and the transmittance at the time of ON decreases because the cycle of the selection pulse is long, resulting in a decrease in contrast.

[0006] There is known a method of increasing the frame frequency in order to suppress the occurrence of the frame response and thereby shortening the period of the selection pulse, but this has a serious drawback. That is, when the frame frequency is increased, the frequency spectrum of the applied waveform is increased, which causes non-uniform display and increases power consumption. Therefore, the upper limit of the frame frequency is limited in order to prevent the selection pulse width from becoming too narrow.

[0007] In order to solve this problem without increasing the frequency spectrum, new driving methods have recently been proposed. This is a method of simultaneously selecting a plurality of row electrodes (selection electrodes), such as a multiple line simultaneous selection method. This method can simultaneously select a plurality of row electrodes and independently control the display pattern in the column direction, and can shorten the frame period while keeping the selection width constant. That is, high-contrast display with suppressed frame response can be performed.

[0008]

In the multiple line simultaneous selection method, in order to control the column display pattern independently,
A fixed voltage pulse train is applied to each row electrode applied simultaneously. The reason why the voltage needs to be applied as a pulse train is as follows.

In the driving method for simultaneously selecting a plurality of lines, a voltage pulse is applied to a plurality of row electrodes at the same time. At this time, in order to simultaneously and independently control the display patterns in the column direction, it is necessary to apply pulse voltages having different polarities to the row electrodes. A pulse having polarity is applied to the row electrode several times, and a voltage corresponding to the data is applied to the column electrode. Thus, an effective voltage corresponding to ON and OFF is applied to each pixel in total.

The selection pulse voltage group applied to each row electrode can be represented as a matrix of L rows and K columns (hereinafter referred to as a selection matrix (A)). Since the selection pulse voltage sequence can be expressed as a group of vectors orthogonal to each other, a matrix including these as column elements is an orthogonal matrix. Each row vector in this matrix is orthogonal to each other. The number L of rows corresponds to the number of simultaneously selected rows, and each row corresponds to each line. For example, L
The element of the first row of the selection matrix (A) is applied to the line 1 of the selection lines, and the selection pulse is applied in the order of the element of the first column and the element of the second column.

In the present specification, in the notation of the selection matrix (A), 1 means a positive selection pulse, and -1 means a negative selection pulse. FIG. 3 shows a Hadamard matrix as a representative example of the selection matrix (A). FIG. 3A shows the case of 4 rows and 4 columns, FIG. 3B shows the case of 8 rows and 8 columns, and FIG.
7 rows and 8 columns excluding the first row of the columns.

A voltage level corresponding to each column element of the matrix and the column display pattern is applied to the column electrodes. That is, the column electrode voltage sequence is determined by the matrix that determines the row electrode voltage sequence and the display pattern.

The sequence of the voltage waveform applied to the column electrode is determined as follows. FIG. 2 is an explanatory diagram showing the concept. The case where the selection matrix is a Hadamard matrix having 4 rows and 4 columns will be described as an example. It is assumed that the display data on the column electrode i and the column electrode j are as shown in FIG. The column display pattern is represented as a vector (d) as shown in FIG. Here, when the column element is −1, it indicates on display, and when it is 1, it indicates off display. Assuming that a row electrode voltage is sequentially applied to the row electrodes in the order of the columns of the matrix, the column electrode voltage level becomes a vector (v) shown in FIG. 2B, and its waveform is shown in FIG. become that way. In FIG. 2C, the vertical axis and the horizontal axis are arbitrary units.

In the case of selecting a partial line, in order to suppress the frame response of the liquid crystal display element, it is preferable to apply voltages in a distributed manner within one display cycle. Specifically, for example, the first simultaneously selected row electrode group (hereinafter, referred to as
Then, the first element of the vector (v) for the second simultaneously selected row electrode group is applied, and so on. Take the sequence of

Therefore, the voltage pulse sequence actually applied to the column electrodes determines how the voltage pulses are dispersed in one display cycle, and what selection matrix for each of the simultaneously selected row electrode groups. It is determined by whether (A) is selected.

In the case of displaying a window pattern which is used very frequently recently, a phenomenon called crosstalk occurs, which causes a display problem.

The case where the influence of crosstalk is most remarkable appears when a bar is displayed. This phenomenon is caused by the distortion of the driving waveform as described in Japanese Patent Application Laid-Open No. 8-62574.

Another major problem is crosstalk in halftone display. As a method of halftone display,
There are a frame rate control (FRC) method, an amplitude modulation method, a combination with a dither method, and the like. The FRC method is most frequently used as a driving method of a liquid crystal display device. At this time, in order to make the occurrence of flicker less noticeable, a combination with spatial modulation that spatially (between adjacent pixels) makes a phase difference and cancels flicker is often used. In this case, unlike the solid display based on the binary display, the spatial frequency of the image may be extremely high for each frame. For this reason, crosstalk occurs and the quality of the image is degraded. Similarly, when the dither method is used, the spatial frequency is high and there is a problem of crosstalk.

Further, when displaying a moving image such as a video display, there is a problem of image deterioration. In a video display, unlike a basically geometric display such as a window, many spatially complicated displays (that is, high spatial frequencies) appear. Therefore, in particular, when an attempt is made to display a video display in a specific window, there arises a problem that not only the quality of the video display itself is degraded due to the generated crosstalk but also the surrounding windows are affected.

[0020]

The present invention provides the following image display device in order to solve the above-mentioned problems.

That is, N (N is an integer of 200 or more)
An image display device having a scan electrode and a plurality of data electrodes, and optically responding to an effective value of a voltage applied to a pixel determined as an intersection of the scan electrode and the data electrode, wherein the number of scan electrodes is L (L is an integer of 2 or more and 8 or less). Each of the sub-groups is divided into a plurality of sub-groups, and the sub-groups are collectively selected based on a signal obtained by expanding a column vector of an orthogonal matrix having L rows in a time series. Means for applying a voltage to scan electrodes belonging to one subgroup, and means for applying to the data electrodes voltages having three or more levels based on signals obtained by orthogonally transforming display data using the orthogonal matrix, Among the levels, the current supply capacity of a power supply that supplies a part of the voltage level with a high load according to the display is set to be larger than the current supply capacity of a power supply that supplies another voltage level. That is an image display device.

In particular, the current supply capability of a power supply for supplying a data voltage level selected when the display pattern in the sub-group is a repetition pattern of all on, all off, or on / off for each pixel is set large. An image display device is provided.

The multiple line simultaneous selection method is characterized in that there are a plurality of data voltage levels as described above, and the actual waveform is determined by the display data and the orthogonal matrix used. For this reason, transitions between the voltage levels frequently occur, and this greatly affects the occurrence of crosstalk.
The formation of a waveform by the plurality of data voltage levels makes it difficult to perform crosstalk control in the multiple line simultaneous selection method.

What is important in reducing this crosstalk is that the crosstalk involves the driving waveform, the load on the power supply (such as the capacitance and electrode resistance of the liquid crystal) and the ability of the power supply to supply a current to the liquid crystal. In that they interact with each other.

According to the present invention, in an image display apparatus using the simultaneous selection method for a plurality of lines, in order to provide a high-quality image, an optimum configuration in which a driving waveform and a power supply form in the simultaneous selection method for a plurality of lines are comprehensively viewed. It shows.

[0026]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, the following conditions are constituent elements.

(1) The number of simultaneous selections in the simultaneous selection of a plurality of lines is 2 or more and 8 or less.

(2) Among the data voltage levels, the current supply capability of the power supply that supplies a part of the voltage level with a high display load is set large. That is, the intensity of the power supply that supplies each data voltage level is changed according to the frequency of use of the data voltage level.

The first condition is determined from conditions for preventing the number of data voltage levels and the maximum value of the data voltage from becoming too large. As L, the number of lines selected at the same time, increases, the number of data voltage levels generally becomes (L
+1), and the maximum voltage increases in proportion to L1 ^{/ 2} . Therefore, if L is too large, the waveform becomes complicated and its voltage amplitude increases, and crosstalk increases. Therefore, the first condition is set.

This will be described in detail. In the multiple line simultaneous selection method, a selection pulse, which was conventionally one per line in one display frame, is applied a plurality of times and a data voltage is determined so as to correspond thereto. Therefore, the voltage balance of selection and data changes according to the number of simultaneously selected lines, and the state of occurrence of crosstalk changes.

For a simple understanding, a case will be described in which the selection voltage and the data voltage have an optimum bias ratio at which the voltage ratio between the ON waveform and the OFF waveform is theoretically maximum. When the data voltage in the APT method which is the conventional line sequential driving method is set to 1, the selection voltage V _r in the multiple line simultaneous selection method,
The maximum value of the data voltage V _c, to the simultaneous selection number L, V
_r = ^N1 / ² / L1 / ² and _Vc = L1 ^{/ 2} . Where N
Is the total number of lines.

As is apparent from the above equation, the selection voltage V _r as the increase in L is reduced, the data voltage V _c increases. Therefore, when L changes, the intensity of crosstalk changes. The degree of the change differs depending on the type of crosstalk.

The second condition is based on the inventor's finding that the load on the power supply is generally different for each data voltage level, and the level with a large load and the level with a small load are generally separated. Things.

That is, when driving is performed by the multiple line simultaneous selection method, a data voltage proportional to the inner product of the display data pattern vector and the column vector of the selection matrix is applied, but the display pattern has 2 L powers. Of the patterns, generally continuous on, continuous off,
There are many regular patterns, such as once on / off and twice on / off. Therefore, the data voltage can easily take a certain value, and the load tends to concentrate on a certain voltage level. If the power supply for this voltage level is strengthened compared to the other voltage levels, waveform distortion due to load imbalance is reduced and a display with less crosstalk as a whole is obtained.

For example, when L = 4, when the selection matrix shown in FIG. 4 is used, the display according to FIG. 2 is performed. In the case of continuous ON display, (d) (-1, -1, -1) , -1) (v) (-2, -2, -2, -2) In the case of the continuous OFF display, (d) (1, 1, 1, 1, 1) (v) (2, 2, 2, 2, 2) (D) (-1, 1, -1, 1) (v) (2, -2, 2, -2) Twice on / off display, (d) ) (-1, -1, 1, 1) (v) (2, 2, -2, -2).

On the other hand, the power supply circuit is generally as shown in FIG. That is, each voltage level produced by resistance division is output as V ₀ ~V ₄ via the operational amplifier. A capacitor for voltage smoothing is interposed between the line supplying each voltage and the ground. OP0 in the figure
OP4 operational amplifier for low impedance output, C ₀ -C ₄ capacity of the smoothing capacitor, R ₀ to R ₄
Is an equivalent series resistance inside the smoothing capacitor.

The element (v) and the voltage level in FIG.
4 = V ₀ , −2 = V ₁ , 0 = V ₂ , 2 = V ₃ , 4 = V ₄
Corresponds as follows. That is, in the case of the above-described pattern which is generally used, the load is concentrated on V ₁ and V _3, and as a result, if the current supply capability of the V ₁ and V ₃ power supplies is increased as compared with V ₀ , V ₂ and V ₄ , Power supply waveform distortion is reduced, and good display with less crosstalk is obtained.

As a method for realizing this, (1) the current supply capability of the operational amplifier connected to V ₁ and V ₃ is made larger than V ₀ , V ₂ and V ₄ . This allows
The recovery speed of the voltage drop due to the load fluctuation is increased, and the waveform distortion is reduced. (2) The capacities of the smoothing capacitors of V ₁ and V ₃ are V ₀ and V
_2, is increased compared to V _4. Due to this capacity increase, the voltage drop due to the load fluctuation is reduced, and the waveform distortion can be reduced.

(3) The equivalent series resistance of the smoothing capacitors of V ₁ and V ₃ is made smaller than V ₀ , V ₂ and V ₄ .
The equivalent series resistance functions to limit the amount of current sent out by the capacitor with respect to an instantaneous load change. The smaller the value is, the greater the instantaneous current supply capability is, and as a result, the waveform distortion is reduced.

As described above, the current supply capability of the power supply that supplies a part of the data voltage level that is heavily loaded by display is set to be larger than the current supply capability of the power supply that supplies the other voltage levels. Thereby, the distortion of the power supply having a large load and the distortion of the power supply having a small load are balanced, and the crosstalk is reduced. Since it is extremely difficult to completely eliminate the distortion of the waveform, this balance effect is more effective than the case where the current supply capability is uniformly increased at each voltage level, as in the present invention.
It is more remarkable to make a difference in the power supply capability depending on the voltage level.

In the present invention, it is possible to drive with a bias ratio different from the conventional one by the characteristic of the waveform of the multiple line simultaneous selection method. Here, the bias ratio is defined by the maximum value of the row voltage / column voltage, and the optimal bias ratio already described is
N ^1/2 / L. In the line sequential driving method, since the column voltage becomes extremely high, the bias ratio is generally used smaller than the optimum bias ratio.

However, in the multiple-line simultaneous selection method, 1) since there is a frame response in the line-sequential driving method, the contrast is higher when the bias is reduced, but in the multiple-line simultaneous selection method, the frame response is suppressed by the method itself. 2)
In the multiple line simultaneous selection method, the row voltage is lower than the line sequential driving method.
It is not necessary to use the bias ratio smaller than the optimum bias ratio.

It is rather desirable to use this voltage ratio larger than the optimum bias ratio. The reason for this is that the contribution rate of the column voltage is reduced, so that the crosstalk due to the change in the column voltage is reduced. As a result, the crosstalk can be reduced without lowering the contrast ratio.

The maximum voltage V _c of the voltage amplitude V _r and column voltage line voltage as described _{above, max} is, since to meet the number one relationship obtain a high-quality image more in advance image display apparatus of the present invention This is a more desirable condition.

[0045]

N ^0.5 / L ≦ V _r / V _{c, max} ≦ 1.4 N ^0.5 / L

In the present invention, the other parts of the drive circuit
It can be easily realized using a conventionally known circuit for simultaneous selection of a plurality of lines. For example, as a gradation method, FR
In the case of using the C method, the input multi-bit data may be converted into 1-bit (one frame) data before storing it in the memory, stored in the memory, and sequentially read to calculate the data voltage waveform. Alternatively, the multi-bit data may be stored as it is in the memory, and may be converted into 1-bit data by referring to a table prepared in advance of the data voltage operation.
The spatial modulation table may be stored in a ROM and read out sequentially for use. However, a configuration using a logic circuit can be easily realized. Display is achieved by inputting a data voltage waveform calculated by these circuits to a data signal driver having a plurality of voltage levels and applying a voltage to the liquid crystal.

[0047]

The liquid crystal panel used had a cell gap of 4 to 6 μm.
It is an STN display panel having a twist angle of 220 to 260 degrees at m.

Example 1 VGA (640 × 480 × 3
(RGB)), the color STN display element was divided into upper and lower two screens, and two screens were driven. The number of row lines in one screen is 240, and the number of simultaneous selections L = 4 (that is, the number of subgroups = 6)
In 0), a plurality of lines are simultaneously selected and driven. The size of the display screen is 10.4 inches diagonally and the transparent electrode used is ITO
And a sheet resistance of 5Ω. The orthogonal matrix used was that shown in FIG. 3, and the FRC method was used for gradation display.

The maximum drive voltage (V _r ) was about 16V. Note that the bias ratio was an optimum bias ratio (3.9). The column voltage has a total of five levels (V ₀ ,
V ₁ , V ₂ , V ₃ , V ₄ ), of which V ₁ , V ₃
The capacitance of the level capacitor is 10 μF, V ₀ , V ₂ , V
The capacity of the capacitor ₄ is 4.7 μF, and the current supply capability of the operational amplifier is 30 m for the V ₁ and V ₃ levels.
The A, V ₀ , V ₂ , and V ₄ levels were 20 mA.

As a result of video display, a delicate gradation display with almost no flicker and crosstalk was obtained. The frame frequency was driven at 120 Hz, the contrast ratio was 50: 1, and the response time (rise time and fall time averaged) was 50 ms. Embodiment 2 SVGA (800 × 600 × 3 (RG
B)) The color STN display element is divided into upper and lower
Screen drive. The line line of one screen is 300,
A plurality of lines were simultaneously selected and driven with the number of simultaneous selections L = 4 (that is, the number of subgroups = 75). The size of the display screen was 12.1 inches diagonally, the transparent electrode used was ITO, and the sheet resistance was 4Ω. The orthogonal matrix used is that shown in FIG. 4, and the FRC method was used for gradation display.

The maximum drive voltage (V _r ) was about 18V. Note that the bias ratio is the optimum bias ratio × 1.2 (=
5.2).

The column voltage has a total of five levels.
The capacitance of the V ₁ and V ₃ level capacitors was 20 μF, the equivalent series resistance value was 1.2Ω, the capacitance of the V ₀ , V ₂ and V ₄ level capacitors was 10 μF and the equivalent series resistance value was 5Ω.

When a video display was performed, a delicate gradation display with almost no flicker or crosstalk was obtained. The frame frequency was driven at 120 Hz, the contrast ratio was 50: 1, and the response time (the average of the rise time and the fall time) was 65 ms.

[Comparative Example] A liquid crystal display was constructed and displayed in the same manner as in Example 2. However, regarding the column voltage level, the capacity of the capacitors of V ₀ , V ₂ and V ₄ is 20 μm.
F, the current supply capability of the operational amplifier is 40 mA, V ₁ ,
Capacitance of the capacitor of V ₃ is 10uF, the current supply capability of the operational amplifier was 20mA.

When a video display was performed, a delicate gradation display with almost no flicker was obtained, but the level of crosstalk was worse than that of the second embodiment. The frame frequency was driven at 120 Hz, the contrast ratio was 30: 1, the response time (average of the rise time and the fall time) was 150 ms, and a strong afterimage was seen in the video display.

[0056]

According to the present invention, the simultaneous selection method of a plurality of lines and the performance of a high-speed liquid crystal display element are fully utilized to realize a high-speed and high-contrast-ratio display with low crosstalk. Enables multi-gradation display of moving images. In addition, the power supply voltage can be reduced as compared with the conventional driving method.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a data voltage supply power supply used in the present invention.

FIGS. 2A to 2C are a conceptual diagram and a waveform diagram illustrating a voltage application method in a multiple line simultaneous selection method.

FIGS. 3A to 3C are explanatory diagrams showing Hadamard matrices.

FIG. 4 is an explanatory diagram showing a selection matrix used in the embodiment.

Claims (4)

[Claims] 1. An effective value of a voltage applied to a pixel having N (N is an integer of 200 or more) scan electrodes and a plurality of data electrodes, and determined as an intersection between the scan electrodes and the data electrodes. In an image display apparatus that optically responds to the above, the scanning electrodes are divided into a plurality of L groups (L is an integer of 2 to 8), and L rows are selected in order to select the subgroups collectively. Means for applying a voltage based on a signal obtained by expanding a column vector of an orthogonal matrix in a time series to scan electrodes belonging to one subgroup;
Means for applying a voltage having more than one level to the data electrode, and supplying a current supply capability of a power supply for supplying a part of a voltage level having a high display load among the data voltage levels to supply another voltage level An image display device characterized by being set to be larger than a current supply capability of a power supply. 2. The display patterns in a subgroup are all on,
2. The current supply capability of a power supply that supplies a data voltage level selected in the case of a repetition pattern of all off or on / off for each pixel is set to be larger than the current supply capability of a power supply that supplies another data voltage level. An image display device according to claim 1. 3. The current supply capability of a power supply is increased by increasing the capacity of a capacitor interposed between a voltage supply line and ground for smoothing a data voltage.
Or the image display device according to 2. 4. The current supply capability of a power supply according to claim 1 or 2, wherein the equivalent series resistance of a capacitor interposed between the voltage supply line and the ground for smoothing the data voltage is reduced to increase the current supply capability of the power supply. The image display device as described in the above.