JPS6258195B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an image display device such as a television using a matrix type liquid crystal display panel.

Various display devices have been devised or put into practical use using the electro-optic effect of liquid crystals. These are due to a combination of the alignment characteristics of liquid crystal molecules, dielectric anisotropy, optical anisotropy, etc., and are commonly referred to as
There are names such as DSM, TN, GH, etc. Common features of these liquid crystals include that they have a light-receiving display effect, that they have relatively high resistance, and that the threshold value of the display characteristics is slow or unstable with respect to each parameter. The light-receiving type and high resistance of liquid crystals are considered to be superior to other display materials, and are the reason why they are put into practical use as display materials.
On the contrary, the threshold characteristics are inferior to those of other display elements, making the driving conditions of the liquid crystal complicated and difficult. Furthermore, the short lifespan compared to DC drive is another factor that makes the drive conditions difficult.

Here, an example of the display characteristics of a display using a TN type nematic liquid crystal is shown in FIG.

Figure 1 is from 1976 DISPLAY Conf.51 page.
This is a quotation from Fig.5. In the figure, the effective value of the voltage applied to the pixel in the selected state (display state) is represented by V _s , and the effective value of the voltage applied to the pixel in the non-selected state (non-display state) is represented by V _NS . When multiple liquid crystal pixels are time-divided and driven using a so-called dynamic scanning matrix addressing method, when the number of scanning pixels is set to N and N is increased, (4) and (5) on page 51 of the literature It is clear from the formula that V _S
and V _NS approaches. In FIG. 1, when V _S and V _NS are brought closer together, the difference in relative amounts of transmitted light corresponding to V _S and V _NS decreases, and as a result, it becomes impossible to obtain a large display contrast between the selected state and the non-selected state. As a specific example, if N is 50, V _S /V _NS 1.15 From Figure 1, V _NS = V _Th 3.0V _rns , then V _S 1.15×V _Th 3.45V _rns In this case, the relative amount of transmitted light is approximately 20. Yes, it is only about one-third of the saturation level. That is, in this type of driving method, as the value of N increases, the contrast of the displayed image becomes difficult to obtain. Also,
If the ratio of the selective applied voltage to the standby applied voltage is b, then b = √ + 1 from equation (3) on page 50 of the literature, and there are drawbacks such as the need to increase the operating voltage as N increases. Furthermore, although detailed description is omitted, the viewing angle also becomes narrower. (Literature
1976Proceeding of the SID Vol 17/1First
Quarter P34) The present invention overcomes these drawbacks of liquid crystal displays,
The purpose is to display images including halftones effectively on a liquid crystal. Furthermore, in order to achieve the above-mentioned object, a long lifespan is ensured without impairing the performance of the liquid crystal, and at the same time, the circuit is simplified.

Furthermore, another object of the present invention is to reduce power consumption by keeping the operating voltage of the circuit low. Prior to describing embodiments of the present invention, conventional embodiments related to the present invention will be described.

FIG. 2 is a circuit diagram around a display panel showing a conventional embodiment related to the present invention, for example, in the document SID77.
Examples can be found in DIGEST P64-65, etc. In the figure, 2-1 is an image signal input such as a TV video signal,
2-2 is a synchronous separation signal, and 2-3 is a circuit for generating a control signal such as a timing clock from the synchronous separation signal. 2-4 and 2-5 are circuits that control the vertical lines or horizontal lines of the matrix display section and distribute and scan display signals to each matrix pixel. 2-4 is 2-1
A drain drive circuit converts the serial image signals inputted from the input into parallel and supplies them to the drain side of the transistor connected in series to each pixel. 2-5 is a gate drive circuit that sequentially controls ON/OFF of the gates of transistors connected in series to each pixel line by line using the 2-3 output clock to read image signals into the pixels. The output side drains 2-6 of the transistors arranged in each matrix section are coupled to each pixel electrode of the liquid crystal display. LiteratureSID78DIGEST
As described on pages 96-97, in the conventional matrix display using the circuit shown in FIG. 2, the liquid crystal was driven by direct current. In Figure 2, among the electrodes that sandwich the liquid crystal in the liquid crystal matrix display section, the electrodes facing each pixel electrode are common electrodes that span the entire display surface, and the potential is set at the GND level. It matches the common side electrode potential of the substrate of the MOS transistor and the capacitor arranged in parallel. For this reason, it has become necessary to take measures such as doping liquid crystal materials with redox agents in order to maintain a long DC life.

Here, the relationship between the signal waveform and potential in the circuit of FIG. 2 is shown in FIG. 3-1 is an image signal supplied to terminal 2-1, and 3-2 is an image signal supplied to block 2.
-4 is a synchronizing signal when sampling the image signal for each data line of each matrix. The horizontal axis t represents time, and the vertical axis V represents voltage. 3-3 represents the black level of the image signal, and 3-4 represents the white level, which correspond to the threshold voltage and saturation voltage of the liquid crystal, respectively. Voltage O corresponds to GND in FIG. 2 and is the substrate and common electrode potential.

Furthermore, another conventional circuit example related to the present invention is shown in FIG. Specifically, examples can be found in SID78DIGEST P94-95. In FIG. 4, 4-1 corresponds to 2-1 in FIG. 2 and is an image signal input. 4-2 is a low-pass filter, 4-3 is an amplifier, 4-4 is an A/D converter, 4-5 is a data encoder, 4-
8 is a serial/parallel conversion shift register. The image signal input 4-1 is converted into an image signal of a band corresponding to the display performance of the corresponding display panel through a low-pass filter and an amplifier, and then converted into a digital code by an A/D converter. 4-8 outputs the converted image digital image data to each data line of the matrix in parallel. The parallel output data is input to a D/A converter provided for each data line and restored to an analog image signal. At this time, the gain of the D/A converter output signal is controlled by the gain control circuit 4-9, and adjusted so that the (voltage-contrast) correlation characteristic of the liquid crystal matches the contrast of the image signal. Furthermore, the D/A output is buffer amplifier 4-12.
is input. 4-12, an offset bias level adjustment circuit 4-10 adjusts the reference level of the image signal so that it corresponds to the vicinity of the threshold value of the liquid crystal, and outputs the image signal to the data line. 4-6 is a synchronous separation circuit, 4-7 is a timing signal generation circuit, 4-13
is a circuit that controls the clock line of the matrix display section, and corresponds to 2-5. 4-16 is a display matrix section, and the configuration is the same as 2-7 in Fig. 2, so
Illustrations are omitted. In the circuit example shown in FIG. 4 as well, the liquid crystal is driven by direct current, as in FIG. 2. Further, in FIG. 4, a gain control circuit 4-
9 and offset bias level control circuit 4-10
This makes it possible to adapt the signal level to the characteristics of the liquid crystal display. The method of control in this case is to provide a D/A converter that adjusts the gain and a buffer amplifier that adjusts the offset for each data line, and adjust each using the same control signal line. As is clear from the figure, the number of D/A converters and buffer amplifiers equal to the number of data lines is required, making the data line driving circuit extremely complicated. Furthermore, the amplifiers constituting each D/A or buffer must have the same gain and other amplification characteristics. It is nearly impossible to match the characteristics of each amplifier without adjustment in terms of element manufacturing, and therefore adjustment must be made for each amplifier in advance. Needless to say, in FIG. 4, as in FIG. 2, since the liquid crystal is driven by direct current, deterioration of the display becomes a problem.

The present invention aims to improve the conventional drawbacks and provide an image display device that fully utilizes the display performance of liquid crystal. According to the invention, the document Display
Even if the number of matrix scanning lines is greatly increased compared to the so-called dynamic drive method of Conf.1976 P51, the display contrast performance of the liquid crystal will not be impaired in the slightest, and the applied voltage required for display drive will not increase. Furthermore, in contrast to the embodiments shown in FIGS. 2 and 4, since the present invention drives the liquid crystal with alternating current, it is possible to maintain the life of the liquid crystal for a long time, and it is possible to maintain the life of the liquid crystal for a long time. There will be no need for it. The circuit implementing the present invention is greatly simplified compared to the one shown in FIG. There are no factors that cause such variations.

Hereinafter, the present invention will be explained by specific examples using the drawings. Of course, the present invention can be implemented using other circuit systems, which naturally belong to the present invention.

FIG. 5 is a block diagram showing an overall view of a television receiver configured using the liquid crystal image display device according to the present invention. In the figure, 5-1 is a tuner section that selects the frequency of a predetermined channel from received radio waves input from an antenna. 5-2
is the circuit from the intermediate frequency amplifier to the video detection, 5-
4 is a circuit for audio intermediate frequency, detection, output, etc., 5-5
is a circuit that separates horizontal, vertical, etc. synchronization signals from the video signal. Reference numeral 5-3 denotes a video amplification circuit block according to the present invention, which outputs a liquid crystal display image signal to the matrix display section data signal latch circuit 5-8 at the subsequent stage. 5-9 is a data line driving circuit. 5-
6 and 5-7 receive the output of the synchronization signal separation circuit 5-5 and send a data latch signal to 5-8, respectively.
-10 supplies a timing signal for driving the matrix display clock line (horizontal line). Reference numeral 5-11 is a power supply, which supplies a common electrode voltage to be described later to the common electrode 5-13 (dotted chain line). 5-12 represents a matrix type liquid crystal display panel, the details of which are shown in FIG. 6-1 is a gate drive circuit, 6
-2 is a drain drive circuit to which a transistor for selectively supplying an image signal to a pixel electrode is connected to each pixel 6-3 of the matrix display section. All pixel electrodes to which the outputs of each transistor are connected are
It is located on one of the pair of flat plates that sandwich the liquid crystal.
Each electrode is electrically separated and independent on the flat plate on which the electrode is arranged. Among the flat plates sandwiching the liquid crystal, a single common electrode is provided on the flat plate facing the above flat plate over the entire display section.
Here, the substrate potential of each transistor and the one-sided electrode potential of the capacitor provided for each pixel are commonly equal to the GND potential, but the liquid crystal display section common electrode potential 6-4 is not the GND potential.

FIGS. 7 and 8 show partial views of an example in which transistors and capacitors are configured for each pixel as shown in FIG. 5.

FIG. 7 is a sectional view of a pair of flat plates sandwiching a liquid crystal, the side having electrodes separated for each pixel and arranged in a matrix. In the figure, 7-1 is a silicon substrate. 7-2 is a diffusion layer of a conductivity type opposite to that of 7-1, and 7-3 is a diffusion layer of the same conductivity type as 7-1, which serves as a stopper and a capacitor electrode. Further, 7-4 is a gate oxide film, and its film thickness is approximately 400 to 2000 Å. 7-5 is polysilicon, 7-5a is the gate electrode of the MOS transistor, and 7-5b is the electrode of the capacitor. 7
-6 is a field oxide film, 7-7 is an insulating film, 7-
8 is an aluminum electrode. In Figure 7, the transistors that switch each pixel are:
It is composed of a silicon gate MOS transistor, and the electrodes of the capacitor arranged in parallel with each pixel of the liquid crystal are the silicon substrate itself and polysilicon 7-5b. In this case, the silicon substrate
It is held at the GND potential, and as shown in FIG. 6, the one side electrode of the capacitor and the substrate potential of the transistor match and become the GND level.

FIG. 8 shows a plan view of the drive circuits arranged in a matrix, and the sectional view taken along the line AA' in the figure corresponds to FIG. In the figure, 8-2 to 8-8 correspond to 7-2 to 7-8, respectively. Also, the 8th
In the figure, the drain electrodes 7-8b in FIG. 7 are omitted so as not to complicate the figure. In FIG. 8, the pixels are areas indicated by two-dot chain lines. Therefore, the so-called pixel electrode that applies voltage to the liquid crystal is insulated from the transistors or the signal lines 8-5a, 8-8a running vertically and horizontally, and is shown approximately as a chain double-dashed line above the pattern in FIG. It will be placed. As mentioned earlier, 7-1 is a monolithic silicon crystal substrate, but there are various other methods of configuring the circuit shown in FIG. 6, including thin film technology. In Figure 6, each transistor is
Although it is composed of MOSFETs, other switching elements may be used.

Next, FIG. 9 shows an example of a signal waveform according to the present invention. In the figure, both 9-1 and 9-2 are image signals. The dashed line 9-6 indicates the potential on the common electrode side of the liquid crystal matrix display body, and the voltage polarity of the image signal applied to each matrix pixel electrode of the liquid crystal is repeatedly inverted at a certain period. For example, in the case of image signals for television broadcasting, one screen of video signal is taken as one frame, one frame is further divided into two fields, and interlaced scanning of the screen is performed for each field. Here, in FIG. 9, for example, 9-1 corresponds to one horizontal scanning line of one frame of image signal including the first and second fields. And 9-2 is the above 1
This is an image signal corresponding to the same display portion of the image signal for one frame following the signal for a frame. 9-3 is an image sampling synchronization signal,
The period indicated by 9-4 corresponds to the period during which one display pixel is displayed in the matrix display panel in the horizontal direction.
9-5 corresponds to the horizontal retrace period of the television image signal. On the vertical axis of FIG. 9, the O potential, that is, 9-11
For example, let 9-1 be the potential of the display substrate 7-1, and
Let 0 be the display body circuit voltage corresponding to 9-11. In this case, the switching transistors arranged for each pixel in FIG. 6 can be configured with, for example, a P-channel enhancement MOSFET. 9
-10 as the potential of the substrate 7-1, the switch transistor may be constructed of an N-type MOS.

The amplitude of the image signal 9-1 is between the dotted line 9-7 and the dotted line 9-8. 9-7 is the black image signal, 9-
8 corresponds to white. When using a liquid crystal having the characteristics shown in Figure 1, 9-7 is near V _th and 9-8 is V th.
It is taken near _S. FIG. 1 OV _rns corresponds to 9-6. Regarding the linearity of the signal, it is sufficient to prevent the linearity of the original image signal from being distorted by the liquid crystal by interposing an amplifier that is corrected according to the correlation characteristic between the applied voltage of the liquid crystal and the display contrast. The purpose of alternately applying the image signals 9-1 and 9-2 to each liquid crystal display pixel electrode is to extend the life of the display by AC driving the liquid crystal. When converting the signal into an alternating current signal to drive the liquid crystal, flickering of the displayed image occurs due to the alternating voltage driving of the liquid crystal. This is because the direction in which the electric dipole of the liquid crystal molecules faces also changes in response to the reversal of the applied voltage polarity. The following methods can be considered as methods for reducing flickering or making it effectively negligible. In other words, the phase may be reversed at a cycle faster than the eye can respond.

(1) Invert the phase at the frame period and make the frame period approximately 30 Hz or more.

(2) Invert the phase in units of pixels or scanning lines within one frame period to increase the effective inversion period.

Furthermore, various methods can be considered depending on the above application. When displaying TV images in a matrix,
There are cases where it is attempted to realize the number of pixels constituting the matrix with a number smaller than the effective number of pixels (or resolution) of the television video signal. At this time, for example, if a liquid crystal image is constructed from a matrix of one field (1/2 frame) of a TV video signal, the signals of the first field and the second field are phase-inverted, and the signals of two fields are transmitted to the same pixel. can be displayed at a frequency of 60Hz. Although the resolution as an image quality is reduced, the flicker caused by the difference in cost signals is canceled by the response performance of the liquid crystal itself, and an average image of the first field and the second field is displayed. Furthermore, in the method (2), the polarity direction is switched on a pixel-by-pixel basis, and the image display signal within one frame is selected to be both positive and negative polarity signals.
If the AC cycle of each pixel is set to one frame, the linearity of the amplifier or the linearity of the switching characteristics of the transistor provided in each pixel will vary within the operating voltage range (range from 9-11 to 9-10). Even if a sufficient amount cannot be obtained, the nonlinearity from the viewpoint of display effect can be effectively ignored.

The common electrode potential is set differently for waveform 9-1 and waveform 9-2 as shown in 9-6a and 9-6b.
That is, the common electrode potential is alternated between 9-6a and 9-6b at the polarity reversal period of the signal that drives the liquid crystal. By setting the potential and inverting the image signal as described above, the operating voltage or power supply voltage of the liquid crystal display driving circuit can be set to approximately the voltage V _S +α required for driving the liquid crystal, thereby enabling AC driving. Here, the elements of α include the threshold voltage of the drive circuit transistor shown in FIG. 6 and the channel resistance value when the transistor is ON. In order to reliably write a signal corresponding to an image within the sampling period, it is necessary that the gate voltage level is higher than the threshold voltage with respect to the drain signal level. There is a difference in the response speed of image signal writing when the drain potential is 9-8a and 9-8b with respect to the gate potential 9-10. Therefore, the present invention proposes to set the potential of 9-10 to more than twice the threshold potential of the transistor compared to 9-6a or 9-8b. In particular, when outputting a television video signal to the liquid crystal display device according to the present invention, one horizontal scanning signal time is
It is 63.5 μsec, and this inner retrace time is about 10 μsec. Therefore, the writing time to each pixel is 10μ
It is taken within the retrace period of sec, and is limited to about several μsec. Conventionally, for example, published patent publication 50-
10993As shown in Fig. 10, two 2-4 drain drive circuits are installed in parallel, and while data is being sampled in one circuit, the data that has already been sampled in the other circuit is written to the pixel. I found a way to do it. In this case, the write time is 63.5 μsec.

In the present invention, the data sample drain drive circuit 6-2 requires only one sampling circuit for each output, thereby simplifying the circuit. For this purpose, as mentioned above, the gate applied signal voltage is increased to ensure that errors do not occur during writing due to differences in input image signal levels.
And the pixel data is correctly rewritten within the retrace period. The transistor is P
In the case of a channel type MOS, the gate applied signal potential or circuit potential is set to be lower than the lowest level of the drain signal and at least twice the transistor threshold voltage. In the case of N-channel MOS, on the contrary, the potential is set high.

FIG. 10 shows an embodiment of a circuit that implements the above description. 10-2, 10-3, and 10-4 correspond to image signal amplifiers, and 10-5, 10-7, and 10-8 correspond to block 6-2 in FIG. 10-6 is a changeover switch circuit, and the output from 10-6 becomes the image signal input shown in FIG. 6, 6-8.

The operation will be explained below. 10-1 is an original image signal input, 10-2 is an initial stage amplifier, and 10-9 has an amplification factor adjustment terminal. Figure 1 shows the amplification factor characteristics according to the slope of the liquid crystal voltage-contrast characteristics in 10-2.
All you have to do is have it held. 10-3 and 10-4 are differential amplifiers. 10-3 positive polarity input terminal and 1
The same signal, that is, 1, is applied to the negative polarity input terminals of 0-4.
Combine 0-2 outputs. 10-3 negative polarity input terminal and 10-4 positive polarity input terminal are combined, 10
- There is a terminal at 10. 10-3 and 10-4
are set in advance so that substantially the same characteristics as the amplifier can be obtained. The 10-10 terminal is a terminal for adjusting the brightness of the displayed image on the liquid crystal.
A variable DC voltage is applied. For example 10-
The output signals 3 and 10-4 correspond to 9-1 and 9-2 in FIG. 9, respectively. At this time, 10-9 adjusts the difference between 9-8 and 9-7, that is, the amplitude, in other words, the contrast of the surface image. 10-10
adjusts the difference between 9-7 and 9-6. 10-3
The gain of (10-4) may be set appropriately. 10
-6 is a switch circuit, and as mentioned above, when supplying an AC drive signal to the liquid crystal, 10-3 and 10-
This circuit switches each of the four output signals and outputs them selectively. As the switch element, various types such as bipolar or MOS transistors can be considered. However, when a semiconductor is used for the display substrate as shown in FIG. 6 and the block 2-4 is housed inside the semiconductor substrate, 10-6 It is also desirable to have a similar structure, such as a so-called transmission gate. The same applies to the subsequent circuit 10-7. 10-5 is a circuit that sequentially generates signals for controlling each switch element 10-7, for example from left to right, and is constituted by a shift register. 10-8 is a circuit for storing and holding the image sampling signal sampled by the switch and distributing it to each pixel electrode. 10-8 and subsequent parts correspond to the liquid crystal matrix display section including the driving section, that is, FIG. 5.

FIG. 11 shows yet another embodiment. In FIG. 11, the configurations of the amplifiers 10-2, 10-3, and 10-4 in FIG. 10 are changed. 11-1 and 1
1-2 are image signals with substantially the same amplitude and opposite polarities. In the figure, the upper amplifier circuit (transistors 11-3, 11-5) and the lower amplifier circuit (transistors 11-13, 11-14) are designed so that their circuit configurations and amplification characteristics match. . Reference numerals 11-4 and 11-8 are variable resistors for controlling the gain of the amplification system, which adjust the contrast of the liquid crystal display image. 11-4, 11-8 are 11 indicated by the wavy line
-10, and can be adjusted manually from the outside. 11-7 and 11-9 control the output potential level. That is, it is a variable resistor that changes the brightness of the liquid crystal image display, and is linked by the dotted line 11-11, and can be manually adjusted from the outside. However, 11-7 and 11-9
The potential level operates in the opposite direction, and the symmetry of each output is maintained around level 9-6 as shown in FIG. 9 9-1 and 9-2. Reference numeral 11-12 represents an image signal polarity switching circuit corresponding to 10-6 in FIG.

The present invention can also be realized by a configuration other than the circuit described in the embodiment. Furthermore, the contrast and brightness can be adjusted manually as described above, or the gain or bias level can be automatically controlled by automatically detecting light by matching the display degree of the liquid crystal to the reference pattern display signal level. Of course it is also possible to do so. In the description of the embodiments of the present invention, as shown in FIG. 6, a silicon substrate is used as one of the flat plates sandwiching the liquid crystal, and a transistor is formed within the silicon substrate, or a transistor made of, for example, a polycrystalline material is used. It can also be realized by configuring each element on a glass substrate using thin film technology, or by other methods. Fifth
In the figure, the transistor provided to switch each pixel is one MOS type transistor, but in order to improve the linearity, response speed, operating voltage, etc. of the element, two P-type and N-type transistors are used. kind of
Switching can also be performed by coupling MOSFETs in a complementary manner. Of course, it is also possible to configure it with elements other than MOSFET.

In Fig. 5, a capacitor is arranged in parallel with each pixel of the liquid crystal, but in this case, as mentioned earlier, both electrodes of the capacitor are not connected completely in parallel with the liquid crystal pixel electrode, but the common electrode side The potentials are set separately for each. This is because by adopting the structure shown in FIG. 6, the common side electrode of the capacitor can be replaced by the substrate. At this time, the polarity and magnitude of the bias potential applied to the capacitor in accordance with the image signal applied to the liquid crystal pixel are different from the bias potential of the liquid crystal pixel electrode, but the effective electrical characteristics related to display are shown in Figure 2. This has the same effect as the case shown in .

Regarding the liquid crystal used in the display device according to the present invention, only the TN type liquid crystal is explained in Fig. 1, but the DSM, GH, and other liquid crystals mentioned at the beginning have basically different operating performance. isn't it. A liquid crystal is sealed between a pair of substrates, a common transparent electrode is formed on one of the substrates, and a plurality of pixel electrodes arranged in a matrix are formed on the other substrate. In a liquid crystal display device, an image display signal generating means for inverting the polarity of an image display signal in synchronization with a field period of the image display signal and supplying the image display signal with the polarity inverted to the pixel electrode; The common signal generating means generates a two-level voltage that is inverted in synchronization with the image signal, and supplies the inverted two-level voltage to the common electrode as a common electrode signal, and the field period is equal to the frame period of the image signal. Since the period is set to be twice or more, even if the voltage required to drive the display of the liquid crystal is reduced to less than half of the conventional voltage, a good display can be obtained without deteriorating the display quality of the liquid crystal. Furthermore, since the inversion timing of the image signal and the common signal is performed at a field period having a period that is more than twice the frame period, there is no image shift due to inversion or screen flickering caused by alternating drive. This has the effect of allowing stable images to be obtained over a long period of time.

[Brief explanation of the drawing]

Figure 1...Example of voltage-display characteristics of TN type liquid crystal, 2nd
Figure...Example of conventional display circuit, Figure 3...Example of conventional signal,
Fig. 4...Another conventional display circuit example, Fig. 5...A block diagram showing an embodiment of the present invention, Fig. 6...An example of a display circuit diagram according to the present invention, Fig. 7...Partial cross section of a display device. Examples of the figures, FIG. 8... A plan view of FIG. 7, FIG. 9... A signal waveform diagram in an embodiment of the present invention, FIGS. 10, 11... Examples of an implementation circuit of the present invention. 5-12...Matrix display section, 7-1...Silicon substrate, 8-8a...Data line for driving matrix display, 8-5a...Clock line for driving matrix display, 8-5b...Capacitor electrode, 3 ―
1,9-1,9-2...image signal, 10-2...
Image signal amplifier, 10-3, 10-4...differential amplifier.

Claims (1)

[Claims] 1 A liquid crystal is sealed between a pair of substrates, a common transparent electrode is formed on one of the substrates, and a plurality of pixel electrodes arranged in a matrix are formed on the other substrate. In the liquid crystal display device, an image display signal generating means for inverting the polarity of the image display signal in synchronization with a field period of the image display signal and supplying the image display signal with the polarity inverted to the pixel electrode; The common signal generating means generates a two-level voltage that is inverted in synchronization with the period, and supplies the inverted two-level voltage to the common electrode as a common electrode signal, and the field period is equal to the frame period of the image signal. A liquid crystal display device characterized in that the period is twice or more.