JPS63307432A - Matrix driving method for liquid crystal display element - Google Patents

Abstract

PURPOSE:To fix the contrast by including no components different in pulse width in a signal applied in a non-selection time and controlling waveforms on the scanning side and the data side so that pulses having the same polarity are not continuously applied. CONSTITUTION:With respect to bipolar type unit write signals 301 and 302 and write signals 303 and 304 obtained by inverting and connecting signals 301 and 302, the combination of signals 301 and 304 or signals 302 and 303 different in type is used and is divided to the scanning side and the data side on the time base. In case of this division, polarities of first pulses of turning-on and turning-off signals on the data side are equalized and polarities of the last pulses of turning-on and turning-off signals on the data side and the polarity of a write pulse are made opposite to the polarity of first pulses. By these pulses, pulses having the same polarity are not continuously applied in the non-selection time and the contrast is free from variance and fluctuation to stabilize the image quality.

Description

[Detailed description of the invention] (Industrial application field) The present invention is a display device that replaces a CRT for home use.

Alternatively, the present invention relates to a method for driving a liquid crystal display element that has a wide viewing angle and sufficient contrast even in a large-screen lid display as a terminal display device used in advanced information processing for office automation.

(Prior art and its problems) The liquid crystal phase currently used in liquid crystal display elements is a nematic phase, which is a twisted nematic phase in which the direction of liquid crystal molecules changes continuously by 90 degrees between the upper and lower substrates of one display area. (
It has a structure (hereinafter simply referred to as TN type). this is,
Applying an alternating electric field induces a change in the dominant direction of the liquid crystal molecules.

This is extracted and displayed as a change in the amount of light transmitted under an appropriate optical system.

This is a known technique.

A feature of this TN type is that a change in the direction of liquid crystal molecules is induced by the effective value of the applied voltage.

In such an effective value responsive liquid crystal, it is known that increasing the display capacity, that is, increasing the number of scanning electrodes, deteriorates display quality (contrast, viewing angle, etc.) and makes driving conditions more severe. This is the biggest reason why a large TN-type large-screen display has not been realized even now.

However, it is still an effective value response type liquid crystal and has a large capacity.

Techniques for obtaining high-quality displays have also been developed. Typical examples include TPT drive and super twisted type liquid crystal display elements, but both have their disadvantages. The current situation is that it is not yet available.

Under these circumstances, chiral smectic liquid crystal (hereinafter simply referred to as S
mC") was discovered, and its application to displays is being actively considered (N.

A. Clark and S. T. Lagerval, APp.
i, Phys, Lett, 36,899 (1980
)) The greatest features of this liquid crystal include pulse responsiveness, memory effect, and ultra-high-speed response.

It has properties that are suitable for use in liquid crystal displays. However, there are still technical problems to be solved for practical use, such as the need to make the cell gap thinner than that of the TN type and the formation of a monodomain phase of SmC'' (orientation control).

Pulse responsiveness and memory effect are essentially different properties from the effective value response of a TN type liquid crystal. These are considered to be the fundamental reasons why the image quality remains stable without deterioration even if the display capacity is increased. However, in order to display a screen without contrast unevenness or flickering in a large-capacity display using a liquid crystal display element, it is necessary to solve problems such as material properties such as response speed and threshold characteristics of the liquid crystal, orientation control, and applied waveform. It is important to consider driving methods such as scanning and scanning methods. For the same display element, the driving method is of decisive importance.

Next, we will discuss the pseudo-applied waveform for matrix drive and the optical response characteristics of the liquid crystal to it. Figure 1 shows S.
The typical pulse-type application waveform (a) and the change in light transmission amount (b) for mC''' are shown.To explain in more detail, (101) and (102) are write pulses, which are bipolar pulses. The reason why the liquid crystal is bipolar is to prevent DC components from being included in the liquid crystal, and this fact is also maintained in the bias section (103).

There are two types of prediction pulses: an on write pulse (101) and an off write pulse (102), and in both cases, the latter half of the pulse is effective for state formation. The ratio of the amplitude of this write pulse and the bias section (103) is called the bias ratio, and is usually set to 3:1. Write pulse (1
01) (102) and the pulse width τ are appropriately selected to separate it into two bands, a dark state (104) and a bright state (105), as shown in FIG. 1 (b).

It is thought that writing in matrix drive is possible.

Within this band, there is a stable state formed by the write signal! ! Around (106) and (107), the amount of light transmission oscillates regularly in synchronization with the bias signal (103). Actually, when C31014 (manufactured by Chisso ■) is sealed in a cell with a cell gap of 2 μm, V = 10 volts and τ-
Writing was possible within the range of 100 to 300 μsec. When (2) is constant, when the pulse width τ is large, the amplitudes of the two bands become large, and the contrast tends to decrease accordingly.

In the case of matrix driving, it is necessary to divide the pulse (10) and pulse (102) signals into the scanning side and the data side so that they are applied to each intersection. Regarding this division, there are voltage modulation performed on the voltage axis and pulse modulation performed on the time axis. Although FIG. 1 including the bias section is an ideal example obtained when performing on the voltage axis, FIG. 2 shows two specific examples of division. In this case, the write signal portion when selected and the combined signal when not selected are also noted. The applied waveform shown in Figure 2 (c) is what is expected from these, but the difference between this and Figure 1 (a) is that in the former, the bias section of the signal train (c) that is applied when not selected is (2
00) includes a component (201) with a different pulse width.

This is extremely important in voltage modulation driving. Although the stability of the band is not compromised even if different components of pulse 11 coexist, the origin of the waveform is set at the center to intuitively clarify that no DC component with large amplitude is included. . ) Therefore, in the case of matrix driving, the ratio of components with different pulse widths in the bias section (200) will vary depending on the state of on and off on the dark line, and the signal train will change accordingly. As a result, the optical transparency 11JI may also vary. This means that the contrast changes depending on the pattern. The worst case is that the data line is alternately on and off (horizontal stripes), and the best case is only on or only off (vertical stripes). In general,
It can be said that there is a difference in shading at each part on the display surface.

This is an inevitable result of the voltage modulation drive format and the pulse response of the liquid crystal, and is unavoidable no matter how good the SmC's phase alignment is. The scanning side and data side signals can be configured so that a part with zero voltage is inserted between S, T,
Lagerbal.

at al,, International Di
spray Re5search Conference
ce, P213 (1985)), has been found to increase scanning time and to be less effective. Furthermore, according to our experiments, it has not been possible to obtain a steep threshold characteristic that responds only to write signals and does not respond to bias signals.

In any case, this is definitely disadvantageous when displaying iI tone.

(Objective of the Invention) Given the SmC phase exhibited by a ferroelectric liquid crystal, the present invention is tasked with configuring a driving waveform in which the contrast is always constant regardless of the display pattern even if it is driven in a matrix. It is a mission.

(Structure of the Invention) For this purpose, the present invention provides an application signal train that is applied during a non-selected time, in which all components with different pulse widths are not included, and pulses with the same polarity are separated by a certain period of time. The waveforms on the scanning side and the data side are specified so that they are never applied continuously.

(Detailed Description of the Invention) Based on examples, the following describes first a method of configuring an applied waveform in which all pulse widths are the same, and second a method for configuring an applied waveform in which pulses of the same polarity are not applied consecutively at a fixed time interval. The method of configuring the applied waveform will be described.

Since the latter is difficult with the voltage modulation mentioned above, the applied waveform will be considered for pulse modulation as well. Pulse modulation is the division and reconfiguration of a write signal in units of on and off into the scanning side and data side along the time axis.

For unit write signals that do not include DC components, bipolar type (301) (30
In addition to 2), (303) and (304) are also possible. It is possible for any waveform to divide the write signal along the time axis so that no DC component is included at any matrix intersection, and no different pulse widths are included. -As an example, only two are shown in Figure 4. Figure 4 (A)
(301) and (302), and Figure 4 (B) shows (303).
) and (304), respectively. The broken line in the figure indicates division at that point, and the arrow indicates movement in that direction. Figure 4(C) shows the fourth
This figure is a reconfiguration of the scanning signal and data signal in FIG. 3(B). It is clear that all pulse widths in the bias portion of these waveforms are the same. The ratio of the write pulse to that of the bias section is 2:1, and the peak value is constant (2:1 pulse modulation).

The characteristic is that the width of all pulses in the bias section is certainly the same, but the pulses of the same polarity are separated by a certain period of time.
There is also a portion (401) where the voltage is applied continuously;
A typical signal train is shown in FIG. 4(D). Figure 5 shows the results of applying this type of applied signal to the cell mentioned above (a cell with a gap of 2 μm and sealing a strongly transparent liquid crystal C31014 (manufactured by Chisso)) and examining the response. . In the part of the birch shown in this figure (501) where the same minute pulse is applied at fixed time intervals, the state is reversed, and writing is impossible in this case. °This response remained the same even when the pulse width and peak value were varied. Further, even if the interval between pulses of the same polarity is increased, the same 0 reversal must occur only in the write portion. This cumulative effect is considered to be a general trend in pulse modulation driving.

Therefore, in pulse modulation, it is not enough to keep the pulse width of the bias section constant; it is necessary to prevent pulses of the same polarity from being applied continuously.This can be done as follows. It is.

(1) The polarity of the first pulse of the data-side ON signal and OFF signal must match.

(2) The polarities of the last pulses of the data-side ON signal and OFF signal are the same and opposite to 1).

(3) The polarity of the last pulse of the writing/writing part is opposite to (1).

The above (1) and (2) are the write pulse (301) and (3
As mentioned earlier, it is difficult to use the same type of write signals such as 02) or write pulses (303) and (304). Therefore, different combinations of write pulses (301) and (304) or (302) and (303) are candidates, but either one is sufficient. Therefore, FIG. 6 shows an example in which the two signals (302) and (303) are divided on the time axis into a data side and a scanning side so as to satisfy the above (1) and (2).

It is clear that the two data signals satisfy the conditions (1) and (2) above. Therefore, even if these signals flow in from the data side when not selected, pulses of the same polarity will not be applied continuously at a fixed time interval.
Consider the write signal part at the time of selection. For a common selection signal, ON and OFF cannot be written at the same time, so they are written successively. As a scanning signal (601
) and (602) can be used. Correspondingly, a dummy zero-volt part is added to the data signal.

The unit signal groups in this case are collectively shown in FIG. 7(A). The last pulse (701) of the write signal part that determines on and off is the on (702) and off (702) of the data signal.
703), and condition (3) is automatically satisfied.

In addition to the pulse (704) shown in the figure, the signal applied when the scanning side is not selected may be a monotonous constant voltage (0 volt). However, the former shown in FIGS. 7(A) and 7(B) had a wider range of displayable combinations of pulse width and voltage, and had better stability against temperature changes.

This shows that in the case of pulse driving, stability is improved if parts other than the write signal part vibrate constantly.

Other combinations of scanning and data signals are possible. As can be easily inferred from the division in FIG. 6, there are eight truly independent combinations, and only one example is shown in FIG. 7(B). FIG. 8 shows a result of applying the pseudo applied waveform (801) obtained from these to the same cell and examining the response. Since pulses of the same polarity are never applied consecutively at intervals of a certain period of time, as is clear from this figure, no cumulative effect is observed and writing is possible.

Additionally, in this method, the voltage does not necessarily need to be constant; it may be varied as much as possible for writing. In particular, changing the voltage of the data signal between on and off states or lowering it depending on the diversity of liquid crystal responses is effective in reducing the amplitude during bias.

For liquid crystals with negative dielectric anisotropy, a monotonous constant voltage (0 volts) applied when the scanning side is not selected, or (
704) other than the first bipolar pulse can be replaced by an alternating electric field with a pulse width shorter than the write pulse width. It is expected that vibrations will be greatly suppressed and contrast will increase (alternating electric field superimposition method) (Effects of the invention) According to the present invention, the contrast unevenness that naturally exists in voltage modulation driving can be eliminated without increasing the scanning time. and fluctuations can be completely eliminated. Therefore, j'jff is stable and
It becomes possible to introduce gradation display.

[Brief Description of the Drawings] Figure 1 (a) is a pulse waveform diagram showing an example of an applied signal train to demonstrate the possibility of matrix drive;
B) is a graph diagram showing changes in the amount of light transmitted through the liquid crystal corresponding to the pulse waveform of FIG. 1(A). FIG. 2 is an explanatory diagram of a unit signal group (scanning signal and data signal) for matrix drive, and in FIG. 2 (a), the non-selection signal is constant (0 volt). Figure 2 (b) is an explanatory diagram showing an example in which both selection and non-selection signals are not constant, and Figure 2 (c) shows an example of the bias portion expected from the data signal shown in Figure 2 (b). FIG. Figures 3 (a) and 3 (b) are examples of the configuration of a writing part that does not include a DC component with the minimum number of pulses to cause switching, and both are pulse waveform diagrams showing examples of switching when the polarity is reversed. 4(A) and 4(B) are explanatory diagrams showing an example in which the write signal portion in FIG. 3 is divided into a scanning side and a data side, and FIG. 4(C) is an explanatory diagram showing an example in which the write signal portion in FIG. FIG. 4 is an explanatory diagram showing an example in which a pulse waveform is reconfigured into a scanning signal and a data signal for matrix driving, and FIG.
) is an explanatory diagram showing a bias portion expected from the data signal of FIG. 4(C), and showing a portion where pulses of the same polarity are continuously applied. Figure 5 (a) is a pulse waveform diagram showing a signal train in a portion where pulses of the same polarity are continuously applied, and Figure 5 (b) shows the corresponding change in the amount of light transmission through the liquid crystal. FIG. Figure 6 shows write signal pulses (303) with different symmetries.
(302) into a scanning side and a data side. FIGS. 7(A) and 7(B) are explanatory diagrams showing examples of reconstructed scanning signals, data signals, and composite waveforms for matrix driving by pulse modulation of the present invention. FIG. 8(a) is a pulse waveform diagram showing an example of the pulse modulation drive waveform of the present invention, and FIG. 8(b) is a graph diagram showing the corresponding change in the amount of light transmission of the liquid crystal. (101) Write pulse (on) (102)
Write pulse (off) (103) Bias signal part (104) Light transmission spectrum of liquid crystal (on) (10
5) Light transmission spectrum of liquid crystal (off) (106) (
107) Bistable state (200) Bias signal part (201) Wide pulse width part (301) (302) Bipolar type on and off write pulses (303) (304) Bipolar type inverted and connected Type of write pulse (401) Part where pulses of the same polarity are applied continuously after a certain period of time (501) Application signal part where state reversal has occurred (6,0
1) Contribution to the scan waveform from 303 (602) Contribution to the scan waveform from 302 (700) Last pulse of write pulse (701) (702) On and off write signal in pulse modulation (701) Combined signal when selected (702) (703) Combined signal when not selected (704)
When a non-selective scanning signal (801) containing a bipolar pulse at the beginning is a pseudo-applied waveform of pulse modulation
Figure 4 (C)

Claims (1)

[Claims] In a matrix driving method for driving a liquid crystal cell in which a ferroelectric liquid crystal is sandwiched between a pair of transparent electrode substrates having a scanning electrode group on one side and an information electrode group on the other side facing each other by line sequential scanning. , the signal train applied to the ferroelectric liquid crystal during the non-selected time within one period consists of pulses of the same width, and the pulses of the same polarity are not applied consecutively with a certain time width apart. A method for driving a matrix of a liquid crystal display element, characterized in that: