Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1998-08-05
2001-10-23
Liang, Regina (Department: 2674)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S097000, C345S210000
Reexamination Certificate
active
06307533
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATION
The present application is related to and claims priority from Japanese Patent Application No. Hei-9-225182, filed on Aug. 21, 1997, the content of which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to a liquid crystal display device, and more particularly to a liquid crystal display device which has a matrix electrode structure to drive n×m pixels.
2. Description of Related Art
JP-A-5-119746 discloses a liquid crystal display device with a matrix electrode structure. As the liquid crystal for the display, an anti-ferroelectric material is used. The anti-ferroelectric liquid crystal of this kind has at least one anti-ferroelectric state (a first stable state AF) and two ferroelectric states (second and third stable states F+, F−), and each of these states can be attained stably.
The liquid crystal display device disclosed in the above-mentioned publication displays picture images on the panel by sequentially scanning its scanning electrodes which constitute a matrix together with signal electrodes. A selection voltage for writing images in combination with a signal voltage supplied to the signal electrodes is sequentially supplied to the scanning electrodes, and then a holding voltage to maintain the written images is supplied to the scanning electrodes. The selection voltage is supplied to each scanning electrode with a predetermined phase shift. However, there are problems that the images may be displayed as ghost images and that moving images are difficult to be displayed in a good condition. This is because a response time for changing the state of the anti-ferroelectric liquid crystal from the ferroelectric state (F+ or F−) to the anti-ferroelectric state (AF) is more than 10 times longer than a response time for changing from AF to F+ or F−, and, accordingly, time required for switching images displayed becomes considerably long. In other words, displayed images are influenced by the optical response time of the anti-ferroelectric liquid crystal when they are eliminated, and, accordingly, the state of the liquid crystal immediately before application of the selecting voltage is different by pixel by pixel and luminance of each pixel may not be uniform even a same level of the selecting voltage is applied to pixels. This problem occurs not only in moving images but also in switching still images.
To solve the above-mentioned problem, some proposals have been made, for example, in JP-A-7-28432 and JP-A-7-43676. JP-A-7-28432 proposes to provide a response period to make the anti-ferroelectric liquid crystal change from the ferroelectric state to the anti-ferroelectric state in the selecting period. However, this driving method requires a longer time to scan one scanning electrode, because the selecting period is a total of both periods for writing images and for changing the state of the anti-ferroelectric liquid crystal from the ferro-electric state to the anti-ferroelectric state. Therefore, in the device having a large number of the scanning electrodes, moving images cannot be displayed properly. JP-A-7-43676 proposes to provide an eliminating period in which the anti-ferroelectric liquid crystal changes its states from the ferro-electric to the anti-ferroelectric between the selecting period and the holding period. This driving method enables to display the moving images in the device having a large number of scanning electrodes. However, because the level of the voltage applied in the eliminating period is zero, the response time from the ferroelectric state to the anti-ferroelectric state becomes longer, and, accordingly, the eliminating period has to be made longer. Therefore, there are such problems that the display luminance is low and that flicker appears when the display panel is driven by a low frequency. Also, in both driving methods disclosed in the above publications, the response of a particular pixel selected is influenced by image signals determining a display condition of other pixels to which the image signals are applied in an eliminating period before the selecting period for the particular pixel. This results in a phenomenon called a cross-talk in the longitudinal direction of the signal electrodes.
SUMMARY OF THE INVENTION
The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a liquid crystal display device using an anti-ferroelectric liquid crystal or a liquid crystal having a similar optical characteristic and having a matrix electrode structure, which has an improved driving system that attains a good display with a short eliminating period while suppressing the cross-talk.
A liquid crystal display panel is composed of a plurality of scanning electrodes, a plurality of signal electrodes and an anti-ferroelectric liquid crystal disposed between both electrodes. The signal electrodes are disposed perpendicularly to the scanning electrodes so that both electrodes form a matrix structure. Each intersection of both electrodes constitutes a pixel together with the anti-ferroelectric liquid crystal. Scanning voltages are sequentially supplied to the scanning electrodes from a scanning electrode driving circuit while signal voltages are sequentially supplied to the signal electrodes from a signal electrode driving circuit in synchronism with the scanning voltages. The scanning and signal voltages are combined on the pixels thereby displaying picture images on the display panel.
In a scanning process, selecting, holding and eliminating periods are provided in this order. The picture images are written on the pixels in the selecting period, maintained in the holding period and eliminated in the eliminating period. During the holding period the polarity of the holding voltage supplied from the scanning electrode driving circuit is reversed at least one time. Preferably, a refresh pulse voltage which is higher than the holding voltage is imposed on the scanning electrode at a time the polarity of the holding voltage is reversed, so that flicker of the display is suppressed even when the panel is driven with a relatively low frequency.
To eliminate the picture images maintained in the holding period as quickly as possible, a pulse voltage having a polarity opposite to that of the holding voltage is imposed on the scanning electrode at the beginning of the eliminating period, and then a standard voltage having a lower level than the pulse voltage is imposed. The anti-ferroelectric liquid crystal changes its state from a positive or negative ferroelectric state to an anti-ferroelectric state in the eliminating period, i.e., the picture images change from a bright state to a dark state. Then the next selecting period follows. By imposing the pulse voltage at the beginning of the selecting period, the picture images are quickly eliminated without sacrificing brightness of the display. The problems seen in conventional devices, such as the ghost image or the cross-talk, are avoided at the same time.
The level and width of the pulse voltage imposed in the eliminating period are chosen so that the picture images can be eliminated as quickly as possible. Preferably, the width of the pulse voltage is selected to be shorter than a response time of the anti-ferroelectric liquid crystal from the ferroelectric state to the anti-ferroelectric state. The pulse voltage may be imposed in a form of a bipolar pulse which is a pair of pulse consisting of a first pulse having a polarity opposite to the polarity of the preceding holding voltage and a second pulse having a reversed polarity. The level of the pulse voltage may be selected at the same as that of either the selecting or holding voltage. In this case, the number of voltage levels supplied from the power source does not have to be increased to supply the pulse voltage in the elimination period.
Other objects and features of the present invention will become more readily apparent from
Nakamura Koji
Souda Norifumi
Yamamoto Toshio
Denso Corporation
Liang Regina
Pillsbury & Winthrop LLP
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