Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1998-07-31
2001-11-20
Shalwala, Bipin (Department: 2673)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S089000, C345S098000, C345S099000, C349S045000, C349S048000
Reexamination Certificate
active
06320562
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display device. More particularly, the present invention relates to a driving circuit which solve the problem of deterioration in a display quality, thereby realizing a uniform display quality in an active matrix type liquid crystal display device used in a variety of office automation devices including personal computers and word processors, multimedia information terminals, audio vidual devices, game machines, and the like.
2. Description of the Related Art
In recent years, due to the advent of the highly-informationalized society, demand for a display capable of displaying a large amount of information at once has been rapidly increased. Conventionally, a CRT (cathode ray tube) has been generally used for displaying a large amount of information. However, the CRT is generally large in size and consumes a large amount of power. Since the CRT is made as a stationary type device, the CRT is not suitable for use as a portable device. On the other hand, a flat display such as a liquid crystal display device is thin, and is light in weight. Such characteristics of the flat display are attracting attention.
Liquid crystal display devices are roughly classified into two groups, i.e., a passive matrix type and an active matrix type. A super twisted nematic (hereinafter, referred to as “STN”) liquid crystal display device which is a typical passive matrix type liquid crystal display device and a thin film transistor (hereinafter, referred to as “TFT”) liquid crystal display device which is a typical active matrix type liquid crystal display device will be described hereinafter.
The TFT liquid crystal display device includes switching elements such as TFTs which are positioned at intersections of row electrodes and column electrodes arranged in an matrix. Display is performed by controlling the switching element so as to independently apply a voltage to a liquid crystal layer in each of pixels. In such a TFT liquid crystal display device, liquid crystals are operated in a TN mode. Thus, it is possible to realize both a high contrast and high-speed response.
According to the STN liquid crystal display device, on the other hand, a liquid crystal layer is interposed between a pair of glass substrates on which row electrodes and column electrodes are provided so as to cross each other. A display is realized by changing optical state of the liquid crystal layer depending on an RMS voltage level of a driving voltage which is applied between the row electrodes and the column electrodes.
If these two types of the liquid crystal display devices are compared with each other in terms of their costs, the STN liquid crystal display device is superior to the TFT liquid crystal display device due to its simple panel structure and fabrication process.
As to their display performances, however, the TFT liquid crystal display device has an advantage over the STN liquid crystal display device which has no switching element associated with a pixel. In particular, the STN liquid crystal display device tends to have a deteriorated display quality as its display capacity increases. This is because as its display capacity increases, its driving margin reduces, thereby reducing its contrast ratio, and display unevenness which depends on its display pattern, i.e., cross-talk, occurs.
Regarding their optical response performances, the optical response speed of the STN liquid crystal display device is generally about 300 ms, whereas that of the TFT liquid crystal display device is about 50 ms. Therefore, the STN liquid crystal display device has an optical response speed slower than that of the TFT liquid crystal display device, and thus is not suitable for displaying a moving picture. Moreover, in the STN liquid crystal display device, its contrast ratio tends to reduce as its response speed increases.
As described above, both types of the liquid crystal display devices have their advantages and disadvantages. Along with an increase in the use of multimedia, however, even the relatively inexpensive STN liquid crystal display device came to be required to display a moving picture (e.g., a video picture, a picture, or the like). Needs for a high-speed responsiveness and a high picture quality are increasing.
Hereinafter, the cause of a reduction in its contrast in the STN liquid crystal display device having a high-speed responsiveness and a technique for improving the problem will be described.
The STN liquid crystal display device conventionally employs a line-sequential driving method. This driving method sequentially scans a group of row electrodes one line at a time during one frame period. Upon scanning, a high level scanning pulse is applied to each of the row electrodes only once in the one frame period. Synchronizing with the application of the high level scanning pulse, a data voltage which complies with display data in each pixel related to the scanned row electrode is applied to a column electrode.
It is intended that the liquid crystal display device employing the conventional line-sequential driving method mainly displays a still picture or the like. Such a liquid crystal display device conventionally uses a liquid crystal material having a relatively low response speed. In such a case, the liquid crystal molecules respond to an applied RMS (root-mean-square) voltage (i.e., effective voltage), thereby obtaining a practical contrast ratio. However, if high speed responsiveness of the liquid crystal layer is realized by reducing the liquid crystal viscosity or reducing a thickness of the liquid crystal layer in order to realize the display of a moving picture, the liquid crystal molecules respond not to the RMS voltage (i.e., effective voltage) but to a driving waveform itself according to the line-sequential driving method. As a result, the phenomenon in which a transmittance varies for each of the frames becomes prominent. This phenomenon is referred to as a “frame response phenomenon”. The frame response phenomenon leads to a significant reduction in the contrast ratio.
In order to improve such a problem, unlike the line-sequential driving method in which the high level scanning pulse is applied only once in the one-frame period, a driving method in which the high level scanning pulse is divided into a plurality of low level scanning pulses and applied in the plurality of times in one frame, thereby suppressing the frame response phenomenon to prevent the reduction in the contrast ratio has been suggested. Such a driving method is referred to as a multiline selection driving method. Such a driving method is characterized in that a plurality of row electrodes are simultaneously scanned using an orthogonal matrix. Hereinafter, the fundamental operation thereof will be briefly described.
After performing an orthogonal transformation operation for input image data by using the orthogonal matrix, a data voltage based on its arithmetic data is applied to a column electrode. In synchronization with the application of the data voltage, a scanning voltage based on a column vector of the orthogonal matrix is applied to all of the simultaneously-selected row electrodes at the same time. In this manner, the orthogonal inverse transformation of an image data is performed on the liquid crystal panel. As a result, the input image can be reproduced. Depending on the number of the row electrodes which are simultaneously selected, their scanning order, or the like, the following three driving methods have been suggested. However, the fundamental principals thereof are as described above.
The first driving method is an active addressing method in which all of row electrodes for an entire display screen are simultaneously scanned. This method is disclosed in T. J. Scheffer et al. (SID '92, Digest, p. 228), Publication for Opposition No. 7-120147, and the like.
The second driving method is a sequency addressing method in which a plurality of row electrodes which are fewer than the total number of row electrodes
Furukawa Hiroyuki
Taniguchi Koki
Ueno Satoshi
Yasunishi Norio
Nguyen Jimmy H.
Shalwala Bipin
Sharp Kabushiki Kaisha
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