Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1991-10-31
2001-04-24
Mengistu, Amare (Department: 2778)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S182000
Reexamination Certificate
active
06222515
ABSTRACT:
BACKGROUND OF THE INVENTION
1) Field of the invention
The present invention relates to an apparatus for controlling the data voltage of liquid crystal display unit to achieve multiple gray-scale, levels and particularly to the apparatus for controlling the data voltage of an active matrix liquid crystal display unit that is useful for a field of flat display panels to achieve digital multiple gray-scale levels.
2) Description of the Related Art
In recent years, information terminals such as personal computers and word processors have become small-sized, high performance, and high function. Many compact information processors of the desktop type, laptop type, and even smaller notebook type and palmtop type are being marketed.
To reduce the size and weight of these information terminals, light-weight and thin display units are required. For this purpose, liquid crystal display units are frequently employed in place of cathode ray tubes (CRTs), which are usually employed for desktop information terminals. The liquid crystal display units include simple matrix liquid crystal display units such as TN (Twisted Nematic) display units and STN (Super Twisted Nematic) display units. Compared with these simple matrix display units, active matrix liquid crystal display units such as TFT (Thin Film Transistor) and MIM (Metal Insulator Metal) display units are frequently used in a field for displaying high quality color images with multiple gray-scale levels, because the active matrix display units are capable of precisely controlling intermediate gray-scale levels, ensuring high contrast ratios, and providing a high response speed, compared with the simple matrix display units.
For the active matrix liquid crystal display units, presently available digital driver ICs for selecting voltage levels for assigning gray-scale levels can handle only eight gray-scale levels. Even driver ICs presently being developed can operate only up to 16 gray-scale levels. To assign multiple gray-scale levels of more than 16 levels, expensive analog drivers must be employed to drive data. This prevents reducing the cost of liquid crystal display units.
Various studies have been made to increase the number of gray-scale levels with proper use of digital driver ICs. For example, a driver having a capacity of eight gray-scale levels may be employed. First, groups each involving eight power source voltages are prepared and temporarily switched from one to another and combined together, to realize a number of gray-scale levels greater than the number of the power sources.
In this case, if a difference between voltages that are combined together is extremely large, transmissivity based on an average of the combined voltages may deviate from a required value, or a gray-scale level may be swapped with another one, to deteriorate the quality of the gray-scale level assignment.
It is required, therefore, to provide an inexpensive digital data driver that can correctly assign multiple gray-scale levels.
Conventional active matrix liquid crystal display units that achieve good display quality include those employing TFTs.
The TFT display units involve many thin film transistors and pixel capacitance between two electrode layers to form a liquid crystal panel, and a display voltage is written in an optional capacitance through corresponding thin film transistors.
Brightness of the pixel depends on the written voltage. For example, “n” pieces of write voltages may be prepared to provide “n” gray-scale levels.
The n write voltages may be generated as follows:
(1) A predetermined constant voltage is amplified through an operational amplifier whose amplification factor is variable in multiple steps (in this case, n steps). (This will be referred to as the first generation method.)
(2) Two constant voltages are divided by resistances into n kinds of voltages, and one of which is selected by a switching element. (This will be referred to as the second generation method.)
Since the first generation method amplifies the predetermined constant voltage through the operational amplifier, it has the problem that a minimum variable width of the amplification factor is determined by the accuracy of the operational amplifier.
Since the second generation method selects one of the two constant voltages through the switching element, it has the problem that the number of the voltage dividing resistances and switching elements must be increased to expand a circuit scale.
Namely, any one of the above methods cannot easily increase the number (n) of generated voltages, and therefore, cannot further increase the number of gray-scale levels.
According to the development of GUI (Graphic User Interface) in recent years, requirements for multiple colors in computer displays have escalated from the conventional eight or 16 colors to 4096 colors with 16 gray-scale levels, or over 260,000 colors with 64 gray-scale levels. At present, however, about 512 colors with 8 gray-scale levels is the maximum, due to the reasons mentioned above. This is completely insufficient to meet the above requirements for multiple colors.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an apparatus for controlling the data voltage of a liquid crystal display unit that can realize a multiple gray-scale level display of high quality without increasing a circuit scale.
Accordingly to the present invention is an apparatus for controlling data voltage of a liquid crystal display unit having a liquid crystal display panel, scan driver, and data driver to display an image of one frame composed of a plurality of fields. The apparatus of the present invention is composed of a timing signal generating means for generating at least timing signals for the scan driver and the data driver, and a timing signal for switching each field; a data voltage selecting signal generating means for generating a data voltage selecting signal by which data voltages applied on liquid crystal elements become different for each field in accordance with an input data signal and outputting the same to the data driver; and a voltage applying means for generating more data voltage levels than the number of fields of one frame, to apply different data voltage levels for each of the field in accordance with the data voltage selecting signal and outputting the same to the data driver and a common electrode provided in the liquid crystal display panel. In addition, the data driver selectively applies a data voltage among the applied voltage levels to the liquid crystal element, in accordance with the data voltage selecting signal, thereby assigning a gray-scale level according to a mean effective voltage of the voltage levels applied to each of the fields of the one frame.
In this way, the present invention assigns a gray-scale level according to a mean effective voltage of voltage levels applied to fields of one frame. This technique easily assigns multiple gray-scale levels without increasing circuit scale.
The invention changes the combination and/or phase of voltage levels applied to adjacent pixels, to suppress flickering. The invention may increase a scan frequency according to the number of fields in a frame, to further suppress the flickering.
In addition, the invention restricts a voltage difference applied to each frame below a predetermined value, to surely reduce flickering between a plurality of cells.
The invention substantially equalizes the averages of combinations of voltage levels set for a plurality of fields, thereby suppressing differences in the combined voltages, and securing the quality of a gray-scale level assignment.
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patent:
Oda Masami
Takahara Kazuhiro
Yamaguchi Hisashi
Yamaguchi Tadahisa
Fujitsu Limited
Mengistu Amare
Staas & Halsey , LLP
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