Computer graphics processing and selective visual display system – Display driving control circuitry
Patent
1996-04-17
1999-09-14
Hjerpe, Richard A.
Computer graphics processing and selective visual display system
Display driving control circuitry
345147, 345 89, G09G 500
Patent
active
059530022
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a driving method for a passive addressing type liquid crystal display device.
2. Discussion of the Background
As a basic driving method for a passive (multiplexed) addressing type liquid crystal display element, there has been proposed a line successive selection method (for instance, APT: Alt Pleshko Technique) or IAPT (Improved Alt Pleshko Technique as an improvement of APT). This technique is very useful as a multiplex driving method since ON-OFF levels can be easily driven. However, since the direct addressing type liquid crystal display device does not use active elements such as TFTs, there was a problem of reduction of contrast ratio due to frame response when a liquid crystal display element of fast response was used.
In order to solve such problem, a multiple line selection method has been proposed whereby it has been possible to display a picture having a high contrast ratio at a high speed. Further, in order to achieve the same purpose as described above, an attempt of using a whole line simultaneous selection method (AA: Active addressing) has been reported. Thus, a new addressing technique has been developed with the result of improving a quality of display.
There has been an increased demand for displaying pictures with many gradation levels for personal computers, TVs etc. and liquid crystal display devices as well. Several methods have been used for displays with gradation. In an active type driving method using transistors, diodes, or the like, an amplitude modulation can be easily achieved by using voltage pulses whose pulse height is varied depending on gradation levels of data to be displayed. This is because voltages applied to liquid crystal are basically of a static waveform.
In a passive multiplexed type driving method which typically uses a STN (super-twisted nematic) liquid crystal element and so on, however, there is a voltage change in a non-selection time when voltage pulses whose pulse height is varied depending on gradation levels of data to be displayed are simply applied to the element. Under the circumstances, there have been used or proposed several methods to display gradation levels in the passive multiplexed type driving method.
In the conventional driving methods of driving STN, there have been proposed and used a frame rate control method (FRC) and a pulse width modulation method (PWM) in order to obtain a display with gradation. Recently, an amplitude modulation method (AM) has been proposed. In the following, description will be made briefly on the proposed methods, and then, description will be made on problems caused when these methods are applied to the multiple line selection method.
(1) Frame rate control (FRC)
A gradation display is made with use of a plurality of frames. Namely, an intermediate tone is formed in response to the number of ON and OFF as a binary state. For instance, when three frames are used, four states, ON/ON/ON, ON/OFF/ON, OFF/ON/OFF and OFF/OFF/OFF can be displayed.
However, when a picture having many gradation levels is to be displayed with use of the FRC method, there may cause a flicker because an increased number of frames takes a long time to complete a display. Practically, the FRC method is combined with a spatial modulation method for shifting spatially phases to thereby avoid the occurrence of the flicker. However, the proposed method is considered to be difficult to obtain a picture having more than 16 gradation levels.
Another important problem in the FRC method resides in difficulty in applying it to a video display. For instance, in a display of dynamic picture, the display should be completed in a period in which a dynamic picture is changed. Accordingly, it is impossible to use many frames, and a display of many gradation levels is difficult.
For instance, when a frame frequency of 120 Hz (a generally used frequency, and the length of a frame is 8.3 ms) is used and a dynamic picture of 30 pictures per sec.(30 Hz) is to be displayed, it i
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Hirai Yoshinori
Kawaguchi Kazuyoshi
Kuwata Takeshi
Motegi Hiroyuki
Nagai Makoto
Asahi Glass Company Ltd.
Hjerpe Richard A.
Nguyen Francis N.
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