Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
2000-03-03
2002-11-26
Shankar, Vijay (Department: 2673)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S100000
Reexamination Certificate
active
06486864
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display device for use in a television set, a computer, a word processor, an OA (Office Automation) apparatus, or the like, and a method for driving such a liquid crystal display device.
2. Description of the Related Art
FIG. 20
illustrates an equivalent circuit diagram of a typical active matrix type licquid crystal display device.
The liquid crystal display device includes a plurality of gate electrodes
61
and a plurality of source electrodes
62
crossing the gate electrodes
61
. A switching element
63
, e.g., a thin film transistor, is provided in the vicinity of each intersection of the gate electrode
61
and the source electrode
62
. The display area is divided into a plurality of pixel regions arranged in a matrix, which are partitioned from one another by the gate electrodes
61
and the source electrodes
62
. A pixel electrode
64
is provided in each of the pixel regions. The pixel electrode
64
is connected to the source electrode
62
via the switching element
63
. A liquid crystal layer (not shown) is interposed between the pixel electrode
64
and a common electrode
65
. A liquid crystal capacitor
66
is provided between the pixel electrode
64
and the common electrode
65
. An image is displayed by holding an intended voltage in each liquid crystal capacitor
66
.
FIG. 21
illustrates typical voltage waveforms used for driving such an active matrix type liquid crystal display device.
A pulse waveform of ON voltage Vgh for turning ON the switching element
63
is applied as a gate voltage Vg to the gate electrodes
61
for each field period. A plurality of such pulses are sequentially applied to scan the entire frame. A voltage corresponding to an image signal for the row for which the ON voltage Vgh is being input to the gate electrode
61
is applied as a source voltage Vs to the source electrode
62
. A plurality of such image signals are applied sequentially. As a common voltage Vc, ±Vcac is applied to the common electrode
65
. Each time the ON voltage Vgh is applied to the gate electrode
61
, a new source voltage Vs is applied to the pixel electrode
64
as a pixel voltage Vp. As a result, a liquid crystal voltage Vlc, which is equal to the difference between the pixel voltage Vp and the common voltage Vc, is applied through the liquid crystal layer.
FIG. 22
illustrates the relationship between the liquid crystal voltage Vlc and the transmission.
FIG. 22
shows an example for a TN (twisted nematic) type normally white mode which has been used in the art as a typical liquid crystal display mode.
The output range ±Vs
max
of the source voltage Vs is normally set to the voltage difference between a voltage Vlc
a
at which the 100% transmission is obtained and a voltage Vlc
c
at which about 1% transmission is obtained. Thus, the output range of the source voltage Vs is set to a minimum range required to obtain a practically sufficient contrast. When the output range of the source voltage Vs is set to be higher than this, a source driver having a high voltage resistance is required, thereby increasing the cost of the device. Therefore, the source voltage Vs and the common voltage Vc with respect to the liquid crystal voltage Vlc are set as shown in the following Expression 1.
Vlc
a
=┌+Vs
max
±Vcac|
Vlc
b
=└+Vcac|
Vlc
c
=└+Vs
max
±Vcac|
Expression 1
FIG. 23
illustrates transmission response characteristics of the liquid crystal panel. In
FIG. 23
, a solid line shows the change in the transmission obtained when the image signal is changed from white display→black display→white display, and a broken line shows the change in the transmission obtained when the image signal is changed from white display→gray-level display→white display.
FIG. 23
shows that when the image signal is changed from white display→black display→white display, each transmission response is substantially completed within one field period. However, when the image signal is changed from white display→gray-level display→white display, the transmission does not change to a transmission corresponding to the image signal within one field period. Thus, between two image signals where the voltage difference is small, the transmission response may be slow.
Liquid crystal display devices have been wide spread as thin display devices as the image qualities thereof, e.g., the contrast, the brightness, and the color reproducibility, have been improved and are now comparable to those of other types of display devices such as CRTs. However, liquid crystal display devices have a relatively slow response as described above. Therefore, when displaying a motion picture on a liquid crystal display device, the displayed picture may be blurred or the after-image phenomenon may occur. This drawback has prevented liquid crystal display devices from replacing CRTs in some applications.
Japanese Laid-Open Publication No. 56-27198 discloses a display device which produces a color display by using a black and white liquid crystal panel in combination with a light source whose output color is switched among red, blue and green. This is called a “field sequential color method”.
In this method, the output color of the light source is switched among red, blue and green while an image corresponding to each output color is synchronously displayed. Therefore, when displaying a color image, the image signal changes for each display operation even if the image is stationary. Thus, when the response of the liquid crystal panel is slow, the color information for one display operation and the color information for the next display operation may be mixed together, thereby reducing the color reproducibility. In such a case, it is difficult to realize a sufficient display performance.
As described above, the slow response of the liquid crystal panel has been a drawback which deteriorates the display performance. In order to address such a drawback, various methods have been proposed in the art as follows.
For example, Japanese Laid-Open Publication No. 4-42211 proposes a method in which a voltage corresponding to an assist signal, which is different from an image signal, is applied before applying a voltage corresponding to the image signal. This method is based on the fact that the effective liquid crystal response speed (i.e., the speed of the response of the liquid crystal molecules to an applied voltage) can be increased by applying a voltage which is larger or lower than the voltage corresponding to the image signal through the liquid crystal layer before applying the voltage corresponding to the image signal therethrough. This method improves the motion picture display quality.
“SID 98 DIGEST P. 143 A Novel Wide-Viewing Angle Motion-Picture LCD” proposes another method in which a voltage corresponding to an assist signal is applied before applying a voltage corresponding to an image signal. This method improves the motion picture display quality by erasing the displayed image before displaying the next image.
Japanese Laid-Open Publication No. 9-138421 proposes a driving method in which a voltage corresponding to an assist signal is applied by activating all scanning lines and then providing the assist signal by varying the common voltage at each common electrode before applying the voltage corresponding to the image signal.
It is possible to prevent the color reproducibility from being reduced by applying the above-described improvements to the field sequential color method.
As described above, it is possible to increase the liquid crystal response by providing a period for the application through the liquid crystal panel of a voltage corresponding to an assist signal, which is not an image signal, before the period for the application of a voltage corresponding to the image signal. In order to effectively improve the liquid crystal response, it is import
Miyachi Koichi
Nakajima Mutsumi
Conlin David G.
Dike Bronstein Roberts & Cushman
Patel Nitin
Shankar Vijay
Sharp Kabushiki Kaisha
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