Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
2000-07-07
2004-05-25
Liang, Regina (Department: 2674)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S094000, C345S099000
Reexamination Certificate
active
06741229
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present Invention relates to a display device and a method for driving the same. More particularly, the present invention relates to a display device including a thin film transistor (hereinafter referred to as TFT) as a switching element for each pixel, such as an active matrix liquid crystal display device, and a method for driving the same.
2. Description of the Related Art
Conventionally, liquid crystal display (hereinafter referred to as LCD) devices have been widely used for televisions, graphic displays, etc. Of the LCD devices, an active matrix LCD device has an excellent display image without crosstalk between adjacent pixels even when the number of pixels is increased. For this reason, the active matrix LCD device has been widely used as a display for digital systems such as a computer.
Such an active matrix LCD device includes an LCD panel
10
and a driving circuit
356
, for example, as shown in FIG.
11
.
The LCD panel
10
includes a pair of electrode substrates having a liquid crystal material therebetween. A polarizer is attached on the outer surface of each electrode substrate. One of the electrode substrates is a TFT array substrate. The other electrode substrate is a counter substrate.
The TFT array substrate is made of a transparent, insulative substrate such as glass. On the TFT array substrate, a plurality of signal lines S(
1
), S(
2
), . . . , S(i), . . . , S(N), and a plurality of scanning lines G(
1
), G(
2
), . . . , G(j), . . . , G(M) are provided in a matrix. A switching element
102
, such as TFT, is provided at each intersection of a signal line
201
and a scanning line
301
. The switching element
102
is connected to a pixel electrode
103
. An alignment film is provided on substantially an entire surface of the TFT array substrate, covering those lines and elements.
The counter substrate is made of a transparent, insulative substrate such as glass, as is the TFT array substrate. A counter electrode
105
and an alignment film are successively provided on an entire surface of the counter substrate. A display cell (pixel)
1
is a portion of a liquid crystal layer sandwiched between the pixel electrode
103
and the counter electrode
105
. A matrix of such pixels is provided in the LCD panel
10
.
The driving circuit
356
includes a write circuit
250
and a timing control circuit
400
. The write circuit
250
includes a scanning line driving circuit
300
connected to the scanning lines
301
, a signal line driving circuit
200
connected to the signal lines
201
, and a counter electrode driving circuit (not shown) connected to the counter electrodes
105
. The timing control circuit
400
is connected to the signal line driving circuit
200
and the scanning line driving circuit
300
.
The scanning line driving circuit (gate driver)
300
, for example, includes a shift register and a select switch. The shift register includes M flip-flops in cascade connection. The select switch is switched in response to an output from each flip-flop. A gate scanning voltage Vgh, which is sufficient to switch the TFT
102
to the ON state, or a gate holding voltage Vg
1
, which is sufficient to switch the TFT
102
to the OFF state, is input to the scanning line driving circuit
300
. The voltage Vgh or Vg
1
is successively propagated through the flip-flops while being output from the respective select switches. In response to the voltage Vgh, the select switch outputs the voltage Vgh to the scanning line
301
in a scanning period of time (TH) to switch the TFT
102
to the ON state. In response to the voltage Vg
1
, the select switch outputs the voltage Vg
1
to the scanning line
301
to switch the TFT
102
to the OFF state. Timing of the output is controlled by the timing control circuit
400
.
Such an operation writes into a display cell (pixel)
1
a video signal output onto the TFT via the signal line
201
from the signal line driving circuit
200
.
In this way, the video signal is written into the pixel
1
via the TFT
102
in the scanning period of time (which is typically equal to a horizontal synchronization period, e.g., several tens of micro seconds). Thereafter, the voltage is held in the pixel
1
until a next write operation starts, i.e., a vertical synchronization period (a frame period). This allows the video signal to be displayed on the display device.
Recently, LCD devices have been commonly used for displaying not only still pictures but also moving pictures, owing to high-performance computers, etc. Further, large-size liquid crystal televisions have come into practice. Accordingly, high-quality display performance is required for the LCD devices.
Unfortunately, conventional LCD devices do not have satisfactory display performance for moving pictures.
For example, consider the following case. Referring to
FIG. 12A
, a white quadrangle is displayed in the black background, and the quadrangle is moved from the left to the right. In the conventional LCD devices. the contour of the moving quadrangle is blurred as shown in FIG.
12
B.
This is caused because the conventional LCD devices have a response time of as great as 50 ms. Such devices are not suitable for visual devices dealing mainly with moving pictures, since the moving pictures have unclear contours, resulting in poor picture quality.
Picture quality may be evaluated on the following two scales: (1) a transit response time which is a period of time during which a display changes from white to black or from black to white, i.e., a change in luminance from 10% to 90% or 90% to 10%; and (2) a human perceptive response time which is a period of time during which a human perceives a change in a luminance level from 0% to 100% or 100% to 0%. For display devices exhibiting moving pictures, although the transit response time (1) is conventionally used, the human perceptive response time has more important meaning. The reason is that even when the luminance level is changed from 10% to 90% in a short time, if it takes a long time to change 90% to 100%, the blurred contour of a moving picture in perceived as shown in FIG.
12
B.
For convenience of explanation, a response time which it takes for a human to perceive a change in a luminance level from 0% to 100% or 100% to 0% is defined as Td_LCD (black display to white display) or Tr_LCD (white display to black display), respectively. When Tr_LCD is not conditionally separated from Td_LCD, the change in a luminance level is defined as T_LCD (from black display to white display or from white display to black display). The response time of a display device which is required for non-blurred moving picture display is not strictly defined, since it significantly varies depending on the size of a moving picture and the background or among individuals. In the present invention, it is assumed that a moving picture response limit time Tmov is equal to about 20 ms. The moving picture response limit time Tmov is applied to the case of the luminance change from black display to white display as well an the case of the luminance change from white display to black display.
The response time of a liquid crystal material used in the above-described LCD device is defined on the following scales: (1) Tr_LC which is a period of time which is it takes a liquid crystal molecule to change the orientation toward the vertical direction due to an applied electric field; and (2) Td_LC which is a period of time which it takes a liquid crystal molecule to return to the original state due to an intermolecular force in the absence of an applied electric field. Tr_LC and Td_LC are given by
Tr
—
LC=&eegr;d
2
/{(|∈
p−∈s
|)
V−K&pgr;
2
} (1)
Td
—
LC=&eegr;d
2
/K&pgr;
2
(2)
where K=K
1
+(K
3
−2×K
2
)/4 where K
1
, K
2
, and K
3
are the divergent, torsional, and flexural elastic coefficients of the liquid crystal material, respectively; ∈s is the dielectric constant in the major axis direction of a
Kawaguchi Takafumi
Yanagi Toshihiro
Conlin David G.
Jensen Steven M.
Liang Regina
Sharp Kabushiki Kaisha
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