Television – Special applications – Manufacturing
Reexamination Certificate
2001-12-05
2004-07-06
Chow, Dennis-Doon (Department: 2675)
Television
Special applications
Manufacturing
C345S213000, C345S215000
Reexamination Certificate
active
06760059
ABSTRACT:
This application claims the benefit of Korean Application No. P2001-56235 filed on Sep. 12, 2001, which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly, to a method and apparatus of driving a liquid crystal display. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for improving a picture quality.
2. Discussion of the Related Art
Generally, a liquid crystal display (LCD) controls a light transmittance of each liquid crystal cell in accordance with a video signal, thereby displaying a picture. An active matrix LCD including a switching device for each liquid crystal cell is suitable for displaying a moving picture. The active matrix LCD uses a thin film transistor (TFT) as a switching device.
The LCD has a disadvantage in that it has a slow response time due to inherent characteristics of a liquid crystal, such as a viscosity and an elasticity, etc. Such characteristics can be explained by using the following equations (1) and (2):
&pgr;
r
∝&ggr;d
2
/&Dgr;&egr;|V
a
2
−V
F
2
| (1)
where &pgr;
r
represents a rising time when a voltage is applied to a liquid crystal, V
a
is an applied voltage, V
F
represents a Freederick transition voltage at which liquid crystal molecules begin to perform an inclined motion, d is a cell gap of liquid crystal cells, and &ggr; represents a rotational viscosity of the liquid crystal molecules.
&pgr;
f
∝&ggr;d
2
/K
(2)
where &pgr;
f
represents a falling time at which a liquid crystal is returned into the initial position by an elastic restoring force after a voltage applied to the liquid crystal was turned off, and K is an inherent elastic constant of a liquid crystal.
A twisted nematic (TN) mode liquid crystal has a different response time due to physical characteristics of the liquid crystal and a cell gap, etc. Typically, the TN mode liquid crystal has a rising time of 20 to 80 ms and a falling time of 20 to 30 ms. Since such a liquid crystal has a response time longer than one frame interval (i.e., 16.67 ms in the case of NTSC system) of a moving picture, a voltage charged in the liquid crystal cell is progressed into the next frame prior to arriving at a target voltage. Thus, due to a motion-blurring phenomenon, a moving picture is blurred out on the screen.
Referring to
FIG. 1
, the conventional LCD cannot express desired color and brightness. Upon implementation of a moving picture, a display brightness BL fails to arrive at a target brightness corresponding to a change of the video data VD from one level to another level due to its slow response time. Accordingly, a motion-blurring phenomenon appears from the moving picture and a display quality is deteriorated in the LCD due to a reduction in a contrast ratio.
In order to overcome such a slow response time of the LCD, U.S. Pat. No. 5,495,265 and PCT International Publication No. WO99/05567 have suggested to modulate data in accordance with a difference in the data using a look-up table (hereinafter referred to as high-speed driving scheme). This high-speed driving scheme allows data to be modulated by a principle as shown in FIG.
2
.
Referring to
FIG. 2
, a conventional high-speed driving scheme modulates input data VD and applies the modulated data MVD to the liquid crystal cell, thereby obtaining a desired brightness MBL. In the high-speed driving scheme, |V
a
2
−V
F
2
| is increased from the above equation (1) on the basis of a difference of the data so that a desired brightness can be obtained in response to a brightness value of the input data within one frame interval, thereby rapidly reducing a response time of the liquid crystal. Accordingly, the LCD employing such a high-speed driving scheme compensates for a slow response time of the liquid crystal by modulating a data value in order to alleviate a motion-blurring phenomenon in a moving picture, thereby displaying a picture at desired color and brightness.
In other words, the high-speed driving scheme compares most significant bits MSB of the previous frame Fn−1 with those of the current frame Fn. If there is a change in the most significant bits, the corresponding modulated data Mdata are selected from the look-up table to modulate the data as shown in FIG.
3
. The high-speed driving scheme modulates only several most significant bits to reduce a memory size upon implementation of hardware equipment. A high-speed driving apparatus implemented in this manner is as shown in FIG.
4
.
Referring to
FIG. 4
, a conventional high-speed driving apparatus includes a frame memory
43
connected to a most significant bit bus line
42
and a look-up table
44
commonly connected to the most significant bit bus line
32
and an output terminal of the frame memory
43
.
The frame memory
43
stores most significant bit data MSB during one frame interval and supplies the stored data to the look-up table
44
. Herein, the most significant bit data MSB may be the most significant 4 bits of the 8-bit source data RGB.
The look-up table
44
compares most significant bits MSB of a current frame Fn inputted from the most significant bit line
42
with those of the previous frame Fn−1 inputted from the frame memory
43
as shown in Table 1 or Table 2, and selects the corresponding modulated data Mdata. The modulated data Mdata are added to least significant bits LSB from a least significant bit bus line
41
.
TABLE 1
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
0
0
2
3
4
5
6
7
9
10
12
13
14
15
15
15
15
1
0
1
3
4
5
6
7
8
10
12
13
14
15
15
15
15
2
0
0
2
4
5
6
7
9
10
12
13
14
15
15
15
15
3
0
0
1
3
5
6
7
8
10
11
13
14
15
15
15
15
4
0
0
1
2
4
6
7
8
9
11
12
13
14
15
15
15
5
0
0
1
2
3
5
7
8
9
11
12
13
14
15
15
15
6
0
0
1
2
3
4
6
8
9
10
12
13
14
15
15
15
7
0
0
1
2
3
4
5
7
9
10
11
13
14
15
15
15
8
0
0
1
2
3
4
5
6
8
10
11
12
13
15
15
15
9
0
0
1
2
3
4
5
6
7
9
11
12
13
14
15
15
10
0
0
1
2
3
4
5
6
7
8
13
12
13
14
15
15
11
0
0
1
2
3
4
5
6
7
8
9
11
12
14
15
15
12
0
0
1
2
3
4
5
6
7
8
9
10
12
14
15
15
13
0
0
1
2
3
3
4
5
6
7
8
10
11
13
15
15
14
0
0
1
2
3
3
4
5
6
7
8
9
11
12
14
15
15
0
0
0
1
2
3
3
4
5
6
7
8
9
11
13
15
TABLE 2
0
16
32
48
64
80
96
112
128
144
160
176
192
208
224
240
0
0
32
48
64
80
96
112
144
160
192
208
224
240
240
240
240
16
0
16
48
64
80
96
112
128
160
192
208
224
240
240
240
240
32
0
0
32
64
80
96
112
128
160
192
208
224
240
240
240
240
48
0
0
16
48
80
96
112
128
160
176
208
224
240
240
240
240
64
0
0
16
48
64
96
112
128
144
176
192
208
224
240
240
240
80
0
0
16
32
48
80
112
128
144
176
192
208
224
240
240
240
96
0
0
16
32
48
64
96
128
144
160
192
208
224
240
240
240
112
0
0
16
32
48
64
80
112
144
160
176
208
224
240
240
240
128
0
0
16
32
48
64
80
96
128
160
176
192
224
240
240
240
144
0
0
16
32
48
64
80
96
112
144
176
192
208
224
240
240
160
0
0
16
32
48
64
80
96
112
128
160
192
208
224
240
240
176
0
0
16
32
48
64
80
96
112
128
144
176
208
224
240
240
192
0
0
16
32
48
64
80
96
112
128
144
160
192
224
240
240
208
0
0
16
32
48
48
64
80
96
112
128
160
176
208
240
240
224
0
0
16
32
48
48
64
80
96
112
128
144
176
192
224
240
240
0
0
0
16
32
48
48
64
80
96
112
128
144
176
208
240
In the above tables, a left column is for a data voltage VDn−1 of the previous frame Fn−1 while an uppermost row is for a data voltage VDn of the current frame Fn. Table 1 is a look-up table information in which the most significant bits (i.e., 2
0
, 2
1
, 2
2
and 2
3
) are expressed by the decimal number format. Table 2 is a look-up table information in which weighting values (i.e., 2
4
2
5
, 2
6
and 2
7
) of the most significant 4 bits are applied to 8-bit data.
However, the conventional high-speed driving scheme is problematic. Since it has been studied on the assumption that a driving frequency of the data is fixed like a television, the conventional scheme is difficult to be applied in a frequency-variable display device which receives different driving frequencies such as a computer monitor. More
Chow Dennis-Doon
Morgan & Lewis & Bockius, LLP
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