Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1999-10-13
2003-05-06
Shankar, Vijay (Department: 2673)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S087000, C345S094000
Reexamination Certificate
active
06559823
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of driving liquid crystals and a display apparatus therefor, and in particular to a driving method of displaying STN (Super Twisted Nematic) liquid crystals with high contrast and a display apparatus therefor.
2. Description of the Related Art
As a conventional driving method of a liquid crystal display apparatus having a matrix structure, there is known a technique described in “Ultimate Limits for Matrix Addressing of RMS-Responding Liquid-Crystal Displays,” IEEE Transactions on Electron Devices, Vol. ED-26, No. 5, May 1979 (pp. 795-802) and “Active Addressing Method for High-Contrast Video-Rate STN Displays,” SID 92 DIGEST, pp. 228-231. According to this technique, each row electrode is provided with a voltage depending upon an orthogonal function, whereas each column electrode is provided with a voltage depending upon a function obtained as a sum of products of every display information of that column and a function of the scanning side. The driving method will hereafter be described in detail by referring to
FIGS. 1
to
4
.
FIG. 1
shows the structure of a liquid crystal display panel having a matrix structure consisting of N rows by M columns. An intersection of a row electrode and a column electrode forms a dot D(i,j). A voltage represented by a function f(i) (i=1, 2, . . . N) is supplied to each of the N row electrodes. A voltage represented by a function g(j) (j=1, 2, . . . M) is supplied to each of the M column electrodes. U(i,j) denotes a voltage supplied to the dot D(i,j). The voltage U(i,j) is a difference between values of the voltage functions f(i) and g(j). In the ensuing description, voltage is normalized.
FIG. 2
is a diagram showing an example of orthogonal function voltages supplied to row electrodes to drive STN liquid crystal displays. This example is generally used at the present time. Assuming now that the function f(i) is represented by
FIG. 2
, the functions f(i) and g(j) can be represented by equations (1) and (2), respectively.
f
(
i
)=
FP·&dgr;
(
i,t
) (1)
g
(
j
)
=
1
N
∑
i
=
1
N
P
(
i
,
j
)
f
(
i
)
(
2
)
In the equations (1) and (2), &dgr;(i,t) is 1 for i=t and 0 for i≠t. FP is a constant given by the following equation (3).
FP
=
N
N
2
(
N
-
1
)
(
3
)
P(i,j) denotes display information of the dot D(i,j). P(i,j) is −1 for a display-on state and 1 for a display-off state. By using equations (1), (2) and (3), the effective voltage U
rms
(i,j) applied to the dot D(i,j) at this time can be represented by the following equation (4).
U
rms
(
i
,
j
)
=
A2
=
[
1
T
∫
0
T
f
(
i
)
2
ⅆ
t
+
1
T
∫
0
T
g
(
j
)
2
2
T
∫
0
T
f
(
i
)
g
(
j
)
ⅆ
t
]
1
/
2
(
4
)
Letting T=N and rewriting (4) gives
1
T
∫
0
T
f
(
i
)
2
ⅆ
t
=
1
N
∑
t
=
1
N
(
FP
·
δ
(
i
,
t
)
)
2
=
N
2
(
N
-
1
)
(
5
)
1
T
∫
0
T
g
(
j
)
2
=
1
N
∑
t
=
1
N
(
1
N
∑
i
=
1
N
P
(
i
,
j
)
f
(
i
)
)
2
=
1
N
∑
t
=
1
N
[
1
N
∑
i
=
1
N
P
(
i
,
j
)
N
N
2
(
N
-
1
)
·
δ
(
i
,
t
)
]
2
=
1
N
·
1
N
·
N
N
2
(
N
-
1
)
∑
t
=
1
N
[
∑
t
=
1
N
P
(
i
,
j
)
δ
(
i
,
t
)
]
2
=
1
N
N
2
(
N
-
1
)
·
N
=
N
2
(
N
-
1
)
(
6
)
2
T
∫
0
T
f
(
i
)
g
(
j
)
ⅆ
t
=
2
N
∑
t
=
1
N
f
(
i
)
∑
i
=
1
N
1
N
P
(
i
,
j
)
f
(
i
)
=
2
N
∑
t
=
1
N
N
N
2
(
N
-
1
)
·
&AutoLeftMatch;
δ
(
i
,
t
)
∑
i
=
1
N
1
N
P
(
i
,
j
)
N
N
2
(
N
-
1
)
·
δ
(
i
,
t
)
=
2
N
N
·
N
N
2
(
N
-
1
)
·
∑
t
=
1
N
δ
(
i
,
t
)
∑
i
=
1
N
P
(
i
,
j
)
δ
(
i
,
t
)
=
2
2
(
N
-
1
)
·
P
(
i
,
j
)
(
7
)
From equations (5), (6) and (7), therefore, the effective voltage U
rms
(i,j) can be written as
U
rms
(
i
,
j
)
=
[
N
2
(
N
-
1
)
+
N
2
(
N
-
1
)
-
2
2
(
N
-
1
)
P
(
i
,
j
)
]
1
/
2
=
[
2
N
2
(
N
-
1
)
-
2
P
(
i
,
j
)
2
(
N
-
1
)
]
1
/
2
(
8
)
Assuming that the dot D(i,j) is in the display-on state, P(i,j)=−1 and the effective voltage U
rms
(i,j) is represented by equation (9). Assuming that the dot D(i,j) is in the display-off state, P(i,j)=1 and the effective voltage U
rms
(i,j) is represented by equation (10).
U
rms
(
i
,
j
)
=
[
2
N
2
(
N
-
1
)
-
-
2
2
(
N
-
1
)
]
1
2
=
[
N
+
1
N
-
1
]
1
2
(
9
)
U
rms
(
i
,
j
)
=
[
2
N
2
(
N
-
1
)
-
2
2
(
N
-
1
)
]
1
2
=
1
(
10
)
The voltage applied to the dot D(i,j) is (f(i)−g(j)) and has a waveform as shown in
FIG. 3
on the basis of equations (1) and (2). In
FIG. 3
, S
1
, S
2
and S
3
are represented by the following equations.
S1
=
N
N
2
(
N
-
1
)
+
N
2
(
N
-
1
)
(
11
)
(
When
D
(
i
,
j
)
=
display
on
)
N
N
2
(
N
-
1
)
-
N
2
(
N
-
1
)
(
12
)
(
When
D
(
i
,
j
)
=
display
off
)
S2
=
N
2
(
N
-
1
)
(
13
)
S3
=
-
N
2
(
N
-
1
)
(
14
)
Assuming now that N=240, we get S
1
=12.1 (when D(i,j)=display on), S
1
=10.6 (when D(i,j)=display off), S
2
=0.73, and S
3
=−0.73. As a result, a large voltage is applied once (i=t) during one frame (i.e., a period of t=1 to N) and a low voltage is applied during the remaining intervals. In fast responding STN liquid crystal displays, the display luminance lowers while this low voltage is being applied.
As a driving method for avoiding this, a method described below has been proposed.
FIG. 4
shows orthogonal functions called Walsh functions. In the example shown in
FIG. 4
, the number of divisions (time intervals) of the Walsh functions is 8. Assuming now that Walsh functions with the number of divisions being equivalent to T are used as the function f(i) of the voltage applied to row electrodes of the liquid crystal display panel of
FIG. 1 and N
Walsh functions are selected out of T Walsh functions (T≧N) and used as the function f(i), the effective voltage value U
rms
(i,j) of the dot D(i,j) will be is derived.
It is assumed that the functions f(i) and g(j) are represented by the following equations (15) and (16).
f
(
i
)=
FP·W
(
i,t
) (15)
g
(
j
)
=
1
N
∑
i
=
1
N
P
(
i
,
j
)
f
(
i
)
(
16
)
In these equations, W(i,t) is a Walsh function and has a value of 1 or 31 1. FP is a constant indicated by equation (17).
FP
=
N
2
(
N
-
1
)
(
17
)
In equation (4),
1
T
∫
0
T
f
(
i
)
2
ⅆ
t
=
1
T
∑
t
=
1
T
(
FP
·
W
(
i
,
t
)
)
2
=
1
T
{
FP
2
W
(
i
,
1
)
2
+
FP
2
W
(
i
,
2
)
2
+
…
+
FP
2
W
(
i
,
T
)
2
}
=
1
T
·
FP
2
·
T
(
±
1
)
2
=
FP
2
=
N
2
(
N
-
1
)
(
18
)
1
T
∫
0
T
g
(
j
)
2
ⅆ
t
=
1
T
∑
Kitajima Masaaki
Mano Hiroyuki
Nishitani Shigeyuki
Tanaka Toshio
Antonelli Terry Stout & Kraus LLP
Hitachi , Ltd.
Shankar Vijay
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