Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
2002-06-12
2003-05-27
Shankar, Vijay (Department: 2673)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S095000, C345S100000
Reexamination Certificate
active
06570551
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method of driving a liquid crystal display device of an opposing signal line structure in which active three-terminal elements, each of which having a gate electrode connected to a scanning line, a drain electrode connected to a pixel electrode and a source electrode connected to a reference line, are arranged on a first substrate, signal lines are arranged on a second substrate facing the first substrate, and an electric field is applied to a liquid crystal layer between the pixel electrodes and the second substrate.
BACKGROUND OF THE INVENTION
In recent years, a liquid crystal panel has been often used as a display element of a word processor, personal computer, television set, etc. In order to produce such a liquid crystal panel, first, a number of films of metals, semiconductors or the like are formed on a light transmitting substrate such as glass. These films are patterned in a desired design by a photolithography technique to form two pieces of electrode substrates. The electrode substrates are then disposed to face each other and fastened with a predetermined space therebetween, and a liquid crystal is sealed in the space to provide the liquid crystal panel.
FIG. 11
shows a structure of a liquid crystal display device incorporating a generally used TFT (thin film transistor). Scanning lines
71
, signal lines
72
, TFTs
74
and pixel electrodes
75
are formed on a single glass substrate (first substrate). Moreover, as shown by an alternate lone and two sort dashes line, a common electrode
76
common to all pixels is formed on a surface of a glass substrate (second substrate, not shown), which surface faces the first substrate. The second substrate is disposed to face the first substrate with a liquid crystal layer (not shown) therebetween. Additionally, auxiliary capacitors CS (not shown) and reference lines (not shown) may be formed on the glass substrate (first substrate) having thereon the TFTs
74
.
Regarding a driving method for providing a high-quality image with a liquid crystal display device of such a structure, for example, see “Drive System for TFT-LCDs Using Digital Drivers having Gray-Scale Interpolative Function, Hisao Okada, the Journal of the Institute of Image Information and Television Engineers, Vol.51, No.10, pp.1768-1776(1997), published October, 1997.
According to this reference, when a TFT is in an ON state, an equivalent circuit of a single pixel of a liquid crystal display device of the above-mentioned structure is as shown in FIG.
12
(
a
). On the other hand, when a TFT is in an OFF state, the equivalent circuit is as shown in FIG.
12
(
b
).
When the TFT changes from the ON state to the OFF state, the voltage of the pixel electrode is lowered due to the effect of a transition of a gate voltage through a gate-drain parasitic capacitance Cg. Such a change of the electric potential of the pixel electrode causes the apparent non-symmetry of the transmissivity of liquid crystal with respect to positive and negative drive voltages. Thus, a high-quality image display is prevented.
Therefore, in order to display a high-quality image on the liquid crystal display device, the above reference discloses conditions to be satisfied by the drive voltages of the scanning lines, signal lines and common electrode. More specifically, the conditions include that the average of the common electrode drive voltage is lower than the average voltage of the signal line drive voltages by a predetermined amount &Dgr;V, and the average voltage of the signal line drive voltages is increased with a decrease in the absolute value of a voltage to be applied to the liquid crystal (liquid crystal applied voltage), i.e., a decrease of the relative voltage difference between the signal line drive voltage and the common electrode drive voltage. The apparent non-symmetry of the transmissivity of the liquid crystal with respect to the positive and negative voltages are compensated by satisfying these conditions.
FIGS. 13 and 14
are given to explain the above contents. First,
FIG. 14
shows the relationship between a common electrode drive voltage (common voltage) Vcom and a signal line drive voltage (gray-scale voltage) V
0
. In
FIG. 14
, V
0
A is a maximum value of V
0
, while V
0
B is a minimum value of V
0
. VcomH is a maximum value of Vcom, while VcomL is a minimum value of Vcom. As shown in
FIG. 14
, the average voltage of the common electrode drive voltage Vcom is lower than the average voltage of the signal line drive voltage V
0
by &Dgr;V (&Dgr;V>0).
Further,
FIG. 13
shows the relationship between the common electrode drive voltage Vcom and four signal line drive voltages (V
0
, V
2
, V
5
and V
7
) in respect of the phases and &Dgr;V, in accordance with the contents of the above reference. As shown in
FIG. 13
, the phases of V
0
and V
2
are inverted with respect to the phase of Vcom, while the phases of V
5
and V
7
are the same as the phase of Vcom. When Vcom is VcomL, among the whole signal line drive voltages, V
0
applies a voltage V
0
A to the liquid crystal, while V
7
applies a voltage V
7
A to the liquid crystal. V
2
and V
5
apply voltages (V
2
A, V
5
A) between V
0
A and V
7
A to the liquid crystal. Furthermore, when Vcom is VcomH, among the whole signal line drive voltages, V
0
applies a voltage V
0
B to the liquid crystal, while V
7
applies a voltage V
7
B to the liquid crystal. V
2
and V
5
apply voltages (V
2
B, V
5
B) between V
0
B and V
7
B to the liquid crystal.
Here, a gray-scale number is represented by n (n=0, 1, 2, . . . , 7), a liquid crystal applied voltage VLC is given by |Vn−Vcom|. For instance, V
0
A−VcomL. It is clear from
FIG. 13
that the larger the gray-scale number n, the lower the liquid crystal applied voltage VLC.
Moreover, a curved line C
1
in
FIG. 13
connects the averages of the respective signal line drive voltages. Furthermore,
FIG. 13
shows the average of Vcom by a straight light C
2
. It can be understood from the curved line C
1
which rises toward the right that the greater the gray-scale number n, the higher the average of the signal line drive voltages and the larger the difference between the average of the signal line drive voltages and the average of Vcom.
Here, one reason why the results shown in
FIGS. 13 and 14
are obtained is that the liquid crystal applied voltage VLC becomes lower as the gray-scale number n is increased, and consequently the amount of lowering of the voltage of the pixel electrode is increased. In other words, as the liquid crystal applied voltage VLC is lowered, the difference between the average of the positive and negative voltages of the liquid crystal applied voltage VLC and the average of Vcom as a reference is increased. Therefore, in order to minimize this difference, the technique disclosed in the above reference sets Vcom and the respective signal line drive voltages so that &Dgr;V is increased in accordance with the difference More specifically, Vcom is set for each signal line drive voltage so that the average of Vcom is lower than the average of each signal line drive voltage by just an amount of &Dgr;V.
FIG. 15
shows a structure of a basic circuit corresponding to one output of a 3-bit digital driver (this circuit will be hereinafter referred to as a “unit drive circuit”). Data to be displayed is fetched in a sampling memory Msmp by a sampling pulse Tsmp, and then transferred to a holding memory MH by an output pulse LP. Next, the data stored in the holding memory MH is decoded in a decoder DEC. Then, an analog switch (ASW
0
, ASW
1
, . . . , or ASW
7
) corresponding to the value of the data is turned on, and the data is converted into a corresponding voltage and output as a signal line drive voltage (V
0
, V
1
, . . . , or V
7
). For instance, when the value of data is 0, the analog switch ASW
0
is turned on, and the signal line drive voltage V
0
supplied from an external device of the 3-bit digital driver is output to a corresponding signal line of the liquid crysta
Fujiwara Koji
Ichioka Hideki
Inoue Naoto
Tanaka Keiichi
Yamamoto Tomohiko
Conlin David G.
Daley, Jr. William J.
Edwards & Angell LLP
Said Mansour M.
Shankar Vijay
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