Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
2000-10-13
2003-09-02
Mengistu, Amare (Department: 2673)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S095000, C345S096000, C345S101000
Reexamination Certificate
active
06614416
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a driving method and driving device of a liquid crystal panel having a switching element, for example, such as a Thin Film Transistor (TFT), and in particular to the driving method and driving device of a liquid crystal panel which compensates for the shortage of charges supplied to respective pixels of the liquid crystal panel.
BACKGROUND OF THE INVENTION
In recent years, a liquid crystal display device adopting a TFT (hereinafter referred to as TFT-LCD) has been increased in size and become higher definition, and there is such a tendency that a demand for higher display quality of the TFT-LCD has been increased as the TFT-LCD is developed with multimedia.
In such a development of the TFT-LCD which has been increased in size and become higher definition, problems such as shortened time for writing a picture signal onto a pixel and a serious wiring delay due to respective buses have been noted.
Here, driving methods of the TFT-LCD can roughly be divided into two methods: line reversal driving and dot reversal driving.
The line reversal driving refers to driving which reverses a polarity of a signal line in one to several horizontal time (=about 16.7 mS
umber of scanning lines), which has the characteristic of resisting crosstalk in a vertical direction (crosstalk in the vertical direction occurs less often). In addition, since a voltage is applied to a liquid crystal by synthesizing a common electrode potential and a signal line potential, the line reversal driving has such an advantage that the voltage of not more than a threshold value of the liquid crystal can be obtained from the common electrode potential, and thereby an amplitude of the signal line potential is required to have as low a voltage as a dynamic range Vdy of the liquid crystal.
On the other hand, the dot reversal driving refers to driving which reverses the polarity of the signal line in one to several horizontal time (=about 16.7 mS
umber of scanning lines) while also reversing a polarity of an adjacent signal line, which has the characteristic of resisting crosstalk in both of the vertical and horizontal directions. Despite its superior display quality, however, this driving method requires that the common electrode potential be constant, and thus has such a drawback that the amplitude of the signal line potential needs to have a high voltage of 2×(Vth+Vdy), where Vth is the threshold value of the liquid crystal.
Meanwhile, the difference in amplitude of the signal line potentials in the line and dot reversal driving corresponds to difference in endurance of their source drivers, which results in difference in driver cost between the two driving methods. Therefore, when attempting to provide a low-cost and high-quality TFT-LCD, it is more effective to compose the TFT-LCD by using a driver which conducts the line reversal driving in which the signal line potential is more voltage-saving compared with the dot reversal driving.
However, in the line reversal driving, a charge supplying ability of a common electrode line which supplies respective pixels of the TFT-LCD with charges becomes deficient at early stages of writing. Here, the common electrode line is provided parallel to a scanning line which is provided on a pixel substrate, on a counter substrate opposite to the pixel substrate on which the TFT is provided.
FIG. 6
shows an equivalent circuit of the common electrode line. This equivalent circuit is, though having inputs at both ends, basically made up of an integration circuit (low-pass filter) of FIG.
7
. When a rectangular wave is inputted to this circuit, as shown in
FIG. 7
, an output waveform thereof grows dull due to a characteristic of the integration circuit, i.e. this means the shortage of the charge supplying ability at the early stages of writing. Note that, in
FIG. 6
, the waveform grows duller as it moves from the both ends toward the center.
When the charge supplying ability of the common electrode line becomes thus insufficient, crosstalk appears in the horizontal direction, i.e. the direction parallel to the scanning line. This crosstalk in the horizontal direction, as shown in
FIG. 8
, appears particularly when, for example, displaying a black square
52
at the center of a screen having a background of a half tone part
51
(part of the screen indicated by vertical hatching). More specifically, the sides of the black square
52
become whiter than the half tone part
51
.
Here, by forming a thick common electrode line for example, crosstalk in the horizontal direction can be settled on the side of a panel. However, this method results in reducing a numerical aperture, thereby being less preferable.
Accordingly, for example, Japanese Unexamined Patent Publication No. 191821/1992 (Tokukaihei 4-191821 published on Jul. 10, 1992) discloses a technique of reducing crosstalk in the horizontal direction without reducing the numerical aperture of the panel by incorporating a panel impedance which is connected to the common electrode of the TFT-LCD into a negative feedback circuit adopting an operational amplifier. The structure of the circuit according to the publication is shown in FIG.
9
.
In this driving circuit, an original input (signal inputted to the operational amplifier OP) is amplified by a value obtained by dividing a synthetic impedance which is composed of resistors R
1
and R
2
, a capacitor C, and a panel impedance of a liquid crystal panel
55
, by a resistor R
3
. Utilizing the characteristic of the negative feedback of the operational amplifier OP, a voltage which is determined in anticipation of a reduction in voltage inside of the panel is inputted to the common electrode line of the liquid crystal panel
55
, thereby reducing crosstalk in the horizontal direction. The original input, waveforms of the output and input of the liquid crystal panel
55
at that time are shown in FIGS.
10
(
a
) through
10
(
c
).
Note that, a resistor R
4
which is connected to a ground input of the operational amplifier OP is to determine an offset voltage of the counter electrode of the liquid crystal panel
55
. In addition, respective values (circuit constants) of the resistors R
1
, R
2
, R
3
and R
4
, and the capacitor C, etc., need to be optimized.
However, a problem arises when the charge supplying ability of the common electrode line is considerably low, i.e. the driving above cannot fully settle crosstalk in the horizontal direction. The reason is that, when the charge supplying ability of the common electrode line is considerably low, the degree of crosstalk in the horizontal direction becomes different between in the vicinity of the black square and in the vicinity of the input when the black square
52
appears in the middle of the screen having the background of the half tone part
51
.
Thus, as shown in
FIG. 11
, when, for example, the circuit constants of the negative feedback circuit are set so that crosstalk disappears in the vicinity of the input, crosstalk in the horizontal direction still remains in the vicinity of the black square. On the other hand, as shown in
FIG. 12
, when, for example, the circuit constants of the negative feedback circuit are set so that crosstalk disappears in the vicinity of the black square, the crosstalk in the horizontal direction in the vicinity of the input appears in a different form which is blacker than the background of the half tone part
51
. Note that, it has been known that even if, for example, the circuit constants of the negative feedback circuit are set so that crosstalk at an arbitrary position between the vicinities of the input and black square disappears, different types of crosstalk appear in the vicinities of the input and black square.
Note that, in
FIGS. 11 and 12
, hatching density (interval between vertical lines) shows the thickness of the half tone in qualitative terms. Namely, the denser the hatching is, the thicker the half tone is, while the sparser the hatching is, the thinner the half tone is.
The reason for thus resul
Fujiwara Koji
Ichioka Hideki
Inoue Naoto
Tanaka Keiichi
Yamamoto Tomohiko
Conlin David G.
Daley, Jr. William J.
Edwards & Angell LLP
Mengistu Amare
Nguyen Jimmy H
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