Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1997-12-11
2002-03-12
Brier, Jeffery (Department: 2672)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S094000, C345S096000, C345S690000
Reexamination Certificate
active
06356253
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an active-matrix display device using thin film transistors as pixel-driving switching devices and a method for driving the display devices, and in particular, to a technique for improving image quality by eliminating crosstalk (hereinafter referred to as “vertical crosstalk” if necessary) appearing in the vertical direction of a screen.
2. Description of the Related Art
The general structure of an active-matrix display device will be described with reference to FIG.
11
.
FIG. 11
consists of circuit diagrams showing two pixels extracted from the conventional active-matrix device. The active-matrix display device includes rows of gate lines X, columns of signal lines Y, and a matrix of liquid crystal pixels LC arranged in the region where the rows and the columns intersect. There are also formed thin film transistors Tr as switching devices for driving the pixels LC. The gate electrodes G of the thin film transistors Tr are connected to the corresponding gate lines X, and either the source electrodes S or the drain electrodes thereof are connected to the corresponding signal lines Y, with the other electrodes connected to the corresponding liquid-crystal pixels LC. In general, the pixels LC are driven by an alternating current. Thus, the polarity of a video signal to be written in each liquid-crystal pixel LC is inverted. Each drain electrode D and each source electrode S are alternately switched in accordance with this polarity inversion. Here, an electrode (H) having a high voltage is called a “drain electrode”, and an electrode (L) having a low voltage is called a “source electrode S”. A vertical scanning circuit (not shown) is connected to each gate line X. The vertical scanning circuit sequentially scans the gate lines X during one vertical period (
1
F), and selects one row of pixels LC every horizontal period (
1
H). In addition, a horizontal scanning circuit (not shown) is connected to each signal line Y. The horizontal scanning circuit samples a video signal Vsig for each signal line Y, and writes the video signal Vsig in the one row of pixels selected in one horizontal period.
The active-matrix display device has an inferior condition called vertical crosstalk. Thus, when the active-matrix display device is used in an apparatus such as a projector, generated image quality deteriorates, which is a problem to solve. As shown in
FIG. 11
, the vertical crosstalk is caused by the asymmetry of currents leaking from the thin film transistors Tr. In the condition shown in
FIG. 11
, left, the signal line Y is at level L, with the H-level signal written in the pixel LC. In this condition, a leakage current flowing when the gate electrode of the thin film transistor Tr is cut off is represented by I
off1
. In addition, in
FIG. 11
, right, the thin film transistor LC is maintained at level L, the H-level signal is applied to the signal line Y. In this condition, a leakage current flowing when the gate electrode of the thin film transistor Tr is cut off is represented by I
off2
. In general, I
off1
is larger than I
off2
because of the asymmetry of the thin film transistors Tr.
For example, as shown in
FIG. 12
, displaying a black window
30
in the center of a screen
20
generates vertical crosstalk in portions A, and the brightness of the portions A differs from normal portions B. A video signal Vsig to be written into each pixel LC is expressed by VsigC±&Dgr;V where VsigC±&Dgr;V represents a center potential, e.g., 6 volts; the symbol ± means that the video signal Vsig is inverted every horizontal period; and &Dgr;V represents a change of Vsig in reference to VsigC. When the maximum change is represented by &Dgr;V
(MAX)
, &Dgr;V
(MAX)
is, e.g., 4 volts. In normally white mode, VsigC±&Dgr;V
(MAX)
(=6±4 volts) is written in the black window
30
.
Thus, a voltage of 10 or 2 volts is applied to the liquid-crystal pixels LC included in the black window
30
. In addition, an intermediate-level video signal of 6±2 volts is written in the liquid-crystal pixels LC included in the background of the screen
20
excluding the black window
30
. Accordingly, the background is grey, and a voltage of 8 or 4 volts is applied to each pixel LC.
FIG. 13
shows that the potentials of the pixels LC included in the portions A and B shown in
FIG. 12
change during two vertical period (
2
F). During the change, the operating condition of the corresponding thin film transistors Tr chronologically changes. The periods of the change are represented by T
1
to T
4
. The operating condition of the thin film transistors Tr corresponding to the pixels LC included in the portions A changes as shown in periods T
1
, T
2
and T
1
in the initial one vertical period (
1
F), and changes as shown in periods T
3
, T
4
and T
3
in the subsequent one vertical period. The operating condition of the thin film transistors Tr corresponding to the pixels LC included in the portions B changes as shown in period T
1
in the initial one vertical period (
1
F), and changes as shown in period T
3
in the subsequent one vertical period.
FIG. 14
schematically shows the operating conditions of each thin film transistor Tr in periods T
1
to T
4
. In period T
1
, a voltage of 8 volts is applied to the corresponding pixel LC, and the potential of the signal line Y oscillates between 8 and 4 volts every horizontal period. The leakage current at this time flows in the direction of I
off1
. In addition, in period T
3
, the pixel is at 4 volts, and the potential of the signal line Y oscillates between 4 and 8 volts. The leakage current flowing at this time has a polarity identical to that of current I
off2
. The operating condition of the thin film transistors Tr included in portions B is alternately repeated between periods T
1
and T
3
every vertical period (
1
F). The pixel potential caused by the leakage current changes as represented by a dotted line shown in FIG.
13
. The operating condition of the thin film transistors included in portions A is basically similar. However, a video signal of 2 or 10 volts is written in the pixels included in the window
30
during periods T
2
or T
4
, which oscillates the signal line Y between 10 and 2 volts within the writing period. For example, during period T
2
, a voltage of 8 volts is applied to the pixels LC, which changes the potential of the signal line Y between 10 and 2 volts. The amounts of the leakage currents in periods T
1
and T
2
differ due to the asymmetry of the leakage currents. Accordingly, as shown in
FIG. 13
, the pixel potential slightly differs in portions A and B in period T
2
, which causes the vertical crosstalk. Similarly, in period T
4
, the potential of the pixel is maintained at 4 volts, while the potential of the signal line Y oscillates between 10 and 2 volts every horizontal period (
1
H). The leakage currents in the thin film transistors Tr differ in periods T
3
and T
4
, which generates the difference in the pixel potential in portions A and B during period T
4
. In particular, differently from period T
3
, period T
4
includes a condition where the signal line Y is at level L of 2 volts. Thus, the leakage current flowing increases, which causes portions A and B to have an extremely remarkable potential difference.
In addition, the active-matrix display device has a problem of having not only the above-described vertical crosstalk but also vertical fixed-pattern noise, which will be described by referring to FIG.
4
. An example of the active-matrix display device includes rows of gate lines X and columns of signal lines Y, a matrix of pixels LC arranged in the region where the gate lines X and the signal lines Y intersect, and thin film transistors Tr for driving the pixels LC. The active-matrix display device includes a vertical scanner
1
which sequentially scans each gate line X, and selects one row of pixels LC every horizontal period. The active-matrix display device includes a horizontal scanning ci
Maekawa Toshikazu
Uchino Katsuhide
Blackman Anthony
Brier Jeffery
Sonnenschein Nath & Rosenthal
Sony Corporation
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