Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1997-10-24
2001-01-16
Mengistu, Amare (Department: 2778)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
Reexamination Certificate
active
06175351
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image display apparatus of an active driving system and a method for driving the same. More specifically, the present invention relates to an image display apparatus where the scanning is effected by a field sequential scanning system and a method for driving the same.
2. Description of the Related Art
In recent years, brighter and higher quality image displays have been requested for various types of image display apparatuses such as a liquid crystal display (hereinafter, referred to as an LCD) apparatus. At the same time, with the increasing demand for portable information apparatuses, smaller and lighter image display apparatuses have been required.
Before describing conventional image display apparatuses, the terms “field” and “frame” as used herein are clarified as follows.
Frame: An image corresponding to the entire screen of an image display apparatus
Field: a component of the “frame”
In linear sequential scanning, display image data for one field is output by scanning a screen once from the top to the bottom thereof. One field period is defined as that period of time starting from when the scanning is initiated at the top end of the screen until the scanning returns to the top end again after the completion of the scanning at the bottom end of the screen. When interlaced scanning is used, one frame image is obtained by two fields. When non-interlaced scanning is used, one frame image is obtained by one field.
With the above definition, when an input interlaced signal is divided into three signals, red (R), green (G), and blue (B), and scannings are separately conducted for these signals sequentially, six fields are required to complete one frame image. Hereinafter, the field and the frame as used for normal TV signals of the NTSC system and the like are described. Such a field and a frame are hereinafter referred to as a TV field and a TV frame. When interlaced, one TV frame is obtained by two TV fields, completing one picture. One TV field is approximately 16.7 msec (60 Hz) in the case of an NTSC (M) signal, while it is 20 msec (50 Hz) in the case of a PAL·SECAM (B/G/D/K/I/L) signal.
A conventional active matrix LCD apparatus driven by the field sequential scanning system is shown in FIG.
23
. This type of display apparatus is described, for example, in Japanese Laid-Open Patent Publication No. 64-5282. A display apparatus
18
shown in
FIG. 23
is a color LCD apparatus. In the display apparatus
18
, color video signals are supplied to data input terminals
10
R,
10
G, and
10
B as red (R), green (G), and blue (B) image data, respectively. The display apparatus
18
includes A/D converters
20
for converting analog signals supplied as image data into digital signals, a memory
21
composed of a plurality of memory elements for storing data, a memory selection circuit
22
, D/A converters
23
for converting the digital signals to analog signals for display, a data transfer circuit
24
, a data scanning circuit
25
, a control circuit
26
for controlling various circuits, and a pixel display portion
27
. Two memory elements of the memory
21
are allocated for each color so as to complete a double frame memory for storing the image data of one frame and that of the next frame.
The A/D converters
20
and the D/A converters
23
are implemented by A/D conversion ICs (integrated circuits) and D/A conversion ICs, respectively. The memory
21
is implemented by memory elements such as DRAMs (Dynamic Random Access Memories), SRAMs (Static Random Access Memories), ROMs (Read Only Memories), and the like. Also, the data transfer circuit
24
and the data scanning circuit
25
are generally implemented by ICs called a source driver and a gate driver, respectively.
The display apparatus
18
effects color display by dividing the cycle of input image data (a vertical scanning period) into three equal portions. In other words, each of the R, G, and B signals stored in the memory
21
is selectively output from the memory selection circuit
22
one at a time at a cycle one-third that of the input image data. Therefore, the frequency of the signals for display is three times that of the input data signals.
FIG. 24
is an equivalent circuit of a pixel of the conventional display apparatus
18
. The display apparatus
18
includes pixels arranged in a matrix, and signal lines
5
and scanning lines
6
for supplying data signals and scanning signals to the pixels, respectively. Each of the pixels includes a driving element (switching element)
3
composed of a thin film transistor (TFT), a liquid crystal (LC) capacitance
1
(capacitance value C
p
), and an auxiliary capacitance
8
(capacitance value C
s
). A signal line
5
is connected to one of electrodes of the LC capacitance
1
through drain/source terminals of the TFT
3
, and a scanning line
6
is connected to a gate terminal of the TFT
3
.
In such a pixel, the LC capacitance
1
is composed of two electrodes and liquid crystal sandwiched by the two electrodes. One of the electrodes (pixel electrode) is connected to the driving element
3
, while the other electrode (common electrode) is connected to a common electric source line
7
. The scanning line
6
is connected to the data scanning circuit
25
(see
FIG. 23
) which sequentially outputs scanning signals. The signal line
5
is connected to the data transfer circuit
24
(see
FIG. 23
) which transfers data signals. The data transfer circuit
24
outputs a display data signal to the signal line
5
for every scanning line or every pixel. When the scanning line
6
is made active, the TFT
3
is turned on, allowing a charge corresponding to the display data signal on the signal line
5
to be stored in the LC capacitance
1
. The display is maintained by the voltage applied to the liquid crystal according to the charge stored in the LC capacitance
1
.
The LC capacitance
1
is comparatively highly resistive, but, actually, it has a leakage resistance. Accordingly, the charge stored in the LC capacitance
1
leaks, and thus the voltage applied to the liquid crystal decreases before the driving element
3
is next turned on, resulting in the lowering of the quality of the display. In order to prevent the decrease of the applied voltage, the auxiliary capacitance
8
is disposed in parallel with the LC capacitance
1
. Thus, the voltage applied to the liquid crystal is maintained by the LC capacitance
1
and the auxiliary capacitance
8
.
The liquid crystal of the LC capacitance
1
sandwiched by the pixel electrode and the common electrode needs to be driven by an alternate voltage signal. A flicker may be generated by periodically inverting the polarity of the driving voltage to be applied to the liquid crystal. In order to prevent this flickering,
1
H reverse driving is often used where the polarity of the driving voltage is reversed every scanning line. However the
1
H reverse driving raises a problem as follows. Since a large voltage difference is produced between adjacent scanning lines, it is not possible to change the alignment of the liquid crystal molecules following the change in the voltage. This makes the boundaries of pixels between adjacent scanning lines unclear, thus losing the sharpness of the display. In order to solve this problem, a black matrix may be provided so as to be disposed between the scanning lines. This black matrix, however, lowers the numerical aperture of the LCD apparatus, thus darkening the resultant image displayed.
When a color image is displayed, a field sequential scanning system in which the scanning is conducted separately for each color is effective as one of the methods for solving the above problem. The field sequential scanning system is a coloring technique where two or more colors are displayed by temporal mixing. For example, Japanese Patent Application No. 3-77983 (Japanese Laid-Open Patent Publication No. 4-310925) filed by the same applicant as the present application, proposes two examples of driving methods adopting
Ishii Yutaka
Matsuura Manabu
Yamamoto Yoshitaka
Yoneda Hiroshi
Yoshida Shigeto
Mengistu Amare
Nixon & Vanderhye P.C.
Sharp Kabushiki Kaisha
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