Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1997-03-24
2001-02-27
Shankar, Vijay (Department: 2778)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S204000, C345S100000
Reexamination Certificate
active
06195077
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a driving device capable of improving display quality in a liquid crystal display apparatus such as a liquid crystal panel of simple matrix type.
2. Description of The Related Art
FIG. 39
shows a schematic electrical configuration for driving a simple matrix type liquid crystal panel
101
of one prior art. A plurality of segment electrodes of the liquid crystal panel
101
are driven in parallel by a segment side drive circuit
102
, and a plurality of common electrodes are driven by a common side drive circuit
103
while being selected sequentially. Power voltages supplied from a power supply circuit
104
to the segment side drive circuit
102
and to the common side drive circuit
103
are six voltages V
0
, V
1
, V
2
, V
3
, V
4
and V
5
, having a relation of V
0
>V
1
>V
2
>V
3
>V
4
>V
5
. The segment side drive circuit
102
is supplied with four voltages V
0
, V
2
, V
3
and V
5
, and the common side drive circuit
103
is supplied with four voltages V
0
, V
1
, V
4
and V
5
.
Display data which represents an image to be displayed on the liquid crystal panel
101
is given to the segment side drive circuit
102
as serial data by a controller
105
. Data latch clock for latching the display data in synchronization with the display data, horizontal synchronization signal and AC-converting signal are also supplied to the segment side drive circuit
102
from the controller
105
. The controller
105
supplies horizontal synchronization signal, vertical synchronization signal and AC-converting signal to the common side drive circuit
103
. The common side drive circuit
103
selects a common electrode which should display first in response to a vertical synchronization signal, and thereafter scans in the vertical direction by changing the common electrode to be selected successively while synchronizing with the horizontal synchronization signal.
FIG. 40
shows internal configuration of the segment side drive circuit
102
shown in FIG.
39
. The display data supplied from the controller
105
as serial data is converted to parallel data by a shift register
121
, latched by the data latch
122
according to a data latch clock, and latched in a line latch
123
at every horizontal scanning period according to the horizontal synchronization signal (LP). Output of the line latch
123
is sent to a liquid crystal drive output circuit
126
via a level shifter
124
, together with the AC-converting signal which is sent thereto via a level shifter
125
. The level shifters
124
,
125
are provided because the operating voltage of the liquid crystal drive output circuit
126
is different from operating voltage Vcc of the shift register
121
, the data latch
122
and the line latch
123
.
FIG. 41
shows voltage waveforms of various portions and voltage waveform applied to a liquid crystal cell of the liquid crystal panel
101
of the prior art shown in FIG.
39
. Although
FIG. 41
shows a case with seven scan electrodes for the convenience of description, the actual number of scan electrodes is larger than this. The display data stored in the line latch
123
of the segment side drive circuit
102
is given to the liquid crystal drive output circuit
126
via the level shifter
124
. The liquid crystal drive output circuit
126
selects one voltage from among liquid crystal drive voltages V
0
, V
2
, V
3
and V
5
of four levels which are input, on the basis of the display data, and applies the voltage to the segment electrode. The outputs of a segment side drive circuit
102
for one scan electrode are applied to the segment electrodes in parallel. On the other hand, the common side drive circuit
103
supplies liquid crystal drive voltages V
0
and V
5
from among the four liquid crystal drive power voltages V
0
, V
1
, V
4
and V
5
to a selected common electrode, and supplies liquid crystal drive voltages V
1
and V
4
to non-selected common electrodes.
The liquid crystal panel
101
comprises common electrodes and segment electrodes which have non-zero resistance, while the liquid crystal layer interposed between the electrodes acts as a dielectric substance and has a non-zero capacitance. Consequently, electrical resistance of each electrode wire and a capacitor formed by a display dot where the liquid crystal works as a dielectric form a low-pass filter. Due to the low-pass filter, voltage drop and rounding of waveform become more significant as the distance from the segment side drive circuit
102
increases. Accordingly a difference in voltage drop and rounding of waveform is caused between a pixel on a scan electrode near to the segment side drive circuit
102
and a pixel on a scan electrode far therefrom, thereby causing a difference in the effective voltage applied to the liquid crystal cell and resulting in a difference in the display density. This difference in the display density causes an upper portion and a lower portion of the liquid crystal display surface to appear having different display densities.
There is a trend to increase panel sizes of liquid crystal display apparatuses are for such needs as replacing CRT monitors of personal computers. Also the standard display for the so-called PC-AT compatible computer is in the trend of increasing the number of display dots as the display standard evolves from VGA to SVGA, and from XGA to SXGA, causing the pixel pitch to decrease. Increasing display screen size causes the pixel and scan electrodes to become longer. Further, trend toward higher pixel resolution causes the widths of the pixel and scan electrodes to decrease. As a result, electrical resistances of the pixel and the scan electrodes increase, thereby causing the difference in the display density to increase further.
As a solution to these problems, for example, such prior art may be applied as proposed in the Japanese Unexamined Patent Publication JP-A 62-43624 (1987). In this prior art, a liquid crystal drive voltage which changes in a saw-tooth form as shown in
FIG. 42
is used, thereby to change the voltage waveforms of various portions as shown in FIG.
43
. In the case that a high drive voltage is applied at every scanning period, the difference in the density of display between the upper portion and the lower portion of the liquid crystal panel when the segment side drive circuit is installed in the upper portion of the liquid crystal panel can be reduced.
Also for the purpose of driving a simple matrix liquid crystal panel, the present applicant proposed a method of driving the segment side drive circuit with a low voltage, for example to enable it to drive with a single power supply of 5V. Operation with this driving method is shown in FIG.
44
. The segment side drive circuit selects and outputs one of two voltages, VSH and VSL, according to a combination of the AC-converting signal and the display data, and determines whether to turn on or off the display. The common side drive circuit selects and outputs one of three voltages VCH, VCM and VCL according to the combination of the AC-converting signal and selection or non-selection.
Comparison of the voltage applied to each liquid crystal cell of the liquid crystal panel between FIG.
41
and
FIG. 44
shows that the voltages in both driving methods are identical, provided that the following equations hold. This method of driving will be hereinafter called 5V driving method.
V
0
−V
5
=VCH−VSL
V
0
−V
4
=VCH−VSM
V
0
−V
3
=VCH−VSH
(V
4
−V
4
, V
1
−V
1
)=(VCH=VSM)=0
(V
4
−V
5
, V
1
−V
2
)=VCM−VSL
(V
4
−V
3
, V
1
−V
0
)=VCM−VSH
V
5
−V
2
=VCL−VSL
V
5
−V
1
=VCL−VSH
V
5
−V
0
=VCL−VSH
With this 5V driving method, too, there arises differences in the density between pixels in the upper portion and lower portion of the liquid crystal panel, as in the prior art described above. The problem of difference i
Gyouten Seijirou
Ogawa Yoshinori
Said Mansour M.
Shankar Vijay
Sharp Kabushiki Kaisha
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