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Description  |
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BACKGROUND OF THE INVENTION
The present invention relates to a liquid crystal display device in
general, and in particular to a display device with reduced unevenness of
display. While liquid crystal display devices have taken many forms simple
matrix type liquid crystal display devices are generally driven by a
voltage averaging method. The liquid crystal panel, is provided with
scanning and signal electrodes each having a resistance which is greater
than zero (0) and a liquid crystal layer which acts as a dielectric.
Therefore, the effective voltages at the display elements or dots formed
by the intersection of each scanning electrode and signal electrode
changes depending on the nature of the characters and the images displayed
by the liquid crystal panel. This results in unevenness on the display
device.
The problem described above has been known in the art. Many problem solving
techniques have been used in the past, such as, a method of reversing the
polarity of the voltage applied to the liquid crystal panel a plurality of
times in one frame (hereinafter referred to as "line reverse driving
method"). This method is described in Japanese Patent Application
Laid-Opened Official Gazette No. 62-31825, No. 60-19195 and No. 60-19196.
Further, other methods for improving unevenness of display are known in the
art such as the method described in Japanese Patent Application No.
63-159914 proposed by the present inventor, referred to as the "voltage
correcting method".
The line reverse driving method is an effective method of improving
unevenness of display caused by variations of the optical characteristics
of the liquid crystal caused by changes in the frequency component of the
applied voltage. The line reverse driving method is effective at improving
the unevenness of display, but cannot completely remove the unevenness of
display caused by changes in the frequency component of the applied
voltage.
The unevenness of display can be improved by the voltage correcting method
proposed by the present inventor. However, with the application of this
method the unevenness of display such as described below has not been
eliminated.
Referring to FIG. 1 the unevenness of display remaining after application
of the voltage adjusting method is explained. FIG. 1 depicts a liquid
crystal panel generally indicated as 1, composed of a liquid crystal layer
5, a first substrate 2 and a second substrate for sandwiching the liquid
crystal layer 5 therebetween. A plurality of scanning electrodes Y1
through Y6 are formed on substrate 2 in the horizontal direction and a
plurality of signal electrodes X1 through X6 are formed on substrate 3 in
substantially the vertical direction. Each intersection of scanning
electrodes Y1 through Y6 and signal electrodes X1 through X6 forms a
display element (dot) 7. Display elements 7 marked with crosshatching
represent the lighting or illuminated state and blank display elements 7
represent the non-lighting or non-illuminated state. Further, FIG. 1 is
depicted as a checkered pattern or matrix. The display panel of FIG. 1 is
limited to a 6.times.6 matrix or 36 display elements for simplicity,
however, in exemplary embodiments the number of display elements of liquid
crystal panel 1 may be much greater.
In the voltage adjusting method, a scanning voltage correcting wave is
applied to the scanning electrodes. For example, a scanning voltage
correcting wave is applied to the left side of the scanning electrode, to
vary the display pattern. Among the examples of the voltage correcting
method of driving liquid crystal displays proposed by the present inventor
in Japanese Patent Application No. 63-159914, the scanning voltage
correcting wave is applied to every other line display, thereby, improving
the unevenness of the display. Specifically, the correcting voltage is
superimposed upon the non-selective voltage in accordance with the
difference I between the number of lighting display elements on a scanning
electrode and the number of lighting display elements on the following
scanning electrode when the selection is moved from one scanning electrode
to the next scanning electrode. However, in the case of the display
subject shown in this figure, since the difference I is always zero (0),
the correcting voltage is not applied to the non-selective voltage.
The signal voltage wave is fed alternatively to the signal electrodes from
the upper and the lower ends of the signal electrodes, with each
consecutive signal electrode receiving the signal voltage wave from the
opposite direction. The liquid crystal panel 1 displays a "positive
display" which becomes dark when the effective voltage applied to the
display dot becomes higher.
When the display patter depicted in this figure is actually used, the
display dots formed by electrodes X1, X3 and X5 are brighter on the upper
portion, and become darker on the lower portion. On the contrary, the
display dots formed by electrodes X2, X4 and X6 are brighter on the lower
portion, and become darker on the upper portion. In other words, the
effective voltage actually applied to the display dot is greatest in the
dot most proximate to the source of the signal voltage wave, and the
effective voltage decreases as the display dot increases in distance away
from the signal voltage wave.
The following problems have been discovered through further experimentation
on the unevenness of display. Particular reference is made to FIGS. 2(a)
through (c) in order to explain the problems identified.
FIGS. 2(a) through (c) depict examples of actual driving waveforms
(waveforms of applied voltage) applied to the electrodes of the liquid
crystal panel shown in FIG. 1. In FIG. 2(a), the full line shows the
voltage waveform on the signal electrode X3 in the position of display
element D31 (the intersection of X3 and Y1) of FIG. 1. The dotted line
shows the voltage waveform on the signal electrode X2 in the position of
the display dot D21 (the intersection of X2 and Y1). The wave depicted
with a full line and the wave depicted with a dotted line are drawn to be
slightly shifted to distinguish them from each other and to facilitate the
explanation. The two waveforms are actually superposed upon each other.
FIG. 2(b) shows the voltage waveform on signal electrode X1 in the position
of display dot D21 or D31 in FIG. 1. FIG. 2(c) shows the difference
between the voltage wave on scanning electrode Y1 and the voltage wave on
signal electrode X3 in the position of display dot D31 in FIG. 1. The full
line depicts the voltage wave applied to the display dot D31. Similarly,
the dotted line in FIG. 2(c) depicts the voltage wave applied to display
dot D21. The portion with hatching shows the difference in applied voltage
between the lighting dot and the non-lighting dot, which is not the
voltage difference causing the unevenness of display.
In the figure V0, V1, V2, V3, V4 and V5 represent the applied voltages. The
selective and non-selective voltages are applied to the scanning electrode
and the lighting and the non-lighting voltages are applied to the signal
electrodes. The voltages V5, V3, V0 and V4 are defined as the first group
of lighting, non-lighting, selective and non-selective voltages and the
voltages V0, V2, V5 and V1 are defined as the second group of lighting,
non-lighting, selective and non-selective voltages (hereinafter, the
voltage wave applied to the scanning electrode is referred to as scanning
voltage wave and the voltage wave applied to the signal electrode is
referred to as the signal voltage wave). The first and second voltage
groups are periodically switched. In this example, the voltage groups are
switched after all the scanning electrodes Y1 through Y6 are applied with
the selective voltage (this cycle is known as one frame, and it is
represented by F1 and F2 in FIG. 2).
As shown in FIG. 2(a), since the distance between the display element D31
and the end portion applied with the signal voltage wave is short, the
damping of the voltage wave is almost nonexistent and the applied signal
voltage wave is applied as is without any rounding or damping. However, as
shown in FIG. 2(b), since the distance between the dot D21 and the end
portion applied with the signal voltage wave (hereinafter referred to as
the "driving end") is large in the case of signal electrode X2 in the
position of the dot D21, the result is a signal voltage wave having large
damping and rounding.
In other words, the damping and rounding of the voltage wave is caused by
an integrating circuit, which includes the electrical resistance
internally within signal electrodes X1 through X6 and the condenser having
the liquid crystal material as the dielectric. Therefore, when signal
electrodes X1, X3 and X5 are changed from the lighting to non-lighting
voltage and from the nonlighting to lighting voltage, a larger spike type
noise is generated than when signal electrodes X2, X4 and X6 are changed
from the lighting to non-lighting voltage and from the non-lighting to
lighting voltage, when considering scanning electrode Y1. The spike type
noise generated on scanning electrode Y1 by switching signal electrodes
X1, X3 and X5 from the lighting to non-lighting voltage and from the
non-lighting to lighting voltage, thereby, dominates. Therefore, as shown
by the full line waveform shown in FIG. 2(c), the effective voltage
applied to display element D31 decreases, and as shown by the dotted line,
the effective voltage applied to display dot D21 increases.
Alternatively, in the case of scanning electrode Y6 shown in FIG. 14 the
noise generated by the signal electrodes X2, X4 and X6 dominate. Further,
the effective voltage applied to the display dot D26 decreases and the
effective voltage applied to the display dot D3 increases.
Hereinafter we will refer to the mth signal electrode from the left side as
signal electrode Xm, the nth scanning electrode from the upper portion of
liquid crystal as scanning electrode Yn, and the display dot formed at the
intersection of signal electrode Xm and scanning electrode Yn will be
referred to as display element Dmn. Generally, when the selection of
successive scanning electrodes moves from scanning electrode Yn to
scanning electrode Yn+1, and the signal voltage wave applied to those
signal electrodes receiving the signal voltage wave from the end portion
depicted at the upper portion of FIG. 14, is such that the lighting
voltage is successively applied when scanning electrodes Yn and Yn+1 are
scanned (successive lighting display elements), the case is defined as a1.
When the non-lighting voltage is successively applied when scanning
electrodes Yn and Yn+1 are scanned (successive non-lighting display
elements), the case is defined as b1. When the lighting voltage is applied
to the display element formed on scanning electrodes Yn and the
non-lighting voltage is applied to the scanning electrode Yn+1, the case
is defined as cl. When the nonlighting voltage is applied to the display
element formed on scanning electrodes Yn and the lighting voltage is
applied to the scanning electrode Yn+1, the case is defined as d1.
Similarly, when the selection of successive scanning electrodes moves from
scanning electrode Yn to scanning electrode Yn+1, and the signal voltage
wave applied to those signal electrodes receiving the signal voltage wave
from the end portion depicted at the lower portion of FIG. 14, is such
that the lighting voltage is successively applied when scanning electrodes
Yn and Yn+1 are scanned (successive lighting display elements), the case
is defined as a2. When the non-lighting voltage is successively applied
when scanning electrodes Yn and Yn+1 are scanned (successive non-lighting
display elements), the case is defined as b2. When the lighting voltage is
applied to the display element formed on scanning electrodes Yn and the
non-lighting voltage is applied to the scanning electrode Yn+1, the case
is defined as c2. When the nonlighting voltage is applied to the display
element formed on scanning electrodes Yn and lighting voltage is applied
to the scanning electrode Yn+1, the case is defined as d2.
The number of lighting display elements 7 on scanning electrode Yn that are
applied with the signal voltage wave from one end portion of the display
(the upper portion of FIG. 14) is N1.sub.ON and the number of non-lighting
display elements 7 is N1.sub.OFF. Further, the number of lighting display
elements 7 on scanning electrode Yn+1 that are applied with the signal
voltage wave from one end portion (the upper portion of FIG. 14) is
M1.sub.ON and the number of non-lighting display elements 7 is M1.sub.OFF.
Similarly the number of lighting display elements 7 on scanning electrode
Yn that are applied with the signal voltage waveform from one end portion
(the lower portion of FIG. 14) is N2.sub.ON and the number of non-lighting
display elements 7 is N2.sub.OFF. Further, the number of lighting display
elements 7 on scanning electrode Yn+1 that are applied with the signal
voltage wave from one end portion (the lower portion of FIG. 14) is
M2.sub.ON and the number of non-lighting display elements 7 is M2.sub.OFF.
The relationship between scanning electrodes and signal electrodes that
are applied with signal voltage waves from one end portion (the upper
portion of FIG. 14) is as follows:
N1.sub.ON =a1+c1
N1.sub.OFF =b1+d1
M1.sub.ON =a1+d1
M1.sub.OFF =b1+c1
Herein, the numeric value I1 is defined as follows:
##EQU1##
Similarly, the relationship between scanning electrodes and signal
electrodes that are applied with signal voltage waves from one end portion
(the lower portion of FIG. 14) is as follows:
N2.sub.ON =a2+c2
N2.sub.OFF =b2+d2
M2.sub.ON =a2+d2
M2.sub.OFF =b2+c2
Herein, the numeric value I2 is defined as follows:
##EQU2##
Further, the function I(k) is defined as follows:
I(k)=f(k).multidot.I1+f(L-k).multidot.I2.
The function f(k) is a function which decreases as k increases. The
function f(k) shows that the spike type noise generated in the scanning
electrode, by each signal electrode, increases as the signal voltage wave
approaches the driving end (the end where the signal voltage wave is
applied).
The character L indicates the total number of scanning electrodes. The
relationship between k and L is as follows:
1.ltoreq.k.ltoreq.L.
The absolute value of the function I(k) defines the spike type noise
generated on the kth scanning electrode Yk when the selection is moved
from the scanning electrode Yn to scanning electrode Yn+1. Thus, the
function I(k) increase as the noise generated decreases. The direction in
which the noise is generated dependent upon whether the value of the
function I(k) is positive or negative.
In scanning electrode Yk, if the direction of the voltage spike type noise
according to the function I(k) is in phase with the variation of the
voltage wave applied to each signal electrode, then the effective voltage
applied to the display element formed with the signal electrode and the
scanning electrode Yk becomes lower, thereby, making the display element
brighter. Alternatively, if the phases of the voltage spike I(k) and the
signal electrode are reversed, then the effective voltage applied to the
display element will be great, thereby, making the display element darker.
Thus, unevenness of display would remain. The different types of
unevenness remaining after the voltage correcting method will be explained
with reference to FIGS. and 4.
Particular reference is made to FIGS. 3 and 4 , wherein liquid crystal
panels with different display subjects are depicted. FIGS. 3 and 4 depict
the same liquid crystal elements as FIG. 1. Thus, similar numbers are used
to designate similar elements of the liquid crystal panel. The voltage
correcting method of decreasing unevenness of display as described in
Japanese Patent Application No. 63/159914 proposed by the present inventor
describes applying a varied scanning voltage to the left side of the
scanning electrodes Y1 through Y6 according to the pattern of the display
elements 7 upon the display. Accordingly, the scanning voltage wave is
varied by superimposing the correcting voltage upon the selective voltage
in accordance with the number of lighting dots Z on the scanning electrode
selected. In FIGS. 3 and 4 the congruence quadrangle is displayed in the
position shifted to the left end and the right end respectively.
Therefore, the same correcting voltage is applied to each scanning
electrode Y1 through Y6 of the liquid crystal panel 1 when it is in a
condition displaying either FIG. 3 or FIG. 4.
In FIG. 3 there is unevenness of display generated in the display
quadrangle that is manifested in the form of horizontal darkening
resulting from excess correcting voltage. On the contrary, in FIG. 4, the
correcting voltage is not sufficient to cause unevenness by weft pulling.
In FIG. 4 horizontal brightening remains because the condenser formed with
the resistance of each scanning electrode Y1 through Y6 forming a part of
the liquid crystal panel 1 forms an integral circuit and the lighting
display element forms a condenser having larger capacitance as compared
with the non-lighting dot. The interference causing rounding of the
voltage wave of the lighting display element at a greater distance from
the end of the scanning electrode applied with the scanning voltage wave
(hereinafter the "driving end") is larger than the interference of the
non-lighting display element at about the same position. Thus, larger
rounding results in display elements that are more distanced from the
driving end of the scanning voltage wave. Thus, if the lighting dot exists
in a position distanced from the driving end of the scanning electrode Y1
through Y6, the scanning electrode wave including the correcting voltage
is rounded. Therefore, the effective voltage applied to the display dot
decreases.
In a matrix with s scanning electrodes Y1 through Ys, the numerical value
z' representing the unevenness of display by weft pulling, may be
calculated by the following formula:
##EQU3##
wherein, i designates the signal electrode upon which the display dot is
turned on, for example, X.sub.i (i=1, 2, 3, . . ., P) designates the
number of signal electrodes X1 through Xp.
Herein, the letters s and p designate the number of scanning electrodes and
signal electrodes respectively.
The function q(i) is a function that increases as the value i increases.
The function .delta.(i) is 1 when the display element positioned in i on
the selected scanning electrode is lighting, and it is 0 when the display
dot positioned in i is non-lighting.
Therefore the numerical value z' increases greatly on display element most
distanced from the driving end of the scanning electrode.
Thus, the unevenness of display has not been completely removed by the
voltage correcting method of removing unevenness of display by the weft
pulling of conventional displays. The numerical value z may be obtained by
the following formula:
##EQU4##
The unevenness resulting in the mechanisms described above have caused a
decrease in the quality of the display. By this invention applicant seeks
to improve the evenness of display.
SUMMARY OF THE INVENTION
Generally speaking in accordance with the invention, a liquid crystal
display device having an improved ability to prevent unevenness of display
is provided. This is accomplished through determining the specifics of the
pattern to be displayed in terms of mathematical relationships, based on
the positional relationship of the lighted display elements on the display
device. The method for correcting being in accordance with the determined
values in the predetermined mathematical relationships. Thus, resulting in
the unevenness of display being improved, the quality of display being
increased, and the visual quality being increased by considering the
positional relationship between the display pattern and the driving end of
the scanning electrode or the signal electrode taking into consideration
the extracted amount calculated.
A liquid crystal display is formed with two substrates and a liquid crystal
layer formed therebetween in accordance with the invention. A group of
scanning electrodes are formed on one substrate. A group of signal
electrodes is formed on the other substrate. A scanning voltage wave is
applied to at least one end of each scanning electrode and a signal
voltage wave is applied to at least one end of each signal electrode. The
scanning electrodes intersect the signal electrodes, providing display
elements on the liquid crystal display at each intersection. In order to
reduce unevenness of display a correcting voltage is superimposed upon at
least one of the scanning voltage wave or the signal voltage wave in
accordance with patterns with images and characters dictated by the liquid
crystal panel. The value of the correcting voltage that is superimposed
upon the scanning voltage wave or signal voltage wave may be varied in
accordance with at least one of the positions of the display element
relative to the driving end of the scanning electrode or the signal
electrode.
One embodiment corrects the non-selected voltage applied to the scanning
electrodes dependent on the distance of its selected scanning electrode
from the driving end of the signal electrode. A second embodiment corrects
the selective voltage applied to a scanning electrode depending on the
distance of the lighting display elements from the driving end of the
scanning electrodes.
Accordingly it is an object of the present invention to provide an improved
method for driving a liquid crystal display.
Another object of the present invention is to provide a method for driving
a liquid crystal display which prevents unevenness in display density at
the display elements of the matrix.
Still another object of the invention is to prevent unevenness of display
by varying the amount of correcting voltage supplied in every electrode.
Still other objects and advantages of the invention will in part be obvious
and will in part be apparent from the specification.
The invention accordingly comprises the several steps and the relation of
one or more of such steps with respect to each of the others, and the
apparatus embodying features of construction, combinations of elements and
arrangements of parts which are adapted to effect such steps, all as
exemplified in the following detailed disclosure, and the scope of the
invention will be indicated in the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
For a fuller understanding of the invention, reference is had to the
following description taken in connection with the accompanying
drawing(s), in which:
FIG. 1 is a schematic perspective view of a liquid crystal panel showing an
example of a displayed pattern;
FIGS. 2(a)-(c) are waveform diagrams showing the voltage waveforms
conventionally applied to the liquid crystal panel when the pattern shown
in FIG. 1 is displayed;
FIG. 3 is a schematic perspective view of the liquid crystal panel showing
another example of a displayed pattern;
FIG. 4 is a schematic perspective view of the liquid crystal panel showing
still another example of a displayed pattern;
FIG. 5 is a block diagram showing the structure of the first embodiment of
the liquid crystal display device of the present invention;
FIG. 6 is a block diagram and schematic view of the liquid crystal unit of
FIG. 5;
FIG. 7(a) is a circuit diagram of the scanning electrode driving circuit of
FIG. 5;
FIG. 7(b) and FIG. 7(b) (cont'd) are timing diagrams showing the timing
sequence of shift register 207, latch circuit 208 and detect circuit 210;
FIG. 7(c) is a block diagram of detect circuit 210;
FIG. 7(d) is a block diagram of level shifter circuit 212;
FIG. 8 is a block diagram of the correcting circuit of FIG. 5;
FIG. 9 is a circuit diagram of the power source circuit of FIG. 5;
FIG. 10 is a schematic perspective view of a liquid crystal panel showing
an example of a displayed pattern; FIGS. 11(a)-(g) are waveform diagrams
showing an example of the voltage waveform applied to the liquid crystal
panel, which results in the displayed pattern of FIG. 10;
FIG. 12 is a block diagram of the fourth embodiment of the liquid crystal
display device of the present invention;
FIG. 13 is a block diagram and schematic view of the liquid crystal unit of
FIG. 12;
FIG. 14 is a block diagram of the correcting circuit of FIG. 12; and
FIG. 15 is a circuit diagram of the power source circuit of FIG. 12;
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to FIGS. 5-11, a first embodiment of the device and method of
correcting the unevenness of display generated when the checkered pattern
is displayed will be described.
The degree of unevenness of display is determined by subtracting the number
of lighting display elements of selected scanning electrode Yn from the
number of lighting display elements on the following scanning electrode
Yn+1, with consideration given to the distance between the scanning
electrodes considered and the driving end of the signal electrode.
Therefore, wave correcting can be carried out by applying the result of
the above calculation to the liquid crystal display device. This process
will be further displayed through the use of concrete examples.
FIG. 5 depicts a block diagram of the first embodiment of the present
invention. A liquid crystal unit 101 is provided with a liquid crystal
panel 1, a scanning electrode driving circuit 205 and a signal electrode
driving circuit 213. A sequential control signal circuit 102 controls the
operations of the liquid crystal display device, by providing latch signal
LP, frame signal FR, data-in signal DIN, X driver shift clock signal XSCL
and other control signals. Data signal circuit 103 provides the data
signal which determines the display pattern The data signal is varied at
the leading edge of X driver shift clock signal XSCL, and is supplied to
liquid crystal unit 101. A voltage wave correcting circuit (hereinafter
referred to as "correcting circuit") 104 is also varied at the trailing
edge of signal XSCL, and produces a correcting voltage wave, which is
supplied to liquid crystal unit 101 by line 109. Power source circuit 105
produces Y power voltages on line 106 in the form of two group of voltages
which are applied to the scanning electrodes, and X power voltages on line
107 in the form of two groups of voltages which are applied to the signal
electrodes. Latch signal LP and X shift clock signal XSCL are input to
dividing circuit 108, which produces a synchronized clock signal on line
110 (hereinafter referred to as "correcting clock signal").
Reference is next made to FIG. 6, wherein a detailed block diagram of the
liquid crystal unit 101 of FIG. 5 is depicted. Liquid crystal panel 1 is
comprised of scanning electrodes Y1 through Y6 arranged on substrate 2 and
signal electrodes X1 through X6 arranged on substrate 3. A liquid crystal
layer is interposed between substrates 2 and 3. Signal electrodes X1, X3
and X5 have terminals for supplying the signal voltage wave at the upper
end portions thereof, and scanning electrodes X2, X4 and X6 have terminals
for supply | | |
