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Claims  |
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What is claimed:
1. A liquid crystal display device having a plurality of picture elements
which can be placed in lit and unlit states, comprising:
a first substrate;
a second substrate spaced apart from said first substrate;
a plurality of scanning electrodes and signal electrodes;
liquid crystal material disposed in the space between the substrates;
driving means for producing scan voltage waveforms and signal voltage
waveforms, for supplying said scan voltage waveforms and signal voltage
waveforms to said scanning electrodes and signal electrodes, respectively,
whereby picture element voltages are applied across said picture elements,
for periodically inverting the polarity of said scan voltage waveforms and
signal voltage waveforms, and for producing at least one correction
voltage combined with and for varying the voltage level of at least one of
said scan voltage waveforms and signal voltage waveforms which are
supplied to an associated picture element; and
wherein said scan voltage waveforms include at least one select signal and
at least one non-select signal, said at least one correction voltage being
produced at a time when the corresponding scan voltage waveform of a
picture element changes between a select signal and non-select signal at
or following said scan voltage and signal voltage polarity inversion.
2. The liquid crystal display device of claim 1, wherein said plurality of
scanning electrodes include a first scanning electrode and a second
scanning electrode, said at least one select signal being next supplied to
the second scanning electrode following application of said at least one
selected signal to the first scanning electrode, and wherein said at least
one correction voltage is based on a first number of lit picture elements
associated with said first scanning electrode and a second number of lit
picture elements associated with said second scanning electrode.
3. The liquid crystal display device of claim 2, wherein the width of the
at least one correction voltage is based on the sum of said first number
and said second number.
4. The liquid crystal display device of claim 7, wherein said driving means
is further operable for producing at least one additional correction
voltage for further combining with and varying the voltage level of at
least one of said scan voltage waveforms and said signal voltage waveforms
associated with said at least one picture element.
5. The liquid crystal display device of claim 4, wherein said at least one
additional correction voltage is also based on the first number and the
second number.
6. The liquid crystal display device of claim 5, wherein the width of the
at least one additional correction voltage is based on the difference
between said first number and said second number.
7. The liquid crystal display device of claim 1, wherein said driving means
is further operable for producing at least one additional correction
voltage for further varying the voltage level of at least one of said scan
voltage waveforms and said signal voltage waveforms associated with at
least one picture element.
8. The liquid crystal display device of claim 8, wherein said plurality of
scanning electrodes includes a first scanning electrode and a second
scanning electrode, said at least one select signal being next applied to
the second scanning electrode following application of said at least one
select signal to the first scanning electrode, and wherein said at least
one additional correction voltage is based on a first number of lit
picture elements associated with said first scanning electrode and a
second number of lit picture elements associated with said second scanning
electrode.
9. The liquid crystal display device of claim 8, wherein the width of the
at least one additional correction voltage is based on the difference
between said first number and said second number.
10. A method for producing a pattern to be displayed on a liquid crystal
display device having a plurality of picture elements which can be placed
in lit and unlit states, comprising:
producing scan voltage waveforms and signal voltage waveforms;
supplying said scan voltage waveforms and signal voltage waveforms to a
plurality of scanning electrodes and signal electrodes, respectively;
applying picture element voltages across said picture elements;
periodically inverting the polarity of said scan voltage waveforms and said
signal voltage waveforms;
producing at least one correction voltage for combining with and varying
the voltage level of at least one of the scan voltage waveforms and signal
voltage waveforms which are supplied to an associated picture element; and
wherein the scan voltage waveforms include at least one select signal and
at least one non-select signal, said at least one correction voltage being
produced at a time when the corresponding scan voltage waveform of a
picture element changes between a select signal and non-select signal at
or following said scan voltage waveform and signal voltage waveform
polarity inversion.
11. The method of claim 10, wherein the plurality of scanning electrodes
includes at least a first scanning electrode and a second scanning
electrode, the at least one select signal being next supplied to the
second scanning electrode following application of the at least one select
signal to the first scanning electrode, and wherein the at least one
correction voltage is based on a first number of lit picture elements
associated with the first scanning electrode and a second number of lit
picture elements associated with the second scanning electrode.
12. The method of claim 11, wherein the width of the at least one
correction voltage is based on the sum of the first number and the second
number.
13. The method of claim 11, further including producing at least one
additional correction voltage for further varying the voltage level of at
least one of said scan voltage waveforms and said signal voltage waveforms
of said associated picture element, wherein the width of the at least one
additional correction voltage is based on the difference of the first
number and the second number.
14. A liquid crystal display device having a plurality of picture elements
which can be placed in lit and unlit states, comprising:
a first substrate;
a second substrate spaced apart from said first substrate;
a plurality of scanning electrodes and signal electrodes;
liquid crystal material disposed in the space between the substrates;
driving means for producing scan voltage waveforms and signal voltage
waveforms, for supplying said scan voltage waveforms and signal voltage
waveforms to said scanning electrodes and signal electrodes, respectively,
whereby picture element voltages are applied across said picture elements,
for periodically inverting the polarity of said scan voltage waveforms and
said signal voltage waveforms and for producing at least one correction
voltage combined with and for varying the voltage level of at least one of
said scan voltage waveforms and signal voltage waveforms which are
supplied to an associated picture element;
wherein said scan voltage waveforms include at least one select signal and
at least one non-select signal, said at least one correction voltage being
produced at a time immediately following a change in the voltage level of
the signal voltage waveform corresponding to the associated picture
element and at that same time the corresponding scan voltage waveform
changes between a select signal and non-select signal; and
wherein said plurality of scanning electrodes includes a first scanning
electrode and a second scanning electrode, the at least one select signal
being next supplied to the second scanning electrode following application
of the at least one select signal to the first scanning electrode, and
wherein said at least one correction voltage is based on a first number of
lit picture elements associated with said first scanning electrode and a
second number of lit picture elements associated with said second scanning
electrode.
15. The liquid crystal display device of claim 14, wherein the width of the
at least one correction voltage is based on the sum of said first number
and said second number.
16. The liquid crystal display device of claim 15, wherein said driving
means is further operable for producing at least one additional correction
voltage for further combining with and varying the voltage level of at
least one of said scan voltage waveforms and said signal voltage waveforms
associated with said at least one picture element.
17. The liquid crystal display device of claim 16, wherein said at least
one additional correction voltage is also based on the first number and
the second number.
18. The liquid crystal display device of claim 17, wherein the width of the
at least one additional correction voltage is based on the difference
between said first number and said second number.
19. A method for producing a pattern to be displayed on a liquid crystal
display device having a plurality of picture elements which can be placed
in lit and unlit states, comprising:
producing scan voltage waveforms and signal voltage waveforms;
supplying said scan voltage waveforms and signal voltage waveforms to a
plurality of scanning electrodes and signal electrodes respectively;
applying picture element voltages across said picture elements;
periodically inverting the polarity of the picture element voltages; and
producing at least one correction voltage for combining with and varying
the voltage level of at least one of the scan voltage waveforms and signal
voltage waveforms which are supplied to an associated picture element;
wherein the scan voltage waveforms include at least one select signal and
at least one non-select signal, said at least one correction voltage being
produced at a time immediately following a change in the voltage level of
the associated signal voltage waveform and at that same time the
corresponding scan voltage waveform changes between a select signal and
non-select signal; and
wherein the plurality of scanning electrodes includes a first scanning
electrode and a second scanning electrode, the at least one select signal
being next supplied to the second scanning electrode following application
of the at least one select signal to the first scanning electrode, and
wherein the at least one correction voltage is based on a first number of
lit picture elements associated with the first scanning electrode and a
second number of lit picture elements associated with the second scanning
electrode.
20. The method of claim 19, wherein the width of the at least one
correction voltage is based on the sum of the first number and the second
number.
21. The method of claim 19, further including producing at least one
additional correction voltage for further varying the voltage level of at
least one of said scan voltage waveforms and signal voltage waveforms of
said associated picture element wherein the width of the at least one
additional correction voltage is based on the difference of the first
number and the second number. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
This invention relates to a liquid crystal display device and, in
particular, to a liquid crystal display device and method of driving the
display which provides a uniform display with improved contrast.
A known method of driving a liquid crystal display device is the voltage
averaging method shown in FIGS. 17, 18(a)-18(c) and 19(a). A matrix
display liquid crystal cell, shown in FIG. 17, includes a liquid crystal
panel 1 having a layer of liquid crystal material disposed between an
upper substrate 2 and a lower substrate 3. A plurality of parallel spaced
apart scanning electrodes Y1 to Y6 are disposed on the interior surface of
substrate 2 in a lateral direction, and a plurality of parallel spaced
apart signal electrodes X1 to X6 are disposed on the interior surface of
substrate 3. The intersections of scanning electrodes Y1 to Y6 and signal
electrodes X1 to X6 form display elements which may be lit, as depicted
with diagonal lines in FIG. 17, or unlit, which are shown with unshaded
lines. A liquid crystal display generally has more display elements than
the 6.times.6 matrix shown for explanatory purposes in FIG. 17.
Selective voltages or non-selective voltages are applied sequentially to
scanning electrodes Y1 to Y6. Scanning electrodes which are impressed with
selective voltages are known as selected scanning electrodes. The period
for which the particular voltage sequence is applied is known as one
frame.
As the selective or non-selective voltages are applied in the particular
order to scanning electrodes Y1 through Y6, lighting (lit) or non-lighting
(non-lit) voltages are simultaneously applied to signal electrodes X1 to
X6. A display element becomes lit if the corresponding scanning electrode
is selected and a lighting voltage is impressed on the corresponding
signal electrode. If a non-lighting voltage is impressed on the signal
electrode, the intersection of the signal electrode and the selected
scanning electrode is an unlit display element.
FIGS. 18(a)-18(c) and 19(a)-19(c) show the waveform of voltages applied to
a pair of display elements D24 and D23 in FIG. 17,respectively. FIGS.
18(a) and 19(a) show the waveform of the signal voltage applied to signal
electrode X2. FIG. 18(b) illustrates the waveform of the scanning voltage
applied to scanning electrode Y4 and FIG. 19(b) illustrates the waveform
of the scanning voltage applied to scanning electrode Y3. FIG. 18(c)
depicts the waveform of the resulting voltage applied to display element
D24 (a lit state) at the intersection of the signal electrode X2 and the
scanning electrode Y4, and FIG. 19(c) depicts the waveform of the
resulting voltage applied to the display element D23 (an unlit state) at
the intersection of the signal electrode X2 and the scanning electrode Y3.
In FIGS. 18(a)-18(c) and 19(a)-19(c), F1 and F2 represent frame periods.
In frame period F1:
selective voltage=V0, non-selective voltage=V4
lighting voltage=V5, non-lighting voltage=V3
In frame period F2:
selective voltage=V5, non-selective voltage=V1
lighting voltage=V0, non-lighting voltage=V2.
Further, the following relationships are established:
V0-V1=V1-V2=V
V3-V4=V4-V5=V
V0-V5=n.multidot.V ,
when n is a constant. Alternating current is used in the driving process so
that the voltages vary in polarity from period F1 to F2. The time required
to invert the polarity is known as polarity inverting time.
As seen from a comparison of FIGS. 18(a)-18(c) and 19(a)-19(c), a display
element with a corresponding selected scanning electrode is either lit or
unlit depending on whether the voltage applied to the corresponding signal
electrode is a lighting (selecting) voltage or a non-lighting
(non-selecting) voltage. This driving method is known as the voltage
averaging method.
The voltage averaging method is less than completely satisfactory because
clear-cut rectangular waveforms are not in fact applied to the display
dots elements for several reasons. First, the display element has an
electrical capacitance determined by its area, the thickness of the liquid
crystal layer and the dielectric constant of the liquid crystal material.
Second, both the scanning and signal electrodes are made of transparent
conductive films with a typical sheet resistance of several tens of ohms,
which implies that the electrodes have a constant electric resistance.
Accordingly, while the voltages generated by the driving circuit may have
the clear-cut rectangular waveforms of FIGS. 18(a)-18(c) and 19(a)-19(c),
the waveforms become unevenly distorted by the time the voltages are
actually applied to the display elements. Thus, there may be an undesired
difference between adjoining display elements in the effective waveform of
voltages applied thereto, which in turn leads to the problem of uneven
contrast.
Another driving method, known as a line inversion driving method, has been
proposed to overcome the uneven contrast associated with the voltage
averaging method. Disclosed in Japanese Patent Laid-Open Publication Nos.
62-31825, 60-19195 and 60-19196, the line inversion driving method
involves inverting the polarity of the voltage applied to the liquid
crystal panel multiple times during one frame.
FIGS. 20(a)-20(cand 21(a)-21(c) are waveforms utilized in the line
inversion driving method. FIG. 20(a) is the waveform of signal voltage
applied to signal electrode X2 of FIG. 17 and FIG. 20(b) is the waveform
of scanning voltage applied to scanning electrode Y2. The difference
between these two waveforms applied to display element D22 formed by the
intersection of signal electrode X2 and scanning electrode Y2 is shown in
FIG. 20(c). Similarly, FIGS. 21(a) to 21(c) illustrate the waveform of
signal voltage applied to signal electrode X2, the waveform of scanning
voltage applied to scanning electrode Y3, and the difference between these
two waveforms supplied to display element D23.
As is the case in the voltage averaging method, the line inversion driving
method is also less than completely satisfactory. This is due to the fact
that the density or contrast of a display element on the scanning
electrode to which the selective voltage is applied immediately after
inverting the polarity of the voltage applied differs from that of the
display elements along other scanning electrodes. For this reason, the
linear contrast is uneven. When the line inversion drive method is
utilized the position of the scanning electrode undergoing polarity
inversion varies with time and a stream of uneven linear contrast appears.
This phenomenon in turn causes a considerable decline in the quality of
the display of the liquid crystal display device.
Two causes have been determined to explain the uneven linear contrast
associated with these prior art liquid crystal driving methods. These
causes are as follows, referring to the display mode of FIG. 17 and the
waveform of FIG. 21(c) as an example. For explanatory convenience,
scanning electrodes Y1 to Y6 are arranged such that after the selection
sequence from first scanning electrode Y1 to sixth scanning electrode Y6
is complete, the sequence returns to and repeats scanning from electrode
Y1. Also for the example, a polarity inversion based on the line inversion
driving method occurs between scanning electrodes Y3 and Y4, although in
actuality there may be any number and location of polarity inversions
effected.
Liquid crystal display panel 1 provides a so-called positive display
wherein the contrast increases as an effective voltage applied to the
display element rises. Assuming that V is the absolute value of the
difference between the non-selecting voltage and the lighting/non-lighting
voltage and n.multidot.V is the absolute value of the difference between
the selecting voltage and the lighting voltage, where n is a constant
typically having a value between 3 and 50.
The voltage waveform actually applied to display element D23 is illustrated
in FIG. 22, drawn with a solid line 23. Waveform 23 is formed by a
combination of voltage applied to signal electrode X2 and scanning
electrode Y3 on the basis of signal electrode X3 in the display element
matrix of FIG. 17. The voltage waveform indicated by a broken line 23a
represents the voltage applied to scanning electrode Y2 based on signal
electrode X2. As can be seen by comparing the waveform of FIG. 21(c) and
waveform 23 drawn with the solid line in FIG. 22, the waveform of voltage
actually applied to display element D23 is larger than the voltage applied
to signal electrode X2 and scanning electrode Y3.
The reasons for this increase are as follows. Signal voltage waveform 23a
indicated by the broken line in FIG. 22 is applied to display element D22.
Hence, when the selection shifts from scanning electrode Y2 to electrode
Y3, an electric charge amounting to Q.sub.1 is discharged by the capacitor
created by display element D22. Q.sub.1 is waveform 23a indicated by the
broken line in FIG. 22 and is expressed as follows:
Q.sub.1 =nVC-(-VC)=(n+1) VC,
where C is the capacitance of the capacitor. The electric charge quantity
Q.sub.2 absorbed by display element 23 is expressed as follows:
Q.sub.2 =(n-2) VC-VC=(n-3) VC
Hence, the difference .DELTA.Q between Q.sub.1 and Q.sub.2 is given by:
.DELTA.Q=4VC
As shown in FIG. 17 display elements D22 and D23 are next to each other and
form electrically-connected capacitors with a low-valued resistance of the
shorter signal electrode, which in this case is X3 (generally, 1 mm or
less). Therefore, an electric charge, expressed as Q.sub.1 -.DELTA.Q=(n-3)
VC, immediately flows from display element D22 to display element D23,
resulting in almost no voltage drop between the two elements.
However, an electric charge of .DELTA.Q flows from scanning electrodes Y2
and Y3 or an end of signal electrode X3 (i.e., from outside into a portion
to which the voltage is to be applied). When Q is flowing, the resistance
of the scanning electrode and the signal electrode is considerably larger,
even though the electrodes depend on the location of the display elements.
As a result, the flow of electric charge is hindered. Because the electric
charge is not easily discharged, even the voltage on signal electrode X3
is forced to drop when the voltage on scanning electrode Y2 falls from the
level of selecting voltage to a non-selecting voltage. Accordingly, the
effective voltage between signal electrode X3 and scanning electrode Y3
increases.
In other words, if the difference between charge/discharge quantities
before and after the progression is positive, the effective value of the
voltage applied to the display element on the next scanning electrode
increases. Likewise, if the difference is negative, the effective value
decreases. The magnitude of the effective value varies depending on the
absolute value of the charge/discharge quantity. Charge/discharge
quantities before and after the progression are routinely calculated.
Assume K is the number of all display elements on a particular scanning
electrode, N.sub.ON is the number of lit elements, and N.sub.OFF is the
number of unlit elements. Thus, display element number K is as follow:
K=N.sub.ON +N.sub.OFF
Assume M.sub.ON is the number of lit elements on the next scanning
electrode, and M.sub.OFF is the number of unlit elements.
Assume C.sub.ON is the capacitance of the capacitor formed by the lit
element and assume C.sub.OFF is the capacitance of the capacitor formed by
the unlit element. Then, the relationship therebetween is expressed such
as:
C.sub.ON >C.sub.OFF
All display elements on the selected scanning electrode are charged with
the electric charge quantity Q.sub.1 given by:
Q.sub.1 =N.sub.ON n VC.sub.ON +N.sub.OFF (n-2) VC.sub.OFF
The display elements on the next selected scanning electrode are charged
with the electric charge quantity Q.sub.2 given by the formula:
Q.sub.2 =M.sub.ON n VC.sub.ON +M.sub.OFF (n-2) VC.sub.OFF
Accordingly, the difference between electric charge quantities Q.sub.1 and
Q.sub.2 is obtained as follows:
##EQU1##
since N.sub.OFF =K-N.sub.ON and M.sub.OFF =K-M.sub.on, therefore
.DELTA.Q=(N.sub.ON -M.sub.ON) {n (C.sub.ON -C.sub.OFF)+2 C.sub.OFF } V
Assume I is the difference given by (N.sub.ON -M.sub.ON), and B={n
(C.sub.ON -C.sub.OFF)+2 C.sub.OFF } v. The result is:
.DELTA.Q=I.multidot.B (b 1)
The polarity of the waveform then inverts simultaneously as the selection
shifts, so that the display elements on the selected scanning electrode
are charged with the electric charge quantity Q given by:
Q.sub.1 =N.sub.ON n VC.sub.ON +N.sub.OFF (n-2) VC.sub.OFF
The next scanning electrode is then selected. With the inverted polarity,
the display elements on the selected scanning electrode are charged with
the electric charge quantity Q.sub.2 given by:
Q.sub.2 =-(M.sub.ON n VC.sub.ON +M.sub.OFF (n-2) VC.sub.OFF)
The difference Q between Q.sub.1 and Q.sub.2 is expressed by:
##EQU2##
where N.sub.OFF =K-N.sub.On and M.sub.OFF =K-M.sub.ON, so that
##EQU3##
Assume F is the sum of (N.sub.ON +M.sub.ON), and D=2K (n-2) VC.sub.OFF.
The result is:
-Q=F.multidot.B+D
Therefore, taking the polarity inversion into consideration, the electric
charge quantity difference is expressed as:
.DELTA.Q=-F.multidot.B-D (2)
It follows from formulae (1) and (2) that the difference I becomes positive
when the number of lit elements on the scanning electrode selected is
greater than that of lit elements on the subsequently scanned scanning
electrode during a selective shift with no polarity inversion, resulting
in display elements on the subsequently selected scanning electrode having
higher density because of the increased effective voltage. In contrast, if
the number of lit elements in the subsequent scanned scanning electrode is
larger than that of the scanning electrode prior to the selective shift,
the difference I becomes negative, resulting in display elements on the
subsequently scanned scanning electrode having a lower density because of
the decreased effective voltage. These fluctuations correspond to the
absolute value of I.
During a selective shift with polarity inversion, the effective voltage
impressed across the display elements on the subsequently scanned scanning
electrode invariably diminishes by a constant value. At the same time, the
effective voltage decreases by a value corresponding to the difference in
F before and after the selective shift.
In other words, the unevenness in contrast corresponds to the difference I
between the numbers of lit elements before and after a selective shift
with no polarity inversion, whereas if polarity inversion occurs during
the selective shift, the unevenness in contrast corresponds both to the
difference in the number of lit elements before and after the selective
shift as well as to the regular contrasting unevenness.
This first cause of contrast unevenness resulting from a selective shift
with polarity inversion is the subtle difference produced during the step
of changing the polarity between the signal and scanning voltage waveforms
outputted by the actual driving circuit.
The selective voltage is impressed just before inverting the polarity. The
magnitude of the voltage of each signal electrode corresponding to a
non-selective scanning electrode changes immediately after the inversion
has been effected to correspond to the electric charge quantity obtained
from formula (2). This change in the magnitude of the voltage is dragged
(i.e., lags, does not change instantaneously) on the side of the selective
voltage after the polarity inversion.
This phenomenon is shown in FIGS. 23, 24(a)-24(c) and 25(a). FIG. 23
illustrates liquid crystal panel 1 identical with that of FIG. 17 but with
a different display contents. FIGS. 24(a)-24(cand 25(a) illustrate voltage
waveforms for display elements D33 and D43 shown in FIG. 23, respectively.
FIG. 24(a) is the voltage waveform applied to signal electrode X3, FIG.
24(b) is the voltage waveform for scanning electrode Y3, and FIG. 24(c) is
the waveform of voltage applied across a display element D33 formed at the
intersection of signal electrode X3 and scanning electrode Y3. Similarly,
FIG. 25(a) is the voltage waveform applied to signal electrode X4, FIG.
25(b) is the voltage waveform applied to scanning electrode Y3, and FIG.
25(c) shows a voltage waveform applied to an adjacent display element D43
formed at the intersection of signal electrode X4 and scanning electrode
Y3.
Characteristic of what occurs when a lighting voltage is applied to a
signal electrode, the lighting voltage lags on the side of the selecting
voltage just after the polarity inversion, as illustrated in FIG. 24(a).
Eventually the effective voltage applied across display element D33
decreases to a degree coinciding with the lag, as shown in FIG. 24(c).
When a non-lighting voltage is applied to a signal electrode, the
non-lighting voltage also lags on the side of the selecting voltage, as
illustrated in FIG. 25(a). Eventually the effective voltage impressed on
display element D43 increases to a degree coinciding with the lag, as
shown in FIG. 25(c). For this reason, lit element D33 has less display
contrast than other lit display elements, whereas unlit element D43
becomes more visible than other unlit display elements. The unevenness on
the display is proportional to the electric charge given by formula (2).
The second cause of the contrasting unevenness is the unevenness
corresponding to the display contents on the liquid crystal panel.
Uneven contrast in the liquid crystal display can be minimized (such as
disclosed in the '750 application) by compensating the scan voltage
waveform and/or signal voltage waveform according to the characters or
patterns produced on the liquid crystal display. Uneven contrast caused by
differences in the shades of gray of the picture elements associated with
the first and last scanning electrodes compared to the picture elements
associated with the scanning electrodes therebetween can be minimized by
applying appropriate compensating voltages to the picture elements.
Neither compensation technique, however, addresses unevenness in contrast
occurring immediately after the polarity of the voltage applied to the
liquid crystal panel has been inverted.
Accordingly, it is desirable to provide a liquid crystal display apparatus
which counteracts these causes of uneven contrast in the prior art liquid
crystal display devices and, in particular, immediately after the polarity
of the voltage applied to the liquid crystal panel has been inverted.
SUMMARY OF THE INVENTION
Generally speaking, in accordance with the invention, a liquid crystal
display device having a plurality of picture elements which can be lit and
unlit to produce a pattern to be displayed includes a first substrate
including a group of scanning electrodes disposed thereon; a second
substrate spaced apart from said first substrate and including a group of
signal electrodes disposed thereon and liquid crystal material in the
space between the substrate. The device includes driving circuitry for
driving the device by providing scan voltage waveforms to the scanning
electrodes and providing signal voltage waveforms to the signal electrodes
to thereby apply voltages across the picture elements. The driving
circuitry periodically inverts the polarity of voltages applied to the
picture elements and immediately following polarity inversion varies the
voltage level of at least one of the scan voltage waveforms and signal
voltage waveforms which is associated with at least one of the picture
elements.
The unevenness in contrast occurring immediately after polarity inversion
of a voltage applied to the liquid crystal panel is minimized by providing
compensating voltages to the scanning and/or signal electrodes immediately
following polarity inversion.
As used herein, the periodic inversion of polarity of a voltage applied
across a picture element refers to switching the polarity of the voltage
applied across a picture element from one frame to the next frame.
Variation in the voltage level in the scan voltage waveform and/or signal
voltage waveform is based on the number of lit picture elements associated
with a first scanning electrode to which a selecting voltage has been
applied immediately before the polarity inversion and the number of lit
picture elements on a second scanning electrode to which a selecting
voltage is applied immediately following polarity inversion.
The scan voltage waveforms include selecting and non-selecting voltages
which are provided to the scanning electrodes. In one preferred
embodiment, variation in the non-selecting voltages applied to the
scanning electrodes occurs immediately following polarity inversion. In
this case, the selecting voltage is applied immediately before polarity
inversion. Variation in the non-selecting voltage is also based on the
number of lit picture elements on the scanning electrode to which the
selecting voltages have been applied immediately before polarity inversion
and the number of lit picture elements on the scanning electrode to which
the selecting voltage is applied immediately after polarity inversion.
Accordingly, it is an object of the invention to provide an improved liquid
crystal display device which substantially reduces the unevenness in the
contrast of the display.
It is another object of the invention to provide an improved liquid crystal
display device which corrects distortions of the scanning voltage
waveforms and/or signal voltage waveforms based on the pattern or
characters to be displayed by the liquid crystal display device.
It is further object of the invention is to provide an improved liquid
crystal display device which reduces fluctuations in the effective
voltages applied to the picture elements based on cross-talk.
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 a device possessing the features,
properties, and the relation of components which will be exemplified in
the device hereinafter described, 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 drawings,
in which:
FIG. 1 is a block diagram of the circuitry for driving a liquid crystal
display device in accordance with the invention;
FIG. 2 is a block diagram illustrating a liquid crystal display device in
accordance with a first embodiment of the invention;
FIGS. 3(a)-3(e) are timing charts of the control signals and a data signal
to be applied to the device of FIGS. 1 and 2;
FIG. 4 is a block diagram showing one example of a correction circuit for
use in the device of FIGS. 1 and 2;
FIG. 5 is a schematic diagram of a voltage power supply circuit for use in
the device of FIGS. 1 and 2;
FIG. 6 is a diagram illustrating a display matrix on the liquid crystal
panel of the device of FIGS. 1 and 2;
FIGS. 7(a)-7(c) show the voltage waveforms in accordance with a first
embodiment of the invention;
FIG. 8 is a block diagram of the circuitry of a liquid crystal display
device in accordance with another of the invention;
FIG. 9 is a block diagram showing the liquid crystal display device of FIG.
8;
FIG. 10 is a schematic diagram showing the scanning electrode driving
circuit of FIG. 8;
FIG. 11 is a block diagram of a correction circuit for use in the device of
FIG. 8;
FIG. 12 is a schematic diagram of a voltage power supply circuit for use in
the device of FIG. 8;
FIGS. 13, 15, 17 and 23 are diagrams showing various display contents of
the liquid crystal panel of FIG. 6;
FIGS. 14(a)-14(c) and 16(a)-16(c) are voltage waveforms applied to the
panel of FIG. 6;
FIGS. 18(a)-18(c) and 19(a)-19(c) are voltage waveforms applied to the
panel of FIG. 17 in the voltage averaging driving method in accordance
with the prior art;
FIGS. 20(a)-20(c) and 21(a)-21(c) are voltage waveforms applied to the
panel of FIG. 17 in the line inversion method in accordance with the prior
art;
FIG. 22 is a waveform of voltage applied to an element on the line
inversion method of FIGS. 20 and 21; and
FIGS. 24(a)-24(c) and 25(a)-25(c) are voltage waveforms applied to elements
in the display illustrated in FIG. 23.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The following illustrative embodiments in accordance with the invention are
set forth for purposes of illustration and are not presented in a limiting
sense. Embodiments 1-5 are examples of liquid crystal display devices
which overcome the problem of uneven display contrast due to cross-talk.
Embodiments 6-10 are examples of liquid crystal display devices in
accordance with the invention which overcome the problem of uneven display
contrast due to the display contents.
EMBODIMENT 1
The unevenness in contrast is, irrespective of display pattern, caused to a
degree when the polarity of the voltage applied to a display element is
inverted. As discussed above, additional unevenness corresponds to a sum F
of the number of lit elements on a scanning electrode scanned just before
and immediately after the polarity inversion.
When the scanning electrode selected during an operation other than the
polarity inversion shifts to the next electrode, the waveform applied to
the display elements is distorted in accordance with the difference I
between the number of lit display elements on the selected scanning
electrode and the number of lit dots on a subsequently selected electrode.
Hence, waveform corrections corresponding to the sum F and the difference
I may be performed after these values are calculated during an operation
of the liquid crystal display device.
FIG. 1 illustrates one specific embodiment of a liquid crystal display
device 100 constructed and arranged in accordance with the invention for
effecting these corrections. A liquid crystal device 100 includes a liquid
crystal cell 101 which includes appropriate driving circuitry. A series of
control signals 102 for controlling the operation of liquid crystal
display device 100 includes a latch signal LP, a frame signal FR, a
data-in signal DIN, an X-driver shift clock signal XSCL and a data signal
103. A waveform correcting signal generating circuit 104 receives the
control signals and is coupled to a power supply circuit 105 which in turn
is coupled to liquid crystal cell 101.
Correction circuit 104 calculates a numeric value F or I and transmits both
a code signal 108 indicating a positive or negative of the numeric value F
or I and a magnitude signal 109 indicating a magnitude of an absolute
value of F or I to power supply circuit 105. Both code signal 108 and
magnitude signal 109 are correction signals. Magnitude signal 109 is kept
in an active state for a period corresponding to the absolute value of the
numeric value F or I.
Power supply circuit 105 generates a scanning electrode driving power
supply signal 106 which is the Y-power-supply and a signal electrode
driving power supply signal 107 which is the X-power-supply signal.
Specifically for liquid crystal cell 101 in accordance with code signal
108 and magnitude signal 109. Power supply circuit 105 acts to correct the
voltage of Y-power-supply 106.
The fundamental operations of the embodiment shown in FIG. 1 are described
as follows. Correction circuit 104 receives data signal 103 when a
particular scanning electrode is selected and then counts the number
M.sub.ON of lit elements on a subsequently selected scanning electrode.
Then, correction circuit 104 determines the numeric values F and I, i.e.,
the sum of M.sub.ON and the number N.sub.ON of lit elements on the
scanning electrode presently selected, and the difference therebetween.
When the selection is shifted (polarity inversion), the resultant code and
absolute value are outputted in the form of code signal 108 and magnitude
signal 109. With polarity inversion, the numeric value I replaces the
numeric value F and is likewise outputted. Concurrently with this step,
the lit dot number M.sub.ON is taken in for storage for purposes of
determining the number N.sub.ON of the lit dots on the scanning electrode
selected. Power supply circuit 105 makes any corrections necessary for the
Voltage Of Y-power-supply 106 on the basis of code signal 108 and
magnitude signal 109.
The operations described above prevent uneven contrast which appears in the
liquid crystal panel due to the first cause, namely, the difference in
voltages applied to the element and outputted by the driving circuit.
Based on the correcting method in this embodiment, a constant voltage is
impressed in such a direction as to cancel the distortion created in the
driving waveform applied to the liquid crystal display element during the
period corresponding to the magnitude of the distortion. The direction of
the constant voltage is determined by code signal 108, while the
application period depends on magnitude signal 109.
The correction method is explained further with reference to FIGS. 2-5,
which illustrate in detail the components of FIG. 1. FIG. 2 illustrates an
example of a specific construction of liquid crystal display cell 101. A
liquid crystal display panel 201 includes a pair of substrates 202 and 203
with a liquid crystal material in the space between the substrates. A
plurality of scanning electrode lines Y1 to Y6 are arrayed sideways as
rows on upper substrate 202 and a plurality of signal electrode lines X1
to X6 are vertically arrayed as columns on lower substrate 203. Display
elements or pixels are formed at the intersections of scanning electrodes
Y1 to Y6 and signal electrodes X1 to X6. Although this particular liquid
crystal panel is a 6.times.6 matrix for simplicity of explanation, in
reality the matrix may be significantly larger.
A scanning electrode driving circuit 205 includes a shift register circuit
206 coupled to a level shifter circuit 207. Outputs from level shifter
circuit 207 are applied to scanning electrodes Y1 to Y6 liquid crystal
panel 201.
A signal electrode driving circuit 208 includes a shift register circuit
209 coupled to a latch circuit 210, which in turn outputs to a level
shifter circuit 211. Output signals from level shifter circuit 211 are
applied to signal electrodes X1 to X6 in liquid crystal panel 201.
FIGS. 3(a)-3(d) are timing charts showing signals D1N, LP, FR and XSCL,
respectively, which are included within control signals 102. FIG. 2(e)
shows a time chart of data signals 103 corresponding in time to the timing
charts of FIGS. 3(a-3(d).
Signal DIN and Signal LP function as data and shift clocks, respectively
for shift register circuit 206 of scanning electrode driving circuit 205.
Upon a last transition of Signal LP, Signal DIN is input to shift register
circuit 206 and then transferred. At this moment, Signal DIN, which is
active when assuming an "H" level, is outputted once at an interval
defined typically by the number of Signals LP which is equal to or greater
than the number of scanning electrodes Y1 to Y6 in liquid crystal panel
201. Therefore, the data of an "H" level travels through the interior of
shift register circuit 206, and in other cases the signal DIN assumes an
"L" level. If Signal DIN is active, selective voltages are supplied to
scanning electrodes Y1 to Y6 by level shifter circuit 207 according to the
contents of shift register circuit 206. If Signal DIN is inactive,
non-selective voltages are fed to scanning electrodes Y1 to Y6. Selective
voltages and non-selective voltages are both supplied from Y-power-supply
circuit 106.
Data signal 103 and Signal XSCL and Signal LP function as data and shift
clocks of signal electrode driving circuit 208 and shift register circuit
209 and also as a latch clock of latch circuit 210. As shown in FIG. 3,
data signal 103 is active when assuming an "H" level to exhibit a lit
state. Data signal 103 acts as a signal for determining the state, lit or
unlit, of a display element 204 on the next scanning electrode while a
particular scanning electrode on liquid crystal panel 201 is being
selected. During the selecting period of the particular scanning
electrode, data signal 103 is inputted to shift register circuit 209 at a
last transition of signal XSCL so that data signal 103 serves as a signal
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