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Claims  |
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What is claimed is:
1. In a driving method for a matrix type ferroelectric liquid crystal
apparatus which includes two separate display areas, each of the separate
display areas comprising plural pixels arranged in a submatrix each having
more than one row and one column, wherein the separate display areas are
arranged to be driven independently, the improvement comprising the steps
of:
scanning one of said separate display areas; and
simultaneously applying a dummy signal to a signal electrode in the other
of said separate display areas, wherein an image display in said other
display area does not change, whereby a difference in contrast between and
two separate display areas is minimized.
2. A driving method according to claim 1, wherein said dummy signal has a
voltage waveform is an alternating current.
3. A driving method according to claim 1, wherein a voltage applied to a
scanning electrode in which is not being scanned is the same as a voltage
applied to a non-selected scanning electrode of the other display area
which is being scanned.
4. A ferroelectric liquid crystal apparatus, comprising:
a first matrix electrode arrangement including a first group of scanning
electrodes and a first group of signal electrodes, said first groups of
scanning electrodes and signal electrodes crossing each other in
oppositely spaced relationship;
an electrically independent second matrix electrode arrangement including a
second group of scanning electrodes and a second group of signal
electrodes, said second groups of scanning electrodes and signal
electrodes also crossing each other in oppositely spaced relationship;
ferroelectric liquid crystal material interposed between both said first
and second groups of scanning electrodes, and said first and second groups
of signal electrodes;
driving means for driving said first and second matrix electrode
arrangements; and
controlling means for controlling said driving means to apply an
alternating voltage to the portion of said ferroelectric liquid crystal
positioned at the intersection points of one of said first and second
groups of scanning electrodes and a corresponding one of said first and
second groups of signal electrodes while the other group of scanning
electrodes is being partially scanned, said alternating voltage being
insufficient to switch said portion of said ferroelectric liquid crystal.
5. A ferroelectric liquid crystal apparatus, comprising:
a first matrix electrode arrangement including a first group of scanning
electrodes and a first group of signal electrodes, said first groups of
scanning electrodes and signal electrodes crossing each other in
oppositely spaced relationship;
an electrically independent second matrix electrode arrangement including a
second group of scanning electrodes and a second group of signal
electrodes, said second groups of scanning electrodes and signal
electrodes also crossing each other in oppositely spaced relationship;
ferroelectric liquid crystal material interposed between both said first
and second groups of scanning electrodes, and said first and second groups
of signal electrodes;
driving means for driving said first and second matrix electrode
arrangements; and
controlling means for controlling said driving means to apply, a
scanning-electrode non-selecting signal to one group of scanning
electrodes and to simultaneously apply a voltage signal to the group of
signal electrodes which corresponds to said group of scanning electrodes
while the other of said first and second groups of scanning electrodes is
being partially scanned, said voltage signal producing an alternating
voltage which is insufficient to switch said ferroelectric liquid crystal
when said scanning-electrode non-selection signal is combined with said
alternating voltage.
6. A ferroelectric liquid crystal apparatus, comprising:
a first matrix electrode arrangement including a first group of scanning
electrodes and a first group of signal electrodes, said first groups of
scanning electrodes and signal electrodes crossing each other in
oppositely spaced relationship;
an electrically independent second matrix electrode arrangement including a
second group of scanning electrodes and a second group of signal
electrodes, said second groups of scanning electrodes and signal
electrodes also crossing each other in oppositely spaced relationship;
ferroelectric liquid crystal material interposed between said first and
second groups of scanning electrodes and said first and second groups of
signal electrodes;
driving means for driving said first and second matrix electrode
arrangements; and
controlling means for controlling said driving means to apply an
alternating voltage to the portion of said ferroelectric liquid crystal
material which is positioned at each intersection point of one of said
first and second groups of scanning electrodes and a corresponding
intersection point of said first and second groups of signal electrodes
while the other group of scanning electrodes is being partially scanned,
wherein said alternating voltage is insufficient to switch said portion of
said ferroelectric liquid crystal.
7. A ferroelectric liquid crystal apparatus, comprising:
a first matrix electrode arrangement including a first group of scanning
electrodes and a first group of signal electrodes, said first groups of
scanning electrodes and signal electrodes crossing each other in
oppositely spaced relationship;
an electrically independent second matrix electrode arrangement including a
second group of scanning electrodes and a second group of signal
electrodes, said second groups of scanning electrodes and signal
electrodes also crossing each other in oppositely spaced relationship;
ferroelectric liquid crystal material interposed between said first and
second groups of scanning electrodes and said first and second groups of
signal electrodes;
driving means for driving said first and second matrix electrode
arrangements; and
controlling means for controlling said driving means to apply, a
scanning-electrode non-selecting signal to one of said first and second
groups of scanning electrodes and simultaneously to apply a voltage signal
to the group of signal electrodes which corresponds to said one group of
scanning electrodes while the other of said groups of scanning electrodes
is being partially scanned, said voltage signal producing an alternating
voltage insufficient to switch said ferroelectric liquid crystal even when
said scanning-electrode non-selection signal is combined with said
alternating voltage.
8. In a driving method for a matrix-type ferroelectric liquid crystal
apparatus which includes two separate display areas, each of the separate
display areas comprising plural pixels arranged in a submatrix having more
than one row and one column, wherein the separate display areas are
arranged to be driven independently, the improvement comprising the steps
of:
effecting partial scanning of one display area to partially rewrite the
contents displayed in said selected display area, while partially scanning
to corresponding scanning lines in the other display area; and
simultaneously applying an information signal to corresponding signal
electrodes of said other display area to hold the contents currently
displayed in other display area.
9. A ferroelectric liquid crystal apparatus, comprising:
a liquid crystal device having a first matrix electrode arrangement
including a first group of scanning electrodes and a first group of signal
electrodes;
an independent second matrix electrode arrangement including a second group
of scanning electrodes and a second group of signal electrodes, and a
ferroelectric liquid crystal material located between said first group of
scanning and signal electrodes and also between said second group of
scanning and signal electrodes; and
means for partially scanning one of said first and second matrix electrode
while applying dummy signals which do not change the image display to
signal electrodes to the other of said first and second matrix electrode
arrangements, whereby a difference in contrast between said two
independent matrix areas is minimized.
10. A ferroelectric liquid crystal apparatus, comprising:
a liquid crystal apparatus having a first matrix electrode arrangement
including a first group of scanning electrodes and a first group of signal
electrodes, an independent second matrix electrode arrangement including a
second group of scanning electrodes and a second group of signal
electrodes, and ferroelectric liquid crystal material located between the
scanning and signal electrodes; and
means for simultaneously scanning only (i) scanning electrodes in said
first matrix electrode arrangement which correspond to a partial scanning
area in which the contents displayed are to be altered and (ii) scanning
electrodes in said second matrix electrode arrangement which correspond to
a partial scanning area in which the contents displayed are not to be
altered. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a ferroelectric liquid crystal
apparatus having a display portion formed by a pair of display areas and
to a method of driving such a ferroelectric liquid crystal apparatus. More
particularly, the present invention relates to a ferroelectric liquid
crystal apparatus and driving method in which a predetermined voltage
signal is applied to one of the separate display areas during partial
rewriting (i.e., partial scanning) of the contents displayed in the other
display area for partial scanning of the device so as to reduce the
difference in contrast between the display areas. Thereby, improved image
display quality is provided.
2. Description of the Related Art
Recently, ferroelectric and TN-type liquid crystal apparatus have been
intensively developed for use as image displays. Ferroelectric liquid
crystal apparatus are especially advantageous since they provide a
memory-like capability, a fast its response speed, and contrast is
independent of the duty ratio. Accordingly, it would be desirable to apply
the ferroelectric liquid crystal apparatus, for instance, to
large-capacity dot matrix apparatus.
FIG. 2 is a schematic view showing one example of the above-described
ferroelectric liquid crystal apparatus. As illustrated, this example is
provided with a group 1 of scanning electrodes, a group 2 of upper signal
electrodes (as viewed in the figure) and a group 3 of lower signal
electrodes (as viewed in the figure). The group 1 of scanning electrodes
is oppositely spaced apart from groups 2 and 3 of upper and lower signal
electrodes with a ferroelectric liquid crystal interposed therebetween. A
scanning electrode driving circuit 4 is disposed to apply a voltage to the
group 1 of scanning electrodes, while an upper signal electrode driving
circuit 5 and a lower signal electrode driving circuit 6 are disposed to
apply voltages to the group 2 of upper signal electrodes and the group 3
of lower signal electrodes, respectively. The group 1 of scanning
electrodes and the group 2 of upper signal electrodes cross each other to
form an upper display area 7, while the group 1 and the group 3 of lower
signal electrodes cross each other to form a lower display area 8. In each
of the upper display area 7 and the lower display area 8, picture elements
are arranged in a matrix form.
FIG. 3 is a schematic diagram showing exemplary forms of applied voltages
which act to drive the above-described matrix type display apparatus on a
multiplexing basis. As illustrated, scanning pulses 9 are sequentially
applied to the group 1 of scanning electrodes, while information signal
pulses 10 are sequentially applied in parallel to group 2 or 3 of signal
electrodes in synchrony therewith so as to provide a visual image display.
If an electric field is not subsequently applied, the last image which was
displayed is held. Therefore, the image or information which is required
for display on the display apparatus is only rewritten when the necessity
for global rewriting arises or when a particular portion (display area) is
rewritten.
Although the present inventors carried out the above-described conventional
driving method, they found that image contrast markedly differs during a
partial-scanning period than during a non-scanning period. This is
noticeable, for instance, in the case of the panel-shaped display
apparatus of the horizontal split type shown in FIG. 2, which is made of
upper display area 7 and lower display area 8 which are separately driven
by the groups 2 and 3 of upper and lower signal electrodes. That is, when
the upper display area 7 is being scanned in a partial-scanning mode, the
contrast of an image displayed in the lower display area 8 (which is not
being scanned) is noticeably different from an image displayed in the
upper display area 7. This phenomenon, of course, seriously affects the
quality of the displayed image. The inventors believe that the difference
in image contrast between a scanning period and a non-scanning period
arises because, although when no electric field is being applied to a
ferroelectric liquid crystal apparatus, the molecules thereof remain
oriented in one of two stable states so that the molecules assume the
other state if a threshold-exceeding electric field is applied upon the
application of an electric field not exceeding the threshold, the
molecules nevertheless change their positions more or less, although their
state does not substantially change. In FIG. 4, this phenomenon is shown
plotted as the relationship between time and a variation in the quantity
of light transmitted under cross-nicol conditions as an applied voltage 11
not exceeding the threshold is applied over time. Reference numeral 12
denotes a curve which indicates the corresponding variation in the
quantity of transmitted light. It is seen in FIG. 4 that the average
quantity of light transmitted during the application of a voltage differs
from the quality of light transmitted during application of no voltage.
Accordingly, as shown in FIG. 3, during a scanning period, a non-selection
voltage (not exceeding the threshold) shown in FIG. 4 is applied to
picture elements on non-selected scanning electrodes so that the quantity
of light which is transmitted during the scanning period is different than
the quantity of light which is transmitted during the non-scanning period.
As a result, contrast varies between the scanning and non-scanning
periods, as well as between a portion which is being scanned and a portion
which is not being scanned. If variation in contrast over time is to be
suppressed to a minimum, a refresh scanning (driving) method may be
adopted in which information signal pulses are always applied to the
signal electrodes so that the contrast is kept constant.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide a driving
method which enables a ferroelectric liquid crystal apparatus to be
partially scanned without causing any serious difference in contrast
between the scanned display area and the display area which is not being
scanned.
It is another object of the present invention to provide a ferroelectric
liquid crystal apparatus adapted to be driven by the above driving method.
To achieve these objects and others, in accordance with one aspect of the
present invention, there is provided an improved driving method for a
ferroelectric liquid crystal apparatus of the matrix type which has a
display portion made up of two separate display areas arranged to be
independently driven. In accordance with the improvement, a dummy signal
is applied to a signal electrode in one of the separate display areas
while the other separate display areas is being scanned. Otherwise, while
partial scanning PG,6 is being performed to partially rewrite the contents
displayed in one display area, corresponding scanning lines in the other
display area are partially scanned to hold the contents currently
displayed therein.
In accordance with another aspect of the present invention, there is
provided a ferroelectric liquid crystal apparatus having first and second
matrix electrode arrangements, the first matrix electrode arrangement
including a first group of scanning electrodes and a first group of signal
electrodes, the first groups of scanning electrodes and signal electrodes
crossing each other in oppositely spaced relationship, and the second
matrix electrode arrangement including a second group of scanning
electrodes and a second group of signal electrodes, the second groups of
scanning electrodes and signal electrodes crossing each other in
oppositely spaced relationship, the second group of signal electrodes
being electrically independent from the first group of signal electrodes;
a ferroelectric liquid crystal interposed between the first and second
groups of scanning electrodes and the first and second groups of signal
electrodes; driver circuits for driving the first and second matrix
electrode arrangements; and a controller circuit for controlling the
driver circuits so as to apply an alternating voltage to the portion of
the ferroelectric liquid crystal which is positioned at each intersection
point (pixel) of one of the first and second groups of scanning electrodes
and a corresponding one of the first and second groups of signal
electrodes while the other group of scanning electrodes is being partially
scanned, wherein the alternating voltage is at a level which is
insufficient to switch that portion of the ferroelectric liquid crystal.
Further objects, features and advantages of the present invention will
become apparent from the following detailed description of various
embodiments of the present invention with reference to the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a timing chart showing a set of drive pulses in a method of
driving a ferroelectric liquid crystal apparatus in accordance with a
first embodiment of the present invention;
FIG. 2 is a partial schematic view showing a ferroelectric liquid crystal
apparatus of the horizontal split type;
FIG. 3 is a schematic diagram showing a set of drive pulses for driving the
apparatus of FIG. 2 on a multiplexing basis;
FIG. 4 graphically represents variations in the quantity of transmitted
light with respect to voltages not exceeding a threshold voltage of the
liquid crystal;
FIG. 5 is a timing chart showing a set of drive pulses in a refresh-driving
method for the apparatus of FIG. 2;
FIG. 6 is a timing chart showing conventional.drive pulses corresponding to
a partial-scanning mode;
FIG. 7 is a partial schematic view of an apparatus having scanning and
signal electrode driving circuits;
FIG. 8 is a timing chart showing a set of drive pulses in a method of
driving a ferroelectric liquid crystal apparatus in accordance with a
second embodiment of the present invention;
FIG. 9 is a partial schematic view showing a ferroelectric liquid crystal
apparatus which is driven in accordance with a third embodiment of the
present invention;
FIGS. 10A-B are block diagrams showing a peripheral circuit for controlling
the waveforms of the driving signals used in the first embodiment of the
present invention; and
FIG. 11 is a timing chart illustrating the driving signals of FIG. 10.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The inventors believe that the difference in image contrast between a
scanning period and a non-scanning period arises because, although when no
electric field is being applied to a ferroelectric liquid crystal
apparatus, the molecules thereof remain oriented in one of two stable
states so that the molecules assume the other state if a
threshold-exceeding electric field is applied upon the application of an
electric field not exceeding the threshold, the molecules nevertheless
change their positions more or less , although their state does not
substantially change. In FIG. 4, this phenomenon is shown plotted as the
relationship between time and a variation in the quantity of light
transmitted under cross-nicol conditions as an applied voltage 11 not
exceeding the threshold is applied overtime. Reference numeral 12 denotes
a curve which indicates the corresponding variation in the quantity of
transmitted light. It is seen from FIG. 4 that the average quantity of
light transmitted during the application of a voltage differs from the
quality of light transmitted during application of no voltage.
Accordingly, as shown in FIG. 3, during a scanning period, a non-selection
voltage (not exceeding the threshold) shown in FIG. 4 is applied to
picture elements on non-selected scanning electrodes so that the quantity
of light which is transmitted during the scanning period is different than
the quantity of light which is transmitted during the non-scanning period.
As a result, contrast varies between the scanning and non-scanning
periods, as well as between a portion which is being scanned and a portion
which is not being scanned. If variation in contrast over time is to be
suppressed to a minimum, a refresh scanning (driving) method may be
adopted in which information signal pulses are always applied to the
signal electrodes so that the contrast is kept constant.
In a liquid crystal apparatus of the horizontal split type, the upper
display area and the lower display area are scanned at the same time. An
example of a voltage which is applied to each electrode in the refresh
driving method is shown in FIG. 5. In the figure, symbols S.sub.1 -S.sub.n
represent scanning pulses applied to the upper display area, symbols
Sn.sub.+1 -S.sub.2n scanning pulses applied to the lower display area,
symbols I.sub.1 and I.sub.2 signal pulses applied to the upper display
area, and symbols I'.sub.1 and I'.sub.2 signal pulses applied to the lower
display area. If the necessity for partial rewriting arises while refresh
driving is being performed, as shown in FIG. 6, the refresh scanning
(period 13) is tempofarily interrupted and partial-scanning occurs (period
14), that is, the contents on scanning lines are rewritten corresponding
to the scanning pulses S.sub.1 and S.sub.2. On the panel-shaped display
apparatus of the horizontal split type, the lower display area is not
scanned during the partial-scanning period 14 in which the upper display
area is scanned. Accordingly, no information signal pulse I'.sub.1 or
I'.sub.2 is applied to any of the signal electrodes of the lower display
area and a contrast difference therefore occurs between the upper display
area and the lower display area.
In the present invention, however, during the partial-scanning period for
which the upper display area is scanned, dummy information signals are
applied to the signal electrodes of the lower display area while selecting
pulses are not applied to the scanning electrodes of the lower display
area. Accordingly, the contents displayed in the lower display area are
not changed and the contrast of the image displayed in the lower display
area is kept substantially the same as that of the image displayed in the
upper display area. Alternatively, during the partial-scanning period in
which only the upper display area is scanned, the scanning lines in the
lower display area corresponding to the partially scanned lines in the
upper display are also scanned for purposes of refreshment. Accordingly,
as before, the contrasts of the images displayed in the upper and lower
display areas are kept approximately equal.
The above-described partial scanning processe's (for partial rewriting) are
disclosed in, for example, U.S. Pat. Nos. 4,655,561 and 4,693,563.
Specific embodiments of the present invention will be explained below with
reference to the accompanying drawings.
FIG. 1 is a timing chart showing applied voltages according to a first
embodiment of the present invention. In the following description, the
first embodiment is applied to a ferroelectric liquid crystal apparatus of
the horizontal split type which consists of two upper and lower display
areas, as shown in FIG. 2.
In FIG. 1, as mentioned previously concerning FIG. 5, symbols S.sub.1
-S.sub.n represent scanning (selecting) pulses applied to the upper
display area, symbols Sn.sub.+1 -S.sub.2n scanning pulses applied to the
lower display area, symbols I.sub.1 and I.sub.2 signal pulses applied to
the upper display area, and symbols I'.sub.1 and I'.sub.2 signal pulses
applied to the lower display area. As in the conventional example shown in
FIG. 6, a partial-scanning period 14 during which only the upper display
area is scanned is interposed between successive periods 13. allocated for
refresh scanning. The difference, however, between FIG. 1 and FIG. 6 is
that each of the signal pulses I'.sub.1 and I'.sub.2 includes a pulse
train 15 lasting for a time period corresponding to the partial-scanning
period 14 whereby dummy information signals are applied to the signal
electrodes of the lower display area. However, since no scanning pulses
are applied to any of the scanning electrodes of the lower display area
during period 14, the contents of the image displayed in the lower display
are do not change. Accordingly, the device is partially scanned and the
contrasts of the images displayed in the upper display area and the lower
display area are still maintained substantially equal.
FIG. 10 is a block diagram showing a peripheral circuit for controlling the
waveforms of the driving signals used in the first embodiment of the
present invention and FIG. 11 is a timing chart which illustrates the
driving signals. The illustrated peripheral circuit includes a control
circuit 101, a display memory 102, reset pulse generating circuits 103A
and 103B, a scanning clock generating circuit 104A for the upper display
area, and a scanning clock generating circuit 104B for the lower display
area.
During a normal scanning period,.the control circuit 101 generates scanning
enable signals so the upper and lower display areas are scanned at the
same time. In response to the scanning enable signals, the scanning clock
generating circuits 104A and 104B generate scanning clock pulses for the
upper and lower display areas, respectively. In contrast, during the
partial-scanning period 14 during which only the upper display area is
scanned as shown in FIG. 1, the scanning enable signal for the lower
display area is held in a "0" (disable) state wherein the scanning clock
generating circuit 104B stops generating the scanning clock pulses for the
lower display area. During this time, the reset pulse generating circuit
103B operates to hold a data reset pulse for the lower display area in a
"1" (reset) state. In response to a data reset pulse of the 1 level, a
lower-display-segment driving circuit 6 converts all the bit data into
dummy data representing "white", and outputs the white information signal
having the waveform 15 is shown in FIG. 1.
Various waveforms of information signals may be utilized as dummy signals.
For example, information signals representing "white" or "black" may be
applied to all the signal lines, or information signals representing
"white" and "black" may also be alternately applied to the signal lines.
Information signals representing the same color may be applied to the
corresponding portions in the upper and lower display areas. In any case,
it has been found that contrast differences do not occur between the
images displayed in the upper and lower display areas, whereby the quality
of the displayed image is improved.
In the above explanation, the partial scanning of the upper display area
has been referred to. However, it is clear that during a partial-scanning
period in which the lower display area is scanned, the dummy information
signals as described above are applied to the signal electrodes of the
upper display area.
In the above-described horizontally-split panel-like liquid crystal display
apparatus it is preferable for the sake of convenience of the circuit
arrangement that during the period in which the upper display area is
partially scanned, scanning-electrode selecting pulses are applied to
corresponding scanning electrodes in the lower display area. FIG. 7
illustratively shows an arrangement in which a scanning-electrode driving
circuit consists of an upper scanning-electrode driving circuit 4a and a
lower scanning-electrode driving circuit 4b, wherein a control signal S
and other scanning-line address signals which control the operation of the
driving circuits 4a and 4b are shared by upper scanning-electrode driving
circuit 4a and lower scanning-electrode driving circuit 4b. In such an
arrangement, if one of the scanning lines (scanning electrodes) in the
upper display area 7 is selected, a corresponding scanning electrode in
the lower display area 8 is also selected. In this arrangement, shown in
FIG. 8 (as a second embodiment of the present invention) during the
partial-scanning period 14 in which the upper display area 7 is scanned,
an information signal pulse train 17 (which represents the same signal as
an information signal pulse train 16 corresponding to the currently
displayed image) may be applied to prevent a change in the contents
displayed in a corresponding portion in the lower display area. In this
manner, the contrast can be made approximately equal between the upper and
lower display areas, as in the first embodiment.
FIG. 9 is a schematic view showing a panel-like matrix display apparatus of
the vertical split type to which a third embodiment of the present
invention is applied wherein scanning electrodes are grouped into
right-hand and left-hand display areas 21 and 20. This display apparatus
is provided with a group of left-hand scanning electrodes 1a, a group of
right-hand scanning electrodes lb, a scanning-electrode driving circuit 4c
for driving the left-hand scanning electrodes la, a scanning-electrode
driving circuit 4d for driving the right-hand scanning electrodes 1b, a
group of signal electrodes 18 and a signal-electrode driving circuit 19.
In this panel-like display apparatus, during the partial-scanning period
for which the left-hand display area 20 is scanned, dummy information
signal pulses are applied to the signal electrodes 20 in the right-hand
display area 21, thereby maintaining as constant the contrasts of the
images displayed in the right and left-hand display areas.
As is apparent from the foregoing, in accordance with the present
invention, during a partial-rewriting (partial-scanning) period in which
one display area of a display panel having two display areas is scanned,
dummy signals are applied to the signal electrodes of the other of the two
display areas, or the currently displayed contents are repeatedly
displayed again in the other display area by corresponding scanning
electrodes. Accordingly, the present invention makes it possible to effect
partial scanning without any serious difference in contrast between the
display areas, in order to obtain improved quality of the displayed image.
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