The present invention relates to a method of and apparatus for addressing
a ferroelectric liquid crystal device (FLCD) and to an FLCD. Such FLCDs
may be used to provide high resolution display panels, for instance for use
in personal computers and high definition television (HDTV).
Known FLCD display panels comprise rows and columns of picture
elements (pixels) provided with row and column electrodes for passive
matrix addressing. Strobe signals are supplied in sequence to the row
electrodes whereas data signals are supplied simultaneously and in
synchronism with the strobe signals to the column electrodes. Thus, the
display is refreshed by writing display data to the pixels a row at a time.
Once a complete frame of image data has been supplied, the process is
repeated. Such drive schemes rely on the bistability of the ferroelectric
liquid crystal (FLC) to retain the image data i.e. the desired optical state,
between consecutive pixel refreshes.
In general, each row refresh cycle uses a strobe signal which comprises a
blanking pulse for resetting all the pixels of the row to a predetermined
state, such as maximally opaque (black) or maximally transparent (white),
followed by a strobe pulse which is simultaneous with data pulses of the
data signals on the column electrodes. Various addressing or drive
schemes are known for achieving this. For instance, JP-HO 6-1309 and
GB2249653A disclose drive schemes in which an additional pulse is
provided between the blanking pulse and the strobe or main switching
pulse. The purpose of the additional pulse is to improve switching times
for black and white i.e. two grey level displays in which each pixel has a
single switching threshold. WO 95/27971 also discloses a drive scheme
for a two grey level display in which an additional pulse is provided
between a blanking pulse and a switching pulse.
Various other drive schemes are known for FLCs with negative dielectric
anisotropy exhibiting a minimum in their τ-Vmin (slot time-voltage)
characteristics. P.W.H. Surguy et al, Ferroelectrics, 122,63, 1991
discloses a drive scheme known as the JOERS/ALVEY scheme. C.T.H.
Yeoh et al, Ferroelectrics, 132,293, 1992 discloses another type of drive
scheme. J.R. Hughes and E.P. Raynes, Liquid Crystals 13,597, 1993
discloses a strobe pulse expansion type of scheme known as the Malvern
scheme. EP 0 710 945 discloses a pixel pattern independent drive scheme
which can reduce the effects of pixel pattern by using special data signals.
FLCDs are prime contenders for use in HDTV panels and high resolution
display applications, particularly because of the rapid refresh rates which
can be achieved and which allow such panels to operate at video speeds.
However, such applications require the production of grey levels, for
instance a minimum of 256 grey levels for HDTV. Digital techniques
known as spatial dither and temporal dither have been used to produce
grey levels but, even when used in combination, have been limited to 64
grey levels in practical display panels.
In order to achieve additional analogue grey levels, FLCDs having two or
more different threshold levels within each pixel have been proposed, for
instance in JPS 62-145216 and in P.W. Ross et al, SID International
Symposium, Digest of Technical Papers, 147, XXV, 1994. For instance,
the different threshold levels are achieved by subdividing each pixel into
subpixels of different cell thickness. By controlling switching of the two or
more areas of each pixel with different threshold levels independently, it is
possible to achieve more than three additional grey levels. However,
problems arise with independently controlling the different pixel areas or
subpixels as described hereinafter.
According to a first aspect of the invention, there is provided a method of
addressing a ferroelectric liquid crystal device picture element having a
plurality of switching thresholds corresponding to a plurality of grey levels,
comprising applying to the picture element an electric field having a
resetting pulse of a first polarity for resetting the picture element to a reset
grey level, a compensating pulse of a second polarity opposite the first
polarity for reducing τmin shift, and a waveform for achieving a selected
grey level.
The RMS voltage of the compensating pulse may be less than the RMS
voltage of the resetting pulse.
The reset grey level may be a maximally opaque level of the picture
element.
The reset grey level may be a maximally transparent level of the picture
level.
The method may be used for a device of the type comprising a plurality of
picture elements arranged as rows and columns, strobe signals may be
applied in turn to the rows and data signals may be supplied
simultaneously to the columns in synchronism with the strobe signals for
simultaneously selecting the selected grey levels of the picture elements of
each row. Each strobe signal may comprise the resetting pulse, the
compensating pulse and a strobe pulse. The strobe pulse may be of the
second polarity.
The or each picture element may comprise a plurality of regions having
the plurality of switching thresholds. The regions may be of different
thicknesses.
According to a second aspect of the invention, there is provided an
apparatus for addressing a ferroelectric liquid crystal device picture
element having a plurality of switching thresholds corresponding to a
plurality of grey levels, comprising a waveform generator for applying to
the picture element an electric field, characterised in that the waveform
generator is arranged to apply an electric field having a resetting pulse of a
first polarity for resetting the picture element to a reset grey level, a
compensating pulse of a second polarity opposite the first polarity for
reducing τmin shift, and a waveform for achieving a selected grey level.
According to a third aspect of the invention, there is provided a
ferroelectric liquid crystal device characterised by comprising an apparatus
according to the second aspect of the invention, in which the or each
picture element comprises a plurality of regions having the plurality of
switching thresholds.
The regions may be of different thicknesses.
The device may be of passive matrix type.
It is thus possible to provide an FLCD which is capable of displaying one
or more grey levels additional to the "black" and "white" grey levels and
in which the intermediate grey level can be reliably addressed. In
particular, by adopting the compensating pulse, the effects of τmin shift
between different thresholds is reduced so that a larger driving region for
grey scale can be achieved.
By using two bits of spatial dither and two bits of temporal dither, four
analogue grey levels are required to produce 256 grey levels for each
pixel. The four analogue grey levels can be achieved and reliably
addressed by means of the present drive scheme. It is thus possible to
produce display panels which are suitable for use in HDTV and in high
resolution displays operating at video rates.
The invention will be further described, by way of example, with
reference to the accompanying drawings, in which:
Figure 1 is a schematic diagram of an FLCD to which the invention may
be applied; Figure 2 is a schematic diagram of a multi-threshold pixel of the display of
Figure 1; Figure 3 is a graph illustrating the τ-V curves for the two regions of the
pixel shown in Figure 2; Figure 4 is a graph illustrating τ-V curves for three kinds of data voltages
for providing three grey levels from a pixel of the type shown in Figure 2; Figure 5 illustrates addressing waveforms for achieving three kinds of τ-V
curves; Figure 6 illustrates actual τ-V curves achieved using the waveforms of
Figure 5; Figure 7 illustrates a simple monopulse for application to a pixel of the
type shown in Figure 2; Figure 8 is a graph showing the τ-V curves achieved using the waveform
shown in Figure 7; Figure 9 illustrates a waveform comprising a monopulse preceded by a
banking pulse for application to the pixel of Figure 2; Figure 10 is a graph showing the τ-V curves achieved using the waveform
of Figure 9; Figures 11(a) and (b) show schematic τ-V curves obtained where a
switching pulse is preceded by a banking pulse, with the time interval
between the blanking pulse and the switching pulse being large (Figure
11(a)) and small (Figure 11(b)); Figure 12 is a graph illustrating τ-V curves for three kinds of data signals
for providing three grey scales from a pixel divided into two regions; Figure 13 shows a conventional strobe signal for a multi-thickness pixel at
A and waveforms including compensating pulses at B and C; Figure 14 shows alternative waveforms for a conventional strobe signal at
A and for strobe signals including compensating pulses at B and C; Figure 15 shows data signals together with a conventional strobe signal
and a strobe signal including a compensating pulse; and Figures 16(a) and (b) are graphs of τ-V curves illustrating driving windows
achieved by the strobe pulses shown in Figure 15.
Like reference numerals refer to like parts throughout the drawings.
Figure 1 shows an FLCD display panel comprising a 4x4 array of pixels.
In practice, such a display would comprise many more pixels arranged as
a square or rectangular matrix but a 4x4 array has been shown for the sake
of simplicity of description.
The display panel comprises four column electrodes 1 connected to
respective outputs of a data signal generator 2 so as to receive data signals
Vd1 to Vd4. The generator 2 has a data input 3 for receiving data to be
displayed, for instance one row at a time. The generator 2 has a
synchronising input 4 for receiving timing signals so as to control the
timing of the supply of the data signals Vd1 to Vd4 to the column or data
electrodes 1.
The display further comprises four row electrodes 5 connected to
respective outputs of a strobe signal generator 6 so as to receive respective
strobe signals Vs1 to Vs4. The generator 6 has a synchronising input
which is also connected to receive timing signals for controlling the timing
of supply of the strobe signals Vs1 to Vs4 to the row or strobe electrodes
5.
The display further comprises an FLC arranged as a layer between the data
electrodes 1 and the strobe electrodes 5. The FLC has negative dielectric
anisotropy and has a minimum in its τ-V characteristic. The intersections
between the data and strobe electrodes define individual pixels which are
addressable independently of each other. The FLC is bistable and the
display is of the passive matrix addressed type.
One of the pixels of the display shown in Figure 1 is shown in Figure 2 in
more detail. The pixel is divided into subpixels shown as first and second
regions 7 and 8, although each pixel may be divided into more than two
subpixels. The first and second regions 7 and 8 are of different thicknesses
so as to have different switching thresholds. Such an arrangement allows
an additional grey level to be provided by a technique known as the
Multi-Threshold Modulation (MTM) method.
In the embodiment described, the regions 7 and 8 are of different
thicknesses. However, any technique may be used for achieving different
switching characteristics in the regions 7 and 8.
Figure 3 illustrates the switching characteristics of the first and second
regions by unbroken and broken lines forming τ-V curves 9 and 10,
respectively, where τ is the length of a switching signal and V is the
amplitude of the switching signal. For switching signals which occur
above the curve 9, the first region 7 is switched to one of its stable states
whereas, for switching signals below the curve 9, the first region 7 remains
in its other stable state. The switching characteristic for the second region
8 is of the same type. Accordingly, for a switching signal whose period
and amplitude are in a region 11 which is above the curve 9 and below
the curve 10, the first region 7 switches but the second region 10 does not
switch. Similarly, for the region 12, the second region 8 switches but the
first region 7 does not. For the area 13 which is above both the curves 9
and 10, both of the regions 7 and 8 switch. For the area 14 below both
the curves 9 and 10, neither of the regions 7 and 8 switches. Thus, if the
first and second regions 7 and 8 are of the same area, it is possible to
select independently three grey levels corresponding to "black", "white",
and an intermediate grey level. For instance, waveforms whose τ and V
fall within the regions 14, 13 and 11 would achieve this. If the regions 7
and 8 are of different areas, an additional intermediate grey level may be
achieved by also using the area 12 of the τ-V plane illustrated in Figure 3.
As described hereinbefore, one technique for achieving MTM is for the
regions such as 7 and 8 to be of different thicknesses. In general, the
difference between the applied voltages for switching regions of different
thicknesses is almost proportional to the difference in thicknesses. Thus,
varying the thickness of the pixel region results in a Vmin shift in the τ-V
plane as illustrated in Figure 3.
In order to achieve three grey levels from an FLCD with two different
threshold levels for each pixel, three kinds of data voltages are needed and
give rise to three different τ-V curves as illustrated in Figure 4. In
particular, W1, I1 and B1 represent the worst, intermediate and best data
voltages, respectively, for the first region 7 whereas W2, I2 and B2
represent the worst, intermediate and best voltages, respectively, for the
region 8. The shaded region between the curves I1 and I2 illustrates the
driving window for achieving an intermediate grey level from two MTM
regions 7 and 8.
By using a switching signal from the area 14, which is below both the
worst curves W1 and W2, neither of the regions 7 and 8 is switched.
Thus, if the initial pixel state was black, the "worst" voltage leaves the
pixel in its black state. When the best data voltage is applied, the curves
B1 and B2 are observed. The driving window shown at 12 is above both
the curves B1 and B2 so that both MTM regions 7 and 8 are switched to
the white state if the initial state was black. When the intermediate data
voltage is applied, the τ-V curves I1 and I2 are achieved. The driving
window 13 is above the curve I2 but below the curve I1 so that the MTM
region 8 is switched but the region 7 is not switched. This gives the
intermediate (half black and half white) state of the pixel. Thus, if three
types of data voltage giving the τ-V curves shown in Figure 4 are used, the
three grey levels of the MTM pixel can readily be achieved. Figure 5
illustrates data and strobe signals which achieve this performance and
Figure 6 shows actual experimental results achieved by the waveforms
shown in Figure 5 for a standard test cell comprising parallel-rubbed
aligning layers to provide approximately 5 degrees of surface tilt and
ferroelectric liquid crystal type FLC-1 of negative dielectric anisotropy
developed by Sharp K.K. in Japan.
Although the τ-V curves shown in Figures 3 and 4 are represented by
single lines, the τ-V curves actually comprise two curves which are
referred to as the 0% curve and the 100% curve. For example, as the
pulse width is increased while maintaining the pulse height fixed, the
pixel begins to switch at some point, which defines the 0% curve. As the
pulse width increases, the switched area of the pixel increases until finally
the whole area of the pixel is switched to give the 100% curve. Thus,
driving conditions (i.e. combinations of pulse width and pulse height)
above the 100% curve cause full switching of the pixels whereas driving
conditions below the 0% curve give non-switching. The pixel is not
switched at all under driving conditions below the 0% curve for the worst
data voltage. Similarly, applying driving conditions above the 100% curve
for the best data ensures that the pixel is totally switched.
In order to achieve grey scale in an FLCD, a blanking pulse is provided
before the main switching or strobe pulse. All the pixels of the line
currently being strobed are thus reset to a fully switched state by the
blanking pulse. Following this, the resultant between the main or strobe
pulse and the data voltage during the selected period results in the desired
grey level of the pixel being selected. The blanking pulse is necessary in
order to ensure reliability of selection of the grey levels.
Applying strobe signals having blanking pulses to FLCDs with MTM pixels
as shown in Figure 2 causes problems, particularly in the case of multi-thickness
pixels having two or more regions such as 7 and 8 of difference
thicknesses. Figure 7 illustrates the waveform of a strobe signal having no
banking pulse but having main switching on strobe pulses of amplitude
Vs or -Vs having a duration of two slot widths. Figure 8 illustrates typical
τ-V curves of thinner and thicker regions with each pixel comprising one
of each. Thus, only Vmin is changed by variation of the thickness.
Figure 9 illustrates a waveform having the same "monopulse" as in Figure
7 but having a preceding blanking pulse of amplitude -½Vs and duration
of four slot widths. The τ-V curves for this waveform are shown in Figure
10, from which it is apparent that not only does Vmin shift but τmin also
shifts.
This is explained in more detail in Figures 11(a) and (b). If the blanking
pulse (B) precedes the strobe pulse (S) by a certain time interval, the
switching characteristic of the liquid crystal is not affected. This is shown
in Figure 11(a), which shows the τ-V characteristic above the voltage
waveform applied to the liquid crystal. Increasing the time interval
between the blanking pulse and the switching pulse does not affect the τ-V
characteristic.
If, however, the time interval between the blanking pulse and the
switching pulse is small, then the τ-V characteristic is modified. As
shown in Figure 11(b), the minimum switching time is increased, whilst
the voltage at which the switching time is a minimum is decreased. To
prevent the minimum switching time being increased in this way, it is
usual to provide a large time interval (for example, at least ten times
greater than the line address time between the blanking pulse and the
switching pulse.
It will be seen from Figure 10 that the shift in τmin depends on the
thickness of the liquid crystal layer.
The τmin in the thicker region is larger than that in the thinner region.
Thus, although only Vmin was expected to shift, some driving conditions
such as those including blanking pulses cause τmin to shift also. As
illustrated in Figure 12, the drive window 13 for the intermediate grey
level is substantially reduced and in fact may disappear because of the
τmin shift effect. Thus, the presence of the blanking pulses causes the
unexpected τmin shift which makes the driving window 13 narrower.
This is particularly apparent from comparing Figure 12 with Figure 4,
which illustrates the driving window 13 in the absence of such τmin shift.
In order to avoid the problem of narrowing of the intermediate grey level
drive window 13 in the presence of a blanking pulse, a compensating
pulse of opposite polarity to the blanking pulse is provided between the
blanking pulse and the strobe or main switching pulse. It has been found
that the presence of such a compensating pulse increases the width of the
drive window 13 for intermediate grey levels as compared with the use of
a blanking pulse without the compensating pulse.
Figure 13 illustrates at A a conventional waveform for a strobe signal
having a strobe pulse of amplitude Vs occupying two time slots and a
preceding blanking pulse of amplitude -½Vs occupying four time slots.
Figure 13 shows at B a strobe signal which differs from that shown at A in
that the blanking pulse is extended forward by two time slots and a
compensating pulse 20 of amplitude Vs and occupying one time slot
immediately follows the blanking pulse. Figure 14 shows at C another
strobe signal which differs from that shown at A by the provision of the
compensating pulse 20 of amplitude Vs occupying one time slot.
The strobe waveform shown at B is DC balanced whereas that shown at C
is unbalanced. In order to preserve DC balance, the waveform shown at C
may have a small DC offset during part or all of a frame. The waveform
shown at C may be inverted in alternate frame refresh cycles for each row.
Figure 14 illustrates the effective electric field across a pixel corresponding
to the use of the strobe signals shown in Figure 13 together with a data
signal of the type having an amplitude Vd and a positive value in the two
time slots before the strobe pulse and a negative value in the two time
slots occupied by the strobe pulse. These waveforms correspond to a so-called
switching pulse in the JOERS/ALVEY driving scheme referred to
hereinbefore. These waveforms were used to measure the τ-V curves for
FLC cells showing different thickness variations. In particular, a cell A had
two regions of different thickness, one having a thickness of 1 micrometer
and the other a thickness of 1.4 micrometer. A cell B had a region of
thickness 1 micrometer and another region of thickness 1.8 micrometer.
Using the waveforms shown in Figures 13 and 14, the results illustrated in
tables 1 and 2, respectively, were obtained.
| Distance | Blanking C | Blanking B | Blanking A |
Cell A | 1slot | | 3.0us | 3.7us |
| 20.1% | 22.1% |
5slot | | 2.4us | 2.9us |
| 20.7% | 23.7us |
Cell B | 1slot | 6.8us | 6.4us | 8.5us |
40.2% | 37.9% | 45.6% |
5slot | 5.9us | 5.6us | 6.6us |
43.9% | 41.8% | 47.1% |
| Distance | Blanking C | Blanking B | Blanking A |
Cell A | 1slot | | 2.2us | 3.2us |
| 17.5% | 23.7% |
5slot | | 2.0us | 2.6us |
| 22.0% | 27.4% |
Cell B | 1slot | 5.3us | 5.5us | 7.7us |
36.9% | 37.8% | 50.5% |
5slot | 4.9us | 4.8us | 5.8us |
47.3% | 45.7% | 53.7% |
In both tables the upper values show Δτmin in microseconds and the
lower values show Δτmin/τmean as a percentage, where Δτmin is the
difference between the τmin values for the thinner and thicker regions and
τmean is the mean value of the τmin values in the thicker and thinner
regions. As is clear from comparing the table column headed "blanking
A" (prior art) with "blanking B" or "blanking C", the presence of the
compensation pulse decreases the τmin shift effect.
The cells A and B were parallel-rubbed to provide approximately 5
degrees of surface tilt. The FLC material used in the cells was material
known as FLC-1 of negative dielectric anisotropy developed by Sharp K.K.
in Japan. Figure 15 illustrates data and strobe signals for achieving three
grey levels in a pixel of the type shown in Figure 2 having two MTM
regions. The strobe signal labeled "strobe (a)" is of the conventional
blanking pulse type whereas the strobe signal labelled "strobe (b)" is of the
type in which the blanking pulse is followed by a compensating pulse.
These signals were applied to an FLC cell containing FLC-1 and parallel-rubbed
to provide approximately 5 degrees of surface tilt. The thinner
region of the cell or pixel was 1 micrometre thick whereas the thicker
region was 1.4 micrometre thick.
The measured τ-V curves shown in Figures 16(a) and (b) correspond to the
use of the strobe (a) and strobe (b) waveforms, respectively, shown in
Figure 15. As shown in Figure 16(a), the driving window for the
conventional strobe waveform without the compensating pulse is very
narrow so that reliable switching to the intermediate grey level would be
difficult to achieve. As shown in Figure 16(b), the use of the
compensating pulse 20 results in a much wider driving window for the
intermediate grey level, which can therefore be more reliably selected.