EP0838802A2

EP0838802A2 - Method of and apparatus for addressing a ferroelectric liquid crystal device and a ferroelectric liquid crystal device

Info

Publication number: EP0838802A2
Application number: EP97307638A
Authority: EP
Inventors: Michael John Towler; Akira Tagawa; Paul Bonnet
Original assignee: Sharp Corp
Current assignee: Sharp Corp
Priority date: 1996-09-30
Filing date: 1997-09-29
Publication date: 1998-04-29
Also published as: GB2317735A; KR100236433B1; EP0838802A3; KR19980025163A; JPH10153762A; GB9620395D0

Abstract

A ferroelectric liquid crystal device, such as a display panel, has a plurality of pixels arranged as rows and columns in a passive matrix arrangement. Each pixel has a plurality of switching thresholds, for instance by having different regions (7, 8) of different thicknesses, so as to provide a plurality of grey levels. A voltage waveform is applied to each pixel for switching it to a desired grey level. A blanking pulse of a first polarity is applied to reset the pixel to a reset grey level. This is followed by a compensating pulse (20) of polarity opposite the first polarity, after which a waveform for selecting the desired grey level is provided. The effect of the compensating pulse (20) is to reduce the effect of τmin shift so as to provide a relatively wide driving window (13) to allow more reliable selection of an intermediate grey level.

Description

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.

Claims

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 (20) of a second polarity opposite the first polarity for reducing τmin shift, and a waveform for achieving a selected grey level.
A method as claimed in Claim 1, characterised in that the RMS voltage of the compensating pulse (20) is less than the RMS voltage of the resetting pulse.
A method as claimed in Claim 1 or 2, characterised in that the amplitude of the compensating pulse (20) is greater than the amplitude of the resetting pulse.
A method as claimed in any one of the preceding claims, characterised in that the reset grey level is a maximally opaque level of the picture element.
A method as claimed in any one of Claims 1 to 3, characterised in that the reset grey level is a maximally transparent level of the picture element.
A method as claimed in any one of the preceding claims for a device of the type comprising a plurality of picture elements arranged as rows and columns, characterised in that strobe signals (Vs-Vs4) are applied in turn to the rows and data signals (Vd1-Vd4) are supplied simultaneously to the columns in synchronism with the strobe signals (Vs1-Vs4) for simultaneously selecting the selected grey levels of the picture elements of each row.
A method as claimed in Claim 6, characterised in that each strobe signal comprises the resetting pulse, the compensating pulse (20) and a strobe pulse.
A method as claimed in Claim 7, characterised in that the strobe pulse is of the second polarity.
A method as claimed in any one of the preceding claims, characterised in that the or each picture element comprises a plurality of regions (7, 8) having the plurality of switching thresholds.
A method as claimed in Claim 9, characterised in that the regions (7, 8) are of different thicknesses.
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 (2, 6) 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.
An apparatus as claimed in Claim 11, characterised in that the RMS voltage of the compensating pulse (20) is less than the RMS voltage of the resetting pulse.
An apparatus as claimed in Claim 11 or 12, characterised in that the amplitude of the compensating pulse (20) is greater than the amplitude of the resetting pulse.
An apparatus as claimed in any one of Claims 11 to 13, characterised in that the reset grey level is a maximally opaque level of the picture element.
An apparatus as claimed in any one of Claims 11 to 13, characterised in that the reset grey level is a maximally transparent level of the picture element.
An apparatus as claimed in any one of Claims 11 to 15 for a device of the type comprising a plurality of picture elements arranged as rows and columns, characterised in that the waveform generator (2, 6) comprises a strobe signal generator (6) for supplying strobe signals (Vs1-Vs4) in turn to the rows and a data signal generator (2) for supplying data signals (Vd1-Vd4) simultaneously to the columns in synchronism with the strobe signals (Vs1-Vs4) for simultaneously selecting the selected grey levels of the picture elements of each row.
An apparatus as claimed in Claim 16, characterised in that each strobe signal (Vs1-Vs4) comprises the resetting pulse, the compensating pulse (20) and a strobe pulse.
An apparatus as claimed in Claim 17, characterised in that the strobe pulse is of the second polarity.
A ferroelectric liquid crystal device characterised by comprising an apparatus as claimed in any one of Claims 11 to 18, in which the or each picture element comprises a plurality of regions (7, 8) having the plurality of switching thresholds.
A device as claimed in Claim 19, characterised in that the regions are of different thicknesses.
A device as claimed in Claim 19 or 20, characterised by being of passive matrix type.