WO1989005025A1

WO1989005025A1 - Multiplex addressing of ferro-electric crystal displays

Info

Publication number: WO1989005025A1
Application number: PCT/GB1988/001004
Authority: WO
Inventors: Jonathan Rennie Hughes
Original assignee: The Secretary Of State For Defence In Her Britanni
Priority date: 1987-11-18
Filing date: 1988-11-16
Publication date: 1989-06-01
Also published as: JPH03501894A; US5497173A; DE3888202D1; JP2637811B2; GB2232802A; DE3888202T2; EP0391931B1; GB8726996D0; GB2232802B; US5398042A; EP0391931A1; GB9011271D0

Abstract

A ferro-electric liquid crystal display is multiplex addressed by strobe waveform applied in sequence to each electrode in one set of electrodes coincidently with a data waveforms applied to a second set of electrodes. Liquid crystal material in the display is switched by a d.c. pulse of appropriate polarity, amplitude and time. The strobe waveforms have first and second pulse pairs, each pulse pair comprising two pulses of different amplitude and the same or different sign. The pulse pairs are similar but of opposite sign. Data waveforms are rectangular waveforms of opposite sign. The amplitude and ratio of leading pulse to trailing pulse in each strobe pulse pair are adjusted to obtain the desired switching and contrast. Compensation for temperature changes is arranged by measuring the temperature of the liquid crystal material and using the value obtained to adjust the amplitude value of the leading pulse in each strobe pulse pair.

Description

MULTIPLEX ADDRESSING OP FERRO-ELECTRIC LIQUID CRYSTAL DISPLAYS This invention relates to the multiplex addressing of ferroelectric liquid crystal displays. Such displays may use a chiral smectic C, I, and F liquid crystal material.

Liquid crystal display devices commonly comprise a thin layer of a liquid crystal material contained between two glass slides. Electrode structures on the inner faces of these slides enable an electric field to be applied across the liquid crystal layer thereby changing its molecular alignment. Many different types of displays have been made using nematic and cholesteric liquid crystal material. Both these types of material are operated between a field ON state and a field OFF state; i.e. displays are operated by switching a field on and off.

A more recent type of display uses a ferroelectric chiral smectic C, I, and F liquid crystal material in which liquid crystal molecules adopt one of two possible field ON states depending on the polarity of applied field. These displays are thus switched between the two states by pulses of appropriate polarity. In a zero applied field the molecules adopt an intermediate, configuration. Chiral snectic displays offer very fast switching with an amount of bistability. Examples of chiral smectic displays are described in G.B. No. 2,163,273, G.B. No. 2,159,635, G.E. No. 2,166,256, G.E. No. 2,157,451, U.S.A. Patent No. 4,536,059, U.S.A, Patent 4,367,924, G.B. P.A. No 86 08,114 - P.C.T. No. G.B. 87/00,222, G.3. P.A. No 08,115 - P.C.T. No 87/00,221, G.B. P.A. No. 08,116 - P.C.T. 87/00,220.

There are a number of known systems for multiplex addressing chiral smectic displays; see for example article by Harada et al 1985 S.I.D. Paper 8.4 pp 131-134, and Lagerwall et al 1985 I.D.R.C. pp 213-221. In this system a switching pulse is immediately preceeded by an equal and opposite polarity pulse which switches to the opposite state. The purpose of an opposite pulse followed by the wanted switching pulse is to ensure net d.c. at the liquid crystal material. See also GB 2,173,336A and GB 2,173,629 A.

A disadvantage of this system is a reduced switching time. Also the material sometimes fails to switch to the wanted state but stays in an opposite switched state. This gives inverted contrast which under certain conditions could be difficult to control in a complex display.

According to this invention a method of multiplex addressing a ferro electric liquid crystal matrix display formed by the intersections of a first set of electrodes and a second set of electrodes comprises the steps of:- applying a strobe waveform to each electrode in sequence in the first set of electrodes, said strobe waveform comprising a first pair of strobe pulses of different amplitude followed by a second pair of pulses of similar amplitude but different sign to the first pair of strobe pulses, applying one of two data waveforms to each electrode in the second set of electrodes coincidently with strobe waveform, both data waveforms being rectangular waveforms of alternate positive and negative values with one data waveform the inverse of the other data waveform, whereby each intersection is addressed with a d.c. pulse of appropriate sign and magnitude to turn that intersecτion to a desired display state once per complete display address period and an overall net zero d.c. value in each complete display address period. According to this invention a multiplex addressed liquid crystal display comprises: a liquid crystal cell including a layer of ferro-electric smectic liquid crystal material contained between two walls each bearing a set of electrodes arranged to form collectively a matrix of addressable intersections, driver circuits for applying data waveforms to one set of electrodes and strobe waveforms to the other set of electrodes in a multiplexed manner, waveform generators for generating data and strobe waveforms for applying to the driver circuits, means for controlling the order of data waveforms so that a desired display pattern is obtained, Characterised by:- a data waveform generator that generates two sets of waveforms of equal amplitude and frequency but opposite sign, each data waveform comprising d.c. pulses of alternate sign, a strobe waveform generator that generates strobe waveforms comprising a first pair of strobe pulses of different amplitude followed by a second pair of pulses of similar amplitude but different sign to the first pair of strobe pulses.

The strobe waveform may comprise two pairs of strobe pulses separated by a number of time periods when a zero strobe pulse is generated. Alternatively the second pair of strobe pulses may immediately follow the first pair.

Each pair of strobe pulses may be a pulse of one sign followed by a pulse of the opposite sign. Alternatively in each pair both strobe pulses may be of the same sign. The amplitude of one strobe pulse in each pair is greater than, in any proportion, the amplitude of the other strobe pulse.

The amplitude of the smaller strobe pulse in each pair may be the same as or different from the amplitude of the data pulses.

The amplitude and sign of the leading pulse in each strobe pulse pair may be varied to provide satisfactory display operation over a wide range of temperatures.

The invention will now be described by way of example only with reference to the accompanying drawings of which:-

Figure 1 is a diagrammatic view of a time multiplex addressed x, y matrix; Figure 2 is a cross section of part of the display of Figure 1 to an enlarged scale; Figure 3 is a view of an x, y matrix showing one pattern of

ON elements; Figure 4(a), (b), ( c) are waveform diagrams; Figure 5 is a graph showing a boundary between switching and non-switching values of time and applied voltage amplitude . Figure 6 is a graph of applied voltage vs switching times for different values of applied a.c. bias voltage; Figure 7 is a graph of applied voltage vs switching times for different values of leading pulse ratio; Figure 8 shows waveform traces having positive and negative leading pulse ratios as used for measurement of the curves shown in Figure 7; Figure 9 is a graph of applied voltage vs switching times for different liquid crystal temperatures; Figures 10, 11, 12 shows graphs of applied voltage vs switching times at different temperatures and show the effect of varying leading pulse ratios to provide temperature compensation. The display 1 shown in Figures 1 , 2 comprises two glass walls 2, 3 spaced about 1-6 jam apart by a spacer ring 4 and/or distributed spacers.

Electrode structures 5, 6 of transparent tin oxide are formed on the inner face of both walls. These electrodes are shown as row and column forming an X, Y matrix but may be of other forms. For example, radial and curved shape for an r, θ display, or of segments form for a digital seven bar display.

A layer 7 of liquid crystal material is contained between the walls 2, 3 and spacer ring 4.

Polarisers 8, 9 are arranged in front of and behind the cell 1. Row 10 and column 11 drivers apply voltage signals to the cell. Two ets of waveforms are generated for supplying the row and column drivers 10, 11. A strobe wave form generator 12 supplies row waveforms, and a data waveform generator 13 supplies ON and OFF waveforms to the column drivers 11. Overall control of timing and display format is controlled by a contrast logic unit 14.

Temperature of the liquid crystal, layer 7, is measured by a thermocouple 15 whose output is fed to the strobe generator 12. The thermocouple 15 output may be direct to the generator or via a proportioning element 16 e.g. a programmed ROM chip to vary one part of the strobe pulse waveform.

Prior to assembly the walls 2, 5 are surface treated by spinning on a thin layer of polyamide or polyimide, drying and where appropriate curing; then buffing with a soft cloth (e.g. rayon) in a single direction R₁, R₂. This known treatment provides a surface alignment for liquid crystal molecules. The rubbing directions R₁, R₂ are antiparallel. When suitable unidirectional voltages are applied the molecules director align along one of two directors D₁, D₂ depending on polarity of the voltage. Typically the angle between D₁, D₂ is about 45 °. In the absence of an applied electric field the molecules adopt an intermediate alignment directions R₁, R₂ and the directions D₁, D₂.

The device may operate in a transmissive or reflective mode. In the former light passing through the device e.g. from a tungsten bulb is selectively transmitted or blocked to form the desired display. In the reflective mode a mirror is placed behind the second polariser 9 to reflect ambient light back through the cell 1 and two polarisers. By making the mirror partly reflecting the device may be operated both in a transmissive and reflective mode .

Pleochroic dyes may be added to the material 7. In this case only one polariser is needed and the layer thickness may be 4-10 μm.

Suitable liquid crystal materials are:- catalogue references BDH - SCE 3 available from BDH, Poole, Dorset, and 19.6% CM8 (49% CC1 + 51% CC4) + 80.4% H₁

Another mixture is LPM 68 = H1 (49-5%), AS 100 (49-5%), IGS 97(1%) H1 = MB 8.5F + MB 80.5F + MB 70.7F (1 : 1 : 1) AS100 = PYR 7.09 + PYR 9-09 (1 : 2)

For a typical thickness of 2 um this material at 22°C is switched by a d.c. pulse of + or - 50 volts for 100 μs . The two switched states D₁, D₂ may be arbitrarily defined as ON after receiving a positive pulse and OFF after receiving a negative pulse of sufficient magnitude. Polarisers 8, 9 are arranged with their polarisation axes perpendicular to one another and with one of the axes parallel to the director in one of the switched states. In operation strobe waveforms are applied to each row in turn whilst appropriate ON or OFF data waveform are applied to each column electrode. This provides a desired display pattern formed by some x, y intersection in an ON state and other in an OFF state. Such addressing is termed multiplex addressing. The present invention is distinguished from prior art systems by the shape of the applied waveforms.

Figure 3 shows a 4 by 4 x, y matrix with ON intersections indicated by a solid circle, elsewhere the display is OFF.

Figure 4 shows the shape of data ON and OFF plus the shape of strobe waveforms. Each data and strobe pulse lasts for a period of one time slot. As seen the strobe waveform is formed by two sets of pulse pairs separated by a number of time slots where zero voltage is applied. These pairs are of opposite polarity. A +1 pulse is immediately followed by one of -3; zero volts, i.e. earthed, is then applied until the end of a first field period when a -1 volt pulse is followed by a +3 pulse. A string of zero pulses complete a second field. A display is addressed by both fields to provide the desired information. The length of both fields and hence the number of time slots between pairs of pulses is dependent on the number of rows to be addressed. A larger number of rows requires a large number of time slots between the pairs of pulses.

Waveforms applied to each row and column, and to the resulting value at each x, y intersection are shown in tabular form in Table 1. Row 1 is indicated by R1 etc; intersection of row 1 and column 1 is indicated by R1 , C1 etc. The values of applied voltage are adjusted such that +1 or -1 does not switch the display. A +/- 3 or more value will switch the display. However the chiral smetic is sensitive to the amplitude time product as shown in Figure 5. Therefore it is necessary to ensure that when successive time slots are of the same polarity their amplitude time product does not exceed the threshold for switching. The manner in which both voltage and time effect switching is shown in Figure 5; values, above the curve give a switch effect. Note, the curve indicates whether or not switching occurs from either ON or OFF state . The voltage values are modulus voltages.

For the row 1 column 1 intersection a -2 amplitude followed by -1 is obtained in the first field time. Thus the actual value of -2 needs to be kept as low as possible. At the beginning of field 2 a -2 is immediately followed by +4 which is high enough to give a clear switch to an ON state. Similarly, in row 1 column 2, a -4 value gives a clear switch to an OFF state.

Strobe waveforms having values other than +/-1 and +/-3 may be chosen, for example Table 1(b) shows the effect obtained with strobe pulses of 1, -2; -1, 2. Intersections receive maximum values of 3 proceeded by -2, or -3 preceeded by +2. The values -2, (or +2) start to turn the intersection to the OFF (or ON) state whilst the 3 (or -3) fully switches the intersection to the desired ON (or OFF) state.

Various other strobe waveforms and consequential intersection waveforms are shown in Tables 2 to 8.

Table 5-8 show how the two pairs of strobe pulses can be adjacent one another so that only one field is used per frame instead of the two fields of Tables 1 to 4. In all cases the relative values of each strobe pulse and data pulse amplitude can be varied from that shown. Values of 1 and 3 are merely by way of example only. Table 1 (a) Time Data

ON 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 OFF -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1

Strobe

R1 1 -3 0 0 0 0 0 0 -1 3 0 0 0 0 0 0 1 -3

R2 0 0 1 -3 0 0 0 0 0 0 -1 3 0 0 0 0 0 0 R3 0 0 0 0 1 -3 0 0 0 0 0 0 -1 3 0 0 0 0

R4 0 0 0 0 0 0 1 -3 0 0 0 0 0 0 -1 3 0 0

Waveform at column for the display of figure 3

C1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 C2 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1

C3 -1 1 -1 1 1 -1 -1 1 -1 1 -1 1 1 -1 -1 1 -1 1 C4 1 -1 1 -1 -1 1 1 -1 1 -1 1 -1 -1 1 1 -1 1 -1

Waveform at x, y intersection for the display of figure 3 R1C1 0 -2 -1 1 -1 1 -1 1 -2 4 -1 1 -1 1 -1 1 0 -2 R2C2 1 -1 2 -4 1 -1 1 -1 1 -1 0 2 1 -1 1 -1 1 -1 R3C3 1 -1 1 -1 0 -2 1 -1 1 -1 1 -1 -2 4 1 -1 1 1 R3C4 -1 1 -1 1 2 -4 -1 1 -1 1 -1 1 0 2 -1 1 -1 1

Table 1 (b) Data

ON 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1

OFF -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1

Strobe

R1 1 -2 0 0 0 0 0 0 -1 2 0 0 0 0 0 0 1 -2

R2 0 0 1 -2 0 0 0 0 0 0 -1 2 0 0 0 0 0 0

R3 0 0 0 0 1 -2 0 0 0 0 0 0 -1 2 0 0 0 0 R4 0 0 0 0 0 0 1 -2 0 0 0 0 0 0 -1 2 0 0

Waveform at z, y intersection for the display of Figure 5 R1C1 0 -1 -1 1 -1 1 -1 1 -2 3 -1 1 -1 1 -1 1 0 -1 R2C2 1 -1 2 -3 1 -1 1 -1 1 -1 0 1 1 -1 1 -1 1 -1 R5C3 1 -1 1 -1 0 -1 1 -1 1 -1 1 -1 -2 3 1 -1 1 -1

R3C4 -1 1 -1 1 2 -3 -1 1 -1 1 -1 1 0 1 -1 1 -1 1

Table 2 Data

ON -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 OFF 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1

Strobe

R1 -3 1 0 0 0 0 0 0 3 -1 0 0 0 0 0 0 -3 1

R2 0 0 -3 1 0 0 0 0 0 0 3 -1 0 0 0 0 0 0

R3 0 0 0 0 -3 1 0 0 0 0 0 0 3 -1 0 0 0 0

R4 0 0 0 0 0 0 -3 1 0 0 0 0 0 0 3 -1 0 0

Waveforms at columns for the display of Figure 3

C1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1

C2 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 C3 1 -1 1 -1 -1 1 1 -1 1 -1 1 -1 -1 1 1 -1 1 -1

C4 -1 1 -1 1 1 -1 -1 1 -1 1 -1 1 1 -1 -1 1 -1 1

Waveform at x,y intersection for the display of Figure 3 R1C1 -2 0 1 -1 1 -1 1 -1 4 -2 1 -1 1 -1 1 -1 -2 0 R2C2 -1 1 -4 2 -1 1 -1 1 -1 1 2 0 -1 1 -1 1 -1 1

R3C3 -1 1 -1 1 -2 0 -1 1 -1 1 -1 1 4 -2 -1 1 -1 1 R3C4 1 -1 1 -1 -4 2 1 -1 1 -1 1 -1 2 0 1 -1 1 -1

Table 3 Data

ON 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1

OFF -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1

Strobe

R1 -1 -3 0 0 0 0 0 0 1 3 0 0 0 0 0 0 0 0

R2 0 0 -1 -3 0 0 0 0 0 0 1 3 0 0 0 0 0 0

R3 0 0 0 0 -1 -3 0 0 0 0 0 0 1 3 0 0 0 0

R4 0 0 0 0 0 0 -1 -3 0 0 0 0 0 0 1 3 0 0

Waveforms at x,y intersections for the display of Figure 3 R1 C1 -2 -2 -1 1 -1 1 -1 1 0 4 -1 1 -1 1 -1 1 -2 2 R2C2 1 -1 0 -4 1 -1 1 -1 1 -1 2 2 1 -1 1 -1 1 -1 R3C3 1 -1 1 -1 -2 -2 1 -1 -1 1 -1 1 0 4 1 -1 1 -1

R3C4 -1 1 -1 1 0 -4 -1 1 -1 1 -1 1 2 2 -1 1 -1 1

Table 4 Data

ON -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 OFF 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1

Strobe R1 -3 -1 0 0 0 0 0 0 3 1 0 0 0 0 0 0 0 0

R2 0 0 -3 -1 0 0 0 0 0 0 3 1 0 0 0 0 0 0

R3 0 0 0 0 -3 -1 0 0 0 0 0 0 3 1 0 0 0 0

R4 0 0 0 0 0 0 -3 -1 0 0 0 0 0 0 5 1 0 0

Waveforms at x,y intersections for the display of Figure 5

R1 C1 -2 -2 1 -1 1 -1 1 -1 4 0 1 -1 1 -1 1 -1 -2 -2 R2C2 -1 1 -4 0 -1 1 -1 1 -1 1 2 2 -1 1 -1 1 -1 1 R3C5 -1 1 -1 1 -2 -2 -1 1 -1 1 -1 1 4 0 -1 1 -1 1 R3C4 1 -1 1 -1 -4 0 1 -1 1 -1 1 -1 2 2 1 -1 1 -1 Table 5

Data

ON 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1

OFF -1 1 -1 1 -1 1 -1 1 -1 1 . -1 1 -1 1 -1 1 -1 1 -1 1 -1

Strobe

E1 1 -3 -1 3 0 0 0 0 0 0 0 0 0 0 0 0 1 -3 -1 3 0

R2 0 0 0 0 1 -3 -1 3 0 0 0 0 0 0 0 0 0 0 0 0 1

R5 0 0 0 0 0 0 0 0 1 -3 -1 3 0 0 0 0 0 0 0 0 0

R4 0 0 0 0 0 0 0 0 0 0 0 0 1 -3 -1 3 0 0 0 0 0

Waveforms at x,y intersections for the display of Figure 3 R1C1 0 -2 -2 4 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 0 -2 -2 4 -1 R2C2 1 -1 1 -1 2 -4 0 2 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 2 E3C5 1 -1 1 -1 1 -1 1 -1 0 -2 -2 4 1 -1 1 -1 1 -1 1 -1 1

E3C4 -1 1 -1 1 -1 1 -1 1 2 -4 0 2 -1 1 -1 1 -1 1 -1 1 -1

Table 6 Data

Strobe E1 -3 1 3 -1 0 0 0 0 0 0 0 0 0 0 0 0 -3 1

E2 0 0 0 0 -3 1 3 -1 0 0 0 0 0 0 0 0 0 0

R3 0 0 0 0 0 0 0 0 -5 1 5 -1 0 0 0 0 0 0 E4 0 0 0 0 0 0 0 0 0 0 0 0 -5 1 3 -1 0 0

Waveforms at x,y intersections for the display at Figure 3

E1C1 -2 0 4 -2 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 -2 0 R2C2 -1 1 -1 1 -4 2 2 0 -1 1 -1 1 -1 1 -1 1 -1 1 E3C5 -1 1 -1 1 -1 1 -1 1 -2 0 4 -2 -1 1 -1 1 -1 1 R5C4 1 -1 1 -1 1 -1 1 -1 -4 2 2 0 1 -1 1 -1 1 -1 Table 7 Data

ON 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1

OFF -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1

Strobe ,.

R1 -1 -3 1 3 0 0 0 0 0 0 0 0 0 0 0 0 -1 -3

R2 0 0 0 0 -1 -3 1 3 0 0 0 0 0 0 0 0 0 0

R3 0 0 0 0 0 0 0 0 -1 -3 1 3 0 0 0 0 0 0

R4 0 0 0 0 0 0 0 0 0 0 0 0 -1 -3 1 3 0 0

Waveforms at -c,y intersections for the display at Figure 3 R1C1 -2 -2 0 4 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -2 -2 R2C2 1 -1 1 -1 0 -4 2 2 1 -1 1 -1 1 -1 1 -1 1 -1 R3C3 1 -1 1 -1 1 -1 1 -1 -2 -2 0 4 1 -1 1 -1 1 -1 R5C4 -1 1 -1 1 -1 1 -1 1 0 -4 2 2 -1 1 -1 1 -1 1

Table 8 Data

ON -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 OFF 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1

Strobe R1 -3 -1 3 1 0 0 0 0 0 0 0 0 0 0 0 0 -3 -1

R2 0 0 0 0 -3 -1 5 1 0 0 0 0 0 0 0 0 0 0 R3 0 0 0 0 0 0 0 0 -3 -1 3 1 0 0 0 0 0 0 R4 0 0 0 0 0 0 0 0 0 0 0 0 -3 -1 3 1 0 0

Waveforms at x,y intersections for the display at Figure 5 R1C1 -2 -2 4 0 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 -2 -2 R2C2 -1 1 -1 1 -4 0 2 2 -1 1 -1 1 -1 1 -1 1 -1 1 R3C3 -1 1 -1 1 -1 1 -1 1 -2 -2 4 0 -1 1 -1 1 -1 1 R3C4 1 -1 1 -1 1 -1 1 -1 -4 0 2 2 1 -1 1 -1 1 -1 The curve shown in Figure 5 is affected by a number of factors. For good multiplexing a curve with a minimum value of the V.t product is required. The minimum theoretical value of V.t is given as

Emin = Ps/√3 ∑o.△∑ sin²θ

where Ps is spontaneous polarisation coeficient,

∑o = permittivity of free space △∑ = dielectric anisotropy of liquid crystal material θ = cone angle of ferro electric liquid crystal material.

This applies to the case of homogeneous alignment of the liquid crystal molecules. In a practical device where there is likely to be tilt in the bulk of the liquid crystal layer Emin is higher than this value.

Figure 6 shows how the value of Emin is moved upwards and to the left -s the amount of applied A.C. voltage, i.e. the data voltage, is increased. The reason for this is the interaction of the applied field with the negative dielectric anisotropy of the liquid crystal material. Such interaction tends to move the liquid crystal material from a tilted to a more more homogeneous structure. The liquid crystal material used is LPM 68 in a layer 1.7 um thick at a temperature of 20°C.

Figure 7 shows the effect of varying the amplitude and magnitude of the leading pulse in each pair of strobe pulses. The voltage at each electrode intersection, or pixel, is the difference between data and strobe voltages i.e. the resultant waveform. Figure 8(a), (b) show the resultant waveform at a pixel when addressed by a strobe pulse pair and data waveforms. In Figure 8(a) the resultant waveform is a positive first or leading pulse followed by a negative second or trailing pulse; this is defined as a negative leading pulse ratio because the magnitudes are of opposite sign. A negative leading pulse followed by a positive trailing pulse also has a negative leading pulse ratio. In contrast Figure 8(b) shows a waveform with both pulse of the same sign; this is defined as a positive leading pulse ratio. A zero leading pulse ratio will have a zero voltage level leading pulse. Figure 7 shows V.t curves for resultant waveforms with leading pulse ratios of -0.5, -0.2, 0, 0.2, and 0.5. The material and cell are as in Figure 6 but at a temperature of 30°C and with no A.C. bias. Region marked A is non switching (or partial switching), region B is switching by the trailing pulse, and region C is switching by leading pulse .

Figure 9 shows how the V.t curve is affected by temperature. The curves are for temperatures of 10°, 20°, 30°, and 40 °C; the cell material and thickness are as for Figure 7. The value of Emin occurs at lower response times but higher voltages as temperature increases.

Using the above changes in the V.t curve characteristics, temperature compensation can be built into the display of Figure 1. This is achieved by measuring the temperature of the liquid crystal material with the thermocouple 15 (Figure 1) and varying the amplitude and sign of the leading pulse in the strobe pulse pair.

Using a negative leading pulse ratio the value of Emin can be moved to a lower voltage at a correspondingly higher response time. Using a positive leading pulse ratio Emin can be moved to a faster response time at a correspondingly higher voltage.

By way of example a 16 by 16 pixel matrix cell was made using the material LPM 68 in a 1.7 jam thick layer constructed as for Figure 2. The applied waveforms were as in Figure 4 with data voltage Vd of 5 volts amplitude, trailing strobe pulse voltage Tp of 40 volts, a variable leading pulse voltage Lp, and time slots of 60 μs whilst simulating 52 way multiplexing. Temperature and leading pulse Lp were varied as in Table 9. A clear, good contrast, display was obtained at all temperature points with the listed leading pulse voltages.

Table 9

Tem perature Lp volts Lp/Tp Ratio Resultant Waveform Rat io

°C Vx Vy

15 4 0. 1 -0.02 0.26

19.7 -4 -0.1 -0.2 +0.03

25.5 -8 -0.2

30 -1 2 -0.3 -0.38 -0.2

34.1 -1 6 -0.4

36.2 -20 -0.5

38.5 -28 -0.7 -0.73 -0.66

39.4 -32 -0.8

45 -40 -1 .0 -0.78 -1 .0

Vx, Vy = ratio of leading pulse to trailing pulse of resultant waveform in the two strobe pulse pairs. Taking the three temperature values of 19.7, 30, 38.3°C the data, strobe, and resultant waveform are shown in the following table, using the format of Table 1 for a 4 x 4 matrix.

Table 10 Numbers are d.c. voltage levels

Data 5 -5 5 -5 5 -5 5 -5 5 -5 5 -5

Temperature 19.7° C

Strobe -4 40 0 0 0 0 0 0 4 -40 0 0 Resultant -9 45 -5 5 -5 5 -5 5 -1 -35 -5 5

Temperature 30° C

Strobe -1240 0 0 0 0 0 0 12 -40 0 0

Resultant -1745 -5 5 -5 5 -5 5 7 -35 -5 5

Temperature 38.3° C Strobe -2840 0 0 0 0 0 0 28 -40 0 0 Resultant -3345 -5 5 -5 5 -5 5 23 -35 -5 5

From this the result of a strobe pair pulse at 19.75ºC gives a resultant pulse pair of -9, 45 and later -1 , -35. This gives a leading pulse ratio of -9/45 = -0.2, and -1 /-35 = 0.03. Note these two ratios are the same when the inverse of the data waveform is used. The data waveform and its inverse are used depending upon whether a pixel is to be switched to an ON or OFF state. The leading pulse ratios can be calculated for the other temperature values; the results are given in Table 9.

Taking the leading pulse ratios in Table 9 V.t plots have been determined for the three temperatures 19.7, 30, 38.5° C and the results are shown in Figures 10, 11, 12 respectively. Each case curve A shows the response to the first strobe pulse pair, and curve B the response to the second strobe pulse pair. Looking first at Figure 10 the first strobe pulse pair gives a resultant waveform of -9 then 45 volts, i.e. a leading pulse ratio of -0.2, and curve A applies. Thus a voltage of 45 (preceded by -9) for less than about 700 μs will not switch. Looking now at the second strobe pulse pair the resultant waveform is -1 then -55 volts, i.e. a leading pulse ratio of 0.03, and curve B applies. Thus a voltage of (-)35 preceded by (-)1 will switch the material if the slot time is greater than about 80 /is. The voltage levels of 45 and (-)35 are be marked on Figure 10 as vertical lines with a band of time slots. Clear and clean switching is obtained for time slots of about 70 to 400 μs . The bands start slightly below the V.t curves because in practice optical switching is observed at the marked values.

Similarly in Figure 11 curve A applies to the resultant waveform of the first strobe pulse pair where Vx = -0.58, and curve B applies to the second strobe pulse pair where Vy = -0.2. A voltage of 45 volts, preceded by -17 volts, does not switch providing the time slot is less than about 180 μs. A voltage of -55 preceded by 7 volts switches providing the time slot is greater than about 80 μs . Clear and clean switching is available for time slots of about 80 to 180 μs .

Two additional curves are marked C, D for the resultant leading pulse ratios of -0.52 and -0.2 respectively. The C, D curves are plots of the trailing pulse V. t values for resultant pulse pairs that switch the cell on leading pulses. This contrasts with the previous resultant waveforms where the cell always switched on a trailing pulse. It seems unpredictable that a cell should switch on receipt of a small resultant leading pulse and not switch on the larger value trailing pulse. However, this is an observed phenomenon and is due to molecules relaxing immediately prior to receiving the leading pulse. After such relaxation the small leading pulse is able to switch itself fully, but the cell cannot fully switch again within the available time slot of the larger amplitude trailing pulse. For example a given pixel switched by a -35 volts, preceeded by 7 volts (curve B) also receives 45 volts preceeded by -35 volts and no switching on the trailing pulse of 45 volts occurs because it is below curve A. However, 45 volts lies within the switching area of curve C for time slots of about 130-180 μsecs. Thus the leading pulse of -35 volts preceeding 45 volts switches or reinforces the given pixel also switched to the same state by the -35 volts trailing pulse. The net effect of curves C, D in Figure 11 is to reinforce the switching already described for curves A, B within a limited range of time slots.

Again in Figure 12 curve A applies to the resultant waveform of the first strobe pulse pair where Vx = -0.75, and curve B applies to the second strobe pulse pair where Vy = -0.66. A voltage of 45 volts, preceded by -33 volts, does not switch providing the time slot is less than about 80 us. A voltage of -35 preceded by 23 volts switches providing the time slot is greater than about 63 μs. Clear and clean switching is available for time slots of about 63 to 80 μs. Curves C, D show curves for leading pulse switching as in Figure 11. These reinforce the leading pulse switching of curves A, B.

Not shown by Figures but listed in Table 9 are details obtained for the temperature 15° C. This was found to be multiplex addressable for time slot periods of about 70 to 200 μs.

The above shows how a given cell can be satisfactorily addressed over a temperature range of 10 to 40°C merely by changing the amplitude of the leading strobe pulse in each strobe pair from +8 volts to -32 volts, the + or - sign representing the same or opposite polarity as the trailing pulse voltage of +40 volts. These values represent leading pulse ratios Lp/Tp of +0.2 to -0.8. As a further example the above cell with material LPM 68 was operated under the following conditions and the following results obtained:-

Strobe trailing pulse voltage Vs = 15 volts, data pulse Vd = 5 volts, and a 120 μs time slot.

Table 11 Temperature Leading pulse volts Lp/Tp ratio Vx Vy

15 12 0.8 0.35 1 .7

20 5 0.33 0 1 .0

25 0 -0.25 -0.25 0.5

30 -6 -0.4 -0.55 -0.1

35 -1 5 -1 -1 -1

Note the levels of resultant voltages are below Emin on the graphs of Figures 6 to 11. Temperature compensation is applicable for displays operating both above and below Emin.

Thus to provide compensation for liquid crystal temperature variation the strobe waveform generator is programmed to output strobe pulses with a ratio that varies with the liquid crystal temperature. Different materials and cell thickness will have different characteristics that need to be predetermined.

Observation of Tables 9 and 11 show the Lp/Tp ratio to be approximately linearly related to temperature. Thus the output of the thermocouple 15 can be fed to an inverting amplifier for controlling the amplitude of the leading pulse in each strobe pair. Alternatively a ROM chip can be programmed to output the required leading pulse voltage level for a predetermined set of different temperatures inputs. All the above strobe waveforms use identical but opposite polarity first and second pulse pairs. In a modification the strobe leading pulse ratio Lp/Tp is varied between the first and second pulse pair. This has the effect of increasing the separation between the curves A, B in Figures 10 to 12. The resulting small d.c. bias is removed by periodically reversing display polarity.

In a modification the values of the data pulse pair may be varied in field 1 and field 2 to improve the separation of curves A and B in Figures 10-12. This may be achieved either in conjunction with variation of the leading part of the strobe pulse pair or independently of it and may take a number of forms:-

(i) an equal reduction in amplitude of each of the first pair of data pulses with a corresponding increase in the amplitude of the second pair;

(ii) an equal increase in amplitude of each of the first pair of data pulses with a corresponding decrease in the amplitude of the second pair;

(iii) an increase in the amplitude of the first pulse of the first pair of data pulses with a corresponding decrease in amplitude of the first pulse of the second pair;

(iv) a decrease in the amplitude of the first pulse of the first pair of data pulses with a corresponding increase in amplitude of the first pulse of the second pair (v) and (vi) vary second pulse of the pair.

In a further modification the first strobe pair is replaced by a blanking pulse that completely switches to one state a line at a time. Alternatively a group of lines or the whole display can be blanked at one time. Pixels requiring to be switched to the other state are switched by the remaining strobe pulse pair. The resulting d.c. bias is removed by periodically reversing polarity. Use of blanking eliminates the first field in the addressing and reduces the complete addressing time.

Claims

Claims:- 1. A multiple addressed liquid crystal display comprising:- a liquid crystal cell including a layer of ferro-electric smectic liquid crystal material contained between two walls each bearing a set of electrodes arranged to form collectively a matrix of addressable intersections, driver circuits for applying data waveforms to one set of electrodes and strobe waveforms to the other set of electrodes in a multiplexed manner, waveform generators for generating data and strobe waveforms for applying to the driver circuits, means for controlling the order of data waveforms so that a desired display pattern is obtained, characterised by:- a data waveform generator that generates two sets of waveforms of equal amplitude and frequency but opposite sign, each data waveform comprising d.c. pulses of alternate sign, a strobe waveform generator that generates strobe waveforms comprising a first pair of strobe pulses of different amplitude followed by a second pair of pulses of similar amplitude but different sign to the first pair of strobe pulses.

2. The display of claim 1 wherein the strobe waveform comprises two pairs of strobe pulses separated from one another by a number of time periods when a zero strobe pulse is generated.

3. The display of claim 1 wherein the strobe waveform comprises two pairs of strobe pulses immediately following one another.

4. The display of claim 1 wherein the amplitude and sign of the leading pulse in each strobe pulse pair is variable.

5. The display of claim 1 and further comprising a temperature sensing element for sensing the liquid crystal layer temperature, and means for varying the amplitude and sign of the leading pulse voltage in each strobe pulse pair to compensate for temperature variation in the liquid crystal layer.

6. The display of claim 1 wherein the amplitude and sign leading pulse in each strobe pulse can be varied independently to compensate for temperature variation in the liquid crystal material.

7. The display of claim 1 wherein the amplitude of the data waveform can be varied.

8. A method of multiplex addressing a ferro electric liquid crystal matrix display formed by the intersections of a first set of electrodes and a second set of electrodes comprises the steps of:- applying a strobe waveform to each electrode in sequence in the first set of electrodes, said strobe waveform comprising a first pair of strobe pulses of different amplitude followed by a second pair of pulses of similar amplitude but different sign to the first pair of strobe pulses, applying one of two data waveforms to each electrode in the second set of electrodes coincidently with strobe waveform, both data waveforms being rectangular waveforms of alternate positive and negative values with one data waveform the inverse of the other data waveform, whereby each intersection is addressed with a d.c. pulse of appropriate sign and magnitude to turn that intersection to a desired display state once per complete display address period and an overall net zero d.c. value in each complete display address period.

9. The method of claim 8 wherein the leading pulse in each strobe pulse pair is varied in amplitude and sign to compensate for temperature variation in the liquid crystal material.

10. The method of claim 8 wherein the amplitude of the data waveform is varied to compensate for temperature variation in the liquid crystal material.

11. The method of claim 8 wherein the values of applied voltage and time of application product (V.t) are arranged so that the liquid crystal material switches to a given state on receipt of the trailing pulse in one pulse pair and also switches to the same state on receipt of the leading pulse in a different pulse pair.