Display device

Abstract: A method of addressing a display device (2) comprising a matrix of separately operable pixels (6) is provided. The method comprises the step of applying across a given pixel a voltage waveform comprising a latching pulse and an auxiliary pulse of amplitude smaller than the latching pulse. The amplitude of the auxiliary pulse is modulated to determine the latching effect of the latching pulse.

Description: The present invention relates to a method of addressing a display device comprising a matrix of separately addressable pixels arranged in rows, each pixel being latchable into first and second states by applying respective voltage waveforms thereacross, the method comprising, for each row, latching all the pixels into the first state by applying a voltage of a given polarity thereacross and thereafter latching selected pixels of the row into the second state by applying a first voltage waveform thereacross while applying a second voltage waveform across the remaining pixels of the row, the second waveform leaving the corresponding pixels in the first state, the first and second waveforms each including a pulse of the opposite polarity to said given polarity. The invention also relates to a display device including addressing means for performing such a method. GB 2185614A (Canon) discloses a method of this general kind. In a writing period for writing in all or prescribed pixels on a selected scanning electrode, the device is driven in three phases t₁,t₂,t₃. In the first phase t₁, a leading pulse is applied to ensure that a pixel is switched to a blanked state. In the third phase t₃, a trailing pulse of opposite polarity to the leading pulse is applied to effect switching out of that blanked state and latching into an opposite state when required. In the intermediate second phase t₂, a voltage is applied which does not affect the pixel state but which reduces the effect of cross-talk. An example of a waveform scheme from GB 2185614A (Figures 17 and 18) is reproduced in Figures 1 and 2 of the present specification. Figures 1A, 1B, 1C and 1D show respectively the scanning (strobe) selection signal, the scanning (strobe) nonselection signal, the information selection (data 1) signal and the information nonselection (data 0) signal. Figures 2A and 2B show the resultant waveform produced across a pixel from the combination of the scanning selection signal and respectively the data 1 and data 0 signals. Figures 2C and 2D show the resultant waveform produced across a pixel from the combination of the scanning non-selection signal and respectively the data 1 and data 0 signals. In the waveform of Figure 2A, the trailing pulse is preceded by a voltage of the same polarity but of only one third the amplitude. This smaller amplitude pulse is produced by the data and not by the strobe waveform. The amplitude of the trailing pulse is increased by data "1" to effect switching out of the blanked state and decreased by data "0" so as not to effect switching out of the blanked state. Switching or non-switching is determined solely by modulation of the trailing pulse. Modulation of the trailing pulse alone forces the ratio of the strobe and data voltages to be fixed in order to ensure that a non-switching trailing pulse can be achieved. The electro-optic characteristics of a ferroelectric liquid crystal device determine and limit the operating conditions (in terms of pulse voltage and width) for multiplexing. These conditions can be very limited for the voltage ratio given, or for any other fixed voltage ratio scheme. A further problem arises with the possibility of frequent occurrence of double width data pulses in the voltage train across any pixel while the rest of the device is being addressed, either due to the data 1 waveform or accidentally due to data 0 followed by data 1. In conventional schemes, this may result in significant crosstalk i.e. optical noise, thus reducing the device contrast. This accidental occurrence of data pulses forming double width data pulses is common in many multiplex schemes. It is an object of the present invention to enable these disadvantages to be mitigated. According to one aspect the invention provides a method of addressing a display device comprising a matrix of separately addressable pixels arranged in rows, each pixel being latchable into first and second states by applying respective voltage waveforms thereacross, the method comprising, for each row, latching all the pixels into the First state by applying a voltage of a given polarity thereacross and thereafter latching selected pixels of the row into the second state by applying a first voltage waveform thereacross while applying a second voltage waveform across the remaining pixels of the row, the second waveform leaving the corresponding pixels in the first state, the first and second waveforms each including a pulse of the opposite polarity to said given polarity, characterized in that the first waveform, in addition to said pulse of the opposite polarity, includes an auxiliary pulse of a first kind and the second waveform, in addition to said pulse of the opposite polarity, includes an auxiliary pulse of a second kind, each auxiliary pulse having a smaller amplitude than the corresponding said pulse of the opposite polarity, the auxiliary pulse of the first kind being of said opposite polarity and having sufficient amplitude that cooperation of this auxiliary pulse with the corresponding said pulse of the opposite polarity produces latching of the corresponding pixel into the second state, the auxiliary pulse of the second kind having a different amplitude such that cooperation of this auxiliary pulse with the corresponding said pulse of the opposite polarity fails to produce latching of the corresponding pixel into the second state. According to another aspect the invention provides a display device comprising a matrix of separately addressable pixels arranged in rows, each pixel being latchable into first and second states by applying respective voltage waveforms thereacross, and addressing means for addressing said pixels, the addressing means being arranged to address each row by applying a voltage of a given polarity across the pixels of that row to latch the pixels into the first state and thereafter latch selected pixels of the row into the second state by applying a first voltage waveform thereacross while applying a second voltage waveform across the remaining pixels of the row, the second waveform leaving the corresponding pixels in the first state, the first and second waveforms each including, a pulse of the opposite polarity to said given polarity, characterised in that the first waveform, in addition to said pulse of the opposite polarity, includes an auxiliary pulse of a first kind and the second waveform, in addition to said pulse of the opposite polarity, includes an auxiliary pulse of a second kind, each auxiliary pulse having a smaller amplitude than the corresponding said pulse of the opposite polarity, the auxiliary pulse of the first kind being of said opposite polarity and having sufficient amplitude that cooperation of this auxiliary pulse with the corresponding said pulse of the opposite polarity will produce latching of the corresponding pixel into the second state, the auxiliary pulse of the second kind having a different amplitude such that cooperation of this auxiliary pulse with the corresponding said pulse of the opposite polarity will fail to produce latching of the corresponding pixel into the second state. It has been found that more effective selective switching of a pixel from one state to another can be achieved by introducing an auxiliary voltage pulse in addition to the said pulse of the opposite polarity with modulation of the auxiliary pulse determining the latching effect of the said pulse of the opposite polarity. An advantage of the present invention is that a non-switching latching pulse can be achieved other than by reduction of the strobe voltage by data modulation to a data-sized voltage. The modulation of the auxiliary pulse alone can determine whether or not the latching pulse will switch. Consequently there is greater freedom to adjust the data and strobe voltage ratio, pulsewidth and voltage until a suitable set of waveforms for multiplexing is identified. As the present invention ensures that a wide choice of sets of data waveforms is available, it is readily possible to select sets of data waveforms which avoid double data pulses and minimize cross-talk. Preferably the said pulse of the opposite polarity included in the first voltage waveform has a different amplitude to the said pulse of the opposite polarity included in the second voltage waveform. This further enhances the discrimination between the two states of a pixel. In the invention, the auxiliary pulse may be positioned before the said pulse of the opposite polarity or after it and the auxiliary pulse may be immediately adjacent temporally the said pulse or may be spaced temporally therefrom. Additionally there may be provided a further auxiliary pulse which need not be of the same amplitude as the first auxiliary pulse but must be smaller than the said pulse of the opposite polarity. In any of the above variants, preferably the auxiliary pulse of the second kind is of said opposite polarity. Preferably, each row of the matrix is strobed only once per signal corresponding to an image for display. Preferably, temperature compensation is effected by introducing a variable voltage component in the portion of the strobe voltage waveform corresponding to the auxiliary pulses of the first and second kinds; advantageously a variable voltage component is introduced in the portions of the strobe voltage corresponding to both the auxiliary pulse and the latching pulse. It is preferred that the device exhibits a non-linear electro-optic characteristic with an up-turn (e.g. as shown in Figures 18 to 24 and 26). Such a device can be multiplexed, with this invention, in either the normal mode (magnitude of latching pulse greater when switching than when not switching) or the inverse mode (magnitude of latching pulse less when switching than when not-switching). The present invention is application to colour displays and to monochrome displays. In order that the invention may more readily be understood, a description is now given, by way of example only, with reference to the accompanying drawings, in which:- Figures 1 and 2 show a scheme from GB 2185614A; Figure 3 shows schematically part of a display device; Figures 4 to 8 show multiplexing schemes embodying the present invention; Figures 9 and 10 show corresponding line-blanking schemes embodying the present invention; Figures 11 to 13 show electro-optic responses of the scheme of Figure 9; Figures 14 and 15 show further schemes embodying the present invention; Figures 16 and 17 show electro-optic responses of two further schemes embodying the present invention; Figures 18 to 25 illustrate characteristics of the present invention. and Figure 26 shows an electro-optic curve for a monopolar pulse. Figure 3 is a schematic plan representation of part of a matrix-array type liquid crystal cell 2 essentially comprising a layer of a ferroelectric liquid crystal material of thickness in the range of about from 1.5 to 3 µm sandwiched between a first and a second layer of electrodes. Pixels 6 of the matrix are defined by areas of overlap between members 7 of a first set of row electrodes in the first electrode layer and members 8 of a second set of column electrodes in the second electrode layer. For each pixel, the electric field thereacross determines the state and hence alignment of the liquid crystal molecules. Parallel or crossed polarizers (not shown) are provided at either side of the cell 2. The orientation of the polarisers relative to the alignment of the liquid crystal molecules determines whether or not light can pass through a pixel in a given state. Accordingly for a given orientation of the polarizers, each pixel has a first and a second optically distinguishable state provided by the two bistable states of the liquid crystal molecules in that pixel. Voltage waveforms are applied to the row electrodes 7 and column electrodes 8 respectively by row drivers 9 and column drivers 10. The shape of the voltage waveforms that may be applied by the row drivers 9 and the column drivers 10 is determined by waveform generators 11, 12 which may be computer-operated or may comprise solid-state circuitry. The matrix of pixels 6 is addressed on a line-by-line basis by applying voltage waveforms, termed strobe waveforms, serially to the row electrodes 7 while voltage waveforms, termed data waveforms, are applied in parallel to the column electrodes 8. The resultant waveform across a pixel defined by a row electrode and a column electrode is given by the potential difference between the waveform applied to that row electrode and the waveform applied to that column electrode. The row electrode to which a strobe waveform is being applied is termed the 'selected row' or 'selected electrode'. A 'data on' waveform applied to a pixel on a selected row causes the pixel to be put into one of the bistable states whereas a 'data off' waveform causes the pixel to be put into the other of the bistable states. Each electrode can therefore have one of two waveforms - strobe or non-strobe for each row electrode and 'data on' or 'data off' for each column electrode - applied thereto. Which of the two waveforms is applied is determined, in known manner, from the picture signal representing a picture for display. An example of a scheme, referred to hereinafter as the three-component voltage pulse scheme, embodying the present invention is illustrated in Figure 4 which shows the resultant pixel waveform across a pixel. The three components are:- a blanking voltage pulse; an auxiliary voltage pulse, and a latching voltage pulse. The portion of the strobe waveform corresponding to the blanking pulse is chosen to have a sufficiently large voltage-time product to switch and latch the ferroelectric liquid crystal (FLC) molecules into a specified state regardless of their previous state and regardless of the effects of modulation caused by data voltage waveforms on the blanking pulse shape. (Accordingly, for clarity, the effect of data voltage modulation on the shape of the blanking pulse has not been shown.) This latched state is referred to as the blanked state. For the first component, (ie the blanking pulse) ,where T = 0 is defined as the time at the beginning of the blanking pulse, is chosen to be sufficient to switch and latch into the blank state, independent of any data modulation and additional pulses that appear on the sides of the blanking pulse due to data modulation (referred to as parasitic pulses). Also, for "data on", is sufficient for the pixel to switch from the blanked state and to latch into the opposite state. For "data off", is insufficient for the pixel to be unlatched from the blanked state. (For each integral, T = 0 is defined as the time at the beginning of that voltage component.) For on/off data, V A is modulated by data above and below, respectively, a threshold voltage V th . V th is defined as the magnitude of the auxiliary pulse necessary for the combination of the auxiliary and latching pulses to switch the pixel out of the blanked state and latch it into the opposite state. The time interval T₄ can be zero or it can have a positive value; it may contain voltage pulses providing they are not such as to interfere with the function of the three components. The waveform of the three components may take any appropriate form providing that the three integration conditions above are satisfied. It has been found that more efficient switching from one state to another can be achieved by introducing an auxiliary voltage pulse just prior to the latching pulse of the same polarity. An auxiliary voltage pulse of the opposite polarity will inhibit switching. By careful choice of pulse height and width for both the auxiliary pulse and the latching pulse, it is possible to aid or prevent switching and latching by modulating the auxiliary pulse alone with the data voltage waveforms. It is this feature which is embodied in the second and third components of the multiplex scheme of the present invention. Although it is preferable to arrange for the auxiliary pulse to be just prior to the latching pulse with no time separation between the two components, this feature can still be obtained if the scheme is modified, such as if the order of the components is reversed, or time intervals or fixed voltage pulses are introduced between the two components. However, loss of performance in terms of switching speed and width of the multiplex operating conditions window can occur if the scheme is so modified. Component three, i.e. the latching pulse, is arranged to be of the opposite polarity to the blanking pulse. Component two, the auxiliary pulse, and the latching pulse are chosen such that during 'on' data modulation the FLC molecules are switched out of the blanked state and latched into another state referred to as the 'opposite state'. During 'off' data modulation the FLC molecules remain latched in the blanked state. Good high contrast multiplexing can be obtained by modulating the auxiliary pulse alone, without modulating the latching pulse as is used in most multiplexing schemes. Modulation of the latching pulse in addition to the release pulse is optional but can be used if required to improve the discrimination and the width of the operating window. Clearly, a blanking pulse of a single slot width, rather than two slots as shown, can be used provided the pulse satisfies the requirements for a blanking pulse. In this way, the line address time for the four-slot version of Figure 4 is reduced by 25% to give a three-slot version, providing a useful increase in display speed. In Figures 5, 6 and 7, a number of simple 'n-timeslot' multiplex schemes are shown which embody the above requirements. In each of these Figures, a strobe voltage waveform has been shown together with a number of data voltage waveforms which can be used to modulate the strobe voltage waveform. The mode given for each data voltage waveform indicates if the waveform is a 'data on' or a 'data off' waveform for the strobe voltage waveform shown. The number of timeslots between the blanking pulse and the auxiliary pulse can be almost unlimited as long as any intermediate voltage pulses due to the strobe waveform or data modulation do not unlatch the device from its blanked state nor interfere with the combined actions of the auxiliary and latching pulses. It is preferable that all the data sets are DC-compensated although non-compensated sets can be used provided this does not degrade the device performance. The strobe (or row) voltage is not usually compensated. To ensure complete DC compensation the scheme voltages can be inverted in a regular periodic manner for example after every row of the display has been addressed i.e. after each frame. For optimum performance with high contrast, it is preferable that data sets are chosen such that parasitic pulses do not appear on the trailing side of the latching pulse as this might interfere with the discrimination between the select and non-select latching pulses. Also, it is preferable that double pulses and consecutive data pulses of the same polarity are avoided in the data wavetrain, in order to ensure that optical noise due to the data is minimized and the pixel does not become unlatched due to any over-sized VT product. Data sets, i.e. combinations of 'data on' and 'data off' waveforms, satisfying these conditions for the above schemes are as follows:- for the scheme of Figure 5, sets (1,9), (1,11), (2,11), (3,11), (4,11), (5,11), (6,9), (8,9); for the scheme of Figure 6, sets (1,4), (1,7), (1,10), (1,11), (2,4), (2,7), (2,10), (2,11), (3,4), (3,5), (3,9); for the scheme of Figure 7, sets (1,6), (2,6), (3,4). Figure 8 shows the multiplex scheme produced by the combination of the strobe waveform of Figure 5 and the data set (2, 11) of Figure 5. The three component scheme can be adapted and implemented as a line-blanking scheme. The rows of a display are strobed by a unipolar blanking pulse with identical properties to the blanking pulse described above. Hence all the pixels in all rows that have been strobed by the blanking pulse are switched into a fixed and identical state known as the blanked state regardless of the column data voltage. Another unipolar pulse of opposite polarity is strobed down the rows a fixed number of lines behind the blanking pulse. The data voltage pulses are arranged to combine with this second strobe voltage in such a manner that the resultant pixel voltage either switches the pixel out of the blanked state and latches it into the opposite state or leaves the pixel in its blanked state. A two-timeslot line-blanking scheme is illustrated in Figure 9. This scheme corresponds to that shown in Figure 5 with the data set (1,11), but modified to operate as a two-slot blanking scheme. The first component, the blanking pulse, is strobed one to n lines ahead of the combined auxiliary and latching pulse. During operation, it must satisfy the requirements of the general scheme of Figure 5, and V A > V th ; V data > (V A - V th ) T₁= T₂+ T₃ = two time slots. T₄= (2 x integer) time slots. V th depends upon data in timeslot prior to auxiliary pulse and also the time interval between blanking and auxiliary pulse, i.e. the number of lines blanked. Accordingly, V th varies with the voltages produced across a pixel by "off" and "on" cross-talk data voltages prior to the auxiliary pulse; the scheme voltage pulses must be selected to satisfy the variation in V th to ensure that no unwanted crosstalk occurs between neighbouring pixels in the same column. Figure 10 shows another line-blanking scheme which corresponds to the multiplexing scheme of Figure 6 with the data set (3,4), but modified for line-blanking. The following conditions apply: V A < V th ; V data > (V th -V A ); T₁ = T₂ + T₃ = two time slots ; T₄ = (2 x integer)time slots; V A may be positive or negative voltage. Figures 11, 12 and 13 are examples of the electro-optic response during multiplexing using the scheme of Figure 9 for the case where blanking occurs one line ahead of the data addressed line. Figures 11b, 12a, 12b and 13 show the electro-optic response around respectively the points 1, 2, 3 and 4 of Figure 11a. This scheme can be used in the n-line blanked mode if required. The data set satisfies the requirements for optimizing the multiplex performance. In addition no parasitic pulses appear on the trailing side of the latching pulse interfering with the discrimination between the select and non-select latching pulses. One advantage of an 'n-lines' blanked or a multi n-slot scheme is that some time is allowed for the FLC molecules to relax from the fully driven and blanked state to a blanked but relaxed state prior to the application of the auxiliary and latching pulses. Consequently narrower auxiliary and latch pulsewidths can be used to switch from the relaxed to the opposite state. Thus an increased number of lines may be addressed in the display in a given time providing the number of slots required in the scheme have not increased by more than the proportional increase in addressing speed. Figures 14a and 14b each show an n-slot scheme, i.e. a scheme in which the waveform takes up more than four slots, designed to allow some relaxation to occur after the blanking pulse in order to reduce the width of the timeslot. Any chosen voltage pulses between the blanking pulse and the auxiliary and latching pulses must be such as to not interfere with the fundamental operations of the addressing scheme. Any of the schemes of Figures 5, 6 and 7 can be used as the sequence of blanking, auxiliary and latching pulses. A useful advantage of the three component scheme is that some temperature compensation may be readily implemented by introducing a variable voltage component into the auxiliary pulse timeslot part of the strobe voltage (i.e. the portion of the strobe voltage corresponding to the auxiliary pulse) thereby to alter the efficiency of the action of the auxiliary pulse to counter the effect of changes in temperature (see Figure 15). This is used to compensate for and avoid shifts in the data addressing frequency, data voltage, blanking and latching voltage that are often required to maintain multiplexing as the temperature varies. The amount of temperature compensation possible depends greatly upon the liquid crystal material and device parameters; however, a temperature variation of a few degrees centigrade can readily be achieved for most materials by use of the above method. For temperature compensation over a wider range, an additional adjustable voltage component can be introduced into the strobe latching pulse component. In the illustrated example, temperature 1 is greater than temperature 2, and V A1 is less than V A2 to compensate for the difference in temperature. In this way, V data , V₁, V b and the pulse width can be kept constant during multiplexing. Data modulation has been removed from the blanking pulse in this illustration for clarity. Figures 16 and 17 relate to a scheme using a trailing auxiliary pulse. There is no data modulation of the latching pulse. Thus all switching is determined by the auxiliary pulse alone. From the shown results it is clear that time intervals and other fixed intermediate pulses between the auxiliary pulse and the latching pulse are permissible providing they do not interfere with the mechanism causing switching by the auxiliary pulse. The relative position of the auxiliary pulse and latching pulse is not critical for obtaining multiplexing, but it does have a significant effect on the speed and width of the multiplex operating window conditions. These observations highlight the sensitivity of the system to the effect of neighbouring pixel data (crosstalk) following the latching pulse. It is still preferable to position the auxiliary pulse immediately prior to the latching pulse and modulate both with data. This ensures optimum speed and wide operating conditions, the effect of any trailing neighbouring pixel data causing crosstalk is then minimised. The addition of a trailing auxiliary pulse as well as the normal auxiliary pulse, so that the latching pulse is sandwiched between two identical pulses modulated in phase with each other, can be used to back up the preferred scheme (at the expense of an additional timeslot) to widen out the operating conditions even further. It is believed that a device embodying the present invention achieves the desired effect by the auxiliary pulse causing deepening of the blanking pulse electro-optic curve. (The blanking pulse electro-optic curve describes the ability of a given voltage pulse or pulse sequence to switch and latch a pixel out of the blanked state.) Figure 18 shows the curves due to the introduction of a simple auxiliary pulse prior to the latching pulse such as can be provided by data modulation. Thus it is possible to shift the e-o characteristic up and down the pulsewidth axis by modulating the auxiliary pulse. An auxiliary pulse with the same polarity as the latching pulse shifts the e-o curve 'down', i.e. faster switching. A auxiliary pulse with opposite polarity to the latching pulse retards switching and hence shifts the curve 'up', i.e. slower switching. Correct choice of the latching pulse voltage V L , width T L and auxiliary pulse modulation voltage (data voltage) enables multiplexing to occur. Figure 18 shows the curves for V B and V a fixed, while T L (timeslot) and V L (multiplex operating point) are chosen such that, when V A =O , no latching occurs (below "no auxiliary pulse" curve), when V A = V a latching occurs (above "fixed auxiliary" curve). By combining both auxiliary pulse and latching pulse modulation in a multiplex scheme as shown in Figure 19 it is possible to obtain very good discrimination between the select and non-select states and to obtain good wide multiplexing operating condition windows. A measure of the discrimination between select and non-select switching is the time between the non-select operating point and the no auxiliary pulse e-o curve i.e. ΔT₂. The use of an auxiliary pulse effectively increases the discrimination by ΔT₁. Figure 20 shows the effect of temperature on the blanking pulse electro-optic characteristic obtained with V A = 0 for various values of temperature ϑ, and so on where ϑ₁ < ϑ₂ < ϑ₃ < ϑ₄ < ϑ₅. Several important features are to be noted: first, the minimum in the curve deepens with increasing temperature, i.e. the e-o response is faster; second, the minimum voltage increases with temperature; thirdly, the steepness of the upturn in the e-o curve decreases with temperature increase. These changes in the e-o curve with temperature have a significant effect on the voltages required for multiplexing and the discrimination between the select and non-select multiplex states. In order to ensure the device can be multiplexed over some temperature range at a constant addressing rate it is necessary for the latching pulse voltages to 'track' the e-o characteristics, with temperature variation, to ensure that the select and non-select pulses lie in a switching and non-switching region respectively of the e-o characteristic. Hence by applying a variable voltage component to the auxiliary pulse slot independent of the data modulation of the auxiliary pulse it is possible to obtain some degree of temperature compensation by simply shifting the e-o curve up and down the pulsewidth axis. Figure 21 shows a series of blanking pulse e-o curves such that the curve α relates to no auxiliary pulse at a temperature ϑ₁; curve β relates to an auxiliary pulse V A1 at the temperature ϑ₁; curve γ relates to no auxiliary pulse at a temperature ϑ₂ (with ϑ₂ > ϑ₁); curve δ relates to an auxiliary pulse V A1 at temperature ϑ₂; and curve ε relates to an auxiliary pulse V A2 (with V A2 > V A1 ) at temperature ϑ₁. Thus, it can be seen that by increasing the auxiliary pulse voltage as temperature decreases, or vice versa, the e-o curve is maintained so that select operating point still latches and non-select does not. For temperature shifts involving significant variation in the minimum voltage it is necessary to apply an independent voltage component to the latching pulse slot to ensure good tracking of the e-o curve. Figure 22 shows e-o curves indicating temperature compensation using a latching pulse component, such that S₁ is the select operating point at ϑ₁, NS₁ is the non-select operating point at ϑ₁, S₂ is the select operating point at ϑ₂ and NS₂ is the non-select operating point at ϑ₂, with ϑ₂ being greater than ϑ₁. The minimum timeslot, hence maximum addressing rate, of the device is determined by the e-o curve for the lowest temperature at which the device is to operate. Consequently it is beneficial to use a combination of both latching pulse and auxiliary pulse temperature compensation to ensure a 'faster' e-o curve at the lowest temperature. The steepness of the upturn in the e-o curve has a significant effect on the discrimination between the select and non-select multiplex states and consequently the width of the operating conditions window. As the steepness of the upturn decreases with increasing temperature the device eventually reaches a temperature at which it does not multiplex, in the inverse mode (See Figure 23). Figure 23 shows a set of e-o curves for increasing temperature ϑ where ϑ₅ > ϑ₄ > ϑ₃ > ϑ₂ > ϑ₁. For a given ΔV I , the discrimination ΔT decreases with increase in temperature. It is possible to improve the discrimination a little, and hence the ability to multiplexl by increasing the data voltage and thus separating the select and non-select operating points further apart. Thus the non-select operating point lies further below the e-o curve well into the non-latching region (see Figure 19 for example). However, taken too far this has the undesirable effect of increasing the crosstalk thus degrading the contrast of the device - the same net effect as loss in upturn steepness. If, at a fixed temperature, a blanking pulse test is carried out in which the time between the blanking pulse and the latching pulse is increased (see Figure 24) a set of e-o curves can be obtained which are similar in shape to those obtained when the temperature is varied, as in Figure 20. Figure 24 shows the effect of increasing the relaxation time T R on the e-o curve by reference to curves I, II, III and IV with respective relaxation times T R1 , T R2 , T R3 and T R4 wherein T R4 > T R3 > T R2 > T R1 ; it can be seen that if the time between leading and trailing pulses becomes sufficiently large enough the e-o characteristic is the same as obtained in a monopolar pulse experiment (see Figure 26) where the duty cycle becomes very large. The e-o characteristics in Figure 20 and 24 are a consequence of the same phenomenon. When a voltage pulse is applied of sufficient voltage and width to cause a device to switch and latch, such as a blanking pulse, it switches into a 'driven' state. At the end of the voltage pulse the device is then observed to relax back into a latched state, see Figure 25 wherein T R1 is greater than the relaxation time and T R2 is less than the relaxation time, and T L2 is greater than T L1 for latching. In the case of the blanking pulse test and most multiplex schemes consisting of a leading and trailing pulse there is insufficient time for the device to relax after the leading pulse. Consequently the trailing pulse is trying to switch the device into the opposite state from effectively a blanked driven state. Thus the device requires a relatively wide trailing pulse. If sufficient time is allowed for the device to relax some way then it requires a much narrower pulse to switch into the opposite state. Hence introducing extra slots between the blanking and latching pulse in a typical three component scheme means smaller timeslots are needed. However, the device now operates on an e-o curve with an upturn which is reduced in steepness (such as one of the curves in Figure 24 with an increased relaxation period) with a subsequent reduction in discrimination. Similarly using a line blanking scheme means that greater time is allowed for relaxation between the blanking pulse and the select/non-select pulse and thus it is possible to use much narrower timeslots and address the device faster. If the device is blanked enough lines ahead then the device effectively operates with the monopolar pulse test e-o characteristic. Thus it is necessary, if the device is to operate in the inverse mode with good discrimination and a wide operating conditions window, for it to have a monopolar pulse e-o characteristic with an upturn. When in the driven state the torque due to the negative dielectric anisotropy is much greater than when switching from a relaxed state. Consequently a highly non-linear e-o characteristic with a greater upturn is obtained. In the monopolar pulse test there is sufficient time between pulses to allow the device to relax fully into a latched, but relaxed, state. Consequently the opposing torque due to the dielectric anisotropy is smaller and it requires a narrower pulse to switch the device into the opposite state. Thus the upturn in the e-o curve for a monopolar pulse test is not so steep as in the blanking pulse test and the device response is faster. An increase in temperature causes an increase in the relaxation rate so it has the same effect as allowing more time between the blanking and latching pulses. Hence the similarity between Figure 20 and 24 and the eventual match of the monopolar and blanking pulse test e-o characteristics. Figure 26 shows the e-o curve for a monopolar pulse of amplitude V and pulse width T together with the repetitive monopolar pulse waveform used to produce that e-o curve. The voltage and pulsewidth of the blanking pulse at any given temperature is determined by the monopolar pulse e-o curve at that temperature, providing sufficient time has occurred between the last non-data pulse and the blanking pulse to ensure the device is in a relaxed and not driven state (which normally happens in any multi-row matrix device). If the device is to operate over a range of, temperatures at a constant addressing rate (assuming appropriate temperature compensation has been introduced into the latching pulses) then the pulsewidth and voltage of the blanking pulse is determined by the monopolar pulse e-o curve for the minimum operating temperature. Clearly, for the maximum addressing rate the blanking pulse is chosen to lie on the fastest part of the e-o curve.

Claims: A method of addressing a display device comprising a matrix of separately addressable pixels arranged in rows, each pixel being latchable into first and second states by applying respective voltage waveforms thereacross, the method comprising, for each row, latching all the pixels into the first state by applying a voltage of a given polarity thereacross and thereafter latching selected pixels of the row into the second state by applying a first voltage waveform thereacross while applying a second voltage waveform across the remaining pixels of the row, the second waveform leaving the corresponding pixels in the first state, the first and second waveforms each including a pulse of the opposite polarity to said given polarity, characterized in that the first waveform, in addition to said pulse of the opposite polarity, includes an auxiliary pulse of a first kind and the second waveform, in addition to said pulse of the opposite polarity, includes an auxiliary pulse of a second kind, each auxiliary pulse having a smaller amplitude than the corresponding said pulse of the opposite polarity, the auxiliary pulse of the first kind being of said opposite polarity and having sufficient amplitude that cooperation of this auxiliary pulse with the corresponding said pulse of the opposite polarity produces latching of the corresponding pixel into the second state, the auxiliary pulse of the second kind having a different amplitude such that cooperation of this auxiliary pulse with the corresponding said pulse of the opposite polarity fails to produce latching of the corresponding pixel into the second state. A method as claimed in Claim 1, wherein the said pulse of the opposite polarity included in the first voltage waveform has a different amplitude to the said pulse of the opposite polarity included in the second voltage waveform. A method as claimed in Claim 1 or Claim 2, wherein the auxiliary pulse of the second kind is of said opposite polarity. A method as claimed in any preceding claim wherein each auxiliary pulse is temporally adjacent the corresponding said pulse of the opposite polarity. A method as claimed in Claim 4, wherein each auxiliary pulse immediately precedes the corresponding said pulse of the opposite polarity. A method as claimed in any preceding claim, wherein the first and second voltage waveforms each include a further auxiliary pulse. A method as claimed in any preceding claim, wherein the first and second voltage waveforms are produced by simultaneously applying a common strobe voltage waveform to all the pixels of the row and respective data voltage waveform to the individual pixels of the row. A method as claimed in Claim 7, wherein temperature compensation is effected by introducing a variable voltage component in the portion of the strobe waveform corresponding to the auxiliary pulses of the first and second kinds. A display device comprising a matrix of separately addressable pixels arranged in rows, each pixel being latchable into first and second states by applying respective voltage waveforms thereacross, and addressing means for addressing said pixels, the addressing means being arranged to address each row by applying a voltage of a given polarity across the pixels of that row to latch the pixels into the first state and thereafter latch selected pixels of the row into the second state by applying a first voltage waveform thereacross while applying a second voltage waveform across the remaining pixels of the row, the second waveform leaving the corresponding pixels in the first state, the first and second waveforms each including a pulse of the opposite polarity to said given polarity, characterised in that the first waveform, in addition to said pulse of the opposite polarity, includes an auxiliary pulse of a first kind and the second waveform, in addition to said pulse of the opposite polarity, includes an auxiliary pulse of a second kind, each auxiliary pulse having a smaller amplitude than the corresponding said pulse of the opposite polarity, the auxiliary pulse of the first kind being of said opposite polarity and having sufficient amplitude that cooperation of this auxiliary pulse with the corresponding said pulse of the opposite polarity will produce latching of the corresponding pixel into the second state, the auxiliary pulse of the second kind having a different amplitude such that cooperation of this auxiliary pulse with the corresponding said pulse of the opposite polarity will fail to produce latching of the corresponding pixel into the second state.

Inventors: COULSON IAN, COULSON, IAN
Applicants: EMI PLC THORN, THORN EMI PLC
Application year: 1989
Main classification: G09G3
IPC codes: G02F1, G09G3