EP0327627B1 - Apparatus and method for driving a ferroelectric liquid crystal device - Google Patents
Apparatus and method for driving a ferroelectric liquid crystal device Download PDFInfo
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- EP0327627B1 EP0327627B1 EP19880906995 EP88906995A EP0327627B1 EP 0327627 B1 EP0327627 B1 EP 0327627B1 EP 19880906995 EP19880906995 EP 19880906995 EP 88906995 A EP88906995 A EP 88906995A EP 0327627 B1 EP0327627 B1 EP 0327627B1
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- G—PHYSICS
- G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
- G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
- G09G3/00—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes
- G09G3/20—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters
- G09G3/34—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters by control of light from an independent source
- G09G3/36—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters by control of light from an independent source using liquid crystals
- G09G3/3611—Control of matrices with row and column drivers
- G09G3/3622—Control of matrices with row and column drivers using a passive matrix
- G09G3/3629—Control of matrices with row and column drivers using a passive matrix using liquid crystals having memory effects, e.g. ferroelectric liquid crystals
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- G—PHYSICS
- G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
- G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
- G09G2310/00—Command of the display device
- G09G2310/06—Details of flat display driving waveforms
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- G—PHYSICS
- G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
- G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
- G09G2320/00—Control of display operating conditions
- G09G2320/02—Improving the quality of display appearance
- G09G2320/0204—Compensation of DC component across the pixels in flat panels
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- G—PHYSICS
- G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
- G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
- G09G2320/00—Control of display operating conditions
- G09G2320/02—Improving the quality of display appearance
- G09G2320/0209—Crosstalk reduction, i.e. to reduce direct or indirect influences of signals directed to a certain pixel of the displayed image on other pixels of said image, inclusive of influences affecting pixels in different frames or fields or sub-images which constitute a same image, e.g. left and right images of a stereoscopic display
Definitions
- This invention relates to ferroelectric liquid crystal (FLC) devices, and particularly to a method and apparatus for driving the liquid crystal elements of such devices.
- FLC ferroelectric liquid crystal
- a ferroelectric liquid crystal has a permanent electric dipole which interacts with the applied electric field. Hence, ferroelectric liquid crystals exhibit fast response times, which make them suitable for use in display, switching and information processing applications.
- An example of an FLC device is described in a paper by N. A. Clarke et al, entitled “Submicrosecond bistable electro-optic switching in liquid crystals" in Appl. Phys. Lett. Volume 36, 1980, pp 899-901.
- the stimulus to which an FLC device responds is a dc field, and its response is a function of the applied voltage (V) and the length of time (t) for which it is applied.
- the response is not a linear function of V x t, and there may be a voltage level at which, irrespective of the length of time for which the voltage is applied, switching of the device will not occur. There may also be a length of time of application of the voltage which will be too short for switching to occur, irrespective of the magnitude of the voltage.
- An FLC device which can be multiplexed needs to have at least two different states (called latched states) which the liquid crystal can adopt in the absence of an applied field. These can be the same states as the states (called switched states) obtained when a field of either polarity is applied, or they can be different states.
- the liquid crystal can change from one switched state to another switched state when a field is applied thereto, without necessarily going to a latched state when the field is removed.
- the voltage at which the liquid crystal switches from one state to the other by 10% is called the switching threshold at 10% switching (T S10 ).
- the voltage at which the liquid crystal switches fully from one state to the other state is called the switching threshold at 100% switching (T S100 ).
- the voltage at which the liquid crystal will go fully into one of the latched states when the field is removed is called the latching threshold at 100% latching (T L100 ).
- the voltage at which the liquid crystal no longer goes into either of two different states when the field is removed is called the latching threshold at 0% latching (T Lo ).
- a ferroelectric liquid crystal element is switched to one state by the application of a voltage of a given polarity across its electrodes, and is switched to the other state by the application thereto of a voltage of the opposite polarity. It is essential that an overall dc voltage shall not be applied across such an element for an appreciable period, so that the elements remain charge-balanced, thereby avoiding decomposition of the crystal material. Pulsed operation of such elements has therefore been effected, with a pulse of one polarity being immediately followed by a pulse of the other polarity, so that there is no resultant dc polarisation.
- the liquid crystal elements are commonly arranged in matrix formation and are operated selectively by energising relevant row and column lines.
- Time-division multiplexing is effected by applying pulses cyclically to the row (strobe) lines in sequence and by applying pulses, in synchronism therewith, to selected column (data) lines.
- EP-A-0229647 discloses a further example of a 4 time slot system.
- a method and apparatus for driving a ferroelectric liquid crystal device matrix in a time-division multiplex mode in which strobing signals are applied cyclically to strobe lines coupled to two-state liquid crystal elements of the display and data signals are applied selectively to data lines coupled to the elements.
- the strobing signal applied to a strobe line during a line address period comprises a first strobe pulse of one polarity followed by a second strobe pulse of the opposite polarity and of different amplitude from the first strobe pulse.
- the strobing signal also includes a dc voltage to substantially cancel a dc voltage level resulting from the difference between the amplitudes of the first and second strobe pulses, which dc voltage will not switch the liquid crystal elements during its period of application.
- the data signals comprise a first data signal operative in combination with the strobing signal to set a selected liquid crystal element in a first one of its states and a second data signal operative in combination with the strobing signal to set the selected liquid crystal element in the other of its states.
- One or each of the first and second data signals comprises two consecutive pulses of opposite polarities.
- the d.c. voltage extends within the line address period, so that the line address period must be longer than is necessary to accommodate the two strobe pulses.
- the strobing signal applied to a strobe line during a line address period and the data signals each comprise only two time slots
- the d.c. voltage of a strobing signal extends only from the end of the second time slot of that strobing signal to the beginning of the first time slot of the next strobing signal applied to the same strobe line.
- a ferroelectric liquid crystal device such as a display, comprises a matrix of ferroelectric liquid crystal elements 1 coupled to row (strobe) and column (data) lines 2 and 3, respectively.
- a strobe pulse generator 4 is coupled to the strobe lines
- a data pulse generator 5 is coupled to the data lines.
- the strobe pulse generator continuously applies strobing signals to the strobe lines 2 in sequence
- the data pulse generator applies data signals to the data lines 3, in synchronism with the pulsing of the strobe lines, to set the corresponding element 1 in the required state.
- Figure 2 shows waveforms which would be applied to the lines 2 and 3 in a known 4-slot drive system.
- a strobing signal comprises a positive pulse 6 followed by a negative pulse 7 and, later during the same frame period, a negative pulse 8 followed by a positive pulse 9. All of these pulses are of the same amplitude V R , and there is therefore no residual dc level.
- the data signal may comprise a pulse train 10 ( Figure 2(b)) for setting the addressed element in the ON state or a pulse train 11 ( Figure 2(c)) for setting it in the OFF state, where ON and OFF merely indicate two different states.
- the pulse train 10 comprises positive and negative pulses 12 and 13, respectively, coincident with the pulses 6 and 7, and positive and negative pulses 14 and 15, respectively, coincident with the pulses 8 and 9.
- the pulses 12, 13, 14 and 15 are all of amplitude V d .
- the pulse train 11 comprises pulses 16, 17, 18 and 19 of the same amplitude as the pulses 12, 13, 14 and 15 but of opposite polarity thereto.
- the data pulses are also applied via the lines 3 to those liquid crystal elements 1 which are not being addressed by the strobing signal. This leads to crosstalk, which is inherent in any multiplexing scheme. In order to reduce visible crosstalk effects there are certain conditions which a multiplexing scheme must satisfy, as follows
- a small dc voltage 26 is applied to the strobe line between the end of the pulse 21 and the beginning of the pulse 20 of the next frame period.
- Figure 3(d) shows the voltage appearing across the addressed liquid crystal element as a result of the strobe signal and the data signal of Figure 3(b), whilst Figure 3(e) similarly shows the resultant, but for the data signal of Figure 3(c).
- Figure 4 shows an alternative arrangement of data pulses.
- the strobe pulses ( Figure 4 (a)) are similar to those in Figure 3(a), and the data OFF pulses ( Figure 4 (c)) are similar to those in Figure 3(c).
- the data ON signal ( Figure 3(b)) comprises merely a zero voltage level.
- the various voltages must then satisfy the following conditions. V1>T L100 V2 ⁇ T Lo V2 + V d >T L100 V d ⁇ T
- Figure 5 shows another alternative arrangement of data pulses.
- the data ON pulses ( Figures 5(b)) are similar to the data ON pulses of Figure 3(b), but the data OFF signal ( Figure (c)) is merely a zero voltage level.
- the duration of the element-addressing time can be shortened by reducing the period (t) of either of the strobe pulses and by increasing the voltage (V) of each reduced-length pulse, taking into account the criteria mentioned hereinbefore.
- Figure 6 shows one such configuration of strobe and data pulses.
- a first strobe pulse 27 has an amplitude V1 and a period t1
- a second strobe pulse 28 has a period t2 which is shorter than t1
- an amplitude V2 which is larger than V1.
- V1xt1+V2xt2+V dc xt3 must be substantially zero, where t3 is the length of the period between the end of the pulse 28 and the beginning of the next pulse 27.
- the data ON signal shown in Figure 6(b) comprises a positive-going pulse 29 of amplitude V d1 and duration t1, and a negative-going pulse 30 of amplitude V d2 and duration t2.
- the data OFF signal shown in Figure 6(c), is the inverse of Figure 6(b).
- V d1 xt1 In order to avoid subjecting the liquid crystal elements to an overall dc level due to the application of the data pulses, V d1 xt1 must be equal to V d2 xt2 for each data signal.
- the voltages and periods of the strobe and data pulses are preferably selected to obtain optimum working of the liquid crystal elements.
- Figure 7 shows an alternative pulse configuration in which the strobe pulses are the same as in Figure 6, but the first data pulse 31 is of different period from the first strobe pulse 27.
- the pulse 31 begins later than the beginning of the strobe pulse 27, but the pulses end simultaneously.
- the data OFF signal of Figure 7(c) is the inverse of the data ON signal of Figure 7(b).
- V d3 xt4 must equal V d2 xt2, where V d3 and t4 are the amplitude and the period, respectively, of the pulse 31.
- Figure 8 shows another pulse configuration in which the strobe pulses are the same as in Figure 6.
- the first data pulse 32 is the same width as the first strobe pulse 27, but the second data pulse 33 is longer than the second strobe pulse 28.
- the pulse 33 may alternatively be shorter than the pulse 28.
- V d4 xt5 must equal V d1 xt1, where V d4 and t5 are the voltage and period, respectively, of the pulse 33.
- Figure 9 shows another alternative configuration, in which the first data pulse 34 begins simultaneously with the first strobe pulse, but the data pulse is shorter than the strobe pulse.
- the second data pulse 35 is the same length as the second strobe pulse.
- the performance of the FLC device may be improved by including a period of zero voltage between the positive and negative strobe and/or data pulses and/or before and/or after any of those pulses.
- the zero voltage period can be of any suitable length and should be selected to suit the particular liquid crystal elements.
- Such a zero voltage level may be as shown at 36 in the data signal in Figure 9 or as shown in Figure 10, wherein the first and second strobe pulses 37 and 38, respectively, are separated by a period 39 of zero voltage.
- the pulse voltages and lengths will be adjusted to suit the particular type of liquid crystal elements and the particular combination of strobing and data signals.
- the polarity of both the strobe pulses and the data pulses may be reversed.
- a further improvement may be effected by superimposing an ac voltage at, say, 10-100kHz on the pulses. This helps to sharpen the switching thresholds and may also improve the contrast ratio of the data ON and OFF states during multiplexing.
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Abstract
Description
- This invention relates to ferroelectric liquid crystal (FLC) devices, and particularly to a method and apparatus for driving the liquid crystal elements of such devices.
- A ferroelectric liquid crystal has a permanent electric dipole which interacts with the applied electric field. Hence, ferroelectric liquid crystals exhibit fast response times, which make them suitable for use in display, switching and information processing applications. An example of an FLC device is described in a paper by N. A. Clarke et al, entitled "Submicrosecond bistable electro-optic switching in liquid crystals" in Appl. Phys. Lett.
Volume 36, 1980, pp 899-901. - The stimulus to which an FLC device responds is a dc field, and its response is a function of the applied voltage (V) and the length of time (t) for which it is applied. The response is not a linear function of V x t, and there may be a voltage level at which, irrespective of the length of time for which the voltage is applied, switching of the device will not occur. There may also be a length of time of application of the voltage which will be too short for switching to occur, irrespective of the magnitude of the voltage.
- An FLC device which can be multiplexed needs to have at least two different states (called latched states) which the liquid crystal can adopt in the absence of an applied field. These can be the same states as the states (called switched states) obtained when a field of either polarity is applied, or they can be different states.
- The liquid crystal can change from one switched state to another switched state when a field is applied thereto, without necessarily going to a latched state when the field is removed.
- For a given time interval, the voltage at which the liquid crystal switches from one state to the other by 10% is called the switching threshold at 10% switching (TS10). The voltage at which the liquid crystal switches fully from one state to the other state is called the switching threshold at 100% switching (TS100). The voltage at which the liquid crystal will go fully into one of the latched states when the field is removed is called the latching threshold at 100% latching (TL100). The voltage at which the liquid crystal no longer goes into either of two different states when the field is removed is called the latching threshold at 0% latching (TLo).
- A ferroelectric liquid crystal element is switched to one state by the application of a voltage of a given polarity across its electrodes, and is switched to the other state by the application thereto of a voltage of the opposite polarity. It is essential that an overall dc voltage shall not be applied across such an element for an appreciable period, so that the elements remain charge-balanced, thereby avoiding decomposition of the crystal material. Pulsed operation of such elements has therefore been effected, with a pulse of one polarity being immediately followed by a pulse of the other polarity, so that there is no resultant dc polarisation.
- The liquid crystal elements are commonly arranged in matrix formation and are operated selectively by energising relevant row and column lines. Time-division multiplexing is effected by applying pulses cyclically to the row (strobe) lines in sequence and by applying pulses, in synchronism therewith, to selected column (data) lines.
- An example of an FLC display driving system is disclosed in an article by T. Harada, M. Taguchi, K. Iwasa and M. Kai in SID 85 Digest, p 131 et seq. This system uses four pulses per refresh cycle, and can therefore be classified as a 4 time slot system. For a 625-line display at video frame rates this would require a 16us response of the crystal elements.
- EP-A-0229647 discloses a further example of a 4 time slot system. In that document there are described a method and apparatus for driving a ferroelectric liquid crystal device matrix in a time-division multiplex mode, in which strobing signals are applied cyclically to strobe lines coupled to two-state liquid crystal elements of the display and data signals are applied selectively to data lines coupled to the elements. The strobing signal applied to a strobe line during a line address period comprises a first strobe pulse of one polarity followed by a second strobe pulse of the opposite polarity and of different amplitude from the first strobe pulse. The strobing signal also includes a dc voltage to substantially cancel a dc voltage level resulting from the difference between the amplitudes of the first and second strobe pulses, which dc voltage will not switch the liquid crystal elements during its period of application. The data signals comprise a first data signal operative in combination with the strobing signal to set a selected liquid crystal element in a first one of its states and a second data signal operative in combination with the strobing signal to set the selected liquid crystal element in the other of its states. One or each of the first and second data signals comprises two consecutive pulses of opposite polarities.
- The d.c. voltage extends within the line address period, so that the line address period must be longer than is necessary to accommodate the two strobe pulses.
- It is an object of the present invention to provide a method and apparatus for driving the elements of an FLC device matrix, in which only two strobe pulse time slots per line address period are required, and in which the d.c. voltage does not take up part of the line address period, thereby enabling the line address period to be reduced.
- According to the invention, the strobing signal applied to a strobe line during a line address period and the data signals each comprise only two time slots, and the d.c. voltage of a strobing signal extends only from the end of the second time slot of that strobing signal to the beginning of the first time slot of the next strobing signal applied to the same strobe line.
- Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, wherein
- Figure 1 is a block schematic diagram of an FLC device drive system;
- Figure 2 illustrates strobing and data pulses occurring in a known 4-slot drive system;
- Figure 3 illustrates strobing and data pulses occurring in one embodiment of a 2-slot drive system according to the present invention; and
- Figures 4 to 10 illustrate strobing and data pulses occurring in second to eighth embodiments, respectively, of the invention.
- Referring to Figure 1, a ferroelectric liquid crystal device, such as a display, comprises a matrix of ferroelectric
liquid crystal elements 1 coupled to row (strobe) and column (data)lines strobe pulse generator 4 is coupled to the strobe lines, and adata pulse generator 5 is coupled to the data lines. The strobe pulse generator continuously applies strobing signals to thestrobe lines 2 in sequence, and the data pulse generator applies data signals to thedata lines 3, in synchronism with the pulsing of the strobe lines, to set thecorresponding element 1 in the required state. - Figure 2 shows waveforms which would be applied to the
lines positive pulse 6 followed by anegative pulse 7 and, later during the same frame period, anegative pulse 8 followed by apositive pulse 9. All of these pulses are of the same amplitude VR, and there is therefore no residual dc level. The data signal may comprise a pulse train 10 (Figure 2(b)) for setting the addressed element in the ON state or a pulse train 11 (Figure 2(c)) for setting it in the OFF state, where ON and OFF merely indicate two different states. Thepulse train 10 comprises positive andnegative pulses pulses negative pulses pulses pulses pulse train 11 comprisespulses pulses - The data pulses are also applied via the
lines 3 to thoseliquid crystal elements 1 which are not being addressed by the strobing signal. This leads to crosstalk, which is inherent in any multiplexing scheme. In order to reduce visible crosstalk effects there are certain conditions which a multiplexing scheme must satisfy, as follows - 1.
- The data voltage Vd must not be large enough to switch the liquid crystal. Switching the liquid crystal will reduce the contrast of the device.
- 2.
- The strobe voltage plus the data voltage (VS + Vd) must be large enough to switch and latch the liquid crystal so that the correct state (ON or OFF) of the element is achieved.
- 3.
- The strobe voltage minus the data voltage (VS - Vd) can switch the liquid crystal since it occurs only once in every frame scan. However, it must not latch the liquid crystal, since this will reverse the data required, nor must it unlatch the liquid crystal from the original state.
- Since the
strobe pulses small dc voltage 26 is applied to the strobe line between the end of thepulse 21 and the beginning of thepulse 20 of the next frame period. - Figure 3(d) shows the voltage appearing across the addressed liquid crystal element as a result of the strobe signal and the data signal of Figure 3(b), whilst Figure 3(e) similarly shows the resultant, but for the data signal of Figure 3(c). For the system to operate correctly, the following conditions should be satisfied as nearly as possible. The system can operate without their being satisfied, but there is then a loss of contrast.
Vd<TSo
It will be seen that each strobe and data signal comprises only two pulses, so that the liquid crytal elements are addressed in only two time slots during a frame period, as compared to four time slots for the known system. This halves the requirement as regards the speed of switching of the liquid crystal elements. - Figure 4 shows an alternative arrangement of data pulses. The strobe pulses (Figure 4 (a)) are similar to those in Figure 3(a), and the data OFF pulses (Figure 4 (c)) are similar to those in Figure 3(c). In this case, however, the data ON signal (Figure 3(b)) comprises merely a zero voltage level. The various voltages must then satisfy the following conditions.
V₁>TL100
V₂<TLo
Vd<TSo
Figure 5 shows another alternative arrangement of data pulses. In this case the data ON pulses (Figures 5(b)) are similar to the data ON pulses of Figure 3(b), but the data OFF signal (Figure (c)) is merely a zero voltage level. The voltages must then satisfy the following conditions.
V₂>TL100
Vd<TSo
In each of the drive arrangements described above, the duration of the element-addressing time can be shortened by reducing the period (t) of either of the strobe pulses and by increasing the voltage (V) of each reduced-length pulse, taking into account the criteria mentioned hereinbefore. - Figure 6 shows one such configuration of strobe and data pulses. In Figure 6(a), a
first strobe pulse 27 has an amplitude V₁ and a period t₁, whereas asecond strobe pulse 28 has a period t₂ which is shorter than t₁, and an amplitude V₂ which is larger than V₁. It will be apparent that V₁xt₁+V₂xt₂+Vdcxt₃ must be substantially zero, where t₃ is the length of the period between the end of thepulse 28 and the beginning of thenext pulse 27. - The data ON signal, shown in Figure 6(b) comprises a positive-going
pulse 29 of amplitude Vd1 and duration t₁, and a negative-goingpulse 30 of amplitude Vd2 and duration t₂. The data OFF signal, shown in Figure 6(c), is the inverse of Figure 6(b). In order to avoid subjecting the liquid crystal elements to an overall dc level due to the application of the data pulses, Vd1xt₁ must be equal to Vd2xt₂ for each data signal. - The voltages and periods of the strobe and data pulses are preferably selected to obtain optimum working of the liquid crystal elements. The optimum arrangement for the strobe pulses is achieved when V₁ = V₂ and t₁ and t₂ are adjusted to suit the liquid crystal elements. Any discrepancy between V₁xt₁ and V₂xt₂ is then accounted for by selecting the correct value of the
dc voltage 26. - Figure 7 shows an alternative pulse configuration in which the strobe pulses are the same as in Figure 6, but the
first data pulse 31 is of different period from thefirst strobe pulse 27. Thepulse 31 begins later than the beginning of thestrobe pulse 27, but the pulses end simultaneously. Again, the data OFF signal of Figure 7(c) is the inverse of the data ON signal of Figure 7(b). In this case Vd3xt₄ must equal Vd2xt₂, where Vd3 and t₄ are the amplitude and the period, respectively, of thepulse 31. - Figure 8 shows another pulse configuration in which the strobe pulses are the same as in Figure 6. In this case, however, the
first data pulse 32 is the same width as thefirst strobe pulse 27, but thesecond data pulse 33 is longer than thesecond strobe pulse 28. Thepulse 33 may alternatively be shorter than thepulse 28. For dc cancellation, Vd4xt₅ must equal Vd1xt₁, where Vd4 and t₅ are the voltage and period, respectively, of thepulse 33. - Figure 9 shows another alternative configuration, in which the
first data pulse 34 begins simultaneously with the first strobe pulse, but the data pulse is shorter than the strobe pulse. Thesecond data pulse 35 is the same length as the second strobe pulse. - In each of the drive arrangements described herein, the performance of the FLC device may be improved by including a period of zero voltage between the positive and negative strobe and/or data pulses and/or before and/or after any of those pulses. The zero voltage period can be of any suitable length and should be selected to suit the particular liquid crystal elements. Such a zero voltage level may be as shown at 36 in the data signal in Figure 9 or as shown in Figure 10, wherein the first and
second strobe pulses period 39 of zero voltage. - In every case, the pulse voltages and lengths will be adjusted to suit the particular type of liquid crystal elements and the particular combination of strobing and data signals. In any of the configurations described above, the polarity of both the strobe pulses and the data pulses may be reversed.
- In each of the drive arrangements of the present invention a further improvement may be effected by superimposing an ac voltage at, say, 10-100kHz on the pulses. This helps to sharpen the switching thresholds and may also improve the contrast ratio of the data ON and OFF states during multiplexing.
Figure 3 shows waveforms provided in a first embodiment of the present invention. The strobing signal (Figure 3(a)) comprises a
Claims (9)
- A method of driving a ferroelectric liquid crystal device matrix in a time-division multiplex mode, comprising applying strobing signals cyclically to strobe lines (2) coupled to two-state liquid crystal elements (1) of the display and applying data signals selectively to data lines (3) coupled to the elements, wherein the strobing signal applied to a strobe line during a line address period comprises a first strobe pulse (20) of one polarity followed by a second strobe pulse (21) of the opposite polarity and of different amplitude from the first strobe pulse, the strobing signal also including a dc voltage (26) to substantially cancel a dc voltage level resulting from the difference between the amplitudes of the first and second strobe pulses and which dc voltage will not switch the liquid crystal elements during its period of application; wherein the data signals comprise a first data signal (22, 23) operative in combination with the strobing signal to set a selected liquid crystal element in a first one of its states, and a second data signal (24, 25) operative in combination with the strobing signal to set the selected liquid crystal element in the other of its states; and wherein one or each of the first and second data signals comprises two consecutive pulses of opposite polarities; characterised in that the strobing signal applied to a strobe line during a line address period and the data signals each comprise only two time slots; and in that the d.c. voltage of said strobing signal extends only from the end of the second time slot of that strobing signal to the beginning of the first time slot of the next strobing signal applied to the same strobe line.
- A method as claimed in Claim 1, characterised in that said first data signal comprises two consecutive pulses (22, 23) of mutually opposite polarities and of subtantially equal amplitudes, and said second data signal comprises two consecutive pulses (24, 25) of opposite polarities to the pulses of said first data signal and of substantially equal amplitudes.
- A method as claimed in Claim 1, characterised in that said first data signal comprises a constant dc level, and said second data signal comprises two consecutive pulses (24, 25) of polarities opposite to those (20, 21) of the pulses of the strobing signal.
- A method as claimed in Claim 1, characterised in that said first data signal comprises two consecutive pulses (22, 23) of the same polarities as the pulses (20, 21) of the strobing signal, and said second data signal comprises a constant dc level.
- A method as claimed in Claim 1, characterised in that said first and second strobe pulses (27, 28) of the strobing signal are of mutually different time durations.
- A method as claimed in Claim 1, characterised in that each first and second data signal comprises two pulses (29, 30) of mutually opposite polarities and of mutually different time durations.
- A method as claimed in Claim 1, characterised in that the strobing signal and/or each first and second data signal comprises two pulses (34, 35) of mutually opposite polarities separated by a period (36) of zero voltage.
- A method as claimed in any preceding claim, characterised in that a relatively high-frequency ac voltage is superimposed on the strobing signal and/or the data signals.
- Apparatus for driving a ferroelectric liquid crystal device matrix in a time-division multiplex mode, comprising means (4) to apply strobing signals cyclically to strobe lines (2) coupled to two-state liquid crystal elements of the display; and means (5) to apply data signals selectively to data lines (3) coupled to the elements, wherein the strobing signal applied to a strobe line during a line address period comprises a first strobe pulse (20) of one polarity followed by a second strobe pulse (21) of the opposite polarity and of different amplitude from the first strobe pulse, the strobing signal also including a d.c. voltage (26) to substantially cancel a d.c. voltage level resulting from the difference between the amplitudes of the first and second strobe pulses which d.c. voltage will not switch the liquid crystal elements during its period of application; wherein the data signals comprise a first data signal (22, 23) operative in combination with the strobing signal to set a selected liquid crystal element in a first one of its states, and a second data signal (24,25) operative in combination with the strobing signal to set the selected liquid crystal element in the other of its states; and wherein one or each of the first and second data signals comprises two consecutive pulses of opposite polarities; characterised in that the strobing signal applied to a strobe line during a line address period and the data signals each comprise only two time slots; and in that the d.c. voltage of said strobing signal extends only from the end of the second time slot of that strobing signal to the beginning of the first time slot of the next strobing signal applied to the same strobe line.
Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
GB8719078A GB2208559B (en) | 1987-08-12 | 1987-08-12 | Ferroelectric liquid crystal devices |
GB8719078 | 1987-08-12 |
Publications (2)
Publication Number | Publication Date |
---|---|
EP0327627A1 EP0327627A1 (en) | 1989-08-16 |
EP0327627B1 true EP0327627B1 (en) | 1993-12-08 |
Family
ID=10622191
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
EP19880906995 Expired - Lifetime EP0327627B1 (en) | 1987-08-12 | 1988-08-12 | Apparatus and method for driving a ferroelectric liquid crystal device |
Country Status (5)
Country | Link |
---|---|
EP (1) | EP0327627B1 (en) |
JP (1) | JPH02500936A (en) |
DE (1) | DE3886192T2 (en) |
GB (1) | GB2208559B (en) |
WO (1) | WO1989001680A1 (en) |
Families Citing this family (6)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
FR2666923A2 (en) * | 1990-06-22 | 1992-03-20 | Centre Nat Rech Scient | Improvements to nematic liquid-crystal displays, with surface bistability, controlled by flexoelectric effect |
JP2990297B2 (en) * | 1990-09-10 | 1999-12-13 | セイコーインスツルメンツ株式会社 | Liquid crystal light valve device and liquid crystal light valve driving method |
GB2249653B (en) * | 1990-10-01 | 1994-09-07 | Marconi Gec Ltd | Ferroelectric liquid crystal devices |
EP0561135A2 (en) * | 1992-02-08 | 1993-09-22 | Hoechst Aktiengesellschaft | Method of driving bistable displays, in particular ferroelectric liquid crystal displays |
GB2293906A (en) * | 1994-10-03 | 1996-04-10 | Sharp Kk | Liquid crystal display |
GB2333634B (en) | 1998-01-21 | 2002-02-20 | Sharp Kk | Liquid crystal device and method of addressing liquid crystal device |
Family Cites Families (1)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US4770502A (en) * | 1986-01-10 | 1988-09-13 | Hitachi, Ltd. | Ferroelectric liquid crystal matrix driving apparatus and method |
-
1987
- 1987-08-12 GB GB8719078A patent/GB2208559B/en not_active Expired - Fee Related
-
1988
- 1988-08-12 WO PCT/GB1988/000669 patent/WO1989001680A1/en active IP Right Grant
- 1988-08-12 JP JP50664288A patent/JPH02500936A/en active Pending
- 1988-08-12 DE DE88906995T patent/DE3886192T2/en not_active Expired - Fee Related
- 1988-08-12 EP EP19880906995 patent/EP0327627B1/en not_active Expired - Lifetime
Also Published As
Publication number | Publication date |
---|---|
GB2208559A (en) | 1989-04-05 |
EP0327627A1 (en) | 1989-08-16 |
WO1989001680A1 (en) | 1989-02-23 |
GB8719078D0 (en) | 1987-09-16 |
DE3886192T2 (en) | 1994-04-14 |
GB2208559B (en) | 1991-09-04 |
DE3886192D1 (en) | 1994-01-20 |
JPH02500936A (en) | 1990-03-29 |
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