Classifications

• • 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
  • G09G3/3637—Control of matrices with row and column drivers using a passive matrix using liquid crystals having memory effects, e.g. ferroelectric liquid crystals with intermediate tones displayed by domain size control
• • G—PHYSICS
  • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
  • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
  • G09G2300/00—Aspects of the constitution of display devices
  • G09G2300/04—Structural and physical details of display devices
  • G09G2300/0469—Details of the physics of pixel operation
  • G09G2300/0478—Details of the physics of pixel operation related to liquid crystal pixels
  • G09G2300/0482—Use of memory effects in nematic liquid crystals
  • G09G2300/0486—Cholesteric liquid crystals, including chiral-nematic liquid crystals, with transitions between focal conic, planar, and homeotropic states
• • 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
  • G09G2310/04—Partial updating of the display screen
• • 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
• • 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
• • 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
  • G09G2320/0233—Improving the luminance or brightness uniformity across the screen
• • 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

Definitions

• the present invention relates to the field of liquid crystal displays.
• the present invention relates to nematic liquid crystal bistable displays.
• the present invention is particularly applicable to bistable displays with nematic liquid crystals, anchoring breaking, two stable textures differ by a twist of about 180 °.
• the primary object of the present invention is to improve the performance of bistable display devices.
• a second aim is to propose an addressing principle for bistable display devices making it possible to obtain controlled and uniform gray levels throughout the display.
• BiNem nematic liquid crystal bistable displays with anchor break, two stable textures differ by a 180 ° twist. They are described in documents [1] and [2].
• a BiNem display consists of a chiralized nematic liquid crystal layer placed between two substrates formed of two glass slides, one "master” 30, the other "slave” 32. Electrodes line 34 and column 36, respectively disposed on each of the substrates, receive electrical control signals and allow to apply on the nematic liquid crystal an electric field perpendicular to the surfaces. Anchoring layers 38 and 40 are deposited on the electrodes. On the master blade, the anchoring 38 of the liquid crystal molecules is strong and slightly inclined, on the slave blade, the anchoring 40 is weak and flat or very slightly inclined.
• Two bistable textures can be obtained. They differ from each other by a twist of ⁇ 180 ° and are topologically incompatible.
• U is called a uniform or slightly twisted texture and T is a twisted texture.
• the spontaneous pitch of the nematic is chosen substantially equal to a quarter of the thickness of the cell, to make the energies of U and T essentially equal. Without a field there is no other state with a lower energy: U and T have a true bistability.
• the two states U and T are very different and can display black and white images with a contrast greater than 100.
• E an almost homeotropic texture called H is obtained.
• the anchoring is said to be "broken”: at the breaking of the electric field, the cell evolves towards one or other of the stable states U and T (see Figure 1).
• the control signals used induce a strong flow of the liquid crystal in the vicinity of the master blade 30, the hydrodynamic coupling 42 between the master blade and the slave blade induces the texture T. Otherwise, the texture U is obtained by coupling. elastic 44, helped by the possible inclination of the weak anchor.
• the term “switching" of a BiNem screen element will be used for the liquid crystal molecules to pass through the homeotrope (anchoring break), then to evolve towards one of the two stable states U or T at the breaking of the electric field.
• the hydrodynamic coupling [6] between the slave blade and the master blade is related to the viscosity of the liquid crystal.
• the hydrodynamic flow 46 is strong enough, near the slave blade 32, it inclines the molecules in the direction that induces the texture T; they turn in opposite directions on both blades.
• the return to the equilibrium of the molecules near the slave blade is a second motor of the flow, it reinforces it and helps the homogeneous passage of the pixel in texture T.
• the passage of the texture H under field to the texture T is obtained thanks to a flow therefore a displacement of the liquid crystal in the direction where is inclined the anchoring of the molecules on the master blade (see Figure 2).
• the BiNem displays are most often matrix screens formed of N x M pixels, made at the intersection of perpendicular conductive strips deposited on the master and slave substrates.
• An example of a display of 4 rows 50 and 4 columns 52 made according to the state of the art is given in FIG. 3.
• On the row electrodes are applied sequentially the so-called excitation signals making it possible to switch the set of pixels in the line.
• the first part of the line excitation signal makes it possible to break the anchorage on the whole of the line.
• a signal is applied on its column. This signal makes it possible to select the final texture of this pixel, independently of the other pixels of the line.
• the set of column signals is applied simultaneously to all the pixels of the line.
• the display is addressed when all the lines have been successively excited.
• the application of multiplexing signals makes it possible, by the combination of line and column signals, to select the final state of the N x M pixels of the matrix forming the display: the switching voltage applied to the pixel during the excitation time of the line forms a pulse which, in a first phase (VlL, Tl), breaks the anchoring, then in a second phase (V2L, T2) , determines the final texture of the pixel (see Figure 4).
• VlL, Tl first phase
• V2L, T2 the applied voltage either stops abruptly, causing a voltage drop sufficient to induce the twisted texture T, or decreases progressively, possibly by trays, and creates the uniform texture U.
• the amplitude of the pixel voltage determining the speed of the voltage drop is generally low. It is performed by so-called “column” multiplexing signals and contains the image information. It is therefore the column voltage that allows, once the anchor "broken", to select the final texture of the pixel. The amplitude of the pixel voltages allowing the breaking of the anchorage is higher. It is performed by so-called “line” multiplexing signals and is independent of the content of the image. In the following, the lines of the electrodes of the display are used to apply the "line” signals, and columns the electrodes for the application of the "column” voltages.
• the application of the multiplexing signals makes it possible to select the texture of all the pixels of a line, successively scanning each line of the screen, and simultaneously applying the column signals determining the state of each pixel of the excited line.
• An example of multiplexing signals according to the state of the art is given in FIG.
• Ln corresponds to the line n
• C m corresponds to the column m
• C m + i corresponds to the column m + 1
• P (n, m) corresponds to the pixel n
• m + i corresponds to the pixel P (n, m + 1).
• voltage signals VlL and V2L of a duration T 1 and T 2 respectively are applied while on the column m, voltages VC of duration T c are applied.
• the column signals are alternately positive and negative.
• the lines of the multiplexed BiNem display are oriented perpendicular to the direction of the hydrodynamic flow.
• This phenomenon can be used judiciously to produce a bistable display of the BiNem type with gray levels. Indeed, if the pixels operate independently, it is possible to adjust the electrical signal to switch at T part of the pixel and thus obtain shades of gray by gradual variation of the switched surface of the pixel (see Figures 6 and 7) .
• Figure 6 comprises four parts, 6a, 6b, 6c and 6d.
• FIG. 6a there is shown a white square 60 for which the texture of the pixel is T.
• the black parts in FIGS. 6b, 6c and 6d (square 62, 64 and 66) correspond to the texture U.
• Figure 6b corresponds to a level of light gray, for which the texture T in white is predominant
• Figure 6c corresponds to a dark gray level for which the texture U is predominant and
• Figure 6d corresponds to black (U texture).
• FIG. 7 represents the optical state of the pixels of a 160x480 display according to the state of the art as a function of the column voltage Vc addressed.
• the texture T pass-through is clear and the U texture is non-passing and dark.
• the double arrow D1 represents the direction of the row electrodes.
• control signals adapted to control the speed of the displacement of the liquid crystal and thus progressively control the extent of one of the two stable states within each of the pixels, in order to obtain levels of gray checked inside each of these.
• the aforementioned control signals may proceed by modulation of various parameters, and in particular the voltage level of the column signals and / or the duration thereof, as shown in FIG. 8.
• FIG. 8a shows the signals for a line L n
• FIG. 8b represents the signals for the line L n + I
• FIG. 8c represents the column signals Cm with a modulation of the amplitude Vc of the column signal
• FIG. 8d represents the column signal Cm with a modulation of duration Tc of this column signal
• FIG. 8d represents the modulation of the ⁇ Tc phase of the column signal Cm.
• a reference electro-optical curve is defined for the BiNem displays: the optical state after switching or texture percentage T as a function of the voltage V2L (FIGS. 4 and 6).
• the precise display of the gray levels in multiplexing is done by modulating the parameters of the column signals, in particular their level of voltage and / or their duration, in order to move along the optical response curve around the selected operating point.
• FIG. 10 A simplified example of producing gray levels by modulating the amplitude of the voltage of the column signals around V2LD is given in FIG. 10, which comprises two parts,
• V2L D of the operating point makes it possible to obtain, according to the electro-optical response curve, a gray level comprising 60% of texture T inside the pixel.
• Vrmsac IV 2 (t) dt t + ⁇ t (D
• the value of the RMS voltage before switching is variable. Indeed, if we do not use technologies of "active matrix" type based on transistors, a pixel of a given line is subject to all the voltages applied to the column where it is located. FIG. 5 shows that the pixel (n, m) at the intersection of the line n and the column m is subjected to all the voltages applied to the column m.
• the voltage Vrmsac (n, m) also depends on the column and line times, respectively Tcrtip and T ⁇ gn e , according to the following formula (see formula 1):
• Vrmsac (n, m) Root square root mean square RMS as seen by the pixel at the intersection of line n and column m before it is switched,
• the value of Vrmsac typically varies between 0 volts (for example for the pixels on the first line of the screen in the scanning direction) and 3 volts.
• Vrmsac has a constant value.
• the voltage and duration of the column signal are adjusted according to the gray level "g" to be obtained.
• gray level "g" there are 16 distinct voltage values
• Tc g Each level of gray "g” thus brings its specific contribution to the voltage Vrmsac.
• the mean square voltage applied to a given pixel before switching depends on the gray levels displayed on the previous pixels located on the same column according to the formula (2).
• a prototype BiNem definition screen of 16 lines x 16 columns, brushed at 90 ° from the direction of the electrodes line has been realized.
• the width of the column electrodes is about 0.27 mm, their length is about 5 mm, the isolation between columns is about 0.015 mm.
• the width of the lines is about 0.27 mm, their length about 5 mm, the insulation between lines is about 0.015 mm.
• the elementary pixel 80 is shown in Figure 11 which shows an enlarged view of a portion of the prototype. In this figure the direction D2 is the direction of brushing. On the assembled cell, the brushing directions of the master and slave blades are parallel.
• the display is equipped with a rear reflector, a front polarizer and a front lighting device to work in reflective mode: the texture T is passing (it appears clear), the texture U is non-passing (it appears dark).
• An adapted control electronics delivering 16 line signals and 16 column signals, completes the device and allows addressing in multiplex mode of the display.
• the screen is addressed by line signals on line 4 (L4) (FIG. 12a) and by identical column signals on all of the columns 1 to 16 (FIG. 12b).
• the applied signals are shown in Fig. 12.
• VlL 20V;
• T1 500 ⁇ s;
• T2 750 ⁇ s;
• TL 50 ⁇ s;
• This signal makes it possible to simulate column data type pulses, similar to those actually seen by a pixel during multiplex addressing. These pulses will be called "pre-column pulses".
• the adjustment of the voltage V ad: ⁇ thus makes it possible to directly modify the mean square voltage applied to the pixels of the line n before they are switched according to the formula:
• Vrmsac (pixel) (4)
• the specific RMS value of the threshold of disappearance of the texture U is variable according to the type of signals used (for example according to the values of VlL, V2L, T1, T2 ).
• a given pixel When addressing a BiNem screen in multiplex mode, a given pixel sees a mean square voltage due to the signals sent on the column on which it is located when addressing the previous lines.
• VlL 20V
• V2L 6V
• T1 500 ⁇ s
• T2 750 ⁇ s
• TL 50 ⁇ s
• Tc T2
• ie T line 1300 ⁇ s
• Vc5 4V
• FIG. 15 shows the switching results of the multiplexing cell according to these signals when Vci- 4 changes from -2 V to -4V. More precisely, FIG. 15 shows the effects of the Vrmsac voltage on the switching of the line 5 during a multiplex type scan by using the signals shown in FIG. 14 for three values of Vci- 4 . It is clear that the value of the voltage
• Vrmsac applied to the pixels of line 5 during a multiplex type scan influences their response to switching signals.
• This part shows the influence of the mean square voltage applied on a pixel before its commutation, in the case where the switching signal which is applied to this pixel takes different values VC 5 .
• Figure 16 shows the evolution of the crossing threshold of the T texture to the texture U on a pixel of the line 5, depending on the value of the column voltage VC 5 applied thereto, and the voltage applied to Vrmsac the latter before the switching signal.
• abscissa the value Vc5 for line 5 in Volt
• the ordinate the texture percentage T on the pixel.
• the pixel situated at the intersection of the line n, l ⁇ n ⁇ N, and of the column m, 1 ⁇ m ⁇ M, denoted P (n, m), has a gray level which will be noted "g (n, m)".
• the voltage Vrmsac (n, m) 2 applied to the pixel P (n, m) is then the sum of the contributions due to the signals applied to the column m during the addressing of the lines p such that p ⁇ n.
• Vcontrib g ( P , m ) 2 of the column signal (voltage Vc g ( P , m ) and duration Tc g ( P , m )) which was used to address this pixel
• V / rrmmssaacc ((n, m) 2 seen by the pixel P (n) , m) is defined by the following formula
• Line ⁇ 5 The voltage Vrmsac seen by the pixel P (n, m) then depends on the gray levels displayed on the n-1 preceding lines according to the formula:
• a first solution to solve the limitation due to the Vrmsac voltage inherent to addressing in passive multiplexing mode of a bistable screen would be to maintain the variations of this voltage below a sufficiently low value given. It will be possible, for example, to lengthen the interlining time (and hence the Tngn e ) sufficiently that, whatever the column voltage applied, the difference between the contributions to the voltage Vrmsac of each gray level remains less than one tenth of a volt. This method has the drawback of lengthening the addressing time of an image.
• thermotropic nematic liquid crystal mixture used is not sensitive to the absolute value of the applied voltage.
• ⁇ is the characteristic time of reorientation of the average direction of the molecules of the liquid crystal mixture.
• the control of the value of the mean square voltage applied to a pixel (n, m) is therefore a requirement of the nematic liquid crystal displays in the context of a "passive matrix" addressing [11].
• the calculation of the square root of the mean square voltage applied to the pixel (n, m) takes into account, on the one hand, the differences in potentials applied via all the column signals of the column m, and d On the other hand, and contrary to the case of the bistable display, the potential difference on the line n at the time of its excitation, ie when a line signal is applied on this line.
• the present invention proposes a bistable nematic liquid crystal matrix display device in which the average voltage, preferably the square root of the mean square voltage applied to each pixel of the display before its switching is made identical, regardless of the content of the image to be displayed.
• the equalization of a mean voltage calculated differently is also applicable.
• the mean square voltage at the terminals of all the pixels of each line is fixed at a constant value at the instant preceding the excitation thereof. This makes it possible to obtain a texture of the liquid crystal molecules identical to all the pixels of this line before its excitation. In this way, the present invention ensures precise control of the switching of each of the pixels of this line to the chosen texture. This is so for each line.
• the mean squared voltage becomes zero when the whole of the bistable screen has been addressed, or when the part to be refreshed of the same screen has been assaulted.
• Vrmsac The mean square voltage Vrmsac seen by each pixel of the display device before switching can be adjusted beyond the value imposed by the addressing signals of the columns representing the image data and independently of the latter.
• the Vrmsac equalization signals can be applied via the column signals of the display, or via a combination of line and column signals.
• One embodiment of the present invention consists in adding an equalization signal of the voltage Vrmsac at each line time; for example, this equalization signal is applied during the excitation time of the line, in particular at the beginning of the line excitation signal.
• the equalization signal of the voltage Vrmsac is applied via the column signals, for each of the gray levels "g" to be reproduced in the image, a torque (column voltage Vc g / duration d 'impulse column Tc g ) to represent the image data, and a complementary torque (equalization voltage RMS Vcomp g / equalization time RMS Tcomp g ) in order to adjust the voltage Vrmsac to a value common to the entire display, noted Vrmsac *.
• the values of the voltage and the duration of the equalization signal RMS are thus adjusted for each gray level as a function of the desired value Vrmsac *.
• the RMS equalization signal can for example be calculated for all gray levels "g" by keeping the voltage Vcomp constant and adjusting the duration Tcomp g , or keeping the duration Tcomp constant and adjusting the voltage Vcomp g .
• Vcomp g and Tcomp g may be varied in any way. It will also be possible to vary both Vcomp g and Tcomp g , or to vary the value of the voltage applied to all or part of the row electrodes, or a combination of these different possibilities.
• Vrmsac * is greater than or equal to IV.
• Another embodiment of the present invention consists of the addition of a voltage equalization signal.
• Vrmsac all p lines, with p> 1.
• the equalization signal is applied during the excitation time of the line in question (one line every p lines), for example at the beginning of the line excitation signal.
• the Vrmsac equalization signal via the column signals can be performed while no physical line of the screen is addressed, when addressing so-called "virtual" lines.
• the excitation signal of the line is bipolar, so as to limit the average voltage seen by the pixel in order to avoid the electrochemical degradations of the liquid crystal, and the equalization signal is applied during the first polarity of the signal; line excitation.
• the present invention proposes to control the mean square value applied to each pixel of a bistable display before switching to a constant value at a given temperature. The present invention is totally different from what is practiced for standard displays
• TN and STN for example.
• the square root of the RMS voltage must take into account the potential difference applied to the selected line.
• a constant mean square voltage across a pixel is equivalent to obtaining a state that is always identical on the pixel considered.
• a first advantage of the regulation of the square root of the mean square voltage applied to each pixel of the display device before it is switched is to clearly improve the uniformity of the image. Any variation of the switching thresholds due to Vrmsac voltage variations from one pixel to another on the same column is indeed controlled.
• Another advantage of the present invention is that it is not necessary to lengthen the addressing time of a line to obtain a faithful reproduction of the gray levels.
• Another advantage of the present invention is its simplicity of implementation. Indeed, the regulation of the square root of the mean square voltage seen by each pixel of the display device before its switching does not require additional image memory, or taking into account the image information of the preceding lines or the previous frame .
• the invention thus generally relates to a method of addressing a bistable nematic liquid crystal matrix screen having two stable textures without an applied electric field, this screen comprising two substrates between which the liquid crystal is arranged, the first substrate comprising row-addressing electrodes and the second substrate comprising column-addressing electrodes, the addressing of the pixels being of the passive multiplex type, the rows being addressed one by one while all the columns are addressed simultaneously during the time excitation of each line, the switching of each pixel from one state to another being controlled by a switching voltage applied between the substrates at the pixel corresponding to the moment of its switching.
• the method according to the invention is characterized in that an electric voltage applied between the substrates for each pixel is chosen such that a time average value of this voltage, preferably the root mean square value, from the beginning of the command displaying the image until the instant immediately preceding the switching, has a predetermined value independent of the information to be displayed, which is the same for all the pixels of the image.
• the average electrical voltage is at least equal to the maximum average electrical voltage that can be obtained with the display of the uniform gray level giving the highest contribution to the average voltage considered.
• At least one equalization pulse is applied to the column corresponding to the pixel that is to be switched. In this case, according to one embodiment, to obtain the same predetermined value of the average voltage at each line, at least one equalization pulse is provided for each line.
• the at least one equalization pulse is applied to the column corresponding to the pixel during the excitation of the line of the corresponding pixel.
• a selection pulse of the desired texture is applied to the column corresponding to the pixel, which is preceded by at least one equalization pulse, the selection pulse and the at least one equalization pulse having voltages such that the average corresponds to the predetermined average voltage value.
• the equalization pulse is, for example, applied during the excitation of the line of the pixel to be switched, in particular at the beginning of the excitation of the line of the pixel to be switched.
• the line excitation signal has two successive portions of different polarities and the equalization signal is applied during the first part of the excitation signal.
• the at least one equalization pulse is applied to the column corresponding to the pixel during the excitation of a line preceding that of the corresponding pixel.
• the equalization pulses are applied during the excitation of a line on p, where p is a predetermined number greater than 1.
• the at least one equalization pulse is applied between the excitation signals of two consecutive lines, this equalization pulse being thus applied in the absence of signals. of line excitation.
• the equalization pulses are applied in a period corresponding to the period separating a predetermined number p 'of lines.
• at least one equalization pulse is applied to the columns prior to the excitation signal of the first line.
• the desired average value of the voltage on each pixel, immediately before the switching of this pixel is obtained by choosing the amplitude and / or the duration of the equalization pulses applied periodically.
• this determined number of pixels is subjected to equalization pulses.
• the respective twists of the two stable textures of the liquid crystal differ by about plus or minus 180 °.
• the first texture is uniform or slightly twisted.
• the invention also relates to a display device using the addressing method defined above and comprising a bistable nematic liquid crystal matrix screen, this screen comprising two substrates between which the liquid crystal is arranged, the first substrate having row addressing electrodes and the second substrate having column addressing electrodes.
• FIG. 1 shows the operating principle of a BiNem type display
• FIG. 2 represents the hydrodynamic flow present in the cell during a sudden interruption of the electric field
• FIG. 3 represents a BiNem display 4 rows x 4 columns in accordance with the state of the art
• FIG. 4 shows the control signals for the simultaneous switching of the pixels of the same line
• FIG. 5 shows the signals used for the multiplexing of a BiNem screen
• FIG. 6 shows the principle of producing gray levels according to the state of the art
• FIG. 7 shows the optical state of the pixels of a 160x480 display according to the state of the art as a function of the column voltage Vc addressed;
• FIG. 8 represents an example of modulation of the parameters of the column signal for the realization of gray levels by "curtain effect" according to the state of the art,
• FIG. 9 represents an example of an electro-optical curve of a BiNem display
• FIG. 10 shows the principle of obtaining gray levels along the electro-optical curve of a BiNem display by modulation of the amplitude of the column voltages
• FIG. 11 shows the switching of the pixels in multiplex mode with a BiNem display
• FIG. 12 shows the signals applied on the columns and on the line 4 of the 16 ⁇ 16 prototype
• FIG. 13 shows the effects of the voltage Vrmsac on the switching of the line 4, using the signals of FIG. 12,
• FIG. 14 shows the signals used for a multiplex type scan
• FIG. 15 shows the effects of the Vrmsac voltage on the switching of line 5 during a type scan. multiplexed using the signals described in FIG. 14, for 3 values of Vci- 4 ,
• FIG. 16 shows the evolution of the switching thresholds T-> U as a function of the voltage Vrmsac seen by the pixel,
• FIG. 17a shows an example of an addressing scheme implementing the equalization of the Vrmsac voltage according to one embodiment of the invention, where the equalization column pulse is inserted all the lines;
• FIG. 17b shows an example of an addressing scheme implementing the Vrmsac voltage equalization according to an embodiment of the invention, wherein the equalization column pulse is inserted all the lines and the excitation signal of the line is bipolar,
• FIG. 18 shows an example of implementation of the equalization of the Vrmsac voltage according to an embodiment of the invention on a BiNem 160x160 display in multiplex mode
• FIG 2 shows an example of addressing implementing the equalization of the voltage Vrmsac diagram according to still another embodiment of the invention wherein the pulse equalization column is inserted in virtual lines, with a virtual line all the 3 physical lines,
• FIG. 19 shows an example of implementation of the equalization of the voltage Vrmsac according to the invention by the addition of virtual lines and pre-taps columns before the first line of the scan
• FIG. 20 shows an example of a result of the implementation of the equalization of the voltage Vrmsac according to the invention
• FIG. 21 shows an example of nonuniformity of a gray level independent of the Vrmsac equalization
• FIG. 22 shows the effect of the increase of the voltage Vrmsac in zone A according to the invention.
• Vrmsac (max) is defined as the maximum Vrmsac voltage that is obtained by displaying the gray level that gives the highest contribution to the Vrmsac voltage.
• Vcomp a compensation or equalization voltage.
• Tcomp g (Vrmsac * 2, line - Vc g 2 .Tc g ) / Vcomp 2 (11)
• the voltage Vcomp can be chosen equal to any value that allows complete equalization of the voltage Vrmsac for all gray levels.
• Vrms g 2 the contribution to the total square Vrmsac voltage provided by the column signals corresponding to each of the gray levels
• Vrms g 2 (Vc g 2, Tc g / line) + (Vcomp 2. Tcomp g / line)
• Vc g 2, Tc g / line [Vcomp 2 . (Vrms * 2. Line - Vc g 2 .Tc g ) / Vcomp 2 /
• the signal applied to a column of the display is therefore, for each line, composed of two parts: a "useful" part which serves to select the gray level to be displayed, and a part of equalization of the voltage Vrmsac to standardize its value on the entire display.
• part A shows the variation of the line voltage as a function of time t
• Parts C and D show an equalization pulse 84 2 and 84 3 for gray levels respectively "s” and "t”, and a column pulse respectively 86 2 and 86 3 imposing gray levels.
• "S” for pulse 86 2 and "t” for pulse 86 3 It will be noted that in part B concerning line 1, there is no equalization pulse for the level "h".
• part E shows the signal seen by the pixels of line 3.
• This signal is equal to Vlign - Vcolonne line 3.
• the variant shown in Figure 17b is similar to that shown in Figure 17a; it is distinguished by the fact that the line excitation signal is bipolar as shown in part A of Figure 17b.
• the other parts B, C, D, E correspond to the parts of the same references in FIG. 17a.
• equalization pulses 84 2 and 84 3 and pulses 86 2 and 86 3 are provided to impose the gray levels, respectively "s" and "t".
• Vrmsac (max) is equal to 1.5 V and obtained for gray 0 or 7.
• Vcomp 3V
• the voltage Vc g to be applied to the columns to obtain the gray level g is determined experimentally.
• the Vrmsac EQ column pulse is inserted at each line time.
• the position of the Vrmsac equalization column pulse may be chosen anywhere during the line time, provided that it does not overlap the selection column signal representing the image data.
• the equalization column signal is applied near the beginning of the line excitation signal, as shown in FIGS. 17a and 17b.
• the voltage Vcomp (or more generally the voltages Vcomp g for each gray level) may for example be chosen equal to the maximum voltage allowed by the column drivers (which will be called Vdriver_ma ⁇ ). However, it will be noted that, depending on its position , the column signal due to Vcomp may interfere with the signals dedicated to the addressing. This is the case if it is located at the beginning of the line signal, during the anchor breaking phase (VlL, Tl). It is understood that when the voltage Vcomp is present on the columns, the liquid crystal is subjected to a total voltage equal to the difference between Vligne and Vcomp.
• Vcomp g , Tcomp g the total absolute voltage seen by a pixel is greater than the voltage of anchor break represented by VlL.
• I Vcomp I is the absolute value of Vcomp.
• Vrmsac equalization signals in order to limit the effects of migration of electric charges within the liquid crystal, and thus to increase the lifetime of the display.
• This embodiment is particularly desirable in the case of a high rate display, for example to display video.
• the alternating mode of the polarity of the Vrmsac equalization column pulses may be chosen, according to the state of the art, at each frame, at each line, or according to any time period.
• the excitation signal of the line may be bipolar, so as to limit the average voltage seen by the pixel, in order to avoid electrochemical degradation of the liquid crystal, and the equalization signal is applied during the first polarity of the line excitation signal, as shown in FIG. 17b.
• the shape of the first polarity is not limited to the shape shown in Figure 17b, for example a two-plate form is also possible.
• the dimensions of the pixels are identical to those of the previously described prototype.
• FIG. 18a shows the image obtained when the equalization of the voltage Vrmsac is not activated: it is observed that the transition to the texture T is not complete. All lines theoretically to be 100% T (clear) have a non-zero and variable proportion of texture U, in the form of small dark bands.
• FIG. 18b shows the image obtained when the equalization of the voltage Vrmsac according to the invention is activated. We choose :
• the equalization column pulse is inserted every p lines.
• Figure 18i shows the implementation of Vrmsac equalization according to this option.
• p 4: nothing is inserted when addressing the lines n, n + 1, n + 2, then the equalization signal is inserted in the column signal during the addressing of line n + 3, and so on until the last line.
• the pulses 92 and 94 are column pulses for compensation of the voltage Vrmsac.
• the parameters of the equalization voltage are different because they are calculated taking into account the contributions of the column voltages on p lines and not for a line.
• the first option previously described makes it possible to equalize the mean square voltage applied to the pixels prior to the application of the line selection signal. It relies on the addition of pulses on the columns at times such that they do not interfere with the "useful" impulses (the image information). This technique becomes tricky when the addressing time of a line is comparable to the duration of the texture selection column signal. In this case, it is impossible to overlay the influence of the selection pulse with that of the equalization pulse.
• a third option is to use the addressing time of a line to apply to the columns an equalizing voltage of Vrmsac, and not applying any line selection voltage during this line period.
• This technique amounts to addressing a "virtual" line (with an equalization voltage) for each block of p physical lines, p> 1.
• the lines L n + 3 and L n + 7 are virtual.
• the advantage of this embodiment is also to make possible the equalization of the Vrmsac value of the voltage applied to the pixels before the application of a switching signal, even in cases where the line period is less than the sum of the durations the selection pulse and the equalization pulse.
• a disadvantage of this embodiment is to lengthen the refresh time of the entire screen by a duration proportional to the refresh time of a line and the ratio of the number of lines of a block p to the total number of lines of the screen.
• pre-pulses applied before the excitation of the first line of a display is partly a function of this method.
• Vcomp g 2 (Vrmsac * 2, line - Vc g 2 .Tc g ) / Tcomp (14)
• This embodiment may be more judicious for a simplified control of the management of the column drivers.
• This mode is compatible with the various options previously described: insertion of the equalization all the lines, all the p lines, or during virtual lines.
• Variant 3 Adjustment of the voltage Vrmsac seen by the pixels of the display to a value Vrmsac *> Vrmsac (max)
• the maximum value present in the image data is used as the chosen value Vrmsac *. It is possible to adjust this value to a higher Vrmsac * voltage. An advantage of doing so is to control the position of the switching threshold from the T to the U to optimize the quality of the display.
• Vrmso is a freely chosen value for fitting Vrmsac *.
• the rest of the calculations are then identical to the one given by the formula (11) in the case of an adjustment to constant Vcomp, or to the formula (14) in the case of an adjustment to constant Tcomp.
• This mode is compatible with the various options previously described: insertion of the equalization all the lines, all the p lines, or during virtual lines.
• Variant 4 Equalization of the RMS voltage seen by the pixels of the first addressed lines
• the first line of the screen sees a zero voltage prior to the application of the switching signal, even when the equalization of RMS is used.
• the column pre-impulsions have a temporal distribution such that they correspond to virtual lines before the first line of the image, with a period equal to Tii gne .
• the values of the voltages and times of the column signals applied during these virtual lines can be taken as values identical to those of the first line, or any other value which would be suitable for the desired image quality.
• the virtual lines can be replaced by a single column pre-pulse of a duration and a voltage adapted to the desired voltage value.
• Figure 20 shows the result on the beginning of the display of a display of 160 lines by 160 columns as described above.
• the signals used are the same as those in FIG. 18.
• the reference 100 corresponds to the beginning of the display of the lines.
• the first lines of the display do not receive any equalization signal. We note that they do not have a 100% T texture as expected but have a non-zero proportion of parasitic U (dark) texture.
• the first lines of the display receive an equalization signal of 10 column prepulses. There is a decrease in the proportion of parasitic U texture.
• the first lines of the display receive an equalization signal of 20 column prepulses.
• the proportion of parasitic U texture has become almost zero.
• RMS equalization pre-pulses before the start of the display can also be performed via the row electrodes.
• the first lines of the display may selectively receive the RMS equalization signals before starting to scan the image.
• Variant 5 case of a partial refreshing of the image
• the principle of equalizing the RMS before the start of the scan can be extended to the case of a partial refresh of the image.
• This mode is compatible with the various options previously described: insertion of the equalization all the lines, all the p lines, or during virtual lines.
• the local value of the left and right operating points of a BiNem type display may differ from one pixel to another in the case, for example, of non-uniformity of the anchor layers due to poor control of the parameters. deposition or brushing. It can also be affected by gap variations of the cell (due for example to particles).
• zone A a zone darker corresponding to pixels having a texture amount T less than that desired.
• This nonuniformity can be due to poor control of the manufacturing parameters of the display.
• a solution to remedy this non-uniformity may then consist of a modification of the voltage RMS Vrmsac A * seen by the pixels of the zone A (in the case of this example it will take Vrmsac A *> Vrmsac *), using the regulation RMS voltage according to the invention, so as to change the switching threshold of the pixels of the zone A to values of voltage and column time compatible with those of the rest of the display (see Figure 22).
• the column voltage Vc is on the abscissa, and on the ordinate the texture percentage T.
• the gray level in zone A is indicated by reference 116 and the desired gray level "g" in numeral 118.
• the RMS voltage seen by a pixel of the display depends on the column signals which were used to address the pixels of the preceding lines located on the same column. It is typically necessary to take into account from ten to twenty previous lines to evaluate the RMS voltage seen by a pixel at the time of its switching.
• the regulation of the RMS voltage Vrmsac A * in a given area A of the display such as Vrmsac A * ⁇ Vrmsac * (Vrmsac * being the equalized RMS voltage according to the invention for the pixels of the remainder of the display), in using the means of the invention, can be done in the continuity of the display, providing a gradual variation of the RMS voltage Vrmsac * to Vrmsac A *.
• it will be done by introducing virtual lines. Precise control of the switching thresholds on the entire display is thus allowed, with a slight increase in the refresh time of an image.

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