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
• • 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
  • G09G3/3648—Control of matrices with row and column drivers using an active matrix
• • G—PHYSICS
  • G02—OPTICS
  • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
  • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
  • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
  • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
  • G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
• • 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
• • 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
  • G09G2310/02—Addressing, scanning or driving the display screen or processing steps related thereto
  • G09G2310/0235—Field-sequential colour 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
  • G09G2310/00—Command of the display device
  • G09G2310/02—Addressing, scanning or driving the display screen or processing steps related thereto
  • G09G2310/0243—Details of the generation of driving signals
  • G09G2310/0259—Details of the generation of driving signals with use of an analog or digital ramp generator in the column driver or in the pixel circuit
• • 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
  • G09G3/2007—Display of intermediate tones
  • G09G3/2014—Display of intermediate tones by modulation of the duration of a single pulse during which the logic level remains constant

Definitions

• the invention relates to matrix liquid crystal display micro-screens, and in particular those which are produced on a monolithic silicon substrate in which the electronic control circuits of a matrix network of liquid crystal cells are integrated.
• the liquid crystal displays which are targeted here are those which are capable of displaying intermediate levels of gray and not only binary black / white information. When we speak of gray levels, these are luminance levels in reflection or in transmission, and this vocabulary "gray levels" will be used here even if the light considered is colored as is the case in color displays.
• the luminance of the image point corresponding to the elementary cell depends in fact on the level of the voltage applied to the cell.
• the DC voltage which corresponds to the gray level desired for this pixel is first briefly applied to each pixel of the line. This voltage is put in memory in a local storage capacity, at the pixel level, then we isolate this capacity from the circuits which served to charge it, and we pass to the next line to apply to the storage capacities of the following line d 'other DC voltages desired for the pixels of this new line.
• the storage capacity is connected to the liquid crystal cell; the latter therefore receives (except for a capacitive division ratio) a voltage corresponding to the desired gray level, and it retains this voltage without discharging. This voltage is thus maintained across the terminals of the liquid crystal cell throughout the duration of an image frame.
• Such a pixel will receive on its liquid crystal cell the voltage Vdd throughout the frame duration and will be a "black” pixel, for a type of matrix called “normally white”, that is to say providing a maximum level of light in the absence of voltage applied to the cell, whether in reflection mode or in transmission mode.
• Such another pixel will receive the voltage Vdd on its cell during a zero or insignificant fraction of the frame duration and will be "white”.
• such another pixel will receive the voltage Vdd on its cell during a given fraction of the frame duration; the eye integrates, if the frame frequency is at least 25 Hz, the duration of application of the voltage Vdd and the duration of non-application of this voltage and sees an equivalent gray level which is proportional to the ratio between the duration of application of the voltage Vdd and the total duration of the frame.
• the value of the voltage which will be applied to the cell be fixed (Vdd) and therefore independent of the dispersion of the values of the cell capacities or of the storage capacities, but moreover this voltage will be as high as possible, which is advantageous for reasons of reaction time and image contrast.
• a typical dimension of elementary display cell is 10 micrometers by 10 micrometers and it is necessary to house in this surface the electronic circuit associated with the cell.
• the number of transistors used to control each cell must be limited, and an object of the invention is to propose a method and a circuit which minimize the number of transistors locally associated with each pixel.
• the invention provides for this purpose a method of controlling a liquid crystal display matrix which consists in briefly applying to a storage capacity, associated with an elementary liquid crystal cell, an analog DC voltage corresponding to a level of gray desired, to connect a terminal of the capacitance to the gate of a transistor whose source is then connected to a ground and whose drain is connected to a voltage source Vdd through a current source, and to apply to the Another terminal of the storage capacity is a continuous voltage ramp varying monotonously during the duration of a frame.
• the cell is connected to the drain of the transistor and its "black" or “white” brightness state depends on the high or low level present on this drain.
• the monotonous ramp is in principle essentially linear; however it may not be perfectly linear; one can in particular envisage that it is not perfectly linear in the cases where one would like to correct certain nonlinearities of the system by acting on the profile of the ramp.
• Such a correction by a non-linear ramp profile can serve, for example, to improve ocular perception in certain ranges of luminances.
• the method according to the invention acts as follows: the voltage ramp applied to the capacitance is transferred by the capacitance to the gate of the transistor; the grid therefore receives a voltage ramp which starts from a level that is higher the higher the voltage stored in the capacitor (voltage corresponding to the desired gray level) because the voltage of the ramp is added to the voltage stored beforehand in the capacity; the voltage ramp on the grid extends over the frame duration; at the start, the transistor is blocked, the voltage on its gate being insufficient compared to its source which is grounded (or more generally at a fixed potential).
• the drain of the transistor, supplied through a current source which cannot conduct current until the transistor is conductive, is at a potential level equal to Vdd, the cell therefore being in a first state (for example "black").
• the transistor When the voltage on the gate reaches a threshold voltage VT of the transistor, the transistor starts to conduct and brings the drain potential of the transistor to zero; this moment depends on the voltage level which was initially stored in the capacitor and which is related to the desired gray level.
• the liquid crystal cell is connected to this drain and suddenly changes state (it takes for example the "white” state) and remains in this state during the rest of the frame.
• the average luminance of the cell, integrated by the eye therefore depends on the voltage level initially stored in the capacitor.
• the voltage ramp preferably varies between a zero voltage level and a voltage level substantially equal to the value of the threshold voltage VT of the transistor, the threshold voltage conventionally being the gate-source voltage value above which the transistor is conductive and below which it is not conductive.
• the analog DC voltage representing the gray level and applied to the storage capacities varies between 0 volts (the reference 0 volts being the source voltage of the transistor for the duration of the frame) and the same threshold voltage value VT.
• the liquid crystal cell receives, for a variable duration at each frame, either the supply voltage Vdd or the voltage 0 volts.
• the invention therefore provides a liquid crystal matrix display, comprising an active matrix of image points or pixels and peripheral circuits, the matrix comprising a crossed network of lines.
• an elementary electronic circuit for controlling a cell with an elementary liquid crystal located at this crossing comprising: - at least one storage capacity for storing for the duration of an image frame an analog voltage applied by the column, a first terminal of the storage capacity being connected to the transistor gate, - in series between two voltage supply terminals, an elementary current source and a switching transistor, the drain of the switching transistor being connected to the liquid crystal cell, the peripheral circuits comprising means for receiving a periodic voltage ramp, common to all the cells of at least one line, the ramp being applied to a second confine myself to the storage capacity of the cells that line.
• the ramp preferably has an amplitude of VT: it varies from 0 to VT, or from VT to zero, on the duration of an image frame.
• the analog voltage representing the gray level varies in principle between 0 and VT.
• the voltage ramp is produced by a ramp generator which is inside or outside the monolithic integrated circuit comprising the display matrix and its control circuits.
• the invention can be used for displays in which each image point is associated with an elementary electronic circuit with double memory in which there are not one but two storage capacities and two switching transistors connected to the same cell with liquid crystal and operating alternately every other frame, a voltage value being applied to a capacitor during an odd frame while the other capacitor retains the voltage it received during the previous even frame, and vice versa; the conduction of the transistor connected to the first capacitor is then inhibited during the odd frame and authorized during the even frame.
• a ramp generator can be used to generate a ramp to all the image points of the matrix. The ramp is periodic and has the period of the image frames as a period.
• each image frame corresponds to the display of a single color, a light of said color being emitted in front of the matrix during this frame to be spatially modulated by the matrix according to information specific to this color; the light of a color is obtained by a source of this color (then sources of different color for the following frames, in synchronism with the application of the ramp to the storage capacities which contain the information corresponding to this color); or else the light of a color is obtained from a white light in front of which passes, always in synchronism with the application of the ramp to the storage capacities which contain the information relating to the chosen color, a filter of this color (then filters of other colors for the following frames).
• FIG. 1 represents the general organization of the control electronics of a liquid crystal display micro-screen.
• the matrix includes individual image points or pixels P11, P12, P21, P22, etc., organized in rows and columns.
• the gray level (or, of course, color level) information is supplied by column conductors C1, C2, etc., in the form of an analog voltage varying between a minimum level of 0 volts and a maximum level VT.
• the diagram in FIG. 1 is valid both in the case where the pixels comprise two storage capacities for this analog voltage, operating alternately during successive even and odd frames, as in the case where the pixels comprise only one storage capacity, the content of which is renewed every frame.
• the level of the voltage applied at a given instant to a column represents the gray level to be displayed in a pixel located at the crossroads of this column and of a line activated at this instant by a row selection register RL.
• a line conductor L1, L2, etc., specific to each line makes it possible to activate all the pixels of this line at a given instant, the pixels of the other lines being deactivated so that only one line at a time is activated.
• the line conductor L1 is subdivided into two line conductors Lia, L2a for matrices with double memory, but all the pixels of a line are always activated. simultaneously.
• the pixels of the activated line receive the voltage present at this time on their respective column conductor and store it in an internal storage capacity at each pixel; the deactivated pixels do not receive it but keep in memory the voltage they may have previously stored.
• the analog voltage applied to a column when selecting a line can be established from an analog-digital conversion as follows: a digital register RC contains, for each column, a digital value (coded on 8 bits for example) representing the gray level to be applied at the point located at the crossroads of the column and the line selected at this time; the RC register is reloaded at each new line selection and synchronization circuits not shown are of course used to synchronize the line and column operations.
• the digital output of the register (one output per column) is applied to a comparator CMP1, CMP2 ...
• the comparator also receives the content of a CPT counter which periodically and regularly counts from 0 to the maximum value that can be contained in the register RC (the maximum value is 255 for an eight-bit register per column); when the content of the counter reaches the value contained in the register for a determined column, the comparator associated with this column provides a single short pulse; the CPT counter is the same for all columns.
• the pulse provided by a comparator CMP1, CMP2, ... associated with a column closes a switch K1, K2, ..., located on the column conductor C1, C2, respective; by this closing, the switch applies to this column an analog voltage which, as we will see, represents the desired gray level.
• the period of counter is the line period, that is to say that the counter starts counting again each time a new line is selected to store gray levels in the pixels of this line.
• the analog voltage applied to the column by the switch K1, K2, ... comes from a linear voltage ramp generator acting in synchronism with the CPT counter, and producing a voltage varying linearly from zero to a maximum value (VT ).
• This ramp is renewed with each new line selection. It is common to the whole matrix of points. Thus, as the counter counts from 0 to a maximum content, the ramp increases from 0 to its maximum value. The instantaneous voltage of the ramp is therefore proportional to the content of the meter.
• the switch closing impulse occurs when the content of the counter is equal to a desired value and the ramp has at this time a value proportional to this value. It is the instantaneous value of the ramp at this time which is applied to the column conductor to load into memory into the pixel of the selected line a value representing the desired gray level from the column register RC.
• the ramp generator can, for example, simply be constituted by a digital-analog converter DAC receiving the content of the counter CPT.
• another ramp generator GR possibly divided into two ramp generators Gra, GRB in the case where the pixels of the matrix are with double memory.
• This ramp generator provides each frame with a voltage ramp in principle linear having a rise time, from zero to a maximum voltage, equal to the duration of an image frame. It is used to apply a voltage ramp in principle linear to all the pixels of the matrix during a phase of control of the voltage applied to the electrodes of the elementary liquid crystal cell present locally at each line and column crossing. Note, however, that in the case of single memory pixels, the ramp generator must be capable of producing as many time-shifted ramps as there are lines in the matrix, each ramp being applied to a respective line, then that in the case of pixels with double memory, it suffices that the generator produces a single ramp for all the points of the matrix according to modalities which will be explained later.
• the ramp generator can be produced on the integrated circuit carrying the display matrix or outside of this integrated circuit, and in the latter case the integrated circuit has an input reserved for receiving a ramp signal.
• FIG. 2 represents the constitution of the elementary electronic circuit associated with a pixel located at the crossing of a line L1 and of a column C1, this circuit being located at the location of this crossing; the constitution shown corresponds to an embodiment in which each pixel comprises a double analog voltage memory representing a gray level locally stored in the pixel.
• the operation of a dual memory pixel is as follows: during an odd frame, the operation of storing a respective gray level is carried out in the first memory of each of the pixels and is used to control the display from the cell a gray level which had been previously stored, during the previous even frame, in the second memory; during the even frame which follows the odd frame, the voltage previously stored in the first memory is used to control the display by the liquid crystal cell associated with each pixel, and during this time a new gray level is stored in the second memory associated with the same cell.
• the entire duration of each frame can thus be used for a cell display control operation, whereas if there was only one storage memory per pixel, part of the frame would have to be used for the storage operation and another part of the frame for the actual control of the cells.
• the first memory is constituted by a first storage capacity Ca and the second memory is constituted by a second storage capacity Cb.
• the capacitance Ca can be connected by a first terminal to the column conductor C1 via a line selection switch KL1a and the capacitance Cb can be connected by a first terminal to the same conductor of column C1 by another switch line selection KL1b.
• the switch KL1a is closed to establish this connection only during the odd frames, and only when it is the line L1 which is selected by the line selection register RL for an operation of storage of a new gray level in the pixels of this line.
• the switch KL1b is closed only during the frames pairs and only when it is the turn of line L1 to receive gray levels.
• the second terminal of the capacitor Ca is grounded, so that the analog voltage present on column C1 to this moment is applied, through the switch KL1a across the capacitance Ca.
• This voltage comes from a ramp sampled by the switch K1 (figure 1) at the moment when the voltage level of the ramp corresponds to a value defined numerically by the column register RC.
• the switch KL1a is controlled by a first line conductor Lia and the switch KL1b is controlled by a second line conductor L1b.
• Line L1 is defined by these two conductors, and the line selection register determines the choice of the line conductor used for a given frame: Lia for odd frames, L1b for even frames, but these are always pixels of the row of pixels L1.
• the corresponding line selection switch KL1a or KL1b is open and the Ca or Cb capacity, therefore isolated, retains a constant charge during the rest of the frame (that is to say during the loading of the other lines) and during the following frame (that is to say during the display operation proper).
• the sequencing of the line selection register selects the next line.
• the line selection for closing the switch acts only on switches KL1a during odd frames and only on switches KL1b during even frames.
• the first terminal of the storage capacity Ca (that is to say the terminal which is connected to the switch KL1a) is also connected to the gate of a transistor M OS designated by the reference Ta, while the first capacitance terminal Cb is connected to the gate of a MOS transistor Tb.
• the source of the transistor Ta is connected to ground (that is to say a potential reference which can be considered as zero), but only during the even frames.
• a switch KT1a is interposed between the source of the transistor Ta and the ground to inhibit the conduction of current by the transistor Ta during the odd frames.
• the switches KT1a of all the pixels of the matrix are controlled simultaneously to be closed for the entire duration of the even frames but open for the duration of the odd frames.
• the source of the transistor Tb is connected to ground by a switch KT1 b closed for the entire duration of the odd frames and open during the even frames.
• the drain of the transistor Ta and the drain of the transistor Tb are connected to a first electrode of the liquid crystal cell LC corresponding to the pixel with which the elementary circuit of FIG. 1 is locally associated. Indeed, the cell will be controlled by applying a voltage to the electrodes of the cell either during the even frames by the drain of the transistor Ta or during the odd frames by the drain of the transistor Tb.
• the cell has a second electrode which is generally common to the whole matrix and which will be considered initially as being brought to the ground potential 0 volts.
• the drains of the transistors Ta and Tb are also connected to the same constant current source SC1 constituted by a PMOS transistor connected between the general supply Vdd and the drains, this transistor having its gate connected to a potential Vpol such as the current in the transistor is fixed; in particular, the gate potential can be determined by a conventional circuit with current mirror such that the current in this transistor is the copying of the current from a fixed current source not shown.
• the value of the constant current is conventionally determined by the potential Vpol and by the geometry of the transistor channel.
• the constant current sources of all the pixels are identical.
• This current source SC1 supplies the transistor Ta or the transistor Tb according to whether the frame is odd or even with a fixed current, for example of the order of 100 nanoamperes, provided however that the transistor Ta (or Tb) is in a passing state and not in a blocked state.
• a fixed current for example of the order of 100 nanoamperes, provided however that the transistor Ta (or Tb) is in a passing state and not in a blocked state.
• the state of the transistor is determined by the potential applied to its gate by the capacitance Ca or Cb.
• the potential applied to the second terminal of the capacitance Ca is zero, but during the even frames a potential determined by the linear voltage ramp generator mentioned with reference to FIG. 1 is applied to this second terminal. and which is common to all cells in the matrix.
• the same voltage ramp is applied to the second terminal of the capacitor Cb, while during the even frames a zero potential is maintained on this terminal.
• the ramp generator produces a linear analog voltage ramp which starts from 0 at the start of the frame and arrives at the end of the frame at a maximum value which is preferably equal to the threshold voltage VT for switching on the transistor. Ta or Tb.
• This threshold voltage VT is the limit of a voltage applied between the gate and the source of the transistor such that a value greater than VT makes the transistor conductive and a value less than VT blocks the conduction of the transistor. It can conventionally be around 1 volt but it is possible to produce transistors having threshold values chosen at will.
• the analog voltage stored in the storage capacity has in principle a value which can vary between a minimum value equal to zero and a maximum value which is in principle equal to VT, any intermediate value being intended to allow an illumination to be generated with a gray level intermediate between the white level (for the minimum value 0) and the black level (for the maximum value VT).
• the display matrix operates as follows: after having loaded line by line during an odd frame all the capacities Ca of the matrix with analog voltage values Vi of between 0 and VT and representing the desired gray level for each pixel, the switch KT1a is closed at the start of the next even frame to ground the source of the transistor Ta, and the linear voltage ramp starting from zero and reaching VT is applied to the second terminal of the capacitance Ca after a time equal to the duration of the frame.
• the voltage present on the gate of the transistor Ta is then the sum of the voltage Vr of the ramp at a given instant and the voltage Vi initially charged in the capacitor. This sum of voltage Vr varies linearly starting from Vi and going up to Vi + VT.
• the voltage Vr applied to the gate of the transistor Ta is less than the value VT which is the conduction threshold of the transistor Ta, the latter remains blocked so that the current source SC1 does not conduct current and the drain voltage of the transistor (also that which is applied to the first electrode of the liquid crystal) is equal to Vdd, the second electrode or counter-electrode being at 0 volts.
• the liquid crystal is in a "black” state for a so-called “normally white” matrix.
• the transistor Ta becomes conductive and earths the electrode; the liquid crystal goes to the "white” state.
• the ratio between the time during which the cell is black and the time during which it is white is directly proportional to the gray level value Vi stored in the capacitor Ca.
• Vdd maximum possible value for Vi
• the transistor becomes conductive from the start of the frame, and the voltage applied to the cell is 0 throughout the frame.
• the cell is white for 100% of the frame time.
• the cell is black (application of Vdd) during a proportion Vi / VT of the frame time and white (application of 0 volts) during a fraction (VT-Vi) ⁇ / T of the frame time; the frame period is short (typically 1/25 of a second) and the eye integrates the variations between black and white; the equivalent gray level perceived by the eye is directly represented by the value Vi / VT therefore by the value Vi (gray all the lighter as Vi is large for a normally white cell).
• the switches are made by MOS transistors.
• Capacities Ca and Cb are in principle also produced by MOS transistors whose drain and source are combined and form with the channel a first capacitance electrode and whose insulated gate forms a second electrode. It will be noted that with the diagram according to the invention, the circuitry associated with a pixel comprises a small number of elements, so that the overall size of this circuitry is limited. Operation is based in part on the ability of the capacity
• the circuit according to the invention means that there are few current leakage paths which would cause the charge of the capacitor to be lost.
• the liquid crystal cell has a first electrode connected to the drain of the transistors Ta and Tb and a second electrode or counter electrode connected to ground.
• it is generally necessary to "depolarize" the liquid crystal by arranging for it to have a zero mean voltage at its terminals, which would not be the case if the second electrode was still grounded and if the first oscillated between 0 volts and Vdd. This is why provision is conventionally made, and the invention is compatible with this precaution, to periodically reverse the direction of the voltage applied to the liquid crystal.
• the counter electrode For example, if in a first frame or a first series of frames the counter electrode is at 0 volts, one can provide that in a second frame or a second series of frames the counter electrode will be at Vdd. But if the counter-electrode is at Vdd, then the cell will be black provided that the first electrode is at 0 volts and white provided that the first electrode is at Vdd.
• the ramp applied during the even frame to the capacitance Ca could be a downward ramp starting from VT at the start of the frame and decreasing linearly to 0 volts at the end of the frame.
• the alternation of the polarizations, by alternating the direction of the ramps at the same time as the polarization 0 or Vdd applied to the second electrode of the liquid crystal is alternated, can be done periodically at all the frames or every two frames.

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