KR20050116160A - Methods for driving bistable electro-optic displays - Google Patents

Abstract

A gray scale bistable electro-optic display is driven by storing a look-up table containing data representing the impulses necessary for transitions, storing data representing at least an initial state of each pixel of the display, receiving an input signal representing a desired final state of at least one pixel of the display; and generating an output signal representing a pixel voltage to be applied to the pixel. Compensation voltage data representing a compensation voltage is stored for each pixel, the compensation voltage for each pixel being calculated dependent upon at least one impulse previously applied to that pixel, and the pixel voltage is the sum of a drive voltage determined from the initial and final states of the pixel and the look-up table, and a compensation voltage determined from the compensation voltage data for the pixel. Other similar methods for driving such displays are also disclosed.

Description

METHODS FOR DRIVING BISTABLE ELECTRO-OPTIC DISPLAYS}

The present invention relates to international application PCT / US02 / 37241. The entire contents of publication number WO / 03/044765 are incorporated herein by reference.

The present invention relates to a method for driving an electro-optic display, in particular a bistable electro-optic display, and the present application also relates to an apparatus for use in such a method. More specifically, the present invention relates to a driving method and apparatus (controllers) intended to allow more accurate control of the gray state of a pixel of an electro-optical display. The invention also relates to a method which enables long-term direct current balancing of drive impulses applied to an electrophoretic display. In particular, the invention is not excluded, but is intended for use in particle-based electrophoretic displays in which one or more electrically charged particles are suspended in a liquid phase and move through the liquid to change the display of the display under the influence of an electric field. do.

The term "electro-optical", when applied to a material or display, has here first and second display states that differ in one or more optical properties in the field of imaging, the first display state when an electric field is applied to the material. It is used in the conventional sense to refer to a substance which changes from to a second display state. Optical properties are typically visually recognizable colors, but in the case of displays for machine reading, they are pseudo-color in terms of light transmission, reflectance, luminescence, or changes in reflectance of electromagnetic wavelengths outside the visible region. -color), and other optical properties.

The term " gray state " is used in the conventional sense to refer to a state that mediates the optical state of the extremes of the pixels in the field of imaging, and need not imply a black-white transition between these extremes. For example, a number of patents and published applications mention electrophoretic displays in which the state of the extremes is white and dark blue so that the intermediate "gray state" is actually light blue. Indeed, as mentioned above, the transition between the anodic states does not need to change color at all.

The terms "bistable" and "bistability" include display elements having a first and second display state that differ in one or more optical properties in the art, and have a constant duration. By addressing a pulse of, a given element is driven to take either its first display state or its second display state and after termination of pulse addressing the minimum duration of pulse addressing necessary for the state to change the state of the display element. It has been used in their conventional sense to refer to displays that last at least several times, for example four times. Published US Patent Application 2002/0180687 discloses a constant particle-based electrophoretic display that is capable of stable not only when the gray scale is in its extreme black and white states, but also in its intermediate gray state. And other certain forms of electro-optic displays. Here, the term "bistable" may be used to encompass both bistable displays and multi-stable displays for convenience, but this type of display is strictly referred to as "multi-stable" rather than bistable.

The term "gamma voltage" is used to refer to an external voltage reference used by a driver to determine the voltage to be applied to a pixel of a display. The bistable electro-optic medium does not display a type of one-to-one correlation between the applied voltage and the optical state characteristics of the liquid crystal, where the use of the term "gamma voltage" means that the gamma voltage It should be understood that it is not exactly the same as a conventional liquid crystal display that determines the conversion point.

The term "impulse" was used in the conventional sense of the integration of voltage over time. However, some bistable electro-optical media behave as charge transducers, and alternative definitions of impulses with such media, i.e., integration of current over time (same as the total applied charge) may be used. Depending on whether the medium acts as a voltage-time impulse converter or as a charge impulse converter, the proper definition of impulse should be used.

Several forms of electro-optic displays are known. One type of electro-optic display is described, for example, in US Pat. No. 5,808,783; 5,777,782; 5,760,761; 6,054,071; 6,055,091; 6,097,531; 6,128,124; 6,137,467; And a rotating bichromal member type as described in US Pat. No. 6,147,791 (although this type of display is often referred to as a "rotating bichromal ball" display, in some of the patents mentioned above, The term "rotary dichroic member" is preferred because it is not spherical). Such displays use a number of (usually spherical or cylindrical) small bodies and one internal dipole having two or more portions with different optical properties. These bodies are suspended in a liquid-filled vacuole in the matrix, and the fear is filled with liquid so that the bodies are free to rotate. Appearance of the display changes by applying an electric field there and thereby rotating the bodies to various positions and changing the parts of the body visible through the display screen. This type of electro-optic medium is typically bistable.

Another type of electro-optic display is an electrochromic medium, for example an electrode formed at least partially of a semiconductor metal oxide and a number of dyes attached to the electrode and capable of reversible color change. An electrochromic medium in the form of a nanochromic film containing molecules is used, for example, O'Regan, B., et al., 1991 Nature 353, 737 and Wood, D., Information Disply , 18 (3), 24 (March 2002). In addition, Bach, U., et al., Adv. See Mater., 2002, 14 (11), 845. Also, for example, US Pat. No. 6,301,038 and International Application Publication No. WO 01/27690, and US Application No. 2003/0214695, disclose nanochromic films of this type. Also, this type of media is typically bistable.

Another type of electro-optical display, which has been the subject of intensive research and development for many years, is a particle-based electrophoretic display, in which a plurality of charged particles move through a suspended fluid under the influence of an electric field. Electrophoretic displays can have excellent brightness and contrast, wide viewing angles, state bistable, and low power consumption when compared to liquid crystal displays. Nevertheless, problems with picture quality for long-term use of such displays have hindered their widespread use. For example, particles that make up electrophoretic displays tend to settle, causing inadequacies in the service-life of these displays.

Numerous patents and applications assigned to the Massachusetts Institute of Technology (MIT) and to E INK disclose encapsulated electrophoretic media. Such encapsulated media comprises a number of small capsules, each of which comprises an inner phase comprising electrophoretic-moving particles suspended in a liquid suspending medium, the capsule surrounding the inner phase. Includes walls. Typically, the capsules are themselves fixed in a polymeric binder to form a coherent layer located between two electrodes. Encapsulated media of this type are described, for example, in US Pat. No. 5,930,026; 5,961,804; 6,017,584; 6,067,185; 6,118,426; 6,120,588; 6,120,839; 6,124,851; 6,130,773; 6,130,774; 6,172,798; 6,177,921; 6,232,950; 6,249,721; 6,252,564; 6,262,706; 6,262,833; 6,300,932; 6,312,304; 6,312,971; 6,323,989; 6,327,072; 6,376,828; 6,377,387; 6,392,785; 6,392,786; 6,413,790; 6,422,687; 6,445,374; 6,445,489; 6,459,418; 6,473,072; 6,480,182; 6,498,114; 6,504,524; 6,506,438; 6,512,354; 6,515,649; 6,518,949; 6,521,489; 6,531,997; 6,535,197; 6,538,801; 6,545,291; 6,580,545; 6,639,578; 6,652,075; 6,657,772; 6,664,944; 6,680,725; 6,683,333; 6,704,133 and US Patent Application Publication No. 2002/0019081; 2002/0021270; 2002/0053900; 2002/0060312; 2002/0063661; 2002/0063677; 2002/0090980; 2002/0106847; 2002/0113770; 2002/0130832; 2002/0131147; 2002/0145792; 2002/0171910; 2002/0180687; 2002/0180688; 2002/0185378; 2003/0011560; 2003/0011868; 2003/0020844; 2003/0025855; 2003/0034949; 2003/0038755; 2003/0053189; 2003/0096113; 2003/0102858; 2003/0132908; 2003/0137521; 2003/0137717; 2003/0151702; 2003/0189749; 2003/0214695; 2003/0214697; 2003/0222315; 2004/0008398; 2004/0012839; 2004/0014265; 2004/0027327; And International Patent Application No. WO 99/67678; WO 00/05704; WO 00/38000; WO 00/38001; WO 00/36560; WO 00/67110; WO 00/67372; WO 01/07961; WO 01/08241; WO 03/092077; WO 03 / 107,315.

Many of the patents and applications described above, for example, referring to 2002/0131147 described above, the walls surrounding individual capsules in encapsulated electrophoretic media can be replaced by a continuous phase, so that the electrophoretic media are electrophoretic fluids. The fact that it produces a so-called polymer-dispersed electrophoretic display comprising a plurality of individual droplets of and a continuous phase of polymeric material, and the electrophoretic fluid in such a polymer-dispersed electrophoretic display. It is recognized that individual droplets can be considered as capsules or microcapsules even when individual capsule membranes are each not associated with individual droplets. Thus, for the purposes of this application, such polymer-disperse electrophoretic media are considered as sub-species of encapsulated electrophoretic media.

Encapsulated electrophoretic displays typically do not suffer from the clustering and settling failure modes of conventional electrophoretic devices, such as the ability to print or coat a display on a variety of flexible or rigid substrates. It offers more advantages. (The use of the word "printing" is not limited to pre-metered, such as patch die coating, slot or extrusion coating, slide or cascade coating, curtain coating. ) Coating; roll coating such as knife over roll coating, forward and reverse roll coating; gravure coating; dip coating; spray coating; mens Meniscus coating; spin coating; brush coating; air knife coating; silk screen printing process; electrostatic printing process; thermal printing process; inkjet printing process; and other similar techniques It is intended to include all forms.) Thus, the resulting display is flexible. In addition, since the display medium can be printed (using various methods), the display itself can be manufactured at low cost.

A related form of electrophoretic display is the so-called "microcell electrophoretic display". In microcell electrophoretic displays, the charged particles and suspending fluid are not encapsulated inside the capsule, but instead remain in a plurality of cavities formed in a carrier medium, typically a polymeric membrane. See, eg, International Application Publication No. WO 02/01281 and US Patent Application Publication No. 2002-0075556, assigned to Sipix Image, Inc.

Electro-optical media often operate in opaque and reflective modes (eg, in many electrophoretic media, since the particles substantially block the transmission of visible light through the display), but many electrophoretic displays operate in a single display state. Can be made to operate in a so-called "shutter mode" which is opaque and one is light-transmissive. See, for example, US Pat. Nos. 6,130,774 and 6,172,798 and US Pat. Nos. 5,872,552; 6,144,361; 6,271,823; 6,225,971; See 6,184,856. Dielectrophoretic displays, similar to electrophoretic displays but dependent on electric field intensity changes, can be operated in a similar mode; See US Pat. No. 4,418,346.

The bistable or multistable properties of particle-based electrophoretic displays and other electro-optical displays displaying in a similar manner differ significantly compared to conventional liquid crystal ("LC") displays. Twisted nematic liquid crystals are not bistable or multistable, but behave like voltage converters, so the application of an electric field given to a pixel of such a display creates a specific gray level in that pixel, regardless of the gray level previously provided to that pixel. In addition, the LC display is driven in only one direction (non-transmissive or "dark" to transmissive or "light"), and the reverse transition from bright to dark is affected by the reduction or elimination of the electric field. As a result, the gray level of the pixels of an LC display is not sensitive to the polarity of the electric field, but only to the magnitude of the electric field and, in fact, as a technical factor, commercial LC displays generally invert the polarity of the driving electric field in general.

In contrast, the bistable electro-optical display behaves like an impulse converter, in a first approximation, and the final state of the pixel depends not only on the applied electric field and the time the electric field is applied, but also on the state of the pixel before application of the electric field. In addition, in the case of at least many particle-based electro-optic displays, the impulse required to change a given pixel through the same change in gray level (as measured by the naked eye or standard optics) need not be constant and mutually It is now being discovered that there is no need to be commutative. For example, consider a display where each pixel can display gray levels that are advantageously spaced apart 0 (white), 1, 2, 3 (black). (The spacing between the levels may be linear in percent reflectivity, as measured by the naked eye or by instrument, but other spacings may be used. For example, the spacing may be linear in L *, where L * Has a general CIE definition,

L * = 116 (R / R ₀ ) ^1/3 -16,

Where R is the reflectance and R ₀ is the standard reflectance value) or may be selected to provide a particular gamma, with a gamma of 2.2 often employed for monitors, where current displays are used as replacements for monitors and Use may be desirable.) The impulse required to change the pixel from level 0 to level 1 (hereinafter referred to as "0-1 transition" for convenience) is often not the same as that required for 1-2 transitions or 2-3 transitions. Was found. Also, the impulse required for a 1-0 transition need not be equal to the inverse of the 0-1 transition. In addition, some systems display a "memory" effect such that (say) the impulse required for a 0-1 transition, whether a particular pixel undergoes a 0-0-1, 1-0-1 or 3-0-1 transition. Depends somewhat on (In the "xyz" notation, x, y, z are all optical states, and 0, 1, 2, 3 represent a sequence of optical states that undergo successively from beginning to later with time.) While driving or reducing all pixels of the display to one of the extreme states for quite some time before driving the pixels to another state, the resulting “flash” of solid color is often unacceptable, for example For example, an ebook reader would want the text in the book to scroll down the screen, and if the display had to flash solid black or white frequently, the reader would be disturbed or forgotten where he had read. In addition, the flash of such a display increases energy consumption and reduces the operating lifetime of the display. As a result, at least in some cases, the impulse required for a particular transition is affected by the temperature and the total operating time of the display, that a certain pixel is affected by the time that a given transition has been held in a particular optical state, and these factors It has been found that the compensation for is desirable to ensure accurate gray scale rendition.

Also, as is evident from the foregoing, the driving requirements of bistable electro-optic media may be used to provide an unaltered driver designed to drive an active matrix liquid crystal display (AMLCD) that is not suitable for use of bistable electro-optic media-based displays. to provide. However, such AMLCD drivers are not readily commercially available and expensive off-the-shelf with a widely accepted voltage range and high pin-count package, and such AMLCD drivers are not suitable for driving bistable electro-optical displays. While suitable, on the other hand, similar drivers designed on demand for bistable electro-optic media based displays will be substantially more expensive and will involve significant design and production time. Thus, modification of an AMLCD driver for use in bistable electro-optic displays is advantageous in terms of cost and development time, and the present invention provides a modified driver and method that allows this to be performed.

In addition, as already described, the present invention relates to a method for driving an electrophoretic display that enables long-term DC-balancing of drive impulses applied to the display. Encapsulated or other electrophoretic displays need to be driven with precisely DC-balanced waveforms to retain image stability, maintain symmetrical switching characteristics, and provide maximum useful operating life time for the display. (Ie, the integration of current over time for any particular pixel of the display must be held to zero during the extended operating period of that display). Conventional methods for maintaining accurate DC-balance require a precision-regulated power supply, a precision voltage-regulated driver for gray levels, and a crystal oscillator for timing, and the provision of these and similar components provides Very costly.

(Strictly speaking, DC-balance should be measured "internally" taking into account the voltage experienced by the electro-optic medium itself. However, in practice, such an internal measurement can be achieved in a running display that may contain hundreds of thousands of pixels. This is impractical and in practice the DC-balance is measured using an "external" measurement, i.e., the voltage applied to the electrodes disposed on the opposite side of the electro-optical medium.When referring to DC-balance, there are usually two assumptions: First, of course, of course, the conductivity of the electro-optic medium is not a function of polarity, so it is naturally assumed that the pulse length is a suitable way to track DC balance when a constant voltage is applied. The conductivity of the electro-optic medium is proportional to the applied voltage and it is assumed that impulses can be used to track the DC balance).

The term " superframe " is then used to denote a sequence of consecutive display scan frames needed to achieve all the necessary gray level changes from the initial image to the final image. Typically, the update of the display is initiated only at the start of the superframe.

The aforementioned WO 03/044765 discloses a method of driving a bistable electro-optic display having a plurality of pixels, each of which can display three or more gray levels (as is conventional in the field of display, extreme black and white states For the purposes of counting gray levels, they are considered two gray levels.)

Storing a lookup table containing data representing impulses required to convert the initial gray level to a final gray level;

Storing data representing at least an initial state of each pixel of the display;

Receiving an input signal indicative of a desired final state of one or more pixels of the display; And

Generating an output signal representing the impulse required to convert the initial state of the one pixel to its desired final state as a signal determined from a lookup table.

This method is referred to as " basic lookup table method "

Depending on the number of previous states stored, the lookup table used in the lookup table method can be very large. As an extreme example, consider a lookup table method for 256 (2 ⁸ ) gray level displays using an algorithm that takes into account the initial, last, and two previous states. The necessary fourth order lookup table has 2 ³² inputs. If each input requires 64 bits (8 bytes), the total size of the lookup table will be approximately 32 Gbytes. While the storage of this amount of data does not cause any problems on desktop computers, it can also be problematic on portable devices. In another aspect, the present invention provides a method of driving a bistable electro-optic display that achieves results similar to the lookup table method but does not require storage of very large lookup tables.

One aspect of the present invention relates to a method and apparatus for driving a bistable electro-optic display such that a portion of the display operates at a different bit depth (ie, different number of gray scale levels) than the rest of the display. From the foregoing description of the sawtooth driving method shown in FIGS. 11A and 11B of WO 03/044765, the transition between successive images of a typical image flow of a bistable electro-optic display with multiple gray scale levels is: It will be apparent to those skilled in the art that the same display can be substantially longer than when driven in monochrome mode. Typically, gray scale transitions can be up to four times the corresponding monochrome transition. Relatively slow gray scale transitions are undesirable when the display is used to present a series of images, such as a series of photos or a continuous page of electronic books. However, there are times when it is useful to achieve a quick update of a defined area of such a display. For example, if a user employs such a display for reviewing a series of photos stored in the database to enter in each photo a keyword or other indexing term intended to facilitate later retrieval of images from the database. Consider. In this case, relatively slow transitions between successive pictures are acceptable, for example, if the user spends 1 to 2 minutes searching each picture and determining its indexing term. The one to two second transition between photos does not significantly affect the user's productivity. However, as anyone who wants to run a word processing program on a computer with poor processing power knows, a delay of 1 to 2 seconds in the update of a dialog box in which indexing terms entered by the user are displayed is very frustrating and leads to many typing errors. Easy to trigger Thus, in such or similar situations, it is advantageous to run the dialog box in monochrome mode allowing fast transitions, and to continue the remainder of the display in gray scale mode to allow the image to be reproduced correctly, and the present invention Provides a method and apparatus that allow this to be achieved.

Another aspect of the invention relates to a method that can achieve fine control of the gray level of an impulse driven image medium without the need for fine voltage control. As mentioned above, electrophoresis and some other electro-optic displays are bistable, and this bistable is not unlimited, and the image on the display will slowly fade over time and thus should be maintained for an extended period of time. In turn, the image may need to be refreshed periodically to restore the image to the optical state in which the image was originally written.

However, such refreshing of the image may itself cause problems. As disclosed in the aforementioned U.S. Pat.Nos. 6,531,997 and 6,504,524, problems may arise, and if the method used to drive the display does not have a net time-averaged electric field applied across the electro-optic medium to zero, The operating life time of the display is reduced. The drive method in which the net time-average is zero for which an electric field is applied across the electro-optical medium is referred to as "DC balancing" or "DC balancing" for convenience. If the image is to be maintained for an extended period of time by applying refresh pulses, these pulses need to be of the same polarity as the addressing pulses originally used to drive the relevant pixels of the display to the holding optical state, which is a DC imbalanced drive scheme. Brings about.

The effort to achieve the correct gray scale level in the impulse drive medium is to apply the appropriate voltage impulse to achieve the desired gray tone. Satisfactory transitions between optical states can be achieved by fine control of the voltage of all drive waveforms or portions thereof. The need for precision can be understood from the following example. Suppose the current image consists of a screen that is half black and half white, and the next image you want is a uniform gray that mediates between black and white. In order to achieve a uniform gray level, the impulses used to proceed from black to gray and white to gray must be finely adjusted so that the gray level achieved from black and the gray level achieved from white match. Fine adjustment is further needed if the final gray level achieved is a function of the previous gray level history of the display. For example, as discussed above, the optical state achieved when moving from black to gray can be a function of the state that has reached before the current black state as well as a function of the applied waveform. It is then desirable for the display module to remember certain aspects of the display history, such as the previous image state, and to allow fine tuning of the waveform to compensate for this previous state history (see below for more details on this).

Fine tuning of the impulse can be achieved using only three voltage levels (0, + V, -V) by adjusting the width of the applied pulse with high precision. However, this is undesirable for active matrix displays, since the frame rate must be increased to achieve high pulse width resolution. High frame rates increase the power consumption of the display and place more demands on the control and driving of the electronics. Thus, it is not desirable to operate an active matrix display at a frame rate of substantially about 60-75 Hz or higher.

Fine tuning of the impulse can be achieved when a number of finely-spaced voltages are available. In active matrix driving, this requires the source driver to be able to output one of the multiple sets of available voltages at least in a subset of the available voltages. For example, for a driver that outputs between -10 and +10 volts, the 16 total voltage levels between -10 and -7V and the 16 individual voltage levels between 7 and 10V require the total number of voltage levels required. Having 33 (see Table 1), it is advantageous to have two bands of usable 0V and voltages between -10 and -7V and between 7 and 10V. Then fine control of the optical final state can be achieved, for example, by varying the voltage between +7 and +10 V or between -10 and -7V during the last one or more scan frames of the addressing period. This method is an example of a voltage-modulated technique to achieve acceptable display performance.

<Table 1> Examples of voltages required for voltage regulation drive

The disadvantage of using voltage-regulation techniques is that the driver must have a certain range of fine voltage control. Display adjustment costs can be reduced by using drivers that provide only two to three voltages.

In another aspect, the present invention is a gray using a driver with only a small set of available voltages, especially when the control of the impulse is too coarse to achieve the fine adjustment necessary for acceptable display performance. It is intended to provide a method of achieving fine control of levels. Thus, this aspect of the present invention seeks to provide a method for achieving fine control of the gray level of an impulse driven image medium without the need for fine voltage control. This aspect of the invention can be applied, for example, to an active matrix display having a source driver capable of outputting two to three voltages.

In another aspect, the present invention relates to a method of driving an electro-optic display using a drive scheme comprising at least some direct current (DC) balanced transitions. For reasons detailed in the co-pending application described above, when driving an electro-optic display, a DC balance is maintained, i.e. a voltage applied each time the final optical state matches the initial optical state for any sequence of optical states. It is preferable to use the drive system which has the characteristic that the integral of 0 is zero. This ensures that the net DC imbalance experienced by the electro-optic layer is limited to known values. For example, a 15V, 300ms pulse can be used to drive the electro-optic layer from white to black. After this transition, the image layer experiences a DC unbalanced impulse of 4.5V. To drive the film back to white, if a -15V, 300ms pulse is used, the image layer is DC balanced across a series of white to black transitions and black to white transitions.

It is also found that at least some of the transitions themselves are preferred to use a DC balanced drive scheme, and such transitions are hereinafter referred to as "DC balanced transitions". DC balanced transitions do not have net voltage impulses. The drive scheme waveform employing only DC balanced transitions allows the electro-optic layer to be DC balanced after each transition. For example, a -15V, 300ms pulse followed by a 15V, 300ms pulse can be used to drive the electro-optic layer from white to black. During this transition, the net voltage impulse across this electro-optic layer is zero. The electro-optical layer can then be driven from black to white using a 15V, 300ms pulse followed by a -15V, 300ms pulse. Again, the net voltage impulse over this transition is zero.

The driving scheme that constitutes all DC balanced transition elements is necessarily DC balanced waveforms. As described in detail below, it is also possible to form a DC balanced drive scheme comprising a DC balanced transition and a transition in which DC imbalance is not maintained.

In one aspect, the present invention provides a method of driving a bistable electro-optic display, each having a plurality of pixels capable of displaying three or more gray levels, the method comprising:

Storing a lookup table containing data representing impulses required to convert the initial gray level to a final gray level;

Storing data representing at least an initial state of each pixel of the display;

Storing compensation voltage data indicative of a compensation voltage for each pixel of the display, wherein the compensation voltage for any pixel is calculated according to one or more impulses previously applied to that pixel;

Receiving an input signal indicative of a desired final state of one or more pixels of the display; And

Generating an output signal indicative of a pixel voltage to be applied to said one pixel, said pixel voltage being a drive voltage determined from initial and final states of said pixel and a lookup table, and a compensation voltage determined from compensation voltage data for that pixel; And generating said sum.

This method is hereinafter referred to as the "compensation voltage" method of the present invention for convenience.

In this compensation voltage method, the compensation voltage for each pixel can be calculated according to at least one of the temporal prior states of the pixels and the gray level prior states of that pixel. Further, the compensation voltage for each pixel can be applied to the pixel while the drive voltage is applied to that pixel and while the drive voltage is not applied to that pixel.

For the reasons detailed below, it is necessary to periodically update the compensation voltage used in the compensation voltage method of the present invention. The compensation voltage for each pixel can be updated during each superframe (period required for complete addressing of the display). The compensation voltage for each pixel may comprise: (1) changing a previous value of the compensation voltage using a constant algorithm independent of pulses applied during the associated superframe; And (2) increasing the value from step (1) by an amount determined by the pulses applied during the associated superframe. In a preferred variant of this update process, the compensation voltage for each pixel comprises: (1) dividing a previous value of the compensation voltage by a constant constant; And (2) increasing the value from step (1) by an amount substantially proportional to the total area under the voltage / time curve applied to the electro-optical medium during the associated superframe.

In the compensation voltage method of the present invention, the compensation voltage can be applied in the form of an exponentially decaying voltage applied to the ends of one or more drive pulses.

The present invention also provides an apparatus controller for use in such a compensating voltage method. The controller

A storage table configured to store both a lookup table comprising data representing the impulses required to convert the initial gray level to a final gray level and data representing the initial state of each pixel of the display, and compensation voltage data for each pixel of the display. Way;

Input means for receiving an input signal indicative of a desired final state of at least one pixel of said display;

Determining a driving voltage required to change the initial state of the one pixel to a desired final state from the input signal, the data representing the stored initial state of the pixel, and a lookup table; and further, the compensation voltage data for the pixel. Calculating means for determining a compensation voltage for the pixel from the sum and adding the driving voltage and the compensation voltage to determine the pixel voltage; And

Output means for generating an output signal indicative of said pixel voltage.

In this controller, the calculating means may be configured to determine the compensation voltage in accordance with at least one of a temporary before state of the pixel and a gray level before state of the pixel. Further, the output means may be configured to apply the compensation voltage to the pixel during a period during which a drive voltage is being applied to the pixel and during a hold period during which no drive voltage is being applied to the pixel.

Also in this controller, the calculation means may be configured to update the compensation voltage for each pixel during each superframe necessary for complete addressing of the display. For such an update, the calculation means comprises: (1) changing the previous value of the compensation voltage using a constant algorithm independent of the pulses applied during the associated superframe; And (2) increasing the value from step (1) by the amount determined by the pulses applied during the associated superframe by updating the compensation voltage for each pixel. In a preferred variant of this process, the calculation means comprises: (1) dividing the previous value of the compensation voltage by a constant constant; And (2) increasing the value from step (1) by an amount substantially proportional to the total area under the voltage / time curve applied to the electro-optical medium during the associated superframe. Configured to update the voltage.

The output means of the controller may be configured to apply the compensation voltage in the form of an exponentially decaying voltage applied at the end of one or more drive pulses.

In another aspect, the present invention relates to a plurality of pixels arranged in a plurality of rows and columns so that each pixel is uniquely defined by the intersection of a particular row and a particular column, and independent of each pixel to change the display state of the pixel. A method of updating a bistable electro-optic display having a drive means for applying an electric field is provided, wherein each pixel has at least three different display states,

Storing area data representing a defined area that includes a portion, but not all, of the display;

Determining for each pixel whether the pixel is inside or outside the prescribed area;

And applying a first driving scheme to the pixels in the prescribed region, and applying a second driving scheme different from the first driving scheme to the pixels other than the prescribed region.

This method is hereinafter referred to as the "defined area" method of the present invention for convenience.

In this defined area method, the first and second driving schemes have different bit depths, in particular, one of the first and second driving schemes may be monochrome, and the other has four or more different gray levels. May be gray scale. The defined area may comprise a text box used for the writing of text on the display.

In another aspect, the present invention provides a method of driving a bistable electro-optic display, each having a plurality of pixels capable of displaying three or more gray levels, the method comprising:

Storing a lookup table containing data representing impulses required to convert the initial gray level to a final gray level;

Storing data representing at least an initial state of each pixel of the display;

Receiving an input signal indicative of a desired final state of one or more pixels of the display; And

Generating an output signal representing the impulse required to convert the initial state of the one pixel to the desired final state of the pixel as the signal determined from the lookup table,

For one or more transitions from an initial state to a final state, the output signal includes a DC imbalanced fine tuning sequence, the sequence comprising:

(a) has a non-zero net impulse;

(b) is non-contiguous;

(c) change the gray level of the pixel substantially different from the change in the optical state of the DC reference pulse of the sequence, where the DC reference pulse is a pulse of voltage V ₀ , and V ₀ is applied during the fine adjustment sequence Is the maximum voltage but has the same sign as the net impulse G of the fine tuning sequence, the duration of the reference pulse being G / V _0. Is;

(d) changes the gray level of the pixel which is smaller in magnitude than the change in gray level caused by the time-reference pulse of the sequence, where the time-reference pulse is a monopolar of the same duration as the fine adjustment sequence. ) Is defined as a voltage pulse, but the sign of the reference pulse results in a greater change in gray level.

This method (also a similar method defined below) is hereinafter referred to as the " discontinuous addressing " method of the present invention for convenience, and when the two methods need to be distinguished, the " DC unbalanced discontinuous addressing " method and the " DC balanced " Respectively, in the "discontinuous addressing" method.

In a preferred form of this discontinuous addressing method, a fine adjustment sequence changes the gray level of the pixel to less than one half of the change in gray level caused by the time-referenced pulse of the sequence.

The present invention also provides a method of driving a bistable electro-optical display having a plurality of pixels, each of which can display three or more gray levels.

Storing a lookup table comprising data representing impulses required to convert the initial gray level to a final gray level;

Storing data representing at least an initial state of each pixel of the display;

Receiving an input signal indicative of a desired final state of at least one pixel of the display; And

Generating an output signal representing the impulse required to convert the initial state of the one pixel to the desired final state of the pixel as the signal determined from the lookup table,

Wherein for one or more transitions from the initial state to the final state, the output signal comprises a DC balanced fine tuning sequence, the sequence comprising:

(a) has substantially zero net impulse;

(b) at any point in the fine tuning sequence, the gray level of the pixel is changed to greater than about one third of the difference in gray level between the two extreme optical states of the pixel from the gray level at the start of the fine tuning sequence. Don't let that happen.

In two variations of the discrete addressing method of the present invention, the output signal typically includes one or more single polarity drive pulses in addition to the fine tuning sequence. The discontinuous output signal may be aperiodic. For most transitions in the lookup table, the output signal may have non-zero net impulses and may be discontinuous. In one or more transitions using discrete output signals, the output signal may consist of only pulses having voltage levels of + V, 0, -V, preferably 0 voltage level and one of + V and -V voltage levels. It consists of only pulses having In a preferred variant of this method, for one or more transitions using discrete output signals, and preferably for most transitions in the lookup table where the initial and final states of the pixels differ, the output signal is one of + V and -V. Two or more pulses having the same voltage level consist of a pulse having a voltage level of zero preceding or following. Preferably, the transition table is DC balanced. Also, for one or more transitions using discrete output signals, the output signal may consist of a series of pulses that are integer multiples of a single interval.

In addition, the method of discontinuous addressing of the present invention comprises the steps of storing data representing one or more transient states of the one pixel and / or one or more gray level earlier states of the one pixel, and the one or more transient states and / or Or generating an output signal according to one or more gray level previous states of the one pixel.

The present invention provides a method of driving a bistable electro-optical display having a plurality of pixels, each of which can display three or more gray levels, which method provides an output signal effective for changing the pixel from an initial state to a final state. Applying to each pixel of the display, wherein for one or more transitions where the initial and final states of the pixel are different, the output signal has two or more pulses having the same voltage level as one of + V and -V Consisting of pulses with a voltage level of zero preceding or following them.

In another aspect, the present invention provides a method of driving a bistable electro-optic display having a plurality of pixels, each of which can display three or more gray levels, which method changes the pixel from an initial state to a final state. Applying an effective output signal to each pixel of the display, wherein for one or more transitions the output signal is non-zero, but DC balanced.

This method is hereinafter referred to as the " DC balanced addressing " method of the present invention for convenience.

In this DC balanced addressing method, for one or more transitions, the output signal may comprise a pair of first pulses comprising voltage pulses of the same length but following pulses of opposite signs. The output signal may comprise a period of zero voltage between two pulses, or alternatively one or more pulses may be interrupted by a period of zero voltage. Also, the output signal may further include a second pulse pair of the same length but of opposite sign, and the second pulse pair may have a length different from the length of the first pulse pair. The first of the second pulse pair may have a polarity opposite to the first of the first pulse pair. The first pulse pair may occur between the first and second of the second pulse pair.

Also in this DC balanced addressing method, for the foregoing transitions, the output signal may include one or more pulse elements that are effective to drive the pixel into substantially one optical rail.

As described in more detail below, the DC balanced addressing method of the present invention may utilize a combination of DC balanced transitions and DC unbalanced transitions. For example, for each transition where the initial and final states of the pixel are the same, the output signal may be non-zero but DC balanced, and for each transition where the initial and final states of the pixel are not the same, the output The signal may not be DC balanced. In this addressing method, for each transition where the initial and final states of the pixel are not the same, the output signal has the form -x / ΔIP / x, where ΔIP is the impulse potential between the initial and final states of the pixel. And -x and x are pulse pairs of the same length but of opposite signs.

In addition, the DC balanced addressing method of the present invention,

Storing a lookup table comprising data representing impulses required to convert the initial gray level of the pixel to a final gray level;

Storing data representing at least an initial state of each pixel of the display;

Receiving an input signal indicative of a desired final state of one or more pixels of the display; And

Generating an output signal representing the impulse required to convert the initial state of the one pixel to a desired final state as the signal determined from the lookup table.

In addition, the present invention provides a method of driving a bistable electro-optic display having one or more pixels,

Applying waveform V (t) to the pixel such that is less than about 1 volt sec,

Where T is the length of the waveform, integration is over the duration of the waveform, V (t) is the waveform voltage as a function of time t, and M (t) is the dwell resulting from the short pulse at time zero. Memory function, characterized by reducing the effect of residual voltage to induce dwell-time-dependence). This method is hereinafter referred to as the " DTD integral reduction " method of the present invention for convenience. Preferably J is less than about 0.5 volt sec and more preferably less than about 0.1 volt sec. In fact, J should be constructed as small as possible, ideally zero.

J is Is calculated by

Where τ is the decay (relaxation) time and preferably has a value from about 0.7 to about 1.3 seconds.

1A-1E show five waveforms that can be used in the discrete addressing method of the present invention.

Figure 2 illustrates the problem of addressing an electro-optic display using various numbers of frames of monopolar voltage.

3 illustrates one approach to solving the problem shown in FIG. 2 using the discrete addressing method of the present invention.

FIG. 4 shows a second approach to solving the problem shown in FIG. 13 using the discrete addressing method of the present invention.

5 shows waveforms that can be used in the discrete addressing method of the present invention.

FIG. 6 shows a base waveform that can be modified in accordance with the present invention to produce the waveform shown in FIG. 5.

FIG. 7 illustrates a problem when addressing an electro-optic display using various numbers of frames of a single polarity voltage while maintaining DC balance.

8 illustrates one approach to solving the problem shown in FIG. 7 using the discrete addressing method of the present invention.

9 illustrates a second approach to solving the problem shown in FIG. 7 using the discrete addressing method of the present invention.

FIG. 10 illustrates the gray levels obtained in nominally four gray level electro-optic displays, without using the discrete addressing method of the present invention, as described in the examples below.

FIG. 11 shows gray levels obtained from the same display as shown in FIG. 10 using various discrete addressing sequences.

FIG. 12 shows the gray level obtained from the same display as shown in FIG. 10 using a modified drive scheme according to the discrete addressing method of the present invention.

13 shows a simple DC balanced waveform that can be used for driving an electro-optical display.

14 and 15 show two variations of the waveform shown in FIG. 24 for merging periods of zero voltage.

FIG. 16 schematically illustrates how the waveform shown in FIG. 13 is modified to include additional pairs of drive pulses.

FIG. 17 shows one waveform produced by modifying the waveform of FIG. 13 in the manner shown in FIG. 16.

FIG. 18 shows a second waveform produced by modifying the waveform of FIG. 24 in the manner shown in FIG. 27.

FIG. 19 schematically illustrates how the waveform shown in FIG. 18 is further modified to include a third pair of drive pulses.

20 shows one waveform produced by modifying the waveform of FIG. 18 in the manner shown in FIG.

Figure 21 illustrates one preferred DC unbalanced waveform that can be used with the DC balanced waveform to provide a complete lookup table for use in the methods of the present invention.

22 is a graph showing the reduced residence time dependence that can be achieved by the compensation voltage method of the present invention.

FIG. 23 is a graph illustrating the effect of residence time dependence of an electro-optic display.

From the foregoing description, it is clear that the present invention provides a number of different improvements in a method for driving an electro-optic display and in a device controller or other device for performing such a driving method. In the following description, those skilled in the art will appreciate that various different improvements provided by the present invention will be described separately, and indeed a single display will utilize one or more of these key aspects, but, for example, The display using the discontinuous addressing method of the present invention may use a prescribed area method.

An ideal method for addressing an impulse-driven electro-optic display is a so-called "general gray scale," in which the controller adjusts each entry of the image so that each pixel transitions directly from its initial gray level to its final gray level. Image flow ". Inevitably, however, there are some errors in writing images to the impulse-driven display. As already explained in part, some of those errors that we actually face include:

(a) previous state dependence; The impulse needed to switch a pixel to a new optical state depends not only on the initial and desired optical state, but also on the previous optical state of that pixel.

(b) residence time dependence; The impulse needed to switch a pixel to a new optical state depends on the time the pixel spent in its various optical states. The exact nature of this dependency is not known, but more impulse is generally required to keep the pixel longer in its current optical state.

(c) temperature dependence; The impulse required to switch the pixel to a new optical state is strongly dependent on temperature.

(d) humidity dependence; In at least some types of electro-optic media, the impulse needed to switch the pixel to a new optical state depends on the ambient humidity.

(e) mechanical uniformity; The impulse needed to switch the pixel to a new optical state can be affected by mechanical changes in the display, for example by changing the thickness of the electro-optic medium or associated lamination adhesive. Another type of mechanical non-uniformity arises from the inevitable differences between different batches of manufacturing of media, manufacturing tolerances, and material diversity.

(f) voltage error; The actual impulse applied to the pixel is necessarily slightly different than that applied theoretically, due to the inevitable small error in the voltage delivered by the driver.

Typical grayscale image flows suffer from "accumulation of errors." For example, suppose that the temperature dependence causes an error of 0.2 L * in the positive direction for each transition. After 50 transitions, this error will accumulate to 10 L *. More practically, assume that the mean error for each transition expressed as the difference between the theoretical and actual reflectance of the display is ± 0.2 L *. After 100 consecutive transitions, the pixels will display an average deviation of 2L * from its expected state, which deviation is identified to the average viewer of some type of image.

This accumulation of error phenomena applies not only to errors due to temperature, but also to other types of errors. Compensation for such errors is possible, but only with limited accuracy. For example, the temperature error can be compensated for using a temperature sensor and lookup table, but the temperature sensor has a limited resolution and can read a temperature slightly different from the temperature of the electro-optic medium. Similarly, previous state dependencies can be compensated for storing previous states and using a multidimensional transition matrix, but controller memory limits the number of states that can be written and the size of the transition matrix, as discussed above Limit the precision of

Thus, a typical grayscale image flow requires that very accurate control of the applied impulse provide good results, and empirically, in the current state of electro-optic display technology, a typical grayscale image flow is typically not viable in commercial displays. Was found.

Almost all electro-optic media have their extreme (typically black and white) optical states acting as built-in reset (error limiting) mechanisms, ie "optical rails". After a particular impulse is applied to a pixel of an electro-optic display, that pixel cannot be more white (or black). For example, in an encapsulated electrophoretic display, after a particular impulse is applied, all the electrophoretic particles are forced to each other or to the capsule wall and can no longer move, thus limiting the limited optical state or optical rail. Create Because of the distribution of electrophoretic particle size and charge in such media, some particles hit the rails before other particles and exhibit a "soft rail" phenomenon, whereby the final optical state of the transition is its extreme black and white When the state is reached, the required impulse accuracy is reduced, while the optical accuracy required at transitions ending near the center of the pixel's optical range is significantly increased. Clearly, a pure general grayscale image flow driving scheme cannot rely on the use of an optical rail to prevent errors in the gray level, where any given pixel is gray level without touching either optical rail. Because they can go through an infinite number of changes.

As noted above in US Pat. Nos. 6,504,524 and 6,531,997, in many electro-optical media, particularly particle-based electrophoretic media, the drive scheme used to drive such media is passed through a particular pixel over an extended period of time. In the sense that the OR of the currents should be zero or as close to zero as possible, direct current (DC) balance is preferred, and the drive scheme of the present invention should be designed with this criterion in mind. More specifically, the lookup table should be designed such that any sequence of transitions beginning or ending in one extreme optical state (black or white) of the pixel is DC balanced. From the foregoing description, it will be considered that such a DC balance cannot be achieved because the impulse required for any particular gray-gray transition and thus the current through the pixel is substantially constant. However, this applies only in the first approximation, and in experience, at least in the case of particle-based electrophoretic media (also in other electro-optic media), the effect of applying five spaced 50msec pulses to one pixel is the same. It is not the same as in the case of applying one 250 msec pulse of voltage. Thus, there is flexibility in the current flowing through the pixels to achieve a given transition, and this flexibility can be used to help achieve DC balance. For example, the lookup table used in the present invention may store multiple impulses for a given transition, along with the value of the total current provided by each of these impulses, and the controller may store some previous time ( A register configured to store the logical sum of the impulses applied to that pixel (eg, when the pixel was last black). When a pixel is driven from a white or gray state to a black state, the controller examines the register associated with that pixel and determines the current required for the DC balance of the entire sequence of transitions from the previous black state to the coming black state. And necessary white /, which can accurately reduce the associated resistor to zero or at least to the smallest possible remainder (if its associated resistor maintains this residual value and adds it to the current applied during the next transitions). One can select one of a number of stored impulses for the transition from gray to black. Repeated application of this process can achieve accurate long term DC balancing for each pixel.

The following description of various aspects of the invention is believed to be analogous to the entirety of the foregoing WO 03/044765, in particular the various waveforms disclosed therein. It is to be noted that the various methods of the present invention can be modified to include various optical characteristics (eg, temperature compensation, operating lifetime compensation, humidity compensation, etc.) of the basic lookup table method disclosed in WO 03/044765 described above. It is obvious to those skilled in the art. Various methods of the present invention may use the method disclosed in the aforementioned WO 03/044765 to reduce the amount of data that must be stored for the lookup table. In addition, since the data making up the look-up table can be treated as a general multi-dimensional data set, any standard functions, algorithms, and encodings known to those of ordinary skill in the art of data storage and processing can be used for (a) data sets. It may be employed to reduce one or more of the amount of storage required, (b) the computational effort required to extract the data, or (c) the time required to extract or locate a particular element from the set. Such storage techniques include, for example, representation of the data set as a combination of hash function, loss-less and lossy compression, and basic function.

discontinuity Addressing  Way

Fine control of the gray scale level in the methods of the present invention can be achieved using the discrete addressing method of the present invention. As mentioned above, the discontinuous addressing method has two theoretical variants, a DC unbalanced strain and a DC balanced strain. DC unbalanced deformation affects one or more transitions between grail levels using an output signal with a non-zero net impulse (ie, the lengths of the positive and negative segments are not the same), thus internally balancing the DC It is not maintained and is discontinuous (i.e., the pulse contains a portion of zero voltage or opposite polarity). The output signal used in the discrete addressing method may or may not be aperiodic (ie, may or may not consist of repeating units such as + /-/ + /-or ++ /-/ ++ /-).

Such discontinuous waveforms (hereinafter referred to as " fine adjustment " or " FT " waveforms) may not have frames of opposite polarities, and / or only three voltage levels, i.e., for the effective front plane voltage of the display. It can include only + V, 0, -V (as is the case with an active matrix display and typically the entire display, having a pixel electrode associated with each pixel and a common front electrode extending over multiple pixels). Assuming that the electric field applied to any pixel of the electro-optic medium is determined by the voltage difference between its associated pixel electrode and its common front electrode). Alternatively, the FT waveform may include three or more voltage levels. The FT waveform can be configured with any of the types of waveforms described above (such as n-prepulse, etc.), along with the added discrete waveform.

The FT waveform may depend on (or will typically depend on) one or more previous image states and may be used to achieve smaller changes in optical state than can be achieved using reference pulse width modulation (PWM). Can be. (Thus, in contrast to any conventional waveform where varying polarity pulses are employed to prevent electrophoretic particles from adhering to surfaces such as, for example, capsule walls, the exact FT waveform employed is shifted every time in the lookup table. In a preferred variant of the method of discontinuous addressing, a combination of all the waveforms necessary to achieve all the allowable optical transitions in the display is provided ("transition matrix"), where one or more waveforms are of the present invention. It is an FT waveform and the combination of waveforms is DC balanced. In another preferred variant of the discontinuous addressing method, the length of all voltage segments is an integer multiple of a single interval (frame time) and the voltage segment is part of a waveform in which the voltage remains constant.

The discrete addressing method of the present invention, in many impulse-driven electro-optic media, has waveforms with zero net impulses and can therefore be expected to theoretically not affect the overall change in the gray level of the pixel, in fact the characteristics of such media. Because of some of the nonlinear effects, small changes in gray level are affected, which is more subtle than is possible with a simple PWM drive or with a driver with limited ability to change the width and / or height of the pulse. It is based on the finding that it can be used to achieve adjustment of the level. The pulses that may constitute such a "fine adjustment" waveform may be separated from the "major drive" pulses that affect the major change in gray level and may precede or follow such major drive pulses. Alternatively, in certain cases, the fine tuning pulse may be intermingle with the main drive pulses or a separate block of fine tuning pulses at a single point in the sequence of main drive pulses, alone or as a sequence of main drive pulses. Can be interspersed within a small group of multiple points.

Although the discrete addressing method has very general adaptability, it is explained first by using a driving method using a waveform formed from a source driver having three (+,-, 0) voltage outputs and the following three types of waveform elements. (A modification of the present invention necessary for using other types of drivers and waveform elements will be apparent to those skilled in the art of electro-optic display.)

1) saturation pulse: voltage of one sign or one sign and 0 V driving the reflectance to almost one extreme optical state (one of the darkest state, referred to herein as the optical rail, or the black state, or white state here). A sequence of frames of voltages;

2) set pulse: a sequence of frames of voltages or signs of one sign and voltages of 0V driving the reflectance to a near desired gray level (black, white or intermediate gray level); And

3) FT sequence: A sequence of frames with voltages individually selected to be +,-, 0 such that the optical state of the ink is shifted less than a single-signed sequence of equal length. Examples of FT drive sequences having a total length of five scan frames include; [+-+-] (Where the voltages in each frame are successively represented as + for positive voltages, 0 for zero voltages, and-for negative voltages), [--0 ++], [ 00000], [00 + -0], and [0- + 00]. These sequences are schematically illustrated in FIGS. 1A-1E in the accompanying drawings, where circles represent the start and end points of the FT sequence and there are five scan frames between these points.

The FT sequence can be used to allow fine control of the optical state or to produce a change in the optical state with a different net voltage impulse but similar to a sequence of single polarity (single-sine) voltages, where the impulse is applied over time Defined as integral of voltage). The FT sequences in the waveform are thus used as a tool to achieve DC balance.

The use of an FT sequence to achieve fine control of the optical state is described first. In FIG. 2, more frames of optical states achievable using zero, ie 1, 2, 3 or single polarity voltage, are schematically represented as points on the axis individually. From this figure, it can be seen that the length of a single polarity pulse can be selected to achieve the reflectance represented by its corresponding point on this axis. However, although one may wish to achieve a gray level such as that shown by " target " in FIG. 2, it is not well approached by either of these gray levels. The FT sequence can be used to fine tune the final state achieved after a single polarity drive pulse, or to fine-tune the reflectance to the desired state, using the initial state and a single polarity drive sequence.

The first example of the FT sequence shown in FIG. 3 shows the FT sequence used after two-pulse single polarity driving. The FT sequence is used to fine tune the last optical state to the target state. As in FIG. 2, FIG. 3 shows the optical state achievable using various numbers of scan frames, as indicated by solid points. Also shown is the target optical state. As the optical shift induced by the FT sequence, the optical change by applying two scan frames was indicated.

A second example of an FT sequence is shown in FIG. 4; In this case, the FT sequence was first used to fine tune the optical state to a position where a single polarity drive sequence could be used to achieve the desired optical state. The optical state achievable after the FT sequence is indicated by the open circles in FIG. 4.

The FT sequence may also be used with a rail-stabilized gray scale (RSGS) waveform, as shown in FIGS. 11A and 11B of WO 03/044765 described above. The essence of the RSGS waveform is that a given pixel is allowed to make only a limited number of gray-gray transitions before being driven into one of its extreme optical states. Thus, the use of such waveforms often drives the pixel into an extreme optical state (referred to as optical rails) to maintain DC balance (where DC balance is a net voltage pulse of zero, described in more detail below). Reduces optical error during A well resolved gray scale can be achieved using these waveforms by selecting fine tuning voltages for one or more scan frames. However, if these fine tuning voltages are not available, other methods should preferably be used to achieve fine tuning while maintaining good DC balance. FT sequences can be used to achieve these goals.

First, each zero, one, or two saturation pulses (pulses drive the pixel with one optical rail), followed by one set pulse (making the pixel the desired gray level) as described above. Assume a periodic version of a rail-stabilized grayscale waveform of To illustrate how the FT sequence is used within this waveform, we will use symbolic representations for the waveform elements: "sat" represents a saturation pulse, "set" represents a set pulse, and "N" represents an FT drive sequence. The basic types of three periodic rail-stabilized grayscale waveforms are:

set (e.g., transition 1104 of FIG. 11A of WO 03/044765)

sat-set (e.g., transition (1106/1108) in FIG. 11A of WO 03/044765)

sat-sat'-set (eg, the transition of FIG. 11A of WO 03/044765 (1116/1118/1120))

Where sat and sat 'are two separate saturation pulses.

The first modification of these types with one FT sequence gives the following waveform:

N-set

set-N

That is, the FT sequence is followed by one set pulse or the same elements are in opposite order.

A second modification of these types with one or more FT sequences provides the following FT correction waveform, for example:

N-sat-set

sat-N-set

sat-set-N

sat-N-set-N '

N-sat-set-N '

N-sat-N'-set

N-sat-N'-set-N ''

Here, N, N ', and N' 'are three FT sequences, which may be different or the same.

In essence, a second modification of these types can be achieved by placing FT sequences between three waveform elements that follow the above-described forms. An incomplete list of examples includes:

N-sat-sat'-set

N-sat-sat'-set-N '

sat-N-sat'-N'-set-N ''

N-sat-N'-sat'-N ''-set-N '' '

Another basic waveform that can be modified with an FT sequence is a single-pulse slide show gray scale with driving to black (or white). In this waveform, the optical state first becomes the optical rail and then the desired image. The waveform of each transition can be represented as a symbol by one of the following two sequences:

sat-set

set.

Such a waveform can be modified to include the FT drive sequence elements in essentially the same manner as described above for the rail-stabilized gray scale sequence to produce the following sequences:

sat-set-N

sat-N-set

And so on.

The two examples illustrate inserting FT sequences before or after the saturation and set pulse elements of the waveform. FT sequences may be advantageously partially inserted through saturation or set pulses, ie base sequences:

sat-set

This can be modified as follows:

{sat, part I} -N- {sat, part II} -set

or

sat- {set, part I} -N- {set, part II}.

As already indicated, it has been found that the optical state of a number of electro-optic media achieved after a series of transitions is sensitive to the previous optical state and the time spent in these previous optical states, and thus by adjusting the transition waveform. Methods have been described for compensation for state and prior residence time sensitivity. FT sequences can be used in a similar manner to compensate for previous optical states and / or previous residence times.

To illustrate this concept in more detail, consider a sequence of gray levels that will be displayed in a particular pixel; These levels are represented by R ₁ , R ₂ , R ₃ , R _4, etc., where R ₁ represents the next desired (final) gray level of the transition under consideration, R ₂ is the initial gray level for that transition, and R ₃ is the first pre-gray level, R ₄ is the second pre-gray level, and so on. The gray level sequence can be represented as follows:

R _n R _{n -1} R _{n -2} ... R ₃ R ₂ R ₁

The residence time before gray level i is denoted D _i . D _i may represent the number of frame scans of stay at gray level i.

The FT sequences indicated as above may be appropriately selected for the transition from the current gray level to the desired gray level. In simple form, these FT sequences are a function of the current and desired gray level, where FT sequence N is R ₂ And as R ₁ to indicate dependence on:

N = N (R ₂ , R ₁ ).

In order to improve device performance, it is advantageous to make some adjustments to the transition waveform to clearly reduce the residual gray level shifts associated with the previous image. The selection of FT sequences can be used to achieve these adjustments. Various FT sequences generate various final optical states. Different FT sequences may be selected for different optical histories for a given pixel. For example, the first, and can select the previous image FT sequence that depends on R ₃ in order to compensate for the (R _3), it is expressed as follows:

N = N (R ₃ , R ₂ , R ₁ )

That is, the FT sequence may be selected depending on not only R ₁ and R ₂ but also R ₃ .

Generalizing this concept, the FT sequence may be dependent on the number of any previous gray levels and / or the number of any previous dwell times and is represented as a symbol as follows:

N = N (D _m , D _{m -1} , ... D ₃ , D ₂ ; R _n , R _{n -1} , ... R ₃ , R ₂ , R ₁ )

Here, the symbol D _k represents the residence time spent at the gray level R _k , and the number n of optical states does not need to be equal to the number of residence time m required in the FT determination function. Thus, FT sequences can be a function of previous image and / or previous and current gray level dwell times.

As a special case of this general concept, it is found that the insertion of a zero voltage scan frame into another single polarity pulse can change the final optical state achieved. For example, an optical state achieved after the sequence of FIG. 5 with a zero voltage scan frame inserted is achieved after a corresponding single polarity sequence of FIG. 6 without a zero voltage scan frame but with the same total impulse as the sequence of FIG. 5. It will be somewhat different from the optical state.

It has also been found that the effect of a given pulse on the final optical state depends on the length of the delay between this pulse and the previous pulse. Thus, zero voltage frames can be inserted between the pulse elements to achieve fine tuning of the waveform.

The invention extends to the use of FT drive elements and the insertion of 0V scan frames into a single polarity drive element in other waveform structures. Other examples include, but are not limited to, where all of the optical rails are reached in the transition from one optical state to another (at least once in the case of a high number of prepulses) (triple-prepulse, quadruple-). Dual-prepulse slide show gray scale waveforms, including prepulse and the like, and other rail-stable gray scale waveforms. Also, FT sequences are used in normal image flow gray scale waveforms, where direct transitions are made between gray levels.

Insertion of zero voltage frames can be thought of as a specific example of the insertion of an FT sequence, where attention is paid to this particular case because the FT sequence is all zeros and has been found to be effective in the modification of the final optical states.

The foregoing has focused on the use of FT sequences to achieve fine tuning of gray levels. The use of such FT sequences to achieve DC balancing will be described below. FT sequences can be used to change the degree of DC imbalance in the waveform (preferably to reduce or eliminate DC imbalance). By DC balance, it is intended that all full-circuit gray level sequences (sequences starting and ending with the same gray level) have zero net voltage impulses. Using the fact that FT sequences can (a) change the optical state in the same way as saturation or set pulses, but have substantially different net voltage impulses, or (b) cause little change or net DC imbalance in the optical state. By using one or more FT sequences, the waveform can be DC balanced or less strongly DC unbalanced.

The following example shows how FT sequences can be used to achieve DC balancing. In this example, the set pulse is variable in length, i.e. can be one, two, three or more scan frames. The final gray level achieved for each number of scan frames is shown in FIG. 7, where the number next to each point represents the number of scan frames used to achieve the gray level.

FIG. 7 shows single polarity drive specifying the optical states available using scan frames of positive voltage and the number of single polarity frames whose numeric labels were used to produce the final gray level. In this example, it is assumed that a net voltage impulse of two positive voltage frames needs to be applied to maintain DC balance. The desired (target) gray level can be achieved by using three impulse scan frames, but in doing so, the system will remain DC unbalanced by one frame. On the other hand, DC balance can be achieved using two positive voltage scan frames instead of three, but the final optical state will be significantly off the target.

One way to achieve DC balancing is to drive the electro-optic medium near the desired gray level using two positive voltage frames, as indicated by the symbol in FIG. 8, and also to achieve a DC balanced FT sequence (zero FT sequence with net voltage) to make the final adjustment very close to its target gray level, where the target gray level is followed by an FT sequence of zero net voltage impulses selected to give an appropriate change in optical state. Is achieved using two scan frames.

Alternatively, one can use three positive voltage scan frames of single polarity drive to bring reflectivity to the target optical state, and then use a FT sequence with net negative DC imbalance equal to one negative voltage scan frame. . If you select an FT sequence that results in a substantially unchanged optical state, the final optical state will remain accurate and DC-balance will be restored. This example is shown in FIG. It will generally be appreciated that the use of FT sequences will involve some adjustment of the optical state with some effect on DC balancing, and the two examples above depict extreme cases.

The following example is given for illustrative purposes only and illustrates the experimental use of FT sequences in accordance with the present invention.

Example: periodic RSGS Within a waveform FT Sequence of  Use

This example illustrates the use of FT sequences to improve the optical performance of waveforms designed to achieve four gray level (2-bit) addressing of a single pixel display. This display used an encapsulated electrophoretic medium and was constructed substantially as described in the above paragraphs 2002/0180687. Single pixel displays were monitored by photodiodes.

To achieve a sequence of gray levels in a 2-bit (4-state) grayscale, waveform voltages were applied to that pixel according to a transition matrix (search table). As discussed above, a transition matrix or lookup table is a simple set of rules for applying a voltage to a pixel to transition from one gray level to another in the gray scale.

The waveform suffers from voltage and timing constraints. Only three voltage levels, -15V, 0V, + 15V, were applied to the pixel. In addition, to simulate active matrix driving at 50 Hz frame rate, voltages were applied in increments of 20 ms. In order to optimize the waveform, i.e., to achieve a condition in which the spread in the actual optical state of each of the four gray levels across the test sequence is minimized, adjustment of the algorithms is employed repeatedly.

In early experiments, a periodic rail-stabilized grayscale (cRSGS) waveform was optimized using simple saturation and set pulses. Consideration of previous states was limited to the initial (R ₂ ) and desired final (R ₁ ) gray levels in the determination of the transition matrix. The waveform was globally DC balanced. Very poor performance was expected from this waveform because of the coarseness of the minimum impulse available for adjustment, and the absence of states before R ₂ in the transition matrix.

The performance of the transition matrix was tested by switching the test pixel through a "pentad-complete" gray level sequence that included all gray level pented sequences in a random alignment. (The pented sequence element is a sequence of five gray levels, such as 0-1-0-2-3 and 2-1-3-0-3, where 0, 1, 2, and 3 are four grays available. Level.) For a perfect transition matrix, the reflectance of each of the four gray levels is exactly the same for all occurrences of the gray level in the random sequence. The reflectance of each of the gray levels will vary considerably for realistic transition matrices. The bar graph of FIG. 10 shows the poor performance of the voltage and timing constrained transition matrix. The reflectance measured for the various occurrences of each of the target gray levels is very variable. The optimized cRSGS waveform without the FT sequences developed in some of these experiments is hereinafter referred to as the base waveform.

The FT sequences were then merged into the cRSGS waveform, and in this experiment, the FT sequences were limited to five scan frames, and only DC balanced FT sequences were included. The FT sequences were located at the end of the base waveform for each transition, ie the waveform for each transition had one of the following forms:

set-N

sat-set-N

sat-sat'-set-N.

Successful merging of FT sequence elements into the waveform comprises two steps; First, identifying the effects of the various FT sequences on the optical state at each gray level, and second, selecting the FT sequences to add to the various waveform elements.

"FT effect" experiments were performed to confirm the effect of various FT sequences on the optical state of each gray level. First, a consistent starting point was established by repeatedly switching the electrophoretic medium between the black and white optical rails. Thereafter, the film is in one of four gray levels (0, 1, 2, 3) referred to herein as optical state R2. Thereafter, an appropriate base waveform is applied to cause the transition from R ₂ to one of the other gray levels (herein referred to as R ₁ ) with the added FT sequence. This step is repeated for all 51 DC balanced, five frame FT sequences. The final optical state is recorded for each of the FT sequences. After that, the FT sequences were aligned according to their associated final reflectance. This process was repeated for all combinations of initial (R ₂ ) and final (R ₁ ) gray levels. The alignment of the FT sequences for the final gray level 1 (R ₁ = 1) and the current gray levels 0, 2, 3 (R ₂ = 0, 2, 3) are shown in Tables 2 to 4, respectively, "Frame 1". Columns labeled “Frame 5” indicate the potential applied in volts during five consecutive frames of the associated FT sequence. Final optical states achieved for waveforms using various FT sequences are shown in FIG. 11. From this figure, there is no net voltage impulse difference, FT sequences can be used to influence large changes in the final optical state and the choice of five-scan-frame FT sequences gives fine control over the final optical state. It can be seen that.

Table 2 Final Optical State for Gray Levels 0 through 1 for Various FT Sequences

Table 3 Final Optical State for Gray Levels 2 to 1 for Various FT Sequences

Table 4 Final Optical State for Gray Levels 3 to 1 for Various FT Sequences

Next, the cRSGS waveforms are plotted for their final gray levels different from those shown in Tables 2-4 and 11 (specifically, Sequence 33 in Table 2, Sequence 49 in Table 3, and Sequence 4 in Table 4). It is constructed using FT sequences selected using analogs. The region between ˜36.9 and ˜37.5 L * on the y axis of FIG. 11 is between the optical reflectance of the same final (R ₁ ) state with different initial (R ₂ ) states made available using DC balanced FT sequences. Shows an overlap of. Thus, the target gray level for R ₁ = 1 was chosen at 37.2 L *, and the FT sequence for each R ₂ was selected that gives the final optical state close to this target. This process was repeated for the other final optical states (R ₁ = 0, 2, 3).

Finally, the resulting waveform was tested using a pseudo-random sequence containing all five deep state histories described above. This sequence contains 324 significant transitions. The cRSGS waveform modified by the selected FT sequence was used to achieve all transitions in this sequence, and the reflectance of each of the achieved optical states was recorded. The optical state achieved is shown in FIG. 12. By comparing FIG. 10 with FIG. 12, it can be seen that the spread of the reflectance of each of the gray levels is greatly reduced by the merging of FT sequences.

In summary, the discontinuous addressing aspect of the present invention provides for FT to allow (i) a change in optical state, (ii) means to achieve DC balance, or at least allow a change in the degree of DC imbalance of the waveform. Provide sequences. As we have already seen, it is possible, for example, to provide a somewhat mathematical definition of the FT sequence for the DC unbalanced variant of the method:

(a) Application of a DC unbalanced FT sequence that changes an optical state that is substantially different from the change in the optical state of the DC reference pulse. A “DC reference pulse” is a pulse of voltage V ₀ , where V ₀ has the same sign as the net impulse of the FT sequence but corresponds to the maximum voltage amplitude applied during the FT sequence. The net pulse of the sequence is the area under the voltage versus time curve, indicated by the symbol G. The duration of the reference pulse is T = G / V ₀ . This FT sequence is used to introduce a DC imbalance that is significantly different from the net DC imbalance of the reference pulse.

(b) Application of a DC unbalanced FT sequence that changes an optical state that is smaller in magnitude than the optical change achievable with a time reference pulse. A "time-reference pulse" is defined as a single-sign-voltage pulse of the same duration as the FT sequence, but the sign of the reference pulse is chosen to provide the maximum change in optical state. In other words, if the electro-optic medium is near its white state, the negative voltage pulse will drive the electro-optical medium slightly whiter, while the positive voltage will drive the electro-optical medium strongly towards the black. In this case, the sign of the reference pulse is positive. The goal of this type of FT pulse is to adjust the net voltage impulse without strongly affecting the optical state (eg for DC balancing).

The discontinuous addressing aspect of the present invention relates to the concept of using one or more FT sequences that are also inserted between or within pulse elements of the transition waveform, and against the influence of previous gray levels and previous dwell times. It relates to the concept of using FT sequences to balance. One particular example of the invention is the use of zero voltage frames inserted in the center of a pulse element of the waveform or between pulse elements of the waveform to change the final optical state.

The discontinuous addressing aspect of the present invention is also to allow fine tuning of the waveforms to achieve the desired gray levels with the desired precision, especially with only two or three voltage levels using source drivers that do not allow fine tuning of the voltage. By using source drivers with a, it is to achieve means by which the waveform can approach DC balance (ie, zero net voltage impulse during any periodic excursion for various gray levels).

DC  Balanced Addressing  Way

The tooth drive scheme shown in Figs. 11A and 11B of WO 03/044765 described above is well suited for DC balancing applications, in which only a limited number of transitions are required to sequence any given pixel through the black state. It ensures that it can pass between multiple passes, and indeed that the average pixel will pass the black state at half of its transitions.

However, as already explained, the DC balance according to the present invention is not limited to balancing a group of pulses applied to the electro-optic medium during the duration of the transitions, and the pixels of the display according to the DC balanced addressing method of the present invention. At least some of the transitions experienced by these devices are extended to "internally" DC balancing, which method is described in detail below.

The DC balanced addressing method of the present invention relates to DC balanced transitions that are advantageous for driving encapsulated electrophoretic media and other pulse-driven electro-optic media for display applications. This method can be applied, for example, to an active-matrix display with source drivers capable of outputting only two to three voltages. Although other types of drivers may be used, most of the detailed description below will focus on examples using source drivers with three (positive, negative, zero) voltage outputs.

In the following description of the DC balanced addressing method of the present invention, as described in the previous other aspects of the present invention, the gray level of the electro-optic medium will be represented by 1 to N, where 1 is the darkest. The status is displayed and N is the brightest status. The intermediate states increase in number from dark to bright. The drive scheme for impulse drive imaging media utilizes a set of rules for achieving transitions from the initial gray level to the final gray level. The driving scheme can be expressed as a function of time for each transition, as shown in Table 5 for each of the 16 possible transitions in a 2-bit (4 gray level) gray scale display.

TABLE 5

In Table 5, Vij (t) represents the waveform used to make the transition from gray level i to gray level j. DC-balanced transitions are those in which the time integration of the waveform Vij (t) is zero.

The term "optical rails" has already been described above as referring to the extreme optical states of an electro-optic medium. The phrase "pushing a medium towards or into optical rails" will be employed below. "Toward" means that a voltage is applied to move the optical state of the medium towards one of the optical rails. "Pushing" means that the voltage pulse is of sufficient duration and amplitude that the optical state of the electro-optic medium is substantially close to one of the optical rails. While "pushing into optical rails" does not mean that the optical rail state is necessarily achieved at the end of the pulse, it is important to note that substantially the optical state close to the final optical state is achieved at the end of the pulse. . For example, consider an electro-optic medium with optical rails at 1% and 50% reflectance. A 300 msec pulse is found to bring the final optical state to 50% reflectivity (from 1% reflectance). A 200 msec pulse can be referred to as pushing the display to a high-reflectance optical rail even though only a final reflectance of 45% is achieved. This 200 msec pulse is thought to be a pulse when pushing the medium onto one of the optical rails, which lasts 200 msec compared to the time required to traverse a large fraction of the optical range, such as 1/3 to 2/3 of the optical range. This is because the time is long (in this case 200 msec is longer compared to the pulses needed to make the medium from 1/3 to 2/3 of the reflectance range, here 17% to 34% reflectance).

Three different types of DC balanced transitions in accordance with the DC balanced addressing method of the present invention are described below, together with a hybrid drive scheme using both DC balanced transitions and DC unbalanced transitions. In the following description, for convenience, the pulses are represented by numbers, and the magnitude of the numbers indicates the duration of the pulses. If the number is positive, the pulse is positive; if the number is negative, the pulse is negative. Thus, for example, if the available voltage is + 15V, 0V, -15V, and the duration of the pulse is measured in milliseconds (msec), the pulse is specified as x = 300, 300msec, 15V X = -60 represents 60 msec, -15V.

1st type:

In the first and simplest type of DC balanced transition of the present invention, the voltage pulse ("x") follows the same length but opposite sign ("-x"), as shown in FIG. . (x may itself be negative, so the positive and negative pulses may appear in the opposite order to that shown in FIG. 13).

As mentioned above, the effect of the waveform used to affect the transition is dependent on the duration of the zero voltage (actually the time delay) during or before any pulses in the waveform, in accordance with the discrete addressing method of the present invention. Can be modified by presence. 14 and 15 show variations of the waveform of FIG. 13. In FIG. 14, the time delay is inserted between the two pulses of FIG. 13, the time delay in FIG. 15 is inserted into the second pulse of FIG. 13 in the same amount, and the second pulse of FIG. 13 is separated by the time delay. Divided into two separate pulses. As already described, time delays can be merged into a waveform to achieve optical states that cannot be achieved without such delays. Time delay may be used for the fine-tuned final optical state. This fine tuning capability is important because in active matrix driving, the time resolution of each pulse is determined by the scan rate of the display. The temporal resolution provided by the scan rate can be so coarse that a precise optical state cannot be achieved without some additional means of fine tuning. The time delays provide a small degree of fine tuning of the final optical state, and additional properties such as described below provide additional means of approximation and fine adjustment of the final optical state.

2nd type:

The type II waveform is the insertion of positive and negative pulse pairs (indicated by "y" and "-y") at some point within the type I waveform described above, as shown as symbols in FIG. It consists of. The y and -y pulses need not be continuous and may be provided at different locations within the original waveform. There are two special beneficial forms of type II.

Type II: Special Case A:

In this particular form, the "-y, y" pulse pairs precede the "-x, x" pulse pairs. As shown in Fig. 17, when y and x are opposite signs, the final optical state can be finely adjusted even by slight approximation of the duration y. Thus, the value of x can be adjusted for y values for approximation control and fine adjustment of the final optical state of the electro-optic medium. This is expected to occur because the y pulse increases the -x pulse and thus changes the degree to which the electro-optic medium is pushed into one of its optical rails. The degree of pushing to one of the optical rails is known to provide fine tuning of the final optical state after the pulse from that optical rail (in this case provided by the x pulse).

Type II: Special Case B:

For the reasons mentioned above, it has been found advantageous to use waveforms with one or more pulse elements long enough to drive the electro-optical medium with substantially one optical rail. In addition, for a more visually satisfactory transition, it is desirable to reach the final optical state from closer to the optical rail, since reaching a gray level close to the optical rail requires only a short final pulse. Waveforms of this type require one or more pulses long to drive with an optical rail and short pulses to achieve a final state close to that optical rail, and thus cannot have the type I structure described above. However, a special case of the type II waveform can achieve this type of waveform. 18 shows an example of such a waveform, where y pulses lie after -x, x pulse pairs and -y pulses lie before -x, x pulse pairs. In this type of waveform, the final y pulse provides approximation because the final optical state is very sensitive to the magnitude of y. The x pulse provides fine adjustment because the final optical state does not typically depend strongly on the magnitude of the drive into the optical rail.

3rd type:

The third type (DC type III) of the DC balanced waveform of the present invention is another DC balanced pulse pair (denoted by " z ", " -z "), as schematically shown in FIG. Introduce into the waveform. A preferred example of the type III waveform is shown in FIG. 20, which is useful for fine tuning of the final optical state for the following reasons. Assume that there are no z and -z pulses (ie, type II described above). The x pulse element is used for fine adjustment, and the final optical state can be decreased with x increase and can be increased with x decrease. However, it is undesirable to reduce x beyond a certain point because the electro-optic medium is not brought close enough to the optical rail as required for the stability of its waveform. To avoid this problem, instead of reducing x, it is possible to increase the -x pulse (actually) without changing the x pulse by adding -z and z pulse pairs, as shown in FIG. 20, where z Is the opposite of x. The z pulse increases the -x pulse and the -z pulse maintains the transition to zero net impulse, ie, the DC balanced transition.

The waveforms of the above-described I, II, and III types can of course be modified in various ways. Additional pulse pairs can be added to the waveform to achieve a more general structure. The advantage of such additional pairs decreases with increasing number of pulse elements, but such waveforms are a natural extension of waveforms of type I, II, III. In addition, as already described above, one or more time delays may be inserted at various locations within arbitrary waveforms, in the same manner as shown in FIGS. 14 and 15. As already mentioned, the time delay in the pulses affects the final optical state achieved and is useful for fine tuning. In addition, the placement of the time delays can change the visual appearance of the transitions by changing the position of the transition elements with respect to the transition elements of other transitions as well as for other elements of the same transition. In addition, time delay may be used to align certain waveform transition elements, which may be useful in certain display modules having constant controller capabilities. Also, upon recognizing that a small change in the alignment of the applied pulses can substantially change the optical state of the following pulses, the output signal may be transposed by transposing all or part of one of the pulse sequences described above. Or by repetitively transposing the order of all or part of one of the foregoing sequences, or by inserting one or more periods of 0V at any position of one of the foregoing sequences. In addition, these transpose and insert operations can be combined in any order (eg, transpose and 0V insertion after 0V insertion). It is important to note that all these pulse sequences formed by these modifications retain the inherent nature of having zero net impulses.

Finally, DC balanced transitions can be combined with DC unbalanced transitions to form a complete drive scheme. For example, co-pending application 60 / 481,053, filed on July 2, 2003, discloses a preferred waveform of type -TM (R1, R2) [IP (R1) -IP (R2)] TM (R1, R2). Describe. Here, [IP (R1) -IP (R2)] represents the pulse potential difference between the final and initial states of the transition under consideration, and the two remaining terms represent a pair of DC balanced pulses. For convenience, this waveform is hereinafter referred to as -x / ΔIP / x waveform and is shown in FIG. The transition is satisfactory between different optical states, while the waveform is less satisfactory for zero transitions with the same initial and final optical states. For these zero transitions, in this example, waveforms of type II such as those shown in FIGS. 17 and 18 were used. This complete waveform is shown as a symbol in Table 6, where it can be seen that the -x / ΔIP / x waveform was used for non-zero transitions and the Type II waveform was used for zero transitions.

TABLE 6

The DC balanced addressing method is of course not limited to this type of transition matrices, where the DC balanced transitions are limited to "leading daigonal" transitions, to maximize the improvement of the control of gray levels. For this purpose, the initial and final gray levels are the same, and it is desirable to maximize the number of DC balanced transitions. However, depending on the particular electro-optic medium in use, for example, from transitions to extreme gray levels or from extreme gray levels, such as transitions from black and white to gray levels 1 and 4, or transitions from black and white, respectively, DC balanced transitions involving transitions of may be difficult. Also, in selecting transitions to be DC balanced, it is important not to unbalance the entire transition matrix, i.e., a closed loop starting and ending at the same gray level does not produce a transition matrix that is DC unbalanced. For example, it is undesirable to specify that transitions containing only 0 or 1 units of change in the gray level are DC balanced but other transitions are DC imbalance, as shown in the following example, Because it can be imbalanced; A pixel undergoing a sequence of gray levels 2-4-3-2 will experience 2-4 (DC imbalance), 4-3 (balance) and 3-2 (balance) transitions and thus the entire loop will be unbalanced. The real compromise between these two contradictory requirements is to use DC balanced transitions when only intermediate gray levels (levels 2 and 3) are included and transitions start at extreme gray levels (levels 1 or 4). Or terminating may be using DC imbalanced transitions. The intermediate gray levels selected for such a definition may vary depending on the particular electro-optic medium and controller used; For example, in a 3-bit (8 gray level) display, use DC balanced transitions in all transitions starting or ending at gray level 2-7 (or 3-6), and using gray level 1 and 8 ( Or it is possible to use DC imbalanced transitions in all transitions starting or ending in 1, 2, 7 and 8).

From the foregoing, the DC balanced addressing method of the present invention allows fine tuning of waveforms to achieve the desired gray levels, even at high precision, with a source of waveform transitions, in particular only two or three voltage levels. It can be seen that using a source driver that does not allow fine tuning of the voltage, such as drivers, allows the means to have a zero net voltage. It should be understood that DC balanced waveforms provide better performance than DC unbalanced waveforms. The present invention generally applies to displays and not particularly exclusively, but to an active-matrix display module having source drivers providing only two or three voltages. The invention also applies to active-matrix display modules with source drivers that provide more voltage levels.

The DC balanced addressing method of the present invention may provide certain additional advantages. As mentioned above, in some driving methods of the present invention, the transition matrix is a function of variables other than the previous optical state, for example the length of time since the last update or the temperature of the display medium. In the case of unbalanced transitions, it is very difficult to maintain DC balance. For example, consider a display that repeatedly transitions from white to black at 25 ° C and then from black to white at 0 ° C. Slower responses at lower temperatures will typically exhibit longer pulse lengths. As a result, the display will experience net DC imbalance towards white. On the other hand, if all transitions are internally balanced, different transition matrices can be mixed freely without introducing DC imbalance.

Regulated Area Method

As mentioned above, objectionable effects on reset steps can be further reduced by using local update rather than global update, i.e. by rewriting only portions of the display that vary between successive images. The portion to be rewritten can be selected as a "local area" or per individual pixel. For example, finding a series of images in which relatively small objects move across larger still backgrounds, such as in a diagram illustrating parts in the mechanisms or diagrams used for fault reconstruction. It's not rare. In order to use local update, the display controller needs to compare the final image with the initial image and determine whether the area (s) are different between the two images and thus whether rewriting is necessary. The controller can identify one or more local regions, typically rectangular regions with axes aligned by a pixel grid containing pixels that need to be updated, or simply identify individual pixels that need to be updated. Any of the driving schemes already described can be applied to update only the local regions or thus to update individual pixels identified as needing rewriting. Such a local update scheme substantially reduces the energy consumption of the display.

In addition, as already described, the defined area method of the present invention provides a defined area method that permits updating a bistable electro-optic display using different update methods in different areas of the display.

Electro-optic displays are known in which the entire display can be driven in one-bit mode or in grayscale mode. When the display is in one-bit mode, the update is affected using one-bit general image flow (GIF) waveforms, while when the display is in grayscale mode, only one-bit information within a specific area of the display. Even if only the update is in progress, the update is effected using a multi-prepulse slide show waveform or other slow waveform.

As such, the electro-optical display can be modified to perform the defined area method of the present invention by defining two additional commands in the controller, a "DEFINE REGION" command and a "CLEAR ALL REGIONS" command. The DEFINE REGION command takes a position sufficient to completely define a rectangular region of the display, for example, the position of the upper right and lower left corners of the defined region as an argument; In addition, this command may have an additional independent variable specifying the bit depth at which the prescribed area is set, even if this last independent variable is not needed in a simple form of the defined area method where the specified area is always monochrome. It can have Of course, the bit depth set by the last independent variable overrides any bit depth previously set for the specified area. Alternatively, the DEFINE REGION command can specify a series of points that define the vertices of the polygon. The CLEAR ALL REGIONS command has no independent variables, simply resets the entire display to a single predetermined bit depth, or specifies a single independent variable that specifies the various possible bit depths that should be adopted by the entire display after a clearing operation. You can also take

It is to be understood that the defined area method of the present invention is not limited to using only two areas, and more areas can be provided if desired. For example, in an image editing program, a main area representing an image to be edited at full bit depth, an information display area (for example, a box indicating the current cursor position), and a dialog box operating in one-bit mode (user It may be advantageous to have all of these areas (providing a dialog box for entry of text). The present invention will be mainly described in the following two region versions, since the modifications necessary to enable the use of two or more regions are apparent to those skilled in the art of the configuration of the display controller.

To store depths of different regions, the controller can maintain an array of storage elements, with one element associated with each pixel in the display, each element representing a current bit depth for that associated pixel. Save it. For example, an XVGA (800 x 600) display that can operate in either 1-bit or 2-bit mode has an 800 x 600 array of 1-bit elements (including 0 for 1-bit mode and 1 for 2-bit mode, respectively). Can be used. In such a controller, the DEFINE REGION command can set those elements in the defined area of the display to the requested bit depth, and the CLEAR ALL REGIONS command sets all elements of the array to the same value (determined by a predetermined value or an independent variable of the command). One).

Optionally, if an area is defined or cleared, the controller transfers the display from one mode to another to ensure DC balance or adjust the optical states of the pixels involved, for example, by using an FT sequence as described above. In order to do this, an update sequence can be executed for the pixels in the region.

If the display is operating in defined area mode, a new image is sent to the controller, the display must be redrawn, and there are three possible cases:

1. Only pixels within a defined one-bit region are changed. In this case, a one-bit waveform can be used to update the display;

2. Only pixels in non-limiting (grayscale) regions are changed. In this case, a grayscale waveform should be used to update the display (of course, since the pixels do not change within the defined area, the readability of the defined area such as, for example, the dialog box during rewriting ( legibility does not matter); And

3. Pixels in defined and undefined areas are changed. In this case the grayscale pixels are updated using the grayscale waveform, and the one-bit pixels are updated using the one-bit waveform (shorter one-bit waveforms must be zero-padded to match the length of the grayscale update. (zero-padded)).

The controller may determine whether any of these cases exist by performing the following logic test before scanning the display (as described above, assuming a one-bit value associated with a pixel and storing that pixel mode). do):

(Old_image XOR new_image)> 0: pixels are changed in the display

(Old_image XOR new_image) AND mode_arrary> 0: grayscale pixels are changed

(Old_image XOR new_image) AND (NOT mode_array)> 0: monochrome pixels are changed

Because the controller scans the display, you can use one waveform lookup table for all pixels for case 1 or case 2, because unchanged pixels receive 0V, The transition is assumed to be the same as in grayscale mode (ie, both waveforms are updated locally). Instead, if the grayscale waveform is global-updated (every pixel is updated every time the display is updated), the controller performs a test to see if the pixel is in the proper area to determine whether to apply the global-update waveform. Will be needed. In case 3, the controller should check the value of the mode bit array for each pixel when scanning to determine which waveform should be used.

Optionally, if the lightness values of the black and white states achieved in one-bit mode are the same as the brightness values achieved in grayscale mode, then in case 3 the grayscale waveform is used for all the pixels in the display. And thus eliminates the need for transition functions between one-bit and grayscale waveforms.

The defined region method of the present invention may utilize any optional features of the basic lookup table method, as described above.

A major advantage of the defined area method of the present invention is that it enables the use of fast one-bit waveforms in displays displaying previously written grayscale images. Conventional display controllers typically only allow the displays to be in either grayscale or one-bit mode at any moment. While it is possible to write a one-bit image in grayscale mode, the associated waveforms are very slow. In addition, the defined area method of the present invention is evident for a host system (typically a computer) that is essentially providing images to the controller, since the host system does not need to instruct the controller which waveform should be used. . After all, the defined region method allows both one-bit and grayscale waveforms to be used simultaneously in the display, while other solutions require two separate update events if two kinds of waveforms are used.

more Description of Common Waveforms

The aforementioned driving scheme can be varied in various ways depending on the characteristics of the particular electro-optic display used. For example, in certain cases, it may be possible to eliminate multiple reset steps in the driving schemes described above. For example, the electro-optic medium used is bistable for a long time (i.e., the gray level of a written pixel changes very slowly), and the impulse required for a particular transition rapidly during the time the pixel is in its initial gray state. If not changed, the look-up table is grayed out after a significant period of time, resetting the affected display only if a gradual "drift" of pixels from its nominal gray levels causes significant errors in the provided image. The gray state transition can be configured to effect a direct return to the black or white state without any intervention. Thus, for example, if the user is using the display of the present invention as an electronic book reader, it may be possible to display multiple screens of information before resetting of the display is necessary; Experimentally, with the appropriate waveforms and drivers, about 1000 screens of information can be displayed before resetting is needed, and in practice no resetting is necessary during the normal reading time of the e-book reader.

It will be apparent to those skilled in the display art that a single device of the present invention may be usefully provided with a plurality of different driving schemes for use in various conditions. For example, in the driving scheme shown in FIGS. 9 and 10 of WO 03/044765 described above, since reset pulses consume a significant portion of the total energy consumption of the display, the controller frequently resets the display to minimize gray scale errors. A first driving scheme and a second scheme of resetting the display at longer period intervals to allow larger gray scale errors but reducing energy consumption can be provided. Switching between the two approaches can be effected manually or depending on external parameters; For example, where a display is used in a laptop computer, the first drive scheme may be used when the computer is operating at mains electricity and the second drive may be used when the computer is operating at internal battery power. Can be.

Compensation voltage method

Another variation on the basic lookup table method and apparatus of the present invention is provided by the compensation voltage method of the present invention and will be described in detail below.

As mentioned above, the compensating voltage method and apparatus of the present invention seeks to achieve results similar to the basic search table methods described above without the need to store very large lookup tables. The size of the lookup table grows very rapidly depending on the number of previous states associated with the lookup table being indexed. For this reason, as discussed above, there are practical limitations and cost considerations for increasing the number of previous states used to select pulses to achieve a desired transition in a bistable electro-optical display.

In the compensation voltage method and apparatus of the present invention, the size of the lookup table required is reduced, and the compensation voltage data is stored for each pixel of the display, the compensation voltage data being in accordance with one or more impulses previously applied to the relevant pixel. Is calculated. The voltage applied to the pixel, in turn, is the sum of the drive voltage selected from the lookup table in the usual manner and the compensation voltage determined from the compensation voltage data for the associated pixel. Indeed, the compensation voltage data applies a "correction" to the pixel, such as that applied differently by indexing the lookup table for one or more additional previous states.

The lookup table used in the compensation voltage method can be any of the types described above. Thus the lookup table can be a simple two-dimensional table that is only allowed for the initial and final states of the pixel during the associated transition. Alternatively, the lookup table may take into account one or more temporary prior states and / or gray level prior states. The compensation voltage may only consider compensation voltage data stored in the associated pixel, but may optionally also consider one or more transient previous states and / or gray level prior states. The compensation voltage can be applied to the associated pixel not only during the period during which the drive voltage is applied to that pixel, but also during the so-called "hold" states in which no drive voltage is being applied to that pixel.

The exact manner in which the compensation voltage data is determined can vary widely with the nature of the bistable electro-optic medium used. Typically, the compensation voltage data will change periodically in a manner determined by the drive voltage applied to the pixel during the current and / or one or more scan frames. In a preferred form of the invention, the compensation voltage data consists of a single number of (register) values associated with each pixel of the display.

In a preferred embodiment of the invention, the scan frames are grouped into superframes in the manner described above so that the update of the display can only be started at the beginning of the superframe. A superframe consists of 10 display scan frames, for example, with a 50Hz scan rate for one display, the display scan is 20ms long and the superframe is 200ms long. The display is rewritten during each superframe, and the compensation voltage data associated with each pixel is updated. The update consists of two parts in the following order:

(1) changing the previous value using a constant algorithm independent of pulses applied during the associated superframe; And

(2) increasing the value from step (1) by an amount determined by the impulse applied during the relevant superframe.

In a particularly preferred embodiment steps (1) and (2) are carried out as follows:

(1) dividing the previous value by a constant, conveniently 2; And

(2) increasing the value from step (1) by a value proportional to the total area under the voltage / time curve applied to the electro-optical medium during the relevant superframe.

In step (2), the increase may be exactly or only approximately proportional to the area under the voltage / time curve during the relevant superframe. For example, as described below with reference to FIG. 22, the increase may be “quantized” to a limited set of classes for all possible applied waveforms, with each class being between two bounds. Includes all waveforms having a total area in, and the increase in step (2) is determined by the class to which the applied waveform belongs.

The following example will be described. The display used is a 2-bit gray scale encapsulated electrophoretic display, and the driving method employed used a two-dimensional lookup table as shown in Table 7 below, which only considered the initial state and the final state of the desired transition; In this table, the top of the column represents the desired final state of the display, the beginning of the row represents the initial state, and the number in the individual cells represents the voltage applied to the pixel over a period of time in volts.

TABLE 7

In order to allow the implementation of the compensating voltage method of the present invention, a single numeric register is associated with each pixel of the display. The various impulses shown in Table 7 are classified and the pulse class is associated with each impulse as shown in Table 8 below.

TABLE 8

During each superframe, the number register associated with each pixel is divided by two, and then incremented by the number values shown in Table 13 for pulses applied to the relevant pixels during the same superframe. The voltage applied to each pixel during the superframe is the sum of the drive voltage as shown in Table 12 and the compensation voltage Vcomp given by:

V _comp = A * (pixel register)

Here the pixel register value is read from the register associated with the associated pixel, where "A" is a predetermined constant.

In the laboratory experiments of this preferred compensation voltage method of the present invention, a single pixel display using encapsulated electrophoretic medium whose front is formed of ITO and switched between light transmissive parallel electrodes is between its black and white states. Driven by 300 msec +/- 15 V square wave pulses. The display started in its white state is driven black and returns to white after a certain dwell time. As shown in FIG. 22 of the accompanying drawings, it was found that the brightness of the final white state is a function of the retention state. Thus, this encapsulated electrophoretic medium is sensitive to residence time, and L * in the white state changes by about 3 units with residence time.

To demonstrate the effect of the compensation voltage method of the present invention, the experiment was repeated except that a compensation voltage consisting of an exponentially decaying voltage starting at the end of each drive pulse was added to each pulse. The applied voltage was the sum of the drive voltage and the compensation voltage. As shown in FIG. 22, the white state for the various residence times in the case with the compensation voltage was more uniform than for the uncompensated pulses. Thus, this experiment demonstrated that the use of such compensation pulses according to the present invention can significantly reduce the residence time sensitivity of encapsulated electrophoretic media.

The compensation voltage method of the present invention may utilize any optional characteristics of the basic lookup table method described above.

From the foregoing description, it can be seen that the present invention provides a method for controlling the operation of an electro-optical display, which applies well to bistable particle-based electrophoretic displays or similar displays.

From the foregoing description, the present invention provides a method for controlling the operation of an electro-optical display, which often allows precise control of gray scale without the need to uncomfortably flash the entire display in one of its extreme states. It can be seen that. In addition, the present invention allows accurate control of the display despite changes in its temperature and operating time, while lowering the power consumption of the display. Since the controller can be composed of commercially available components, these advantages are achievable at low cost.

DTD  Integral Reduction Method

As described above, it has been found that, at least in certain cases, the impulse required for a given transition in a bistable electro-optic display varies with the residence time of a pixel in its optical state, as previously described in the report. This invisible phenomenon is hereinafter referred to as "stay time dependent" or "DTD". Thus, it is desirable to change the applied pulse for a given transition as a function of the residence time of the pixel in its initial optical state and in some cases is actually necessary.

The dwell time dependence phenomenon will be described in detail below with reference to FIG. 23 of the accompanying drawings, showing that the reflectance of the pixel is a function of time for the sequence of transitions represented by R ₃ → R ₂ → R ₁ , where each R _k term represents the gray level in the sequence of grail levels, where R of a large index value occurs before R of a small index value. R ₃ and R ₂ Transition between R ₂ And R ₁ Degree of transition between Indicated. DTD is the change in the final optical state R ₁ caused by the change in time spent in the optical state R ₂ called retention time. The DTD integration reduction method of the present invention provides a method for reducing the dependency on duration when driving bistable electro-optic displays.

The invention is not limited by any theory, but the DTD appears to be caused in large part by the residual electric field experienced by the electro-optic medium. These residual electric fields are the rest of the drive pulses applied to the medium. It is common to refer to residual voltages resulting from applied pulses, and the residual voltage is a scalar potential that corresponds to the residual electric field in a general manner appropriate to electrostatic theory. These residual voltages can cause the optical state of the display film to drift over time. In addition, the residual voltages change the efficiency of the subsequent drive voltage and thus change the final optical state achieved after that subsequent pulse. In this way, the residual voltage from one transition waveform can cause the final state after the subsequent waveform to be different from what is expected to be, if the two transitions are very separated from each other. "Very separated" means that the voltage is sufficiently separated so that the residual voltage from the first transition waveform is substantially reduced before the second transition waveform is applied.

Measurement of residual voltages resulting from the transition waveform and other simple pulses applied to the electro-optic medium indicate that the residual voltage decreases with time. The decrease in residual voltage appears monotonic, but is not simply exponential. However, as a first approximation, the reduction in residual voltage can be approximated exponentially with respect to the reduction time constant, with a similar reduction for most tested encapsulated electrophoretic media and other bistable electro-optic media in units of one second. It is expected to display the time.

Thus, the DTD integration reduction method of the present invention provides a method of driving a bistable electro-optic display having one or more pixels, comprising applying a waveform V (t) to the pixels,

end,

Where T is the length of the waveform, the integral is calculated over the duration of the waveform, V (t) is the waveform voltage as a function of time t, and M (t) is the dwell time rising from a short pulse at time 0 Memory function characterized by a reduction in the efficiency of the residual voltage to introduce the dependence). Preferably J is less than about 0.5 V sec, most preferably less than about 0.1 V sec. In fact, J should be constructed as small as possible, ideally zero.

The waveforms can be designed to produce very small J values and thus very small DTDs by generating compound pulses. For example, a long negative voltage that precedes a short positive pulse (with the same magnitude of voltage amplitude but with the opposite sign) can result in a very reduced DTD. Two pulses are known to provide residual voltages with opposite signs (though the present invention is not limited to this fact). If the ratio of the lengths of the two pulses is set correctly, residual voltages from the two pulses can be caused to greatly cancel each other. The proper ratio of the length of the two pulses can be determined by the memory function for the residual voltage.

In a preferred embodiment of the invention J is calculated as follows:

Where τ is the experimentally determined decay (relaxation) time.

For encapsulated electrophoretic media, it has also been found experimentally that waveforms that result in an increase in small J values result in an increase in particularly low DTD values, in particular waveforms that have a large J value result in large DTD increases. . In fact, a good correlation is found between the J values calculated by Equation (2) with τ set to 1 second, which is approximately equal to the measured decay time of the residual voltage after the applied voltage pulse.

Thus, each transition from one gray level to another gray level (or at least most of the transitions in the lookup table) is described in the above-described patent and application with waveforms achieved by a waveform that provides a small J value. It is advantageous to apply the method. This J value is preferably zero, but for at least encapsulated electrophoretic media of the aforementioned patents and applications, it has been found experimentally that the resulting residence time dependence is very small if the J value at room temperature is less than about 1 v sec.

Thus, the present invention provides a waveform for achieving transitions between a set of optical states, where for every transition the calculated value of J is small. Perhaps J is computed by a memory function that monotonically decreases. This memory function is not arbitrary but can be eliminated by observing the residence time dependence of the display film for simple voltage pulses or complex voltage pulses. By way of example, a voltage pulse can be applied to the display film to achieve a transition from the first optical state to the second optical state, wait for the residence time, and then transition from the second voltage pulse to the third voltage pulse. A second voltage pulse can be applied to achieve By monitoring the shift in the third optical state as a function of the residence time, an approximate form of the memory function can be determined. The memory function is a function of the residence time and has a form approximately similar to the difference in the third optical state from its value for the long residence time. Thereafter, the memory function is provided with this form, and when its independent variable is zero, it will have a single amplitude. This method only provides an approximation of the memory function, and for various final optical states, the measured form of the memory function is expected to change somewhat. However, the overall characteristics, such as the characteristic time of the attenuation of the memory function, should be similar for the various optical states. However, if there is a very large difference in shape and final optical state, the best memory function shape to employ is that obtained when the third optical state is in the middle third of the optical range of the display medium. The overall characteristic of the memory function may be evaluated by measuring the attenuation of the residual voltage after an applied voltage pulse.

Although the methods described herein for evaluating the memory function are not accurate, it has been found that even J values calculated from approximate memory are excellent guides for waveforms with low DTD values. Useful memory functions represent the overall nature of the time dependence of the DTD described above. For example, an exponential memory function with a decay time of 1 second has been found to work well in the prediction of waveforms that provide low DTD. Changing the decay time from 0.7 to 1.3 seconds does not destroy the efficiency of the resulting J value as a predictor of low DTD waveforms. However, memory functions that do not attenuate but remain uniform indefinitely are less useful as predictors, and memory functions with very short decay times such as 0.05 seconds are not good predictors of low DTD waveforms.

An example of a waveform that provides a small J value is the waveform shown in FIGS. 19 and 20 described above, where the x, y, z pulses are all of duration less than the characteristic decay time of the memory function. This waveform works well when this condition is met, because it consists of successive opposing pulse elements that tend to decay the residual voltages approximately. For x and y values that are not less than the characteristic decay time of the memory function, but not greater than this decay time, waveforms where x and y are opposite signs tend to give a low J value, and x and y pulse durations The times can be found to allow very small J values, since the various pulse elements provide residual voltages that cancel each other or at least greatly cancel each other after the waveform is applied.

The J value of a given waveform can be adjusted by inserting a period of zero voltage into the waveform or by adjusting the length of any period of zero voltage already provided to the waveform. Thus, various variations of the waveform can be used while maintaining a J value close to zero.

The DTD integration reduction method of the present invention has general adaptability. The waveform structure can be thought of as represented by the parameters, their J value calculated for the various values of these parameters, and the appropriate parameter values selected to minimize the J value to reduce the DTD of the waveform.

Claims (50)

A driving method of a bistable electro-optical display having a plurality of pixels, each of which can display three or more gray levels, Storing a lookup table comprising data representing impulses required to convert the initial gray level to a final gray level; Storing data representing at least an initial state of each pixel of the display; Receiving an input signal indicative of a desired final state of one or more pixels of the display; And Generating an output signal indicative of a pixel voltage to be applied to said one pixel, Stores compensation voltage data indicative of a compensation voltage for each pixel of the display, wherein the compensation voltage for any pixel is calculated according to one or more impulses previously applied to that pixel, And the pixel voltage is a sum of initial and final states of a pixel, a driving voltage determined from the lookup table, and a compensation voltage determined from compensation voltage data for the pixel. The method of claim 1, And the compensation voltage for each pixel is calculated according to one or more temporal prior states of the pixels and the gray level prior states of the pixels. The method of claim 1, The compensation voltage for each pixel is applied to the pixel during a period during which a drive voltage is being applied to the pixel and during a hold period during which no drive voltage is being applied to the pixel. Driving method. The method of claim 1, And the compensation voltage for each pixel is updated during each superframe necessary for complete addressing of the display. The method of claim 4, wherein The compensation voltage for each pixel may include (1) changing a previous value of the compensation voltage using a constant algorithm independent of the pulses applied during the associated superframe; And (2) increasing the value from step (1) by an amount determined by a pulse applied during the associated superframe. The method of claim 5, The compensation voltage for each pixel comprises: (1) dividing a previous value of the compensation voltage by a constant constant; And (2) increasing the value from step (1) by an amount substantially proportional to the total area under the voltage / time curve applied to the electro-optic medium during the associated superframe. How to drive the display. The method of claim 1, And the compensation voltage is applied in the form of an exponentially decaying voltage applied at the end of one or more drive pulses. Stores both a lookup table comprising data representing the impulses required to convert the initial gray level to a final gray level, and at least data representing the initial state of each pixel of the display, and compensation voltage data for each pixel of the display. Storage means configured to; Input means for receiving an input signal indicative of a desired final state of at least one pixel of said display; Determining a driving voltage required to change the initial state of the one pixel to a desired final state from the input signal, the data representing the stored initial state of the pixel, and a lookup table; and further, the compensation voltage data for the pixel. Calculation means for determining a compensation voltage for the pixel from the sum and adding the driving voltage and the compensation voltage to determine a pixel voltage; And Output means for generating an output signal indicative of said pixel voltage. The method of claim 8, And said calculating means is arranged to determine said compensation voltage in accordance with at least one of a temporary prior state of said pixel and a gray level prior state of said pixel. The method of claim 8, And the output means is configured to apply the compensation voltage to the pixel during a period during which a drive voltage is being applied to the pixel and during a hold period during which no drive voltage is being applied to the pixel. The method of claim 8, The calculating means is configured to update the compensation voltage for each pixel during each superframe necessary for complete addressing of the display. The method of claim 11, The calculating means comprises the steps of: (1) changing the previous value of the compensation voltage using a constant algorithm independent of the pulses applied during the associated superframe; And (2) updating the compensation voltage for each pixel by increasing the value from step (1) by an amount determined by a pulse applied during the associated superframe. The method of claim 12, The calculating means comprises: (1) dividing a previous value of the compensation voltage by a constant constant; And (2) increasing the value from step (1) by an amount substantially proportional to the total area under the voltage / time curve applied to the electro-optic medium during the associated superframe. The device controller configured to update the voltage. The method of claim 8, The output means is adapted to apply the compensation voltage in the form of an exponentially decaying voltage applied at the end of one or more drive pulses. A plurality of pixels arranged in a plurality of rows and columns so that each pixel is uniquely defined by an intersection of a specific row and a specific column, and driving means for applying an electric field independently of each pixel to change the display state of the pixels; An update method of a bistable electro-optic display having Each pixel has three or more different display states, The method, Storing area data representing a defined area that includes a portion, but not all, of the display; Determining for each pixel whether the pixel is inside or outside the prescribed area; And Applying a first driving scheme to the pixels in the prescribed region, and applying a second driving scheme different from the first driving scheme to the pixels other than the prescribed region. How to update. The method of claim 15, And said first and second drive schemes differ in bit depth. The method of claim 16, Wherein one of the first and second drive schemes is monochrome and the other is gray scale having at least four different gray levels. The method of claim 15, Wherein said defined area comprises a text box used for writing text on said display. A driving method of a bistable electro-optical display having a plurality of pixels, each of which can display three or more gray levels, Storing a lookup table containing data representing impulses required to convert the initial gray level to a final gray level; Storing data representing at least an initial state of each pixel of the display; Receiving an input signal indicative of a desired final state of one or more pixels of the display; And Generating an output signal representing the impulse required to convert the initial state of the one pixel to the desired final state of the pixel, as a signal determined from the lookup table, For one or more transitions from an initial state to a final state, the output signal includes a DC imbalanced fine tuning sequence, the sequence comprising: (a) has a non-zero net impulse; (b) is non-contiguous; (c) change the gray level of the pixel substantially different from the change in the optical state of the DC reference pulse of the sequence, where the DC reference pulse is a pulse of voltage V ₀ , and V ₀ is applied during the fine adjustment sequence Is the maximum voltage but has the same sign as the net impulse G of the fine tuning sequence, the duration of the reference pulse being G / V _0. Is; (d) change the gray level of a pixel that is smaller in magnitude than the change in gray level caused by the time-reference pulse of the sequence, where the time-reference pulse is a monopolar of the same duration as the fine adjustment sequence. A method of driving a bistable electro-optic display, defined as a voltage pulse, but the sign of the reference pulse is the sign that results in a greater change in gray level. The method of claim 19, And the fine adjustment sequence changes the gray level of the pixel to less than one half of the change in gray level caused by the time-referenced pulse of the sequence. The method of claim 19, For the one or more transitions, the output signal includes one or more single polarity driving pulses in addition to the fine tuning sequence. The method of claim 19, And for said one or more transitions, said output signal is aperiodic. The method of claim 19, For most transitions in the lookup table, the output signal has a non-zero net impulse and is discontinuous. The method of claim 19, For the one or more transitions, the output signal consists only of pulses having voltage levels of + V, 0, -V. The method of claim 23, wherein For the one or more transitions, the output signal consists only of pulses having a voltage level of 0 and one of + V and -V. The method of claim 25, For the one or more transitions, the output signal consists of a pulse having a voltage level of zero preceding or following two or more pulses having a voltage level equal to one of + V and -V. Driving method. The method of claim 26, For most transitions in the lookup table where the initial and final states of the pixel are different, the output signal precedes or follows two or more pulses having a voltage level equal to one of + V and -V. A method of driving a bistable electro-optic display, comprising: a pulse having: The method of claim 19, And said transition table is DC balanced. The method of claim 19, For the one or more transitions, the output signal consists of a series of pulses that are integer multiples of a single interval. The method of claim 19, Storing data indicative of at least one transient state of the one pixel and / or at least one gray level earlier state of the one pixel, wherein the output signal is at least one of the at least one transient state and / or A method of driving a bistable electro-optic display, generated according to one or more gray level prior states of the one pixel. A driving method of a bistable electro-optical display having a plurality of pixels, each of which can display three or more gray levels, Applying an output signal effective to the change of the pixel from the initial state to the final state to each pixel of the display, Wherein for one or more transitions in which the initial and final states of the pixel are different, the output signal consists of a pulse having a voltage level of zero preceding or following two or more pulses having the same voltage level as one of + V and -V. Method of driving bistable electro-optic display. A driving method of a bistable electro-optical display having a plurality of pixels, each of which can display three or more gray levels, Storing a lookup table comprising data representing impulses required to convert the initial gray level to a final gray level; Storing data representing at least an initial state of each pixel of the display; Receiving an input signal indicative of a desired final state of one or more pixels of the display; And Generating an output signal representing the impulse required to convert the initial state of the one pixel to the desired final state of the pixel as the signal determined from the lookup table, Wherein for at least one transition from an initial state to a final state, the output signal comprises a DC balanced fine tuning sequence, The sequence is (a) has substantially zero net impulse; (b) At no point in the fine tuning sequence, change the gray level of the pixel to be greater than about one third of the difference in gray level between the two extreme optical states of the pixel from the gray level at the start of the fine tuning sequence. Method of driving a bistable electro-optic display. The method of claim 32, For the one or more transitions, the output signal includes one or more single polarity driving pulses in addition to the fine tuning sequence. A driving method of a bistable electro-optical display having a plurality of pixels, each of which can display three or more gray levels, Applying an output signal to each pixel of the display effective to change the pixel from an initial state to a final state, Wherein the output signal is non-zero but DC balanced for one or more transitions. The method of claim 34, wherein For the one or more transitions, the output signal comprises a first pair of pulses comprising a voltage pulse of equal length but following a pulse of opposite sign. 36. The method of claim 35 wherein And the output signal comprises a period of zero voltage between two pulses. 36. The method of claim 35 wherein Wherein the one or more pulses are interrupted by a period of zero voltage. 36. The method of claim 35 wherein For the one or more transitions, the output signal further comprises a second pair of pulses of equal length but of opposite signs. The method of claim 38, And a second pair of pulses having a length different from the length of the first pair of pulses. The method of claim 38, Wherein the first of the second pair of pulses has a polarity opposite to the polarity of the first of the first pair of pulses. The method of claim 38, And wherein said first pair of pulses occur between a first and a second of said second pair of pulses. The method of claim 34, wherein For the one or more transitions, the output signal includes one or more pulse components effective to drive the pixel with substantially one optical rail. The method of claim 34, wherein For each transition where the initial and final states of the pixel are the same, the output signal is non-zero but DC balanced, and for each transition where the initial and final states of the pixel are not the same, the output signal is DC A method of driving a bistable electro-optic display that is not balanced. The method of claim 43, For each transition where the initial and final states of the pixel are not equal, the output signal has the form -x / ΔIP / x, where ΔIP is the difference in the impulse potential between the initial and final states of the pixel. , -x and x are the same length but are opposite pairs of pulses of opposite signs. The method of claim 34, wherein Storing a lookup table comprising data representing impulses required to convert the initial gray level of the pixel to a final gray level; Storing data representing at least an initial state of each pixel of the display; Receiving an input signal indicative of a desired final state of one or more pixels of the display; And Generating an output signal representing the impulse required to convert the initial state of the one pixel to the desired final state of the pixel as the signal determined from the lookup table. A method of driving a bistable electro-optic display having at least one pixel, Applying waveform V (t) to the pixel such that is less than about 1 volt sec, Where T is the length of the waveform, integration is over the duration of the waveform, V (t) is the waveform voltage as a function of time t, and M (t) is the dwell resulting from the short pulse at time zero. A method of driving a bistable electro-optic display, which is a memory function characterized by reducing the effect of residual voltage to induce dwell-time-dependence. The method of claim 46, J is a method of driving a bistable electro-optic display of less than about 0.5 volt sec. The method of claim 47, J is a method of driving a bistable electro-optic display of less than about 0.1 volt sec. The method of claim 46, J is Is calculated by Where τ is the decay (relaxation) time. The method of claim 49, τ has a value from about 0.7 to about 1.3 seconds.