WO2017146787A1 - Methods and apparatus for driving electro-optic displays - Google Patents

Abstract

The performance of an electro-optic display, for example, a bistable electro-optic display, can be improved by modifying the frame rate of a base waveform used to drive a transition between gray states. Such modifications permit fine control of gray levels with reduced artifacts. The described methods require less memory to store all of the waveforms needed to achieve good performance of an electro-optic display over a range of temperatures.

Description

METHODS AND APPARATUS FOR DRIVING ELECTRO-OPTIC

DISPLAYS

REFERENCE TO RELATED APPLICATIONS

[Para 1] This application claims priority to U.S. Patent Application Serial No. 15/050,997, filed February 23, 2016, which is incorporated by reference in its entirety.

[Para 2] This application is related to U.S. Patent Application Serial No. 14/089,610, filed November 25, 2013, now U.S. Patent No. 9,269,311, issued February 23, 2016, which is a division of U.S. Patent Application Serial No. 13/086,066, filed April 13, 2011, now U.S. Patent No. 8,593,396, issued November 26, 2013, which itself is a division of U.S. Patent Application Serial No. 11/161,715, filed August 13, 2005, now U.S. Patent No. 7,952,557, issued May 31, 2011, which claims benefit of the following provisional Applications: (a) Application Serial No. 60/601,242, filed August 13, 2004; (b) Application Serial No. 60/522,372, filed September 21, 2004; and (c) Application Serial No. 60/522,393, filed September 24, 2004.

[Para 3] The aforementioned U.S. Patent Application Serial No. 11/161,715 is also a continuation-in-part of U.S. Patent Application Serial No. 10/904,707, filed November 24, 2004, now U.S. Patent No. 8,558,783, issued October 15, 2013, which itself claims benefit of provisional Applications Serial Nos. 60/481,711 and 60/481,713, both filed November 26, 2003.

[Para 4] The aforementioned U.S. Patent Application Serial No. 10/904,707 is a continuation-in-part of U.S. Patent Application Serial No. 10/879,335, filed June 29, 2004, now United States Patent No. 7,528,822, issued May 5, 2009, which claims benefit of the following provisional Applications: Serial No. 60/481,040, filed June 30, 2003; Serial No. 60/481,053, filed July 2, 2003; and Serial No. 60/481,405, filed September 23, 2003.

[Para 5] The aforementioned U.S. Patent Application Serial No. 10/879,335 is also a continuation-in-part of U.S. Patent Application Serial No. 10/814,205, filed March 31, 2004, now U.S. Patent No. 7,119,772, issued October 10, 2006, which claims benefit of the following provisional Applications: Serial No. 60/320,070, filed March 31, 2003; Serial No. 60/320,207, filed May 5, 2003; Serial No. 60/481,669, filed November 19, 2003; Serial No. 60/481,675, filed November 20, 2003; and Serial No. 60/557,094, filed March 26, 2004. [Para 6] The aforementioned U.S. Patent Application Serial No. 10/814,205 is a continuation-in-part of U.S. Patent Application Serial No. 10/065,795, filed November 20, 2002, now U.S. Patent No. 7,012,600, issued March 14, 2006, which itself claims benefit of the following provisional Applications: Serial No. 60/319,007, filed November 20, 2001; Serial No. 60/319,010, filed November 21, 2001; Serial No. 60/319,034, filed December 18, 2001; Serial No. 60/319,037, filed December 20, 2001; and Serial No. 60/319,040, filed December 21, 2001.

[Para 7] This application is also related to U.S. Patent Application Serial No. 10/249,973, filed May 23, 2003, now United States Patent No. 7,193,625, issued March 20, 2007, which claims benefit of provisional Applications Serial Nos. 60/319,315, filed June 13, 2002 and Serial No. 60/319,321, filed June 18, 2002.

[Para 8] This application is also related to U.S. Patent Application Serial No. 10/063,236, filed April 2, 2002, now United States Patent No. 7,170,670; U.S. Patent Application Serial No. 10/064,279, filed June 28, 2002, now U.S. Patent 6,657,772; U.S. Patent Application Serial No. 10/064,389, filed July 9, 2002, now United States Patent No. 6,831,769; and U.S. Patent Application Serial No. 10/249,957, filed May 22, 2003, now United States Patent No. 6,982,178.

[Para 9] The aforementioned U.S. Patent Applications Serial Nos. 10/904,707; 10/879,335; 10/814,205; 10/249,973; and 10/065,795 may hereinafter for convenience collectively be referred to as the "MEDEOD" (MEthods for Driving Electro-Optic Displays) applications.

[Para 10] The entire contents of these copending applications, and of all other U.S. patents and published and copending applications mentioned below, are herein incorporated by reference.

BACKGROUND OF INVENTION

[Para 11] This invention relates to methods for driving electro-optic displays, especially bistable electro-optic displays, and to apparatus (controllers) for use in such methods. More specifically, this invention relates to driving methods which are intended to enable more accurate control of gray states of the pixels of an electro-optic display. This invention also relates to driving methods which are intended to enable such displays to be driven in a manner which allows compensation for the "dwell time" during which a pixel has remained in a particular optical state prior to a transition, while still allowing the drive scheme used to drive the display to be DC balanced. This invention is especially, but not exclusively, intended for use with particle-based electrophoretic displays in which one or more types of electrically charged particles are suspended in a liquid and are moved through the liquid under the influence of an electric field to change the appearance of the display.

[Para 12] The electro-optic displays in which the methods of the present invention are used often contain an electro-optic material which is a solid in the sense that the electro-optic material has solid external surfaces, although the material may, and often does, have internal liquid- or gas-filled space. Such displays using solid electro-optic materials may hereinafter for convenience be referred to as "solid electro-optic displays".

[Para 13] The term "electro-optic" as applied to a material or a display, is used herein in its conventional meaning in the imaging art to refer to a material having first and second display states differing in at least one optical property, the material being changed from its first to its second display state by application of an electric field to the material. Although the optical property is typically color perceptible to the human eye, it may be another optical property, such as optical transmission, reflectance, luminescence or, in the case of displays intended for machine reading, pseudo-color in the sense of a change in reflectance of electromagnetic wavelengths outside the visible range.

[Para 14] The term "gray state" is used herein in its conventional meaning in the imaging art to refer to a state intermediate two extreme optical states of a pixel, and does not necessarily imply a black-white transition between these two extreme states. For example, several of the patents and published applications referred to below describe electrophoretic displays in which the extreme states are white and deep blue, so that an intermediate "gray state" would actually be pale blue. Indeed, as already mentioned the transition between the two extreme states may not be a color change at all. The term "gray level" is used herein to denote the possible optical states of a pixel, including the two extreme optical states.

[Para 15] The terms "bistable" and "bistability" are used herein in their conventional meaning in the art to refer to displays comprising display elements having first and second display states differing in at least one optical property, and such that after any given element has been driven, by means of an addressing pulse of finite duration, to assume either its first or second display state, after the addressing pulse has terminated, that state will persist for at least several times, for example at least four times, the minimum duration of the addressing pulse required to change the state of the display element. It is shown in published U.S. Patent Application No. 2002/0180687 that some particle-based electrophoretic displays capable of gray scale are stable not only in their extreme black and white states but also in their intermediate gray states, and the same is true of some other types of electro-optic displays. This type of display is properly called "multi-stable" rather than bistable, although for convenience the term "bistable" may be used herein to cover both bistable and multi-stable displays.

[Para 16] The term "impulse" is used herein in its conventional meaning of the integral of voltage with respect to time. However, some bistable electro-optic media act as charge transducers, and with such media an alternative definition of impulse, namely the integral of current over time (which is equal to the total charge applied) may be used. The appropriate definition of impulse should be used, depending on whether the medium acts as a voltage- time impulse transducer or a charge impulse transducer.

[Para 17] Much of the discussion below will focus on methods for driving one or more pixels of an electro-optic display through a transition from an initial gray level to a final gray level (which may or may not be different from the initial gray level). The term "waveform" will be used to denote the entire voltage against time curve used to effect the transition from one specific initial gray level to a specific final gray level. Typically, as illustrated below, such a waveform will comprise a plurality of waveform elements; where these elements are essentially rectangular (i.e., there a given element comprises application of a constant voltage for a period of time), the elements may be called "voltage pulses" or "drive pulses". The term "drive scheme" denotes a set of waveforms sufficient to effect all possible transitions between gray levels for a specific display.

[Para 18] Several types of electro-optic displays are known. One type of electro-optic display is a rotating bichromal member type as described, for example, in U.S. Patents Nos. 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 6,147,791 (although this type of display is often referred to as a "rotating bichromal ball" display, the term "rotating bichromal member" is preferred as more accurate since in some of the patents mentioned above the rotating members are not spherical). Such a display uses a large number of small bodies (typically spherical or cylindrical) which have two or more sections with differing optical characteristics, and an internal dipole. These bodies are suspended within liquid-filled vacuoles within a matrix, the vacuoles being filled with liquid so that the bodies are free to rotate. The appearance of the display is changed to applying an electric field thereto, thus rotating the bodies to various positions and varying which of the sections of the bodies is seen through a viewing surface. This type of electro-optic medium is typically bistable. [Para 19] Another type of electro-optic display uses an electrochromic medium, for example an electrochromic medium in the form of a nanochromic film comprising an electrode formed at least in part from a semi-conducting metal oxide and a plurality of dye molecules capable of reversible color change attached to the electrode; see, for example O' Regan, B., et al., Nature 1991, 353, 737; and Wood, D., Information Display, 18(3), 24 (March 2002). See also Bach, U., et al., Adv. Mater., 2002, 14(11), 845. Nanochromic films of this type are also described, for example, in U.S. Patent No. 6,301,038, International Application Publication No. WO 01/27690, and in U.S. Patent Application 2003/0214695. This type of medium is also typically bistable.

[Para 20] Another type of electro-optic display, which has been the subject of intense research and development for a number of years, is the particle -based electrophoretic display, in which a plurality of charged particles move through a fluid under the influence of an electric field. Electrophoretic displays can have attributes of good brightness and contrast, wide viewing angles, state bistability, and low power consumption when compared with liquid crystal displays. Nevertheless, problems with the long-term image quality of these displays have prevented their widespread usage. For example, particles that make up electrophoretic displays tend to settle, resulting in inadequate service-life for these displays.

[Para 21] As noted above, electrophoretic media require the presence of a fluid. In most prior art electrophoretic media, this fluid is a liquid, but electrophoretic media can be produced using gaseous fluids; see, for example, Kitamura, T., et al., "Electrical toner movement for electronic paper-like display", IDW Japan, 2001, Paper HCSl-1, and Yamaguchi, Y., et al., "Toner display using insulative particles charged triboelectrically", IDW Japan, 2001, Paper AMD4-4). See also European Patent Publication Nos. EP1429178; EP1462847; and EP1482354; and International Applications WO 2004/090626; WO 2004/079442; WO 2004/077140; WO 2004/059379; WO 2004/055586; WO 2004/008239; WO 2004/006006; WO 2004/001498; WO 03/091799; and WO 03/088495. Such gas-based electrophoretic media appear to be susceptible to the same types of problems due to particle settling as liquid-based electrophoretic media, when the media are used in an orientation which permits such settling, for example in a sign where the medium is disposed in a vertical plane. Indeed, particle settling appears to be a more serious problem in gas-based electrophoretic media than in liquid-based ones, since the lower viscosity of gaseous fluids as compared with liquid ones allows more rapid settling of the electrophoretic particles. [Para 22] Numerous patents and applications assigned to or in the names of the Massachusetts Institute of Technology (MIT) and E Ink Corporation have recently been published describing encapsulated electrophoretic media. Such encapsulated media comprise numerous small capsules, each of which itself comprises an internal phase containing electrophoretically-mobile particles suspended in a fluid, and a capsule wall surrounding the internal phase. Typically, the capsules are themselves held within a polymeric binder to form a coherent layer positioned between two electrodes. Encapsulated media of this type are described, for example, in U.S. Patents Nos. 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; 6,249,271; 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; 6,710,540; 6,721,083; 6,724,519; 6,727,881; 6,738,050; 6,750,473; 6,753,999; 6,816,147; 6,819,471; 6,822,782; 6,825,068; 6,825,829; 6,825,970; 6,831,769; 6,839,158; 6,842,167; 6,842,279; 6,842,657; 6,864,875; 6,865,010; 6,866,760; 6,870,661; 6,900,851; and 6,922,276; and U.S. Patent Applications Publication Nos. 2002/0060321; 2002/0063661; 2002/0090980; 2002/0113770; 2002/0130832; 2002/0180687; 2003/0011560; 2003/0020844; 2003/0025855; 2003/0102858; 2003/0132908; 2003/0137521; 2003/0214695; 2003/0222315; 2004/0012839; 2004/0014265; 2004/0027327; 2004/0075634; 2004/0094422; 2004/0105036; 2004/0112750; 2004/0119681; 2004/0136048; 2004/0155857; 2004/0180476; 2004/0190114; 2004/0196215; 2004/0226820; 2004/0239614; 2004/0252360; 2004/0257635; 2004/0263947; 2005/0000813; 2005/0001812; 2005/0007336; 2005/0007653; 2005/0012980; 2005/0017944; 2005/0018273; 2005/0024353; 2005/0035941; 2005/0041004; 2005/0062714; 2005/0067656; 2005/0078099; 2005/0105159; 2005/0122284; 2005/0122306; 2005/0122563; 2005/0122564; 2005/0122565; 2005/0151709; and 2005/0152022; and International Applications Publication Nos. WO 99/67678; WO 00/05704; WO 00/38000; WO 00/36560; WO 00/67110; WO 00/67327; WO 01/07961; and WO 03/107,315.

[Para 23] Many of the aforementioned patents and applications recognize that the walls surrounding the discrete microcapsules in an encapsulated electrophoretic medium could be replaced by a continuous phase, thus producing a so-called "polymer-dispersed electrophoretic display" in which the electrophoretic medium comprises a plurality of discrete droplets of an electrophoretic fluid and a continuous phase of a polymeric material, and that the discrete droplets of electrophoretic fluid within such a polymer-dispersed electrophoretic display may be regarded as capsules or microcapsules even though no discrete capsule membrane is associated with each individual droplet; see for example, the aforementioned United States Patent No. 6,866,760. Accordingly, for purposes of the present application, such polymer-dispersed electrophoretic media are regarded as sub-species of encapsulated electrophoretic media.

[Para 24] An encapsulated electrophoretic display typically does not suffer from the clustering and settling failure mode of traditional electrophoretic devices and provides further advantages, such as the ability to print or coat the display on a wide variety of flexible and rigid substrates. (Use of the word "printing" is intended to include all forms of printing and coating, including, but without limitation: pre-metered coatings such as patch die coating, slot or extrusion coating, slide or cascade coating, curtain coating; roll coating such as knife over roll coating, forward and reverse roll coating; gravure coating; dip coating; spray coating; meniscus coating; spin coating; brush coating; air knife coating; silk screen printing processes; electrostatic printing processes; thermal printing processes; ink jet printing processes; and other similar techniques.) Thus, the resulting display can be flexible. Further, because the display medium can be printed (using a variety of methods), the display itself can be made inexpensively.

[Para 25] A related type of electrophoretic display is a so-called "microcell electrophoretic display". In a microcell electrophoretic display, the charged particles and the fluid are not encapsulated within capsules but instead are retained within a plurality of cavities formed within a carrier medium, typically a polymeric film. See, for example, International Application Publication No. WO 02/01281, and U.S. Patent Application Publication No. 2002/0075556, both assigned to Sipix Imaging, Inc.

[Para 26] Other types of electro-optic media may also be used in the displays of the present invention.

[Para 27] Although electrophoretic media are often opaque (since, for example, in many electrophoretic media, the particles substantially block transmission of visible light through the display) and operate in a reflective mode, many electrophoretic displays can be made to operate in a so-called "shutter mode" in which one display state is substantially opaque and one is light-transmissive. See, for example, the aforementioned U.S. Patents Nos. 6,130,774 and 6,172,798, and U.S. Patents Nos. 5,872,552; 6,144,361; 6,271,823; 6,225,971; and 6,184,856. Dielectrophoretic displays, which are similar to electrophoretic displays but rely upon variations in electric field strength, can operate in a similar mode; see U.S. Patent No. 4,418,346.

[Para 28] The bistable or multi-stable behavior of particle-based electrophoretic displays, and other electro-optic displays displaying similar behavior (such displays may hereinafter for convenience be referred to as "impulse driven displays"), is in marked contrast to that of conventional liquid crystal ("LC") displays. Twisted nematic liquid crystals are not bi- or multi-stable but act as voltage transducers, so that applying a given electric field to a pixel of such a display produces a specific gray level at the pixel, regardless of the gray level previously present at the pixel. Furthermore, LC displays are only driven in one direction (from non-transmissive or "dark" to transmissive or "light"), the reverse transition from a lighter state to a darker one being effected by reducing or eliminating the electric field. Finally, the gray level of a pixel of an LC display is not sensitive to the polarity of the electric field, only to its magnitude, and indeed for technical reasons commercial LC displays usually reverse the polarity of the driving field at frequent intervals. In contrast, bistable electro-optic displays act, to a first approximation, as impulse transducers, so that the final state of a pixel depends not only upon the electric field applied and the time for which this field is applied, but also upon the state of the pixel prior to the application of the electric field.

[Para 29] Whether or not the electro-optic medium used is bistable, to obtain a high- resolution display, individual pixels of a display must be addressable without interference from adjacent pixels. One way to achieve this objective is to provide an array of non-linear elements, such as transistors or diodes, with at least one non-linear element associated with each pixel, to produce an "active matrix" display. An addressing or pixel electrode, which addresses one pixel, is connected to an appropriate voltage source through the associated nonlinear element. Typically, when the non-linear element is a transistor, the pixel electrode is connected to the drain of the transistor, and this arrangement will be assumed in the following description, although it is essentially arbitrary and the pixel electrode could be connected to the source of the transistor. Conventionally, in high resolution arrays, the pixels are arranged in a two-dimensional array of rows and columns, such that any specific pixel is uniquely defined by the intersection of one specified row and one specified column. The sources of all the transistors in each column are connected to a single column electrode, while the gates of all the transistors in each row are connected to a single row electrode; again the assignment of sources to rows and gates to columns is conventional but essentially arbitrary, and could be reversed if desired. The row electrodes are connected to a row driver, which essentially ensures that at any given moment only one row is selected, i.e., that there is applied to the selected row electrode a voltage such as to ensure that all the transistors in the selected row are conductive, while there is applied to all other rows a voltage such as to ensure that all the transistors in these non-selected rows remain non-conductive. The column electrodes are connected to column drivers, which place upon the various column electrodes voltages selected to drive the pixels in the selected row to their desired optical states. (The aforementioned voltages are relative to a common front electrode which is conventionally provided on the opposed side of the electro-optic medium from the non-linear array and extends across the whole display.) After a pre-selected interval known as the "line address time" the selected row is deselected, the next row is selected, and the voltages on the column drivers are changed to that the next line of the display is written. This process is repeated so that the entire display is written in a row-by -row manner.

[Para 30] It might at first appear that the ideal method for addressing such an impulse- driven electro-optic display would be so-called "general grayscale image flow" in which a controller arranges each writing of an image so that each pixel transitions directly from its initial gray level to its final gray level. However, inevitably there is some error in writing images on an impulse-driven display. Some such errors encountered in practice include:

(a) Prior State Dependence; With at least some electro-optic media, the impulse required to switch a pixel to a new optical state depends not only on the current and desired optical state, but also on the previous optical states of the pixel.

(b) Dwell Time Dependence; With at least some electro-optic media, the impulse required to switch a pixel to a new optical state depends on the time that the pixel has spent in its various optical states. The precise nature of this dependence is not well understood, but in general, more impulse is required that longer the pixel has been in its current optical state.

(c) Temperature Dependence; The impulse required to switch a pixel to a new optical state depends heavily on temperature.

(d) Humidity Dependence; The impulse required to switch a pixel to a new optical state depends, with at least some types of electro-optic media, on the ambient humidity.

(e) Mechanical Uniformity; The impulse required to switch a pixel to a new optical state may be affected by mechanical variations in the display, for example variations in the thickness of an electro-optic medium or an associated lamination adhesive. Other types of mechanical non-uniformity may arise from inevitable variations between different manufacturing batches of medium, manufacturing tolerances and materials variations.

(f) Voltage Errors; The actual impulse applied to a pixel will inevitably differ slightly from that theoretically applied because of unavoidable slight errors in the voltages delivered by drivers.

[Para 31] General grayscale image flow suffers from an "accumulation of errors" phenomenon. For example, imagine that temperature dependence results in a 0.2 L* (where L* has the usual CIE definition:

L* = 116(R/Ro)^1/3 - 16,

where R is the reflectance and Ro is a standard reflectance value) error in the positive direction on each transition. After fifty transitions, this error will accumulate to 10 L*. Perhaps more realistically, suppose that the average error on each transition, expressed in terms of the difference between the theoretical and the actual reflectance of the display is + 0.2 L*. After 100 successive transitions, the pixels will display an average deviation from their expected state of 2 L* ; such deviations are apparent to the average observer on certain types of images.

[Para 32] This accumulation of errors phenomenon applies not only to errors due to temperature, but also to errors of all the types listed above. As described in the aforementioned 2003/0137521, compensating for such errors is possible, but only to a limited degree of precision. For example, temperature errors can be compensated by using a temperature sensor and a lookup table, but the temperature sensor has a limited resolution and may read a temperature slightly different from that of the electro-optic medium. Similarly, prior state dependence can be compensated by storing the prior states and using a multidimensional transition matrix, but controller memory limits the number of states that can be recorded and the size of the transition matrix that can be stored, placing a limit on the precision of this type of compensation.

[Para 33] Thus, general grayscale image flow requires very precise control of applied impulse to give good results, and empirically it has been found that, in the present state of the technology of electro-optic displays, general grayscale image flow is infeasible in a commercial display. [Para 34] Almost all electro-optic medium have a built-in resetting (error limiting) mechanism, namely their extreme (typically black and white) optical states, which function as "optical rails". After a specific impulse has been applied to a pixel of an electro-optic display, that pixel cannot get any whiter (or blacker). For example, in an encapsulated electrophoretic display, after a specific impulse has been applied, all the electrophoretic particles are forced against one another or against the capsule wall, and cannot move further, thus producing a limiting optical state or optical rail. Because there is a distribution of electrophoretic particle sizes and charges in such a medium, some particles hit the rails before others, creating a "soft rails" phenomenon, whereby the impulse precision required is reduced when the final optical state of a transition approaches the extreme black and white states, whereas the optical precision required increases dramatically in transitions ending near the middle of the optical range of the pixel.

[Para 35] Various types of drive schemes for electro-optic displays are known which take advantage of optical rails. For example, Figures 9 and 10 of the aforementioned U.S. Patent Application No. 2003/0137521, and the related description at Paragraphs [0177] to [0180], describe a "slide show" drive scheme in which the entire display is driven to at least one optical rail before any new image is written. Obviously, a pure general grayscale image flow drive scheme cannot rely upon using the optical rails to prevent errors in gray levels since in such a drive scheme any given pixel can undergo an infinitely large number of changes in gray level without ever touching either optical rail.

[Para 36] Before proceeding further, it is desirable to define slideshow drive schemes more precisely. The fundamental slideshow drive scheme is that a transition from an initial optical state (gray level) to a final (desired) optical state (gray level) is achieved by making transitions to a finite number of intermediate states, where the minimum number of intermediate states is one. Preferably, the intermediate states are at or near the extreme states of the electro-optic medium used. The transitions will differ from pixel to pixel in a display, because they depend upon the initial and final optical states. The waveform for a specific transition for a given pixel of a display may be expressed as:

P2 = goali = goak = ... => goal_n = Ri (Scheme 1) where there is at least one intermediate or goal state between the initial state R2 and the final state Ri. The goal states are, in general, functions of the initial and final optical states. The presently preferred number of intermediate states is two, but more or fewer intermediate states may be used. Each of the individual transitions within the overall transition is achieved using a waveform element (typically a voltage pulse) sufficient to drive the pixel from one state of the sequence to the next state. For example, in the waveform indicated symbolically above, the transition from R2 to goali is typically achieved with a waveform element or voltage pulse. This waveform element may be of a single voltage for a finite time (i.e., a single voltage pulse), or may include a variety of voltages so that a precise goali state is achieved. This waveform element is followed by a second waveform element to achieve the transition from goali to goah. If only two goal states are used, the second waveform element is followed by a third waveform element that drives the pixel from the goah state to the final optical state Ri. The goal states may be independent of both R2 and Ri, or may depend upon one or both.

[Para 37] This invention seeks to provide improved slide show drive schemes for electro- optic displays which achieve improved control of gray levels. This invention is particularly, although not exclusively, intended for use in pulse width modulated drive schemes in which the voltage applied to any given pixel of a display at any given moment can only be -V, 0 or +V, where V is an arbitrary voltage. More specifically, this invention relates to two distinct types of improvements in slide show drive schemes, namely (a) insertion of certain modifying elements into base waveforms for such a drive scheme; and (b) arranging the drive scheme so that at least certain gray levels are approached from the optical rail further from the desired gray level.

[Para 38] In another aspect, this invention relates to dwell time compensation in drive schemes for electro-optic displays. As discussed in the MEDEOD applications, it has been found, at least in the case of many particle-based electro-optic displays, that the impulses necessary to change a given pixel through equal changes in gray level (as judged by eye or by standard optical instruments) are not necessarily constant, nor are they necessarily commutative. For example, consider a display in which each pixel can display gray levels of 0 (white), 1, 2 or 3 (black), beneficially spaced apart. (The spacing between the levels may be linear in percentage reflectance, as measured by eye or by instruments but other spacings may also be used. For example, the spacings may be linear in L* or may be selected to provide a specific gamma; a gamma of 2.2 is often adopted for monitors, and when electro-optic displays are be used as a replacement for monitors, use of a similar gamma may be desirable.) It has been found that the impulse necessary to change the pixel from level 0 to level 1 (hereinafter for convenience referred to as a "0-1 transition") is often not the same as that required for a 1-2 or 2-3 transition. Furthermore, the impulse needed for a 1-0 transition is not necessarily the same as the reverse of that needed for a 0-1 transition. In addition, some systems appear to display a "memory" effect, such that the impulse needed for (say) a 0-1 transition varies somewhat depending upon whether a particular pixel undergoes 0-0-1, 1-0-1 or 3-0-1 transitions. (Where, the notation "x-y-z", where x, y, and z are all optical states 0, 1, 2, or 3 denotes a sequence of optical states visited sequentially in time.) Although these problems can be reduced or overcome by driving all pixels of the display to one of the extreme states for a substantial period before driving the required pixels to other states, the resultant "flash" of solid color is often unacceptable; for example, a reader of an electronic book may desire the text of the book to scroll down the screen, and may be distracted, or lose his place, if the display is required to flash solid black or white at frequent intervals. Furthermore, such flashing of the display increases its energy consumption and may reduce the working lifetime of the display. Finally, it has been found that, 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, and that compensating for these factors is desirable to secure accurate gray scale rendition.

[Para 39] As briefly mentioned above, it has been found that, at least in some cases, the impulse necessary for a given transition in a bistable electro-optic display varies with the residence time of a pixel in its optical state, this phenomenon hereinafter being referred to as "dwell time dependence" or "DTD", although the term "dwell time sensitivity" was used in the aforementioned Application Serial No. 60/320,070. Thus, it may be desirable or even in some cases in practice necessary, to vary the impulse applied for a given transition as a function of the dwell time of the pixel in its initial optical state.

[Para 40] The phenomenon of dwell time dependence will now be explained in more detail with reference to Figure 1 of the accompanying drawings, which shows the reflectance of a pixel a function of time for a sequence of transitions denoted R₃ -> R₂ -> Ri, where (generalizing the nomenclature used above) each of the R_k terms indicates a gray level in a sequence of gray levels, with R's with larger indices occurring before R's with smaller indices. The transitions between R₃ and R2 and between R2 and Ri are also indicated. DTD is the variation of the final optical state Ri caused by variation in the time spent in the optical state R2, referred to as the dwell time. One can compensate for DTD by choosing different waveforms for different dwell times or different ranges of dwell times in the previous optical state. This method of compensation is called "dwell-time compensation," "DTC", or simply "time compensation". [Para 41] However, such DTC may conflict with other desirable properties of drive schemes. In particular, for reasons discussed in detail in the MEDEOD applications, with many electro-optic displays it is highly desirable to ensure that the drive scheme used is direct current (DC) balanced, in the sense that, for any arbitrary series of transitions beginning and ending in the same optical state, the applied impulse (i.e., the integral of the applied voltage with respect to time) is zero. This guarantees that the net impulse (also called "DC imbalance") experienced by any pixel of the display is bounded by a known value regardless of the exact series of transitions undergone by that pixel. For example, a 15 V, 300 msec pulse may be used to drive a pixel from a white to a black state. After this transition, the pixel has experienced 4.5 V sec of DC imbalance impulse. If a -15 V, 300 msec pulse is used to drive the pixel back to white, then the pixel is DC balanced for the overall excursion from white to black and back to white. This DC balance should hold for all possible excursions from one original optical state, to a series of optical states the same as or different from the original optical state, then back to the original optical state.

[Para 42] A drive scheme can be dwell-time-compensated by adding or removing voltage features to or from a base drive scheme. For example, one might begin with a drive scheme for a two optical state (black and white) display, the drive scheme including the following four waveforms:

[Para 43] Table 1

[Para 44] This drive scheme is DC balanced, because any series of transitions that brings a pixel back to its initial optical state is DC balanced, that is, the net area under the voltage profile for the entire series of transitions is zero.

[Para 45] Optical errors can arise from DTD of a display. For example, a pixel may can start in the white state, drive to the black state, dwell for a time, and then drive back to the white state. The final white state reflectance is a function of the time spent in the black state. [Para 46] It is desirable to have a very small DTD. If this is not possible for a specific electro-optic display, it is desirable to compensate for DTD, in accordance with one aspect of the present invention, by selecting different waveforms for different ranges of dwell times in the prior optical state. For example, one may find that the final white state in the example just given is brighter after short dwell times in the previous black state than after long dwell times in the previous black state. One dwell-time-compensation scheme would be to modify the duration of the pulse that brings the pixel layer from black to white to counteract this DTD of the final optical state. For example, one could shorten the pulse length in the black-to-white transition when the dwell time in the previous black state is short, and keep the pulse longer for long dwell times in the previous black state. This tends to produce a darker white state for shorter prior- state dwell times, which counteracts the effects of DTD. For example, one could choose a black-to-white waveform that varies with dwell time in the black state according to Table 2 below.

[Para 48] The problem with this approach to DTC of a drive scheme is that the drive scheme as a whole is no longer DC balanced. Because the impulse for a black-to-white transition is a function of the time spent in the black state, and similarly the impulse for a white-to-black transition may be a function of the dwell time in the white state, the net impulse over a black- to-white-to-black sequence is, in general, not DC balanced. For example, suppose this sequence is carried out with a black-to- white transition after a short dwell time in black using a voltage pulse of -15 V for 280 msec = -4.2 V sec impulse, followed, after a long dwell in the white state, by a white-to-black transition using a voltage pulse of 15 V for 400 msec, for an impulse of 6 V sec. The net impulse in this sequence (black-white -black loop) is -4.2 V sec + 6 V sec = 1.8 V sec. Repeating this loop causes a build-up of DC imbalance, which can be detrimental to the performance of the display. [Para 49] Thus, this aspect of the present invention provides a method for dwell time compensation of a DC balanced waveform or drive scheme that preserves the DC balance of the waveform or drive scheme.

[Para 50] Another aspect of the present invention relates to methods and apparatus for driving electro-optic displays which permits rapid response to user input. The aforementioned MEDEOD applications describe several methods and controllers for driving electro-optic displays. Most of these methods and controllers make use of a memory having two image buffers, the first of which stores a first or initial image (present on the display at the beginning of a transition or rewriting of the display) and the second of which stores a final image, which it desired to place upon the display after the rewrite. The controller compares the initial and final images and, if they differ, applies to the various pixels of the display driving voltages which cause the pixels to undergo changes in optical state such that at the end of the rewrite (alternatively called an update) the final image is formed on the display.

[Para 51] However, in most of the aforementioned methods and controllers, the updating operation is "atomic" in the sense that once an update is started, the memory cannot accept any new image data until the update is complete. This causes difficulties when it is desired to use the display for applications that accept user input, for example via a keyboard or similar data input device, since the controller is not responsive to user input while an update is being effected. For electrophoretic media, in which the transition between the two extreme optical states may take several hundred milliseconds, this unresponsive period may vary from about 800 to about 1800 milliseconds, the majority of this period be attributable to the update cycle required by the electro-optic material. Although the duration of the unresponsive period may be reduced by removing some of the performance artefacts that increase update time, and by improving the speed of response of the electro-optic material, it is unlikely that such techniques alone will reduce the unresponsive period below about 500 milliseconds. This is still longer than is desirable for interactive applications, such example an electronic dictionary, where the user expects rapid response to user input. Accordingly, there is a need for an image updating method and controller with a reduced unresponsive period.

[Para 52] This aspect of the present invention makes use of the known concept of asynchronous image updating to reduce substantially the duration of the unresponsive period. It is known to use structures already developed for gray scale image displays to reduce the unresponsive period by up to 65 per cent, as compared with prior art methods and controllers, with only modest increases in the complexity and memory requirements of the controller. [Para 53] Finally, this invention relates to a method and apparatus for driving an electro- optic display in which the data used to define the drive scheme is compressed in a specific manner. The aforementioned MEDEOD applications describe methods and apparatus for driving electro-optic displays in which the data defining the drive scheme (or plurality of drive schemes) used are stored in one or more look-up tables ("LUT's"). Such LUT's must of course contain data defining the waveform for each waveform of the or each drive scheme, and a single waveform will typically require multiple bytes. As described in the MEDEOD applications, the LUT may have to take account of more than two optical states, together with adjustments for such factors as temperature, humidity, operating time of the medium etc. Thus, the amount of memory necessary for holding the waveform information can be substantial. It is desirable to reduce the amount of memory allocated to waveform information in order to reduce the cost of the display controller. A simple compression scheme that can be realistically accommodated in a display controller or host computer would be helpful in reducing the display controller cost. This invention relates to a simple compression scheme that appears particularly advantageous for electro-optic displays.

SUMMARY OF INVENTION

[Para 54] The invention provides for a method of improving performance of an electro-optic display, e.g., an electrophoretic display, over a range of temperatures by adjusting the frame rate of the display to accommodate for changes in the electrophoretic medium due to temperature. This method involves storing a base waveform defining a sequence of voltages to be applied to a pixel during a specific transition by the pixel between gray levels at a first temperature and a base frame rate, and also storing a temperature-dependent multiplication factor, n, where n is a positive number. The specific transition is then effected by applying to the pixel the base waveform at a frame rate that that is n times the base frame rate. The new frame rate may be faster or slower than the base frame rate, for example, a higher temperature will allow operation at a faster frame rate. The temperature-dependent multiplication factor, n, may be stored in a look-up table (LUT), whereby a temperature measurement is obtained and value of n matching that temperature is obtained from the LUT. In some embodiments, the method additionally comprises adjusting the amplitude of the base waveform by a second temperature-dependent factor, p, which may also be stored in a LUT. By adjusting the frame rate, the overall performance of the electro-optic medium is improved, e.g., as indicated by a reduction in the intensity of residual images after a pixel has been changed from a first image to a second image, a phenomenon known as "ghosting."

BRIEF DESCRIPTION OF DRAWINGS

[Para 55] As already mentioned, Figure 1 of the accompanying drawings shows the reflectance of a pixel of an electro-optic display as a function of time, and illustrates the phenomenon of dwell time dependence.

[Para 56] Figures 2A and 2B illustrate waveforms for two different transitions in a prior art three reset pulse slide show drive scheme of a type described in the aforementioned MEDEOD applications.

[Para 57] Figures 2C and 2D illustrate the variations with time of the reflectances of two pixels of an electro-optic display to which the waveforms of Figures 2A and 2B respectively are applied.

[Para 58] Figures 3A and 3B illustrate waveforms for two different transitions in a prior art two reset pulse slide show drive scheme of a type described in the aforementioned MEDEOD applications.

[Para 59] Figures 4A, 4B and 4C illustrate balanced pulse pairs which, in accordance with the BPPSS method of the present invention, may be used to modify prior art slide show waveforms such as those shown in Figures 2A, 2B, 3A and 3B.

[Para 60] Figure 5A illustrates a set of dwell time compensated waveforms used in a first dwell time compensation balanced pulse pair drive scheme of the present invention.

[Para 61] Figure 5B illustrates a set of dwell time compensated waveforms used in a first dwell time compensation balanced pulse pair drive scheme of the present invention.

[Para 62] Figure 5C illustrates a set of dwell time compensated waveforms used in a first dwell time compensation balanced pulse pair drive scheme of the present invention.

[Para 63] Figure 5D illustrates a set of dwell time compensated waveforms used in a first dwell time compensation balanced pulse pair drive scheme of the present invention.

[Para 64] Figure 5E illustrates a set of dwell time compensated waveforms used in a first dwell time compensation balanced pulse pair drive scheme of the present invention.

[Para 65] Figure 6A illustrates a set of dwell-time-compensated waveforms used in a second dwell time compensation balanced pulse pair drive scheme of the present invention.

[Para 66] Figure 6B illustrates a set of dwell-time-compensated waveforms used in a second dwell time compensation balanced pulse pair drive scheme of the present invention. [Para 67] Figure 6C illustrates a set of dwell-time-compensated waveforms used in a second dwell time compensation balanced pulse pair drive scheme of the present invention.

[Para 68] Figure 7 shows a comparison of ghosting in graytone transitions between a standard frame rate (solid line) and a temperature-adjusted frame rate (dashed line) at several temperatures.

DETAILED DESCRIPTION

[Para 69] The invention provides methods for adjusting driving waveforms for electrophoretic displays to improve performance over a range of temperatures. In particular, a base waveform comprising a sequence of voltages and a base frame rate may be stored for a specific transition, along with temperature-dependent multiplication factors. A specific transition at a specific transition is thus driven by applying the base waveform at a framerate equivalent to the base framerate adjusted by a temperature-dependent multiplication factor.

[Para 70] Balanced pulse pair slide show method and apparatus

[Para 71] A Balanced Pulse Pair Slide Show (BPPSS) method is a method for driving an electro-optic display having at least one pixel capable of achieving at least three different gray levels including two extreme optical states. The method comprises applying to the pixel a base waveform comprising at least one reset pulse sufficient to drive the pixel to or close to one of the extreme optical states followed by at least one set pulse sufficient to drive the pixel to a gray level different from said one extreme optical state, the base waveform being modified by at least one of the following:

(a) insertion of at least one balanced pulse pair into the base waveform;

(b) excision of at least one balanced pulse pair from the base waveform; and

(c) insertion of at least one period of zero voltage into the base waveform.

[Para 72] Also, the term balanced pulse pair ("BPP") denotes a sequence of two pulses of opposite polarity such that the total impulse of the balanced pulse pair is essentially zero. In a preferred form of the BPPSS method, the two pulses of the balanced pulse pair are each of constant voltage but of opposite polarity and are equal in length. The term "base waveform element" or "BWE" may be used hereinafter to refer to any reset or set pulse of the base waveform. The insertion of the balanced pulse pair and/or of the zero voltage period (which may hereinafter be called a "gap") may be effected either within a single base waveform element or between two successive waveform elements. All these modifications have the property that they do not affect the net impulse of the waveform; by net impulse is meant the integral of the waveform voltage curve integrated over the time duration of the waveform. Balanced pulse pairs and zero voltage pauses have of course zero net impulse. Although typically the pulses of a BPP will be inserted adjacent each other, this is not essential and the two pulses may be inserted at separate locations.

[Para 73] Where the modification of the base waveform in accordance with the BPPSS method includes excision of at least one BPP, the period formerly occupied by the, or each, excised BPP may be left as a period of zero voltage. Alternatively, this period may be "closed up" by moving some or all of the later waveform elements earlier in time, but in this case it will normally be necessary to insert a period of zero voltage at some later stage in the waveform, typically at the end thereof, in order to ensure that the overall length of the waveform is maintained, since it is normally necessary to ensure that all pixels of a display are driven with waveforms of equal length. Alternatively, of course, the period may be "closed up" by moving some or all of the earlier waveform elements later in time, with insertion of a period of zero voltage at some earlier stage of the waveform, typically at the beginning thereof.

[Para 74] As already indicated, the BPPSS waveforms of the present invention are modifications of base slide show waveforms described in the aforementioned MEDEOD applications. As discussed above, slide show waveforms comprise one or more reset pulses that cause a pixel to move to, or at least close to, one extreme optical state (optical rail); if the waveform includes two or more reset pulses, each reset pulse after the first will cause the pixel to move to the opposed extreme optical state, and thus to traverse substantially its entire optical range. (For example, if the display uses an electro-optic medium that has a range of (say) 4 to 40 per cent reflectance, each reset pulse after the first might cause the pixel to traverse from 8 to 35 per cent reflectance.) If more than one reset pulse is used, successive reset pulses must of course be of alternating polarity.

[Para 75] A slide show waveform further comprises a set pulse which drives the pixel from the extreme optical state in which it has been left by the last reset pulse to the desired final gray level of the pixel. Note that when this desired final gray level is one of the extreme optical states, and the last reset pulse leaves the pixel at this desired extreme optical state, the set pulse may be of zero duration. Similarly, if the initial state of the pixel before application of the slide show waveform is at one of the extreme optical states, the first reset pulse may be of zero duration. [Para 76] Preferred BPPSS waveforms of the present invention will now be described, though by way of illustration only, with reference to the accompanying drawings.

[Para 77] Figures 2A and 2B of the accompanying drawings illustrate t waveforms used for two different transitions in a prior art (base) slide show drive scheme of a type described In the aforementioned MEDEOD applications. This slide show drive scheme uses three reset pulses for each transition. Figures 2C and 2D show the corresponding variations with respect to time in optical state (reflectance) of pixels to which the waveforms of Figures 2A and 2B respectively are applied. In accordance with the convention used in the aforementioned copending Applications Serial Nos. 10/065,795 and 10/879,335, Figures 2C and 2D are drawn so that the bottom horizontal line represents the black extreme optical state, the top horizontal line represents the white extreme optical state, and intervening levels represent gray states. The beginning and end of the reset and set pulses of the waveforms are indicated in Figures 2A and 2B by broken vertical lines, and the various BWE's (i.e., the reset and set pulses) are shown as consisting of ten or less equal length pulses, although in general the BWE's may be of more arbitrary length and if comprised of a series of equal length pulses, more than ten such pulses would normally be used for a maximum length BWE.

[Para 78] The base waveform (generally designated 100) shown in Figures 2A and 2C effects a white-to-white transition (i.e., a "transition" in which both the initial and the final states of the pixel are the white extreme optical state). The waveform 100 comprises a first negative (i.e., black-going) reset pulse 102, which drives the pixel to its black extreme optical state, a second positive (white-going) reset pulse 104, which drives the pixel to its white extreme optical state, a third negative (black- going) reset pulse 106, which drives the pixel to its black extreme optical state, and a set pulse 108, which drives the pixel to its white extreme optical state. Each of the four pulses 102, 104, 106 and 108 has the maximum ten-unit duration. (To avoid the awkwardness of continual references to "units of duration", these units may hereinafter be referred to as "time units" or "TU's".)

[Para 79] Figures 2B and 2D illustrate a waveform (generally designated 150) for a dark gray to light gray transition using the same three reset pulse drive scheme as in Figures 2A and 2C. The waveform 150 comprises a first reset pulse 152 which, like the first reset pulse 102 of waveform 100, is negative and black-going. However, since the transition for which waveform 150 is used begins from a dark gray level, the duration (illustrated as four TU's) of the first reset pulse 152 is shorter than that of reset pulse 102, since a shorter first reset pulse is needed to bring the pixel to its black extreme optical state at the end of the first reset pulse. For the remaining six TU's of the first reset pulse 152, zero voltage is applied to the pixel. (Figures 2B and 2D illustrate the first reset pulse 152 with the four TU's of negative voltage at the end of the relevant period, but this is arbitrary and the periods of negative and zero voltage may be arranged as desired.)

[Para 80] The second and third reset pulses 104 and 106 of waveform 150 are identical to the corresponding pulses of waveform 100. The set pulse 158 of waveform 150, like the set pulse 108 of waveform 100, is positive and white-going. However, since the transition for which waveform 150 is used ends at a light gray level, the duration (illustrated as seven TU's) of the set pulse 158 is shorter than that of set pulse 108, since a shorter set pulse is needed to bring the pixel to its final light gray level. For the remaining three TU's of set pulse 158, zero voltage is applied to the pixel. (Again, the distribution of periods of positive and zero voltage within set pulse 158 is arbitrary and the periods may be arranged as desired.)

[Para 81] From the foregoing, it will be seen that, in the prior art slide show drive scheme shown in Figures 2A-2D, the duration of the first reset pulse and of the set pulse will vary depending upon the initial and final states of the pixel respectively, and in certain cases one or both of these pulses may be of zero duration. For example, in the drive scheme of Figures 2A-2D, a black-to-black transition could have a first reset pulse of zero duration (since the pixel is already at the black extreme optical state which is reached at the ends of the first reset pulses 102 and 152), and a set pulse of zero duration (since at the end of the third reset pulse 106 the pixel is already at the desired extreme black optical state).

[Para 82] In general, it is desirable to keep the overall duration of waveforms as short as possible so that a display can be rapidly rewritten; for obvious reasons, users prefer displays that display new images quickly. Since each reset pulse occupies a substantial period, it is desirable to reduce the number of reset pulses to the minimum consistent with acceptable gray scale performance by the display, and in general one or two reset pulse slide show drive schemes are preferred. Figures 3A and 3B of the accompanying drawings illustrate waveforms for two different transitions in a two reset pulse prior art slide show drive scheme of the type described in the aforementioned MEDEOD applications.

[Para 83] Figure 3A illustrates a white to light gray single reset pulse waveform (generally designated 200) comprising a reset pulse 202, which drives a pixel from its initial white state to black, and a set pulse 208 (identical to pulse 158 in Figure 2B), which drives the pixel from black to a light gray. Although waveform 200 uses only a single reset pulse, it will be appreciated that it is actually part of a two reset pulse slide show drive scheme with a first reset pulse of zero duration, as indicated by the period of zero voltage at the left hand side of Figure 3A.

[Para 84] Figure 3B illustrates a black to light gray two reset pulse waveform (generally designated 250) comprising a first reset pulse 252, which drives a pixel from its initial black state to white, a second reset pulse 254, which drives the pixel from white to black, and a set pulse 208, identical to the reset pulse in Figure 3A, which drives the pixel from black to light gray.

[Para 85] As already mentioned, the BPPSS waveforms of the present invention are derived from base slide show waveforms such as those illustrated in Figures 2A, 2B, 3 A and 3B by insertion of at least one balanced pulse pair into the base waveform, excision of at least one balanced pulse pair from the base waveform, or insertion of at least one period of zero voltage into the base waveform. In the case of excision of a BPP, the resultant gap may be either closed up or left as a period of zero voltage. Combinations of these modifications may be used.

[Para 86] Figures 4A-4C illustrate balanced pulse pairs for use in the BPPSS waveforms. The BPP (generally designated 300) shown in Figure 4A comprises a negative pulse 302 of constant voltage, followed immediately by a positive pulse 304 of the same duration and voltage as pulse 302 but of opposite polarity. It will be apparent that the BPP 300 applies zero net impulse to a pixel. The BPP (generally designated 310) shown in Figure 4B is identical to the BPP 300 except that the order of the pulses is reversed. The BPP (generally designated 320) shown in Figure 4C is derived from the BPP 310 by introducing a period 322 of zero voltage between the positive and negative pulses 304 and 302 respectively.

[Para 87] It should be noted that not all pixels of a display necessarily reach a given goal state (for example, the inverse monochrome projections goal state) at the same point in time during rewriting of a display from an initial image to a desired final image. The time point in a transition at which the goal states are reached are functions of the initial and desired final gray levels, R2 and Rl, respectively. Ideally (and as normally illustrated herein), the time points for R2 and Rl match, with the entire display being driven through various goal states, and these goal states being reached simultaneously by all pixels. However, it is often desirable to shift the relative timing of the various waveforms of a drive scheme. Time shifting of the waveforms may be done for aesthetic reasons, for example, to improve the appearance of the transition or the appearance of the resulting image. Also, modifications such as those discussed below may shift the relative time positions of the goal states, so that for various combinations of Rl and R2, the goal states are reached at different times during a transition.

[Para 88] It is important to realize that such waveform modifications will affect not only the reflectance not only of the final optical state (i.e., the final gray level), but also the intermediate goal states. While the goal states of a basic waveforms are generally near one of the extreme optical states (optical rails), and, by definition, are near the optical rails for the last goal state, or last two goal states in the preferred form of a drive scheme, the modifications described above can shift the reflectance at a goal state away from an optical rail. It is the change in the degree of drive toward an optical rail that gives small adjustments in the final optical state (gray level).

[Para 89] It has been found desirable to keep the impulses of each of the voltage pulses comprising a BPP relatively small. The magnitude of a BPP may be defined by a parameter d, the absolute value of which describes the length of each of the two voltage pulses of a BPP, and the sign of which denotes the sign of the second of the two pulses. For example, the BPP's shown in Figures 4A and 4B can be assigned d values +1 and -1, respectively (while the BPP of Figure 4C is then, in a consistent scheme, assigned a d value of -1 with a gap modification inserted between the two pulses). In some embodiments, all BPP's used have d values whose magnitudes are less than PL, and preferably less than PL/2, where PL (in the same units used to measure the BPP's) is defined as the length of the voltage pulse required to drive a pixel from one extreme optical state to the other, or the average value of this voltage pulse where the lengths for transitions in the two directions are not the same, at a drive voltage characteristic of the drive scheme. In the example just given, d is expressed in units of display scan frames, and the BPP's of Figures 4A and 4B have voltage pulses each one scan frame in length. In this case, PL would also be defined in scan frames. All quantities could of course alternatively be expressed in a time unit, such as seconds or milliseconds.

[Para 90] As described in the aforementioned MEDEOD applications, it is often necessary or desirable to drive electro-optic displays using drive circuitry which can supply only two drive voltages (a.k.a. "picket fence" driving). Since bistable electro-optic media normally need to be driven in both directions between their extreme optical states, it might at first appear that at least three drive voltages would be required, namely 0, +V and -V, where V is an essentially arbitrary drive voltage, so that one electrode for a specific pixel (typically the common front electrode in a conventional active matrix display) could be held at 0, while the other electrode (typically the pixel electrode for that pixel) can be held at +V or -V depending upon the direction in which the pixel needs to be driven. When two- voltage drive circuitry is used, each waveform of a drive scheme is divided into time segments; typically these time segments are of equal duration, but this is not necessarily the case. In a non-picket fence drive scheme, there may be applied to any specific pixel, in any time segment, a positive, zero or negative driving voltage. For example, in a three drive voltage system, the common front electrode may be held at 0, while the individual pixel electrodes are held at +V, 0 or -V. In a picket fence drive scheme, each time segment is in effect divided into two; in one of the two resultant segments, there may be applied to any specific pixel only a negative or zero driving voltage, while in the other resultant segment, there may be applied to any specific pixel only a positive or zero driving voltage. For example, consider a two driving voltage system having driving voltages V and v, where V >v. In the first of each pair of segments, the common front electrode is set to V, and the pixel electrodes to either V (zero driving voltage) or v (negative driving voltage). In the second of each pair of segments, the common front electrode is set to v, and the pixel electrodes to either v (zero driving voltage) or V (positive driving voltage). The resultant waveform is twice as long as the corresponding non-picket fence waveform.

[Para 91] It is also often desirable that an IMP drive scheme be capable of local updates. As described in the aforementioned MEDEOD applications, it is often desirable to drive electro- optic displays in a manner which permits local updating of a specific area of the display which is undergoing changes while the rest of the display remains unchanged; for example, it may be desirable to update a dialogue box in which a user is entering text without updating the background image on the display. A local update version of some drive schemes can be created by removing all non-zero voltages from the waveforms for zero transitions (i.e., transitions from one gray level to the same gray level). For example, the waveform from gray level 2 to gray level 2 normally is composed of a series of voltage pulses. Removing the nonzero voltages from this waveform, and doing so for all other zero transitions, results in a local update version of the waveform. Such a local update version can be advantageous when it is desired to minimize extraneous flashing during transitions.

[Para 92] Balanced pulse pair dwell time compensation method and apparatus

[Para 93] In some embodiments, at least two different waveforms may be used for the same transition between specific gray levels of a pixel of an electro-optic display, depending upon the duration of the dwell time of the pixel in the state from which the transition begins. These two waveforms may differ from each other by at least one insertion and/or excision of at least one balanced pulse pair, or insertion of at least one period of zero voltage, where "balanced pulse pair" has the meaning previously defined. It is very much preferred that in such a method the drive scheme be DC balanced as that term has been defined above.

[Para 94] In such a balanced pulse pair dwell time compensation (BPPDTC) method (as in the BPPSS method already described), the insertion or excision of the balanced pulse pair and/or of the zero voltage period (pause) may be effected either within a single waveform element or between two successive waveform elements. The two waveforms used for the same transition following differing dwell times in the initial state from which the transition begins may be referred to hereinafter as the "alternative dwell time" or "ADT" waveforms.

[Para 95] It should be noted that ADT waveforms may differ from one another by the location and/or duration of a BPP or pause within a waveform (see, for example, the discussion of Figures 5B-5E below), since such movement of a BPP or pause may be formally regarded as a combination of an excision of a BPP or pause at one location and an insertion of the BPP or pause at a different location, or (in the case of a change of duration at the same location) as a combination of an excision of a BPP or pause at the location and an insertion of a different BPP or pause at the same location.

[Para 96] In a BPPDTC drive scheme, the insertion of excision of BPP's and/or pauses raises the same problems, and may be handled in the same way, as in the BPPSS drive schemes described above. Thus, where the difference between the BPPDTC waveform includes excision of at least one BPP, the period formerly occupied by the, or each, excised BPP may be left as a period of zero voltage. Alternatively, this period may be "closed up" by moving some or all of the later waveform elements earlier in time, normally with insertion of a period of zero voltage at some later stage in the waveform, typically at the end thereof, in order to ensure that the overall length of the waveform is maintained. (In any practical display, which will normally have at least several thousand pixels, in any transition there will normally be at least one pixel undergoing every possible transition, and if the waveforms for all pixels are not of the same length, controller logic becomes extremely complicated.) Alternatively, of course, the period may be "closed up" by moving some or all of the earlier waveforms elements later in time, with insertion of a period of zero voltage at some earlier stage of the waveform, typically at the beginning thereof.

[Para 97] Similarly, inserting a BPP adds to the total duration of a waveform unless an existing period of zero voltage can simultaneously be removed. Since all waveforms of a drive scheme very desirably have the same overall length, when one waveform of a drive scheme has a BPP inserted, all the other waveforms of the drive scheme should have a period of zero voltage added to them, or some other modification made, to compensate for the increase in overall waveform length caused by the insertion of the BPP. For example, if a 40 msec BPP is inserted into the black-to-white waveform shown in Table 1 above (which has a waveform length of 420 msec), 40 msec pauses could be added to the remaining three waveforms shown in Table 1 so that all the waveforms have a length of 460 msec. Obviously, if appropriate, BPP's could be added to the other three waveforms rather than pauses, or some combination of BPP's and pauses totaling 40 msec could be used.

[Para 98] Preferred drive schemes and waveforms of the BPPDTC aspect of the present invention will now be described, though by way of illustration only. The balanced pulse pairs used in such drive schemes and waveforms may be of any of the types described above; for example, the types of BPP's shown in Figures 4A-4C may be used.

[Para 99] Figures 5A-5E illustrate alternative dwell time waveforms which may be used for a single transition in accordance with the BPPDTC aspect of the present invention. Figure 5A illustrates the black-to- white waveform mentioned in the third line of Table 1 and the last line of Table 2 above. Since this is the waveform appropriate for the black-to-white transition after a long dwell time in the black state, it may be regarded as the base black-to-white waveform which is modified in accordance with the BPPDTC aspect of the present invention to produce waveforms appropriate for the black-to-white transition after shorter dwell times in the black state. As already noted, the base waveform of Figure 5A consists of a -15V, 400 msec pulse followed by 0 V for 20 msec.

[Para 100]Figure 5B illustrates a modification of the base waveform of Figure 5A which has been found effective to decrease the reflectance of the final white state when a black-to-white transition is effected after only a short dwell time of not more than 0.3 seconds in the initial black state. The waveform of Figure 5B is produced by inserting a BPP similar to BPP 300 shown in Figure 4 A at the end of the -15V, 400 msec pulse of the waveform of Figure 5 A, so that the waveform of Figure 5B comprises a -15V, 420 msec pulse, followed by a +15V, 20 msec pulse and 0 V for 20 msec.

[Para 101]Figures 5C and 5D illustrate two further ADT waveforms for the same black-to- white transition as the waveforms of Figure 5 A and 5B. The waveforms of Figures 5C and 5D have been found effective to standardize the reflectance of the final white state when the black-to-white transition is effected after dwell times of 0.3 to 1 second, and 1 to 3 seconds, respectively, in the black state. The waveforms of Figures 5C and 5D are produced by inserting the same BPP as in Figure 5B into the waveform of Figure 5A, but at locations different from that used in Figure 5B. As noted above, it has been found that the position at which a BPP is inserted into (or excised from) a base waveform has a significant effect on the final optical state following a transition, and hence that shifting the position of insertion of a BPP with a base waveform is an effective means for compensating the waveform for variations in the dwell time of the pixel in the initial optical state.

[Para 102]Figure 5E is a preferred alternative to the waveform of Figure 5A for effecting the black-to-white transition after long dwell times (3 seconds or greater) in the black state. The waveform of Figure 5E is generally similar to those of Figures 5B-5D in that it is produced by inserting the same BPP into the waveform of Figure 5A. However, in Figure 2E, the BPP is inserted at the beginning of the waveform; it has also been found desirable to make the pulses of the BPP 40 msec rather than 20 msec in duration. Since this makes the overall duration of the waveform 500 msec, when the waveform of Figure 5E is used in conjunction with the waveforms of Figures 5B-5D, it is necessary to "pad" the waveforms of Figures 5B- 5D with an additional 40 msec of 0 V at the end of the waveform. Thus, a preferred set of ADT waveforms for the black-to- white transition is as shown in Table 3 below:

[Para 104]Note that the impulse for the black-to-white transition is -15V*400 msec, or 6 V sec for all the ADT waveforms in Table 3, and thus for all initial state dwell times, so that the drive scheme is DC balanced.

[Para 105]As already mentioned, DTC can also be effected by excising BPP's from a base waveform. For example, consider the drive scheme shown in Table 4 below: [Para 106] Table 4

[Para 107]Note that, in this drive scheme, not merely the whole drive scheme but all waveforms are "internally" DC balanced; the desirability of such internal DC balancing is discussed in detail in the aforementioned copending Application Serial No. 10/814,205. Again, the method for DTC will be discussed with reference to the black-to-white transition, although it should be understood that DTC of the white-to-black transition can be effected in a similar manner.

[Para 108]In this case, DTC of the black-to-white transition is effected by excising BPP's, i.e., by removing a portion of one voltage pulse of one polarity and one duration while simultaneously removing a similar portion of one voltage pulse of the opposite polarity and equivalent duration. One can either replace the pulse sections that were excised with a period of zero voltage or the remaining parts of the waveform can be shifted in time to occupy the period previously occupied by the excised pulse pair, and, in order to maintain the total update time, a zero voltage segment matching the duration of the excised pair can be added elsewhere, typically at the beginning or end of the waveform.

[Para 109]Figures 6A, 6B and 6C illustrate schematically this process for modification of the black-to-white waveform listed in the third row of Table 4 above for DTC at short dwell times of less than 0.3 seconds in the black state. Figure 6A illustrates the base waveform from Table 4. Figure 6B shows schematically excision of a BPP formed by the last 80 msec portion of the positive voltage pulse and the first 80 msec portion of the negative voltage pulse from the waveform of Figure 6A, with the resultant gap being eliminated by shifting the negative pulse forward in time, as indicated by the arrow in Figure 6B. The resultant dwell time compensated waveform, which comprises a 320 msec positive pulse, a 320 msec negative pulse and a 180 msec period of zero voltage, is shown in Figure 6C.

[Para 110]In this case, it was found that DTC for all dwell times could be effected simply by varying the length of the excised BPP, and that for long dwell times of 3 seconds or more in the black state the base waveform of Figure 6A was satisfactory. Hence the full list of ADT waveforms for the black-to- white transition in this case is as shown in Table 5 below:

[Para 112] As already mentioned, when a BPP is excised from a base waveform in the manner shown in Figure 6B, it is not essential that the remaining components be shifted in time; the excised BPP can simply be replaced by a period of zero voltage. Table 6 below shows a modified set of ADT waveforms similar to those in Table 5 but with the excised BPP's replaced with periods of zero voltage:

[Para 113] Table 6

[Para 114]Although the BPPDTC aspect of the present invention has been described above primarily with reference to displays having only two gray levels, it is not so limited but may be applied to displays having a greater number of gray levels. Also, although in the specific waveforms illustrated in the drawings, insertion or excision of the two elements of a BPP has been effected at a single point within the waveform, the invention is not limited to waveforms in which insertion or excision of a BPP is effected at a single point; the two elements of a BPP may be inserted or excised at different points, i.e., the two pulses that make up a BPP do not have to be immediately sequential, but could be separated by a time interval. Furthermore, one, or both, pulses of a BPP could be subdivided into sections and these sections could be then inserted into or excised from a waveform for DTC. For example, a BPP may be composed of a +15 V, 60 msec pulse and a -15 V, 60 msec pulse. This BPP could be divided into two components, for example a +15 V, 60 msec pulse followed immediately by a -15 V, 20 msec pulse, and a -15 V, 40 msec pulse, and these two components simultaneously inserted into or excised from a waveform to achieve DTC.

[Para 115]Inserting or excising zero voltage segments from a waveform has also been found to affect the final gray level after a transition, and hence such insertion or excision of zero voltage segments provides a second method for tuning the final gray level to achieve DTC. Such insertion or excision of zero voltage segments may be used alone or in combination with insertion or excision of BPP's.

[Para 116]Although the BPPDTC aspect of the present invention has been described above primarily with reference to pulse width modulated waveforms in which the voltage applied to a pixel at any given time can only be -V, 0 or +V, the invention is not limited to use with such pulse width modulated waveforms and may be used with voltage modulated waveforms, or waveforms using both pulse and voltage modulation. The foregoing definition of a balanced pulse pair can be satisfied by two pulses of opposite polarity with zero net impulse, and does not require that the two pulses be of the same voltage or duration. For example, in a voltage modulated drive scheme, a BPP might be composed of a +15 V, 20 msec pulse followed by a -5 V, 60 msec pulse.

[Para 117]From the foregoing, it will be seen that the BPPDTC aspect of the present invention permits dwell time compensation of a drive scheme while maintaining DC balance of the drive scheme. Such DTC can reduce the level of ghosting in electro-optic displays.

[Para 118]Target buffer methods and apparatus

[Para 119]Target buffers may be used to drive electro-optic displays having pixels capable of achieving at least two different gray levels. The first of these two methods, the non-polarity target buffer method comprises providing initial, final and target data buffers; determining when the data in the initial and final data buffers differ, and when such a difference is found updating the values in the target data buffer in such a manner that (i) when the initial and final data buffers contain the same value for a specific pixel, setting the target data buffer to this value; (ii) when the initial data buffer contains a larger value for a specific pixel than the final data buffer, setting the target data buffer to the value of the initial data buffer plus an increment; and (iii) when the initial data buffer contains a smaller value for a specific pixel than the final data buffer, setting the target data buffer to the value of the initial data buffer minus said increment; updating the image on the display using the data in the initial data buffer and the target data buffer as the initial and final states of each pixel respectively; next, copying the data from the target data buffer into the initial data buffer; and these steps until the initial and final data buffers contain the same data.

[Para 120]In the second of these two methods, the polarity target buffer method, the final, initial and target data buffers are again provided, together with a polarity bit array arranged to store a polarity bit for each pixel of the display. Again, the data in the initial and final data buffers are compared, and when they differ the values in the polarity bit array and target data buffer are updated in such a manner that (i) when the values for a specific pixel in the initial and final data buffers differ and the value in the initial data buffer represents an extreme optical state of the pixel, the polarity bit for the pixel is set to a value representing a transition towards the opposite extreme optical state; and the target data buffer is set to the value of the initial data buffer plus or minus an increment, depending upon the relevant value in the polarity bit array. The image on the display is then updated in the same way as in the first method and thereafter the data from the target data buffer is copied into the initial data buffer. These steps are repeated until the initial and final data buffers contain the same data.

[Para 121]Prior art controllers for bistable electro-optic displays typically use logic similar to that shown in the following Listing 1 (all Listings herein are in pseudocode):

[Para 122] Listing 1

pixel array initial [x_size, y_size]

pixel array final [x_size, y_size]

while()#endless loop

initial := final

if (host has new data)

final := new_image

update_display (initial, final)

[Para 123]With a controller operating in this manner, the display waits to receive new image information, then, when such new image information is received, performs a full update before allowing new information to be sent to the display, i.e., once one new image has been accepted by the display, the display cannot accept a second new image until the rewriting of the display needed to display the first new image has been completed, and in some cases this rewriting procedure may take hundreds of milliseconds cf. some of the drive schemes set out in Sections A-C above. Therefore, when the user is scrolling or typing, the display appears insensitive to user input for this full update (rewriting) time.

[Para 124]In contrast, a controller effecting the non-polarity target buffer method of the present invention operates by logic exemplified by the following Listing 2 (this type of controller may hereafter for convenience be called a "Listing 2 controller"):

[Para 125] Listing 2

pixel array initial [x_size, y_size]

pixel array final [x_size, y_size]

pixel array target [x_size, y_size]

while()#endless loop

initial := target

final := host_frame_buffer

if initial != final

for each pixel in initial

if Initial == final then target := initial

if Initial > final then target := initial + 1

if initial < final then target := initial -1

update_display (initial, target)

[Para 126]In this modified controller logic for an NPTB method, there are three image buffers. The initial and final buffers are the same as in prior art controllers, and the new third buffer is a "target" buffer. The display controller can accept new image data at any time into the final buffer. When the controller finds that the data in the final buffer is no longer equal to the data in the initial buffer (i.e., rewriting of the image is required), a new target data set is constructed by incrementing or decrementing the values in the initial buffer by one (or leaving them unchanged), depending upon the difference between the relevant values in the initial and final buffers. The controller then performs a display update in the usual way, using the values from the initial and target buffers. When this update is complete, the controller copies the values from the target buffer into the initial buffer, and then repeats the differencing operation between the initial and final buffers to generate a new target buffer. The overall update is complete when the initial and final buffers have the same data set. [Para 127]Thus, in this NPTB method, the overall update is effected as a series of sub-update operations, one such sub-update operation occurring when the image is updated using the initial and target buffers. The term "meso-frame" will be used hereinafter for the period required for each of these sub-update operations; such a meso-frame of course designates a period between that required for a single scan frame of the display (cf. the aforementioned MEDEOD applications) and the superframe, or period required to complete the entire update.

[Para 128]The NPTB method of the present invention improves interactive performance in two ways. Firstly, in the prior art method, the final data buffer is used by the controller during the update process, so that no new data can be written into this final data buffer while an update is taking place, and hence the display is unable to respond to new input during the entire period required for an update. In the NPTB method of the present invention, the final data buffer is used only for calculation of the data set in the target data buffer, and this calculation, being simply a computer calculation, can be effected much more rapidly than the update operation, which requires a physical response from the electro-optic material. Once the calculation of the data set in the target data buffer is complete, the update does not require further access to the final data buffer, so that the final data buffer is available to accept new data.

[Para 129]For reasons discussed in the aforementioned MEDEOD applications and further discussed below with regard to waveforms, it is often desirable that pixels be driven in a cyclic manner, in the sense that once a pixel has been driven from away from one extreme optical state by a voltage pulse of one polarity, no voltage pulse of the opposite polarity is applied to that pixel until the pixel reaches its other extreme optical state. This restriction is satisfied by the PTB method of the present invention, which may use a controller operating with logic exemplified by the following Listing 3 (this type of controller may hereafter for convenience be called a "Listing 3 controller"; this Listing assumes a four gray level system with gray levels numbered from 1 for black to 4 for white, although those skilled in the art can readily modify the pseudocode for operation with differing numbers of gray levels): [Para 130] Listing 3

pixel array initial [x_size, y_size]

pixel array final [x_size, y_size]

pixel array target [x_size, y_size]

bit array polarity [x_size, y_size]

while()#endless loop

initial := target

final := host_frame_buffer

if initial != final

for each pixel in initial

if initial == 1 then polarity := 1

if initial == 4 then polarity := 0

if initial != final then target := initial + (polarity-0.5)*2 update_display (initial, target)

[Para 131]This PTB method requires four image buffers, the fourth being a "polarity" buffer having a single bit for each pixel of the display, this single bit indicating the current direction of transition of the associated pixel, i.e., whether the pixel is currently transitioning from white-to-black (0) or black-to-white (1). If the associated pixel is not currently undergoing a transition, the polarity bit retains its value from the previous transition; for example, a pixel that is stationary in a light gray state and was previously white will have a polarity bit of 0.

[Para 132]In the PTB method, the polarity bit array is taken into account when a new target buffer data set is constructed. If the pixel is currently black or white, and a transition to the opposite state is required, the value of the polarity bit is set accordingly, and the target value is set to the gray level closest to black or white respectively. Alternatively, if the initial state for the pixel is an intermediate (gray) state, the target value is calculated by incrementing or decrementing the state by 1, according to the value of the polarity bit (+1 if polarity = 1; -1 if polarity = 0).

[Para 133]It should be noted that, in this drive scheme, the behavior of pixels in the intermediate states is independent of the current value of the final state for that pixel. A pixel, upon starting a transition from black to white or white to black, will continue in the same direction until it reaches the opposite optical rail (extreme optical state, typically black or white). If the desired image and hence the target state changes during the transition, the pixel will then return in the opposite direction, and so on. [Para 134]Preferred waveforms for use in TB methods of the present invention will now be discussed. Table 7 below illustrates one possible transition matrix which can be used for one- bit (monochrome) operation with NPTB and PTB methods of the present invention, this transition matrix using two intermediate states.

[Para 135] Table 7

[Para 136]The structure of this transition matrix, with black, white, and two intermediate gray states, looks very similar to those used in prior art two-bit drive schemes, such as those described in the MEDEOD applications. However, in the TB methods of the present invention, these intermediate states do not correspond to stable gray states, but are only transition states, which exist only between the completion of one meso-frame and the start of the next. Also, there is no restriction on the uniformity of the reflectivity of these intermediate states.

[Para 137]It should be noted that, in the transition matrix shown in Table 7, many of the elements (indicated by the dashes) are not allowed. The controller only allows each transition to change the gray level by one unit in either direction, so that transitions involving multiple changes in gray level (for example a direct 1-4 black- to- white transition) are forbidden. The elements on the leading diagonal of the transition matrix (corresponding to zero transitions) are forbidden for the intermediate states; such leading diagonal elements are not recommended for white and black states, but are not strictly forbidden, as indicated by the asterisks in Table 7.

[Para 138]In a monochrome NPTB method, an update sequence appears as a series of states, starting and ending at the extreme optical states (optical rails), with a sequence of intermediate gray states separated by zero dwell time. For example, a simple transition from black to white would appears as:

1 ^2^3^4

On the other hand, if the final state of the display changes during the update, this transition might become: 1^2^3^2^ 1

Multiple changes in the final state might produce transitions such as:

More generally, there are four possible types of transitions between the extreme black and white optical states:

1^2^302^3) ^4

1^203^2) = 1

4^3^203^2) = 1

4^3(^2^3) ^4

where the parentheses signify zero or more repeats of the sequence within the parentheses.

[Para 139]Optimization ("tuning") of this class of NPTB drive schemes requires adjusting the non-zero elements of the transition matrix to ensure consistent reflectivity values for the 1 (black) and 4 (white) states, independent of the number of repeats of the parenthetical sequences. The waveform must work for arbitrary dwell times in the black and white extreme optical states, but the dwell times in the intermediate states are always zero, so that, as mentioned above, the reflectivities of the transition states are not important.

[Para 140]In general, the time required for any single meso-frame update is equal to the length of the longest element in the transition matrix. Thus, the time for a total update is three times the length of this longest element. In the best case, the black-to-white and white-to- black (1 =>4 and 4= 1 respectively) waveforms can be segmented into three equal-length pieces; this approach will reduce the update latency to one third of the full update time, while maintaining the same duration for the full update. As the length of the meso-frame updates becomes longer, which may be the result of optimizing the waveform, the benefit becomes less substantial. For example, if one element becomes twice as long, then the latency increases to two-thirds of the simple update time, and the full transition will require twice as long as before. It is possible to test to find the longest element present in a given meso-frame, and dynamically adjust the update time to that length, but the benefit of this extra computation is not likely to be significant.

[Para 141]Consideration should be given to what electro-optical properties of a medium make the display using the medium suitable for use with this type of NPTB drive scheme. Firstly, the dwell time dependency of the medium should be zero (ideally, or at least very low), since this waveform combines a series of near zero dwell times between meso-frames with potentially much longer dwell times between transitions. Secondly, the medium should have little or no sensitivity to optical states preceding the initial state of a particular transition, because the direction of a transition may change in mid-stream; for example, a 2= 1 transition might be preceded by either a l= 2 or a 3= 2 transition. Finally, the electro-optic medium should be symmetric in its response, especially near the black and white states; it is difficult to produce a DC balanced waveform that can perform a 1= 2= 1 or 4= 3= 4 transition that reaches the same black or white state, respectively.

[Para 142]For the foregoing reasons, the "intermediate reversals" in NPTB drive schemes make it very difficult to develop optimized waveforms. In contrast, a PTB drive scheme greatly reduces the demands on the electro-optic medium, and hence should alleviate much of the difficulty in optimizing an NTPB drive scheme while still providing improved performance.

[Para 143]Although the structure of the transition matrix for a PTB drive scheme is identical to that for an NPTB drive scheme, a PTB drive scheme permits only two black-to-white and white-to-black transitions, namely:

1=>2=>3=>4; and 4=>3=>2=> 1.

In fact, these two transitions can be the same as the normal 1= 4 and 4= 1 transitions, with the transitions partitioned into three equal parts. Some slight re-tuning may be desirable to account for any delays between the meso-frames, but the adjustment is straightforward. For simple typing input, this drive scheme should result in a two-thirds reduction in latency.

[Para 144]There are some drawbacks to a PTB method. Extra memory is required for the polarity bit array, and a more complex controller is operate this simpler drive scheme because allowing for the direction of the transition at each pixel requires taking account of an extra datum (the polarity bit) in addition to the initial and final states for a transition. Also, while a PTB method does reduce the latency for starting an update, the controller must wait until an update is complete before reversing the transition. This limitation is apparent if a user types a character, and then immediately erases it; the delay before the character is erased is equal to the full update time. This limits the usefulness of the PTB method for cursor tracking or scrolling. [Para 145]Although the NPTB and PTB methods have been described above primarily with regard to monochrome drive schemes, they are also compatible with gray scale drive schemes. The NPTB method is inherently completely gray scale compatible; the gray scale compatibility of a PTB method is discussed below.

[Para 146]From a drive scheme perspective, it will obviously be more difficult to produce a workable gray scale drive scheme for an NPTB method than a corresponding monochrome drive scheme, because in the gray scale drive scheme the intermediate states now correspond to actual gray levels, and thus the optical values of these intermediate states are constrained. Producing a gray scale drive scheme for a PTB method is also quite difficult. To reduce latency, the meso-frame transitions must be appreciably shortened. For example, a 2^>3 transition could be a stand-alone transition, the last stage of a 1= 2= 3 transition, or the first stage of a 2= 3= 4 transition. Thus, there are competing demands to make this transition short (to achieve a shorter overall update), and accurate (in case the transition stops at gray level 3).

[Para 147]A gray scale PTB method may be modified by introducing multiple gray level steps (i.e., by permitting the gray level to change by more than one unit during each meso- frame, corresponding to re-inserting elements more than one step removed from the leading diagonal of the relevant transition matrix, such as that shown in Table 7 above), thus eliminating the degeneracy of the meso-frame steps described in the preceding paragraph. This modification could be effected by replacing the polarity bit matrix with a counter array, which contains, for each pixel of the display, more than one bit, up to the number of bits required for a full gray scale image representation. The waveform would then contain up to a full N x N transition matrix, with each waveform divided evenly into four (or other essentially arbitrary number of meso-frames).

[Para 148]Although the specific TB methods discussed above are two-bit gray scale methods, with two intermediate gray levels, TB methods can of course be used with any number of gray levels. However, the incremental benefit of reduced latency will tend to decrease as the number of gray levels grows.

[Para 149]Thus, the present invention provides two types of TB methods that give significant reductions in update latency in monochrome mode, while minimizing the complexity of the controller algorithms. These methods may prove especially useful in interactive one-bit (monochrome) applications, for example, personal digital assistants and electronic dictionaries, where a fast response to user input is of paramount importance.

[Para 150]Waveform compression methods and apparatus

[Para 151]The amount of waveform data required to be stored in order to drive a bistable electro-optic display can be reduced with certain compression methods described below. Such "waveform compression" or "WC" methods can be used to drive an electro-optic display having a plurality of pixels, each of which is capable of achieving at least two different gray levels. In an embodiment, the method comprises: storing a base waveform defining a sequence of voltages to be applied during a specific transition by a pixel between gray levels; storing a multiplication factor for the specific transition; and effecting the specific transition by applying to the pixel the sequence of voltages for periods dependent upon the multiplication factor.

[Para 152]When an impulse-driven electro-optic display is being driven, each pixel of the display receives a voltage pulse (i.e., a voltage differential between the two electrodes associated with that pixel) or temporal series of voltage pulses (i.e., a waveform) in order to effect a transition from one optical state of the pixel to another, typically a transition between gray levels. The data needed to define the set of waveforms (forming a complete drive scheme) for each transition is stored in memory, generally on the display controller, although the data could alternatively be stored on a host computer or other auxiliary device. A drive scheme may comprise a large number of waveforms, and (as described in the aforementioned MEDEOD applications) it may be necessary to store multiple sets of waveform data to allow for variations in environmental parameters such as temperature and humidity, and non- environmental variations, for example the operating life of the electro-optic medium. Thus, the amount of memory needed to hold the waveform data can be substantial. It is desirable to reduce this amount of memory in order to reduce the cost of the display controller. A simple compression scheme that can be realistically accommodated in a display controller or host computer would be helpful in reducing the amount of memory needed for waveform data and thus the display controller cost. The waveform compression method of the present invention provides a simple compression scheme that is particularly advantageous for electrophoretic displays and other known bistable displays.

[Para 153]Uncompressed waveform data for a particular transition is typically stored as a series of bit sets, each bit set specifying a particular voltage to be applied at a particular point in the waveform. By way of example, consider a tri-level voltage drive scheme, where a pixel is driven toward black using a positive voltage (in this example, +10 V), toward white using a negative voltage (-10 V), and held at its current optical state with zero voltage. The voltage for a given time element (a scan frame for an active matrix display) can be encoded using two bits, for example, as shown in Table 8 below:

[Para 154] Table 8

[Para 155]Using this binary representation, a waveform for use in an active matrix drive and comprising a +10V pulse lasting for five scan frames followed by two scan frames of zero voltage would be represented as:

01 01 01 01 01 00 00.

Waveforms that comprise a large number of time segments require the storage of a large number of bit sets of waveform data.

[Para 156]In accordance with the WC method of the present invention, waveform data is stored as a base waveform (such a binary representation described above) and a multiplication factor. The display controller (or other appropriate hardware) applies to a pixel the sequence of voltages defined by the base waveform for periods dependent upon the multiplication factor. In a preferred form of such a WC method, a bit set (such as that given above) is used to represent the base waveform, but the voltage defined by each bit set is applied to the pixel for n time segments, where n is the multiplication factor associated with the waveform. The multiplication factor must be a natural number. For a multiplication factor of 1, the waveform applied is unchanged from the base waveform. For a multiplication factor greater than 1, the representation of the voltage series is compressed for at least some waveforms, that is, fewer bits are needed to express these waveforms than would be needed if the data were stored in uncompressed form.

[Para 157]By way of example, using the three voltage level binary representation of Table 8, consider a waveform that requires twelve scan frames of +10V followed by nine scan frames of -10V followed by six scan frames of +10V followed by three scan frames of 0V. This waveform is expressed in uncompressed form as:

01 01 01 01 01 01 01 01 01 01 01 01 10 10 10 10 10 10 10 10 10 01 01 01 01 01 01 00 00 00 and in compressed form as: multiplication factor: 3

base waveform 01 01 01 01 10 10 10 01 01 00.

[Para 158]The length of the voltage sequence that must be allocated for each waveform is determined by the longest waveform. For encapsulated electrophoretic and many other electro-optic displays, the longest waveforms are typically required at the lowest temperatures, where the electro-optic medium responds slowly to the applied field. At the same time, the resolution necessary to achieve successful transitions is reduced when the response is slow, so there is little loss in accuracy of optical state by grouping successive scan frames through the WC method of the present invention. Using this compression method, a number of scan frames (or generally time segments) that is appropriate for waveforms at moderate and high temperatures where the update time is short can be allocated to each waveform. At low temperature, where the number of scan frames needed can exceed the memory allocation, multiplication factors greater than unity can be used to generate long waveforms. This ultimately results in reduced memory requirements and costs.

[Para 159]The WC method of the present invention is in principle equivalent to simply changing the frame time of an active matrix display at various temperatures (as discussed below). For example, a display could be driven at 50 Hz at room temperature, and at 25 Hz at 0°C, to extend the allowable waveform time (as discussed below). In some embodiments, the WC method is superior to altering the frame rate because backplanes are designed to minimize the impact of capacitive and resistive voltage artifacts at a given frame rate. As one deviates significantly from this optimum frame rate in either direction, artifacts of at least one type rise. Thus, in some instances, it is better to keep the actual frame rate constant, while grouping scan frames using the WC method, which, in effect, provides a way of achieving a virtual change in frame rate without actually changing the physical frame rate.

[Para 160]Scan rate compression methods

[Para 161]The invention provides a method of improving the performance of an electro-optic display, e.g., a bistable electrophoretic display, over a range of temperatures by adjusting the frame rate of the display to accommodate for changes in the electro-optic medium due to temperature. For example, in an electrophoretic display, decreased temperature results in decreased electrophoretic mobility because the viscosity of the internal phase increases. As a consequence, temperature fluctuations can result in slow updates and/or image effects when the display is driven with a waveform that was optimized at a temperature different than the current operating temperature. To overcome this problem, some display controllers include complete sets (gray_m(T) -> gray_n(T)) of waveforms for a select group of temperatures (Ti, T₂, T3 . . .)· F°^{r a} given operating temperature, the set of gray scale transitions (gray_m(T) -> gray_n(T)) closest to a measured temperature is used to effect a grayscale transition. Nonetheless, at intermediate temperatures, e.g., between Ti and T₂, the performance of the display may be unacceptable because of higher order effects of the temperature change.

[Para 162]The claimed methods can dramatically reduce the amount of memory needed to store waveforms for a given grayscale transition over a range of temperatures. The method involves storing a base waveform defining a sequence of voltages to be applied to a pixel during a specific transition by the pixel between gray levels at a first temperature and a base frame rate, and also storing a temperature-dependent multiplication factor, n, where n is a positive number. The temperature-dependent multiplication factor, n, may be between 0.1 and 100, for example between 0.5 and 10, for example between 0.8 and 3. In some embodiments n is about 0.9, about 0.95, about 1.05, about 1.1, about 1.15, about 1.2, about 1.25, or about 2. The specific transition is then effected by applying to the pixel the base waveform at a frame rate that that is n times the base frame rate. The new frame rate may be faster or slower than the base frame rate, for example, a higher temperature will require operation at a faster frame rate. The temperature-dependent multiplication factor, n, may be stored in a look-up table (LUT), whereby a temperature measurement is obtained and value of n matching that temperature is obtained from the LUT. In some embodiments, the method additionally comprises adjusting the amplitude of the base waveform by a second temperature-dependent factor, p, which may also be stored in a LUT. By adjusting the frame rate, the overall performance of the electro-optic medium is improved, e.g., as indicated by a reduction in the intensity of residual images after a pixel has been changed from a first image to a second image, a phenomenon known as "ghosting." The frame rate can be adjusted using techniques known in the art and described in a number of the patents and patent applications listed in the Background section.

[Para 163]Because each row of an active matrix needs to be individually selected during each frame, in practice the base frame rate does exceed about 50 to 100 Hz. In some instances, frames of this length lead to difficulties in fine control of gray scale with many fast switching electro-optic medium. For example, some encapsulated electrophoretic media substantially complete a switch between their extreme optical states (a transition of about 30 L* units) within about 100 ms, and with such a medium a 20 ms frame corresponds to a gray scale shift of about 6 L* units. Such a shift is too large for accurate control of gray scale; the human eye is sensitive to differences in gray levels of about 1 L* unit, and controlling the impulse only in graduations equivalent to about 6 L* units is likely to give rise to visible artifacts. Such artifacts include "ghosting" due to prior state dependence of the electro-optic medium, that is, if the transition is under-driven, or not completely cleared, the second image will have remnants of the first image, i.e., "ghosts." The base frame rate is typically on the order of 50 Hz, however, in theory, the base frame rate could be anything reasonable, e.g., between 1 Hz and 200 Hz, e.g., between 40 Hz and 80 Hz.

[Para 164]The variation in ghosting due to temperature, and the ability to correct it using the methods of the invention is illustrated in FIG. 7. A standard waveform, optimized for 26 °C is assessed for ghosting by driving an electrophoretic test panel between first and second gray states multiple times, and then measuring the amount of residual reflectance that resides in the second darker state using a standardized optical bench having a calibrated light source and photodiode. When this standard waveform is applied at the same frame rate to the electrophoretic test panel at temperatures different from 26 °C, however, the ghosting worsens because the transition is either under-driven (lower temperature) or over-driven (higher temperature). See solid line in FIG. 7. In contrast, using the technique of the invention, the frame rate is modified by a temperature-dependent factor, n, and the ghosting is dramatically improved using the same standard waveform. See dashed line in FIG. 7. (Note that the solid and dashed lines intersect at 26 °C because they are both using the same, i.e., 26 "C-optimized, frame rate.) Accordingly, it is not necessary to store complete transition sets for 22 °C, 26 °C, and 30 °C. Rather, the same 26 °C base waveform can be used with a slightly different frame rate at 22 °C and 30 °C.

[Para 165]The temperature-dependent multiplication factors, n, can be stored in a look-up table (LUT) that is, for example, stored in flash memory. The display may include a temperature sensor to allow the display to monitor the temperature of the display in real time. Once the temperature is obtained, the corresponding factor, n, can be matched from the lookup table. In principle, an n could be measured for each unit of °C over the operating range, or even for each tenth of °C over the operating range. Overall, this accumulation of n's takes up very little memory as compared to storing complete wave sets for each temperature.

[Para 166]In some embodiments, it is also beneficial to modify the amplitude of the waveform as a function of temperature. In such embodiments, the amplitude of the base waveform may be altered by a second temperature-dependent factor, p. The second temperature-dependent multiplication factor, p, may be between 0.1 and 100, for example between 0.5 and 10, for example between 0.8 and 3. In some embodiments p is about 0.75, about 0.8, about 0.9, about 1.1, about 1.5, about 2, about 3, about 4, or about 5. Thus, the invention allows for the simultaneous adjustment of both the frame rate and the amplitude of the base waveform to counteract performance changes due to environmental conditions, e.g., temperature. It is to be understood that "amplitude" means the magnitude of the voltage of the waveform compared to ground or some other floating voltage. For example, many of the waveforms illustrated in the figures would all have amplitudes of 15 Volts, even though the waveforms include square waves from 0 to 15V and 0 to -15V. By altering both the frame rate and the amplitude of the waveform, it is possible to maintain (or decrease) the overall energy consumption of the electro-optic display with time, without sacrificing performance. The second temperature-dependent factor, p, may also be stored in the same or a different LUT, thus the display controller can adjust the amplitude of the base waveform to optimize performance.

[Para 167]It will be apparent to those skilled in the art that numerous changes can be made in the specific embodiments of the present invention already described without departing from the spirit scope of the invention. Accordingly, the whole of the foregoing description is to be construed in an illustrative and not in a limitative sense.

Claims

1. A method for driving an electro-optic display having a plurality of pixels, each of which is capable of achieving at least two different gray levels, the method comprising:

storing a base waveform defining a sequence of voltages to be applied to a pixel during a specific transition by the pixel between gray levels at a first temperature and a base frame rate;

storing a temperature-dependent multiplication factor, n, where n is a positive number; and

effecting the specific transition by applying to the pixel the base waveform at a frame rate that that is n times the base frame rate.

2. The method of claim 1, wherein the base frame rate is between 1 Hz and 200

Hz.

3. The method of claim 2, wherein the base frame rate is between 40 Hz and 80

Hz.

4. The method of claim 3, wherein the base frame rate is about 50 Hz.

5. The method of claim 1, wherein the base waveform comprises a set of bits.

6. The method of claim 1, wherein the base waveform is DC balanced.

7. The method of claim 1, wherein the temperature-dependent multiplication factor, n, is stored in a look-up table (LUT).

8. The method of claim 7, further comprising obtaining a temperature measurement, and matching a value of n to the measured temperature.

9. The method of claim 1, further comprising: storing a temperature dependent multiplication factor, p, wherein p is a positive number; and

adjusting the amplitude of the base waveform by a factor of p.

10. The method of claim 9, wherein the amplitude of the base waveform is between 2 Volts and 60 Volts.

11. The method of claim 10, wherein the amplitude of the base waveform is between 4 Volts and 21 Volts.

12. The method of claim 11, wherein the amplitude of the base waveform is about 15 Volts.

13. The method of claim 9, wherein the temperature-dependent multiplication factor, p, is stored in a look-up table (LUT).

14. The method of claim 13, further comprising obtaining a temperature measurement, and matching a value of p to the measured temperature.

15. The method of any of claims 1-14, wherein the electro-optic display comprises an electrophoretic medium.