Classifications

• • G—PHYSICS
  • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
  • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
  • G09G3/00—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes
  • G09G3/20—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters
  • G09G3/34—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters by control of light from an independent source
  • G09G3/3433—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters by control of light from an independent source using light modulating elements actuated by an electric field and being other than liquid crystal devices and electrochromic devices
  • G09G3/344—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters by control of light from an independent source using light modulating elements actuated by an electric field and being other than liquid crystal devices and electrochromic devices based on particles moving in a fluid or in a gas, e.g. electrophoretic devices
• • G—PHYSICS
  • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
  • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
  • G09G2230/00—Details of flat display driving waveforms
• • G—PHYSICS
  • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
  • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
  • G09G2310/00—Command of the display device
  • G09G2310/04—Partial updating of the display screen
• • G—PHYSICS
  • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
  • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
  • G09G2310/00—Command of the display device
  • G09G2310/06—Details of flat display driving waveforms
  • G09G2310/061—Details of flat display driving waveforms for resetting or blanking
  • G09G2310/062—Waveforms for resetting a plurality of scan lines at a time
• • G—PHYSICS
  • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
  • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
  • G09G2310/00—Command of the display device
  • G09G2310/06—Details of flat display driving waveforms
  • G09G2310/065—Waveforms comprising zero voltage phase or pause
• • G—PHYSICS
  • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
  • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
  • G09G2310/00—Command of the display device
  • G09G2310/08—Details of timing specific for flat panels, other than clock recovery
• • G—PHYSICS
  • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
  • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
  • G09G2320/00—Control of display operating conditions
  • G09G2320/02—Improving the quality of display appearance
  • G09G2320/0252—Improving the response speed
• • G—PHYSICS
  • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
  • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
  • G09G2320/00—Control of display operating conditions
  • G09G2320/04—Maintaining the quality of display appearance
  • G09G2320/041—Temperature compensation
• • G—PHYSICS
  • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
  • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
  • G09G2340/00—Aspects of display data processing
  • G09G2340/04—Changes in size, position or resolution of an image
  • G09G2340/0407—Resolution change, inclusive of the use of different resolutions for different screen areas
  • G09G2340/0435—Change or adaptation of the frame rate of the video stream

Definitions

• 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.
• 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”.
• 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.
• 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.
• 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.
• impulse is used herein in its conventional meaning of the integral of voltage with respect to time.
• 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.
• 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.
• 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”.
• driver scheme denotes a set of waveforms sufficient to effect all possible transitions between gray levels for a specific display.
• 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.
• 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.
• Electrophoretic display 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.
• electrophoretic media require the presence of a fluid.
• 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.
• 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,
• 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.
• 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.
• the resulting display can be flexible.
• the display medium can be printed (using a variety of methods), the display itself can be made inexpensively.
• a related type of electrophoretic display is a so-called "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.
• a carrier medium typically a polymeric film.
• 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.
• Shutter mode in which one display state is substantially opaque and one is light-transmissive.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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:
• 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.
• a waveform element typically a 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.
• 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.
• this invention relates to dwell time compensation in drive schemes for electro-optic displays.
• the impulses necessary to change a given pixel through equal changes in gray level are not necessarily constant, nor are they necessarily commutative.
• 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.
• 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.
• 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.
• DTC dwell-time compensation
• time compensation time compensation
• 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:
• Optical errors can arise from DTD of a display.
• 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.
• 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.
• 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.
• FIG. 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• the invention provides methods for adjusting driving waveforms for electrophoretic displays to improve performance over a range of temperatures.
• 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.
• 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:
• balanced pulse pair denotes a sequence of two pulses of opposite polarity such that the total impulse of the balanced pulse pair is essentially zero.
• the two pulses of the balanced pulse pair are each of constant voltage but of opposite polarity and are equal in length.
• 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.
• 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.
• 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.
• the period formerly occupied by the, or each, excised BPP may be left as a period of zero voltage.
• 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.
• 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.
• the BPPSS waveforms of the present invention are modifications of base slide show waveforms described in the aforementioned MEDEOD applications.
• 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.
• each reset pulse after the first might cause the pixel to traverse from 8 to 35 per cent reflectance.
• successive reset pulses must of course be of alternating polarity.
• 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.
• 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.
• 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.
• 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”.)
• 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.)
• 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.)
• 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.
• 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).
• 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.
• 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.
• 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.
• 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.
• 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.
• the resultant gap may be either closed up or left as a period of zero voltage. Combinations of these modifications may be used.
• FIGS 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.
• 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.
• 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.
• d the absolute value of which describes the length of each of the two voltage pulses of a BPP
• the sign of which denotes the sign of the second of the two pulses denotes the sign of the second of the two pulses.
• 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).
• 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.
• 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.
• 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.
• 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.
• the common front electrode may be held at 0, while the individual pixel electrodes are held at +V, 0 or -V.
• 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.
• the common front electrode is set to V, and the pixel electrodes to either V (zero driving voltage) or v (negative driving voltage).
• 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.
• an IMP drive scheme be capable of local updates.
• 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).
• 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.
• 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.
• 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.
• 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.
• 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.
• the base waveform of Figure 5A consists of a -15V, 400 msec pulse followed by 0 V for 20 msec.
• 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.
• 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.
• 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.
• 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.
• DTC can also be effected by excising BPP's from a base waveform.
• Table 4 shows the drive scheme shown in Table 4 below:
• 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.
• 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.
• 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.
• 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.
• a BPP might be composed of a +15 V, 20 msec pulse followed by a -5 V, 60 msec pulse.
• 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.
• 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 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.
• 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.
• 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.
• 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”):
• 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.
• 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.
• 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.
• 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 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.
• parentheses signify zero or more repeats of the sequence within the parentheses.
• the time required for any single meso-frame update is equal to the length of the longest element in the transition matrix.
• the time for a total update is three times the length of this longest element.
• the length of the meso-frame updates becomes longer, which may be the result of optimizing the waveform, the benefit becomes less substantial.
• 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.
• 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.
• 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).
• 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.
• 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.
• 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.
• 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.
• a voltage pulse i.e., a voltage differential between the two electrodes associated with that pixel
• temporal series of voltage pulses i.e., a waveform
• 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.
• 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.
• 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.
• 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:
• Waveforms that comprise a large number of time segments require the storage of a large number of bit sets of waveform data.
• 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.
• 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.
• 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).
• 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.
• 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.
• an electro-optic display e.g., a bistable electrophoretic display
• decreased temperature results in decreased electrophoretic mobility because the viscosity of the internal phase increases.
• 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.
• some display controllers include complete sets (gray m (T) -> gray n (T)) of waveforms for a select group of temperatures (Ti, T 2 , 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 T 2 , the performance of the display may be unacceptable because of higher order effects of the temperature change.
• 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.
• 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.
• the frame rate 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• amplitude means the magnitude of the voltage of the waveform compared to ground or some other floating voltage.
• 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.
• 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.

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