Patent Application
20070212022
The invention relates generally to electronic reading devices such as electronic books and electronic newspapers and, more particularly, to a method and apparatus for reducing cross talk when updating a bi-stable display.
Recent technological advances have provided “user friendly” electronic reading devices such as e-books that open up many opportunities. For example, electrophoretic displays hold much promise. Such displays have an intrinsic memory behavior and are able to hold an image for a relatively long time without power consumption. Power is consumed only when the display needs to be refreshed or updated with new information. So, the power consumption in such displays is very low, suitable for applications for portable e-reading devices like e-books and e-newspaper. Electrophoresis refers to movement of charged particles in an applied electric field. When electrophoresis occurs in a liquid, the particles move with a velocity determined primarily by the viscous drag experienced by the particles, their charge (either permanent or induced), the dielectric properties of the liquid, and the magnitude of the applied field. An electrophoretic display is a type of bi-stable display, which is a display that substantially holds an image without consuming power after an image update.
For example, international patent application WO 99/53373, published Apr. 9, 1999, by E Ink Corporation, Cambridge, Mass., US, and entitled Full Color Reflective Display With Multichromatic Sub-Pixels, describes such a display device. WO 99/53373 discusses an electronic ink display having two substrates. One is transparent, and the other is provided with electrodes arranged in rows and columns. A display element or pixel is associated with an intersection of a row electrode and column electrode. The display element is coupled to the column electrode using a thin film transistor (TFT), the gate of which is coupled to the row electrode. This arrangement of display elements, TFT transistors, and row and column electrodes together forms an active matrix. Furthermore, the display element comprises a pixel electrode. A row driver selects a row of display elements, and a column or source driver supplies a data signal to the selected row of display elements via the column electrodes and the TFT transistors. The data signals correspond to graphic data to be displayed, such as text or figures.
The electronic ink is provided between the pixel electrode and a common electrode on the transparent substrate. The electronic ink comprises multiple microcapsules of about 10 to 50 microns in diameter. In one approach, each microcapsule has positively charged white particles and negatively charged black particles suspended in a liquid carrier medium or fluid. When a positive voltage is applied to the pixel electrode, the white particles move to a side of the microcapsule directed to the transparent substrate and a viewer will see a white display element. At the same time, the black particles move to the pixel electrode at the opposite side of the microcapsule where they are hidden from the viewer. By applying a negative voltage to the pixel electrode, the black particles move to the common electrode at the side of the microcapsule directed to the transparent substrate and the display element appears dark to the viewer. At the same time, the white particles move to the pixel electrode at the opposite side of the microcapsule where they are hidden from the viewer. When the voltage is removed, the display device remains in the acquired state and thus exhibits a bi-stable character. In another approach, particles are provided in a dyed liquid. For example, black particles may be provided in a white liquid, or white particles may be provided in a black liquid. Or, other colored particles may be provided in different colored liquids, e.g., white particles in blue liquid.
Other fluids such as air may also be used in the medium in which the charged black and white particles move around in an electric field (e.g., Bridgestone SID2003—Symposium on Information Displays. May 18-23, 2003,—digest 20.3). Colored particles may also be used.
To form an electronic display, the electronic ink may be printed onto a sheet of plastic film that is laminated to a layer of circuitry. The circuitry forms a pattern of pixels that can then be controlled by a display driver. Since the microcapsules are suspended in a liquid carrier medium, they can be printed using existing screen-printing processes onto virtually any surface, including glass, plastic, fabric and even paper. Moreover, the use of flexible sheets allows the design of electronic reading devices that approximate the appearance of a conventional book.
However, cross talk, including image retention and dithering ghosting, can occur when updating a bi-stable display such as an electrophoretic display, particularly when pixels that undergo transitions between the same or substantially similar optical states are near pixels that undergo transitions between opposite or substantially different optical states. A technique is thus needed for reducing cross talk.
The invention addresses the above and other issues.
In a particular aspect of the invention, a method for driving a bi-stable display with reduced cross talk comprises: (a) accessing data defining at least first and second voltage waveforms, (b) generating the first voltage waveform for driving a first portion of the bi-stable display according to the accessed data from a first optical state to a second optical state that is close to the first optical state, and (c) generating the second voltage waveform for driving a second portion of the bi-stable display (310) according to the accessed data from the first optical state to a third optical state that is substantially different than the first optical state, such that the second voltage waveform is set to terminate at a different time than the first voltage waveform by a time difference (t2) of at least one frame period (FT). For example, the image transitions may be terminated sooner when there is a transition to a substantially different state.
In another aspect of the invention, at least part of various voltage pulses in one drive waveform applied to a subject pixel are supplied and arranged in such a way that the electric field on the subject pixel induced by the voltage pulse applied on the neighboring pixel is compensated, e.g., prior to the termination of the drive waveform. For example, when a negative voltage pulse is supplied on the neighboring pixel, a compensating positive pulse may be simultaneously supplied on the subject pixel. The compensating pulse may be at least partially overlapping with the pulse causing the cross talk.
A related electronic reading device and program storage device are also provided.
In the drawings:
In all the Figures, corresponding parts are referenced by the same reference numerals.
Each of the following is incorporated herein by reference:
European patent application EP 02078823.8, entitled “Electrophoretic Display Panel”, filed Sep. 16, 2002 (docket no. PHNL 020844);
European patent application EP 03100133.2, entitled “Electrophoretic display panel”, filed Jan. 23, 2003 (docket no. PHNL 030091);
European patent application EP 02077017.8, entitled “Display Device”, filed May 24, 2002, or WO 03/079323, Electrophoretic Active Matrix Display Device”, published Feb. 6, 2003 (docket no. PHNL 020441); and
European patent application EP 03101705.6, entitled “Electrophoretic Display Unit”, filed Jun. 11, 2003 (docket no. PHNL 030661).
Each picture element 2 has an appearance determined by the position of the charged particles 6 between the electrodes 3 and 4. Electrophoretic media 5 are known per se, e.g., from U.S. Pat. Nos. 5,961,804, 6,120,839, and 6,130,774 and can be obtained, for instance, from E Ink Corporation.
As an example, the electrophoretic medium 5 may contain negatively charged black particles 6 in a white fluid. When the charged particles 6 are near the first electrode 3 due to a potential difference of, e.g., +15 Volts, the appearance of the picture elements 2 is white. When the charged particles 6 are near the second electrode 4 due to a potential difference of opposite polarity, e.g., −15 Volts, the appearance of the picture elements 2 is black. When the charged particles 6 are between the electrodes 3 and 4, the picture element has an intermediate appearance such as a grey level between black and white. An application-specific integrated circuit (ASIC) 100 controls the potential difference of each picture element 2 to create a desired picture, e.g. images and/or text, in a full display screen. The full display screen is made up of numerous picture elements that correspond to pixels in a display.
The reading device controller 330 may be part of a computer that executes any type of computer code devices, such as software, firmware, micro code or the like, to achieve the functionality described herein. Accordingly, a computer program product comprising such computer code devices may be provided in a manner apparent to those skilled in the art. The reading device controller 330 may further comprise a memory (not shown) that is a program storage device that tangibly embodies a program of instructions executable by a machine such as the reading device controller 330 or a computer to perform a method that achieves the functionality described herein. Such a program storage device may be provided in a manner apparent to those skilled in the art.
The display ASIC 100 may have logic for periodically providing a forced reset of a display region of an electronic book, e.g., after every x pages are displayed, after every y minutes, e.g., ten minutes, when the electronic reading device 300 is first turned on, and/or when the brightness deviation is larger than a value such as 3% reflection. For automatic resets, an acceptable frequency can be determined empirically based on the lowest frequency that results in acceptable image quality. Also, the reset can be initiated manually by the user via a function button or other interface device, e.g., when the user starts to read the electronic reading device, or when the image quality drops to an unacceptable level.
The ASIC 100 provides instructions to the display addressing circuit 305 for driving the display 310 by accessing information stored in the memory 320.
The invention may be used with any type of electronic reading device.
Various user interface devices may be provided to allow the user to initiate page forward, page backward commands and the like. For example, the first region 442 may include on-screen buttons 424 that can be activated using a mouse or other pointing device, a touch activation, PDA pen, or other known technique, to navigate among the pages of the electronic reading device. In addition to page forward and page backward commands, a capability may be provided to scroll up or down in the same page. Hardware buttons 422 may be provided alternatively, or additionally, to allow the user to provide page forward and page backward commands. The second region 452 may also include on-screen buttons 414 and/or hardware buttons 412. Note that the frame around the first and second display regions 442, 452 is not required as the display regions may be frameless. Other interfaces, such as a voice command interface, may be used as well. Note that the buttons 412, 414; 422, 424 are not required for both display regions. That is, a single set of page forward and page backward buttons may be provided. Or, a single button or other device, such as a rocker switch, may be actuated to provide both page forward and page backward commands. A function button or other interface device can also be provided to allow the user to manually initiate a reset.
In other possible designs, an electronic book has a single display screen with a single display region that displays one page at a time. Or, a single display screen may be partitioned into or two or more display regions arranged, e.g., horizontally or vertically. Furthermore, when multiple display regions are used, successive pages can be displayed in any desired order. For example, in
Additionally, note that the entire page need not be displayed on the display region. A portion of the page may be displayed and a scrolling capability provided to allow the user to scroll up, down, left or right to read other portions of the page. A magnification and reduction capability may be provided to allow the user to change the size of the text or images. This may be desirable for users with reduced vision, for example.
Problem Addressed
In order to increase the response speed of a bi-stable display such as an electrophoretic display and reduce optical flicker, it is desirable to resistively drive the display. In displays based upon electrophoretic particles in films comprising capsules (E-Ink Corp.) or micro-cups (SiPix Group), adhesive layers and binder layers are required for the construction. To achieve a resistive driving mode and increase the response speed, it is therefore necessary to reduce the conductivity of these components. However, this unavoidably leads to lateral cross talk, where a portion of the electric field associated with one pixel is inadvertently spread to a neighboring pixel, causing the neighboring pixel to become partially switched to the wrong color, e.g., grey level. The neighboring pixel can be an adjacent pixel or other pixel that is sufficiently close to experience cross talk. This is quite visible where a pixel driven to an extreme optical state, e.g., black or white, is situated adjacent to a pixel that is gently driven or not driven at all. This situation is frequently encountered where additional grey levels are achieved using spatial dithering techniques.
For example, consider a portion of a screen that switches from a black and white block image to a checkerboard-like, spatially dithered, mid-grey pattern, with each adjacent pixel being either black or white. In the black region, those pixels that must become white are driven with a negative voltage, while those that should remain black are not driven. However, due to the cross talk effect, a portion of the drive voltage is transferred to these black pixels, which are inadvertently driven toward white, becoming a grey color at the end of the image update. Consequently, the central portion of the checkerboard pattern becomes too light in color. In contrast, in the white region, those pixels that must become black are driven with a positive voltage, while those that should remain white are not driven. However, due to the cross talk effect, a portion of the drive voltage is transferred to these white pixels, which are inadvertently driven toward black, becoming a grey color at the end of the image update. Consequently, the outer portion of the checkerboard pattern becomes too dark in color.
Proposed Solution
In accordance with the invention, a technique is provided for driving a bi-stable display such as an electrophoretic display with reduced cross talk, including reduced color or grey level error, image retention and dithering ghosting. To achieve this, in one aspect of the invention, at least part of various voltage pulses in one drive waveform applied to a pixel are supplied and arranged in such a way that the electric field on a pixel induced by the voltage pulse applied on the neighboring pixel is immediately compensated. For example, when a negative voltage pulse is supplied on the neighboring pixel of a subject pixel, a positive pulse is simultaneously supplied on the subject pixel. In another aspect of the invention, the drive waveforms are aligned in such a way that, during an image update period, the image transitions from a given optical state to a substantially similar or otherwise close optical state (e.g., black-to-black, white-to-white or black-to-dark gray) are terminated at different times than for transitions from the same initial optical state to a substantially different state (e.g., black-to-white or white-to-black). The optical states that are close to one another include states that are the same as well as states that vary by one or a small number of greyscale or color levels. For example, with four greyscale levels, states that differ by one greyscale level are relatively close to one another. Examples include black and dark grey, dark grey and light grey, and light grey and white. With sixteen greyscale levels, states that differ by, e.g., up to three greyscale levels, may be considered close. In another approach, states that differ by a certain fraction of the greyscale, such as one fourth, e.g., four levels out of sixteen, may be considered close.
In a further aspect of the invention, the drive waveforms are terminated later for transitions between the substantially similar or otherwise close states than for transitions between substantially different states. A dithering pattern may be created by the transition to the substantially different state together with the corrected initial state.
The drive waveforms, in particular those applied for updating pixels without a substantial optical state change, include at least two voltage pulses with opposite polarity, where the last voltage pulse brings the particles toward the desired final optical state.
In the waveform diagrams discussed herein, the vertical lines represent frame boundaries and the horizontal axis represents time. The vertical axis represents a voltage.
For example, pulse width modulation (PWM) may be used with voltages of −15 V, 0 V and +15 V. A frame time or period is indicated as FT. Extreme drive pulses refer to drive pulses in a drive waveform that bring the particles to one of the two extreme positions near one of the two electrodes.
In waveform 520, four voltage pulses are used. The first extreme drive (ED1) pulse and the second extreme drive (ED2) pulse drive the display to the desired white state. The additional pulses (A1, A2) are used for increasing the white state stability and reducing the residual voltage on the pixel in the transition. In waveform 500, triple pulses are used, where the first positive extreme drive pulse (ED1) acts to pin or reinforce the black state, e.g., by ensuring the pixel does not drift away from the pure black state, and reduce residual voltage. An additional negative pulse (A) brings the particles away from the electrode, and the second extreme drive pulse (ED2) sends or drives the particles back close to the electrode, i.e., to the desired final black state.
The duration/energy of the first extreme drive pulse (ED1) in waveform 500 is determined by the amount of residual voltage involved in voltage pulses A and ED2, i.e., the sum of the energy involved in A and ED2 pulses, and the pulse configuration of the drive waveform on the neighboring pixel when, for example, applying waveform 520 for a black to white transition. The timing/position of this extreme drive pulse is also determined by the pulse configuration of the drive waveform on the neighboring pixel. For example, in waveform 520, a positive pulse (A1) with duration of two FTs is followed by a negative pulse (ED1) with duration of seven frames, after which a two-frame positive pulse (A2) is applied, followed by a negative pulse (ED2) with duration of eleven frames. When waveform 520 is applied to drive a first pixel, or group of pixels, from B to W, where the first pixel is a neighboring pixel of a second, black pixel that is driven by the waveform 500, the second black pixel will receive portions of the electric field from the waveform 520 as a result of cross talk. The cross talk effect of the first positive pulse (A1) in 520 does not change the optical state of the neighboring first pixel because the second pixel is in the black state. However, the second negative pulse (ED1) in 520 induces a brightness drift on the second pixel toward dark grey.
To avoid this brightness drift, a positive pulse (ED1, waveform 500) is applied on the second pixel to compensate for the cross talk effect from the negative pulse (ED1) in the waveform 520. The positive pulse may be at least partially concurrent, e.g., overlapping, with the negative pulse. After completion of this positive pulse in waveform 500, a larger negative pulse (A) is applied on the black pixel, followed by a positive pulse (ED2) to achieve the desired final brightness and brightness decay curve after addressing. Moreover, the drive waveform 500 for the black-to-black transition is terminated at a different time than for the black-to-white transition, as indicated by the time difference t2, to completely compensate the cross talk. In this example, the drive waveform (500) is terminated substantially later than for the drive waveform (520).
The time period difference (t2) between the termination times of the waveforms 500, 520 is at least one frame time (FT). t1 denotes the duration of the earlier-ending waveform, e.g., waveform 520. t2/(t1+t2)×100% may be larger than about 5-15%, for instance, depending on the ambient temperature. Refer to
In the above example, a pulse-width modulated (PWM) driving scheme is used. In case other driving schemes, e.g., using voltage modulated (VM) or combined VM and PWM are used, the energy involved in the various pulses may be used in place of pulse duration. The energy of a voltage pulse is the product of the voltage amplitude (V) and time duration (t): i.e., t*V.
The second embodiment of the invention is illustrated in
The third embodiment of the invention is illustrated in
A shaking pulse is defined as a voltage pulse representing energy sufficient for releasing particles at their present position but insufficient for moving the particles from their present position to one of the extreme positions. In this example, the shortened frame time (FT′) of these shaking pulses is shorter than the frame time (FT) used for other portions of the waveform, to reduce the flicker induced by shaking pulses. Shaking pulses are discussed in the above-mentioned European patent application EP 02077017.8 (docket no. PHNL 020441). The use of shaking pulses results in a more accurate greyscale because the dwell time and image history effects can be reduced. Again, the time period difference for the termination of both transitions is at least one frame time duration, and t2/(t1+t2)×100% may be larger than about 5-15%, for instance, depending on the ambient temperature.
The fourth embodiment of the invention is illustrated in
General Remarks
The invention is applicable to any image transition to correct the brightness of a pixel affected by cross talk from a neighbouring pixel. In particular, the drive waveforms for the image transitions between substantially similar or otherwise close optical states are terminated at a substantially different time, e.g., later, than drive waveforms for image transitions between substantially different states, which are used to create a spatial dithering pattern. It is not necessary to adjust the termination of the other transitions in this manner. Moreover, shaking pulses are optional. A set of shaking pulses can be used anywhere during a drive waveform, and a set of shaking pulses may include one or more shaking pulses.
Note also that, in the above examples, pulse-width modulated (PWM) driving is used for illustrating the invention, where the pulse time is varied in each waveform while the voltage amplitude is kept constant. However, the invention is also applicable to other driving schemes, e.g., based on voltage modulated driving (VM), where the pulse voltage amplitude is varied in each waveform, or combined PWM and VM driving. The invention is applicable to color as well as greyscale bi-stable displays. Also, the electrode structure is not limited. For example, a top/bottom electrode structure, honeycomb structure, an in-plane switching structure or other combined in-plane-switching and vertical switching may be used. Moreover, the invention may be implemented in passive matrix as well as active matrix electrophoretic displays. In fact, the invention can be implemented in any bi-stable display that does not consume power while the image substantially remains on the display after an image update. Also, the invention is applicable to both single and multiple window displays, where, for example, a typewriter mode exists.
While there has been shown and described what are considered to be preferred embodiments of the invention, it will, of course, be understood that various modifications and changes in form or detail could readily be made without departing from the spirit of the invention. It is therefore intended that the invention not be limited to the exact forms described and illustrated, but should be construed to cover all modifications that may fall within the scope of the appended claims.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IB05/51060 | 3/29/2005 | WO | 9/23/2006 |
| Number | Date | Country | |
|---|---|---|---|
| 60557499 | Mar 2004 | US |
