(12) INTERNATIONAL APPLICATION PUBLISHED UNDER THE PATENT COOPERATION TREATY (PCT)
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`(19) World Intellectual Property Organization
`International Bureau
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`(43) International Publication Date
`6 July 2006 (06.07.2006)
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`.,
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`9:"
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`'
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`(51) International Patent Classification:
`609G 3/34 (2006.01)
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`(21) International Application Number:
`PCT/IB2005/054377
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`(22) International Filing Date:
`22 December 2005 (22.12.2005)
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`(25) Filing Language:
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`(26) Publication Language:
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`English
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`English
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`(30) Priority Data:
`041069873
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`27 December 2004 (27.12.2004)
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`EP
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`(71) Applicant (for all designated States except US): KONIN-
`KLIJKE PHILIPS ELECTRONICS NV.
`[NL/NL];
`Groenewoudseweg 1, NL—5621 BA Eindhoven (NL).
`
`(72) Inventors; and
`(75) Inventors/Applicants (for US only): VAN WOUDEN-
`BERG, Roel [NL/NL]; c/o Prof. Holstlaan 6, NL75656
`AA Eindhoven (NL). GROOT HULZE, Hendrikus, W.
`[NL/NL]; c/o Prof. Holstlaan 6, NL—5656 AA Eindhoven
`(NL). STESSEN, Jeroen, H., C., J. [NL/NL]; c/o Prof.
`Holstlaan 6, NL—5656 AA Eindhoven (NL). SEVO,
`Aleksandar [YU/NL]; c/o Prof. Holstlaan 6, NL—5656
`AA Eindhoven (NL). HEKSTRA, Gerben, J. [NL/NL];
`
`(54) Title: SCANNING BACKLIGHT FOR LCD
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`(10) International Publication Number
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`WO 2006/070323 A1
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`c/o Prof. Holstlaan 6, NL—5656 AA Eindhoven (NL).
`BLANKERS, Hendrik, J. [NL/NL]; c/o Prof. Holstlaan
`6, NL—5656 AA Eindhoven (NL). KOVACEVIC, Vedran
`[BA/NL]; c/o Prof. Holstlaan 6, NL—5656 AA Eindhoven
`(NL). DEN BREEJEN, Jeroen [NL/NL]; c/o Prof. Hol—
`stlaan 6, NL—5656 AA Eindhoven (NL).
`
`Agents: DAMEN, Daniel, M. et al.; Prof. Holstlaan 6,
`NL—5656 AA Eindhoven (NL).
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`Designated States (unless otherwise indicated, for every
`kind of national protection available): AE, AG, AL, AM,
`AT, AU, AZ, BA, BB, BG, BR, BW, BY, BZ, CA, CH, CN,
`CO, CR, CU, CZ, DE, DK, DM, DZ, EC, EE, EG, ES, FI,
`GB, GD, GE, GH, GM, HR, HU, ID, IL, IN, IS, JP, KE,
`KG, KM, KN, KP, KR, KZ, LC, LK, LR, LS, LT, LU, LV,
`LY, MA, MD, MG, MK, MN, MW, MX, MZ, NA, NG, NI,
`NO, NZ, OM, PG, PH, PL, PT, RO, RU, SC, SD, SE, SG,
`SK, SL, SM, SY, TJ, TM, TN, TR, TT, TZ, UA, UG, US,
`UZ, VC, VN, YU, ZA, ZM, ZW.
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`(74)
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`(81)
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`(84)
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`Designated States (unless otherwise indicated, for every
`kind of regional protection available): ARIPO (BW, GH,
`GM, KE, LS, MW, lVIZ, NA, SD, SL, SZ, TZ, UG, ZM,
`ZW), Eurasian (Al\/l, AZ, BY, KG, KZ, MD, RU, TJ, TM),
`European (AT, BE, BG, CH, CY, CZ, DE, DK, EE, ES, FI,
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`[Continued on next page]
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`6/070323A1|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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`102
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`105
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`104
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`(57) Abstract: A method for displaying images on a display having backlight is disclosed, where the images is updated periodically
`c with a period. The method comprises the steps of: generating a signal with a pulse pattern for each period depending on the contents
`9 of an image to be displayed in that period; and activating backlight in accordance with the signal. Further, a display (100) comprising
`a 1sp ay pane
`an 21 ac
`1g t unit, w erein t e ac
`1g t unit comprises a contro er
`an a 1g ting ev1ce1s
`isc ose .
`Nd'l
`l(102)dbkl'h'h'hbkl'h'
`'
`ll(104)dl'h'd"d'ld
`The controller (104) is arranged to generate a control signal, and the lighting device is arranged to provide backlight to the display
`panel (102) according to the control signal, wherein the control signal comprises a pulse pattern depending on contents of displayed
`images.
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`WO 2006/070323 A1
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`Published:
`FR, GB, GR, HU, IE, IS, IT, LT, LU, LV, MC, NL, PL, PT,
`RO, SE, SI, SK, TR), OAPI (BF, BJ, CF, CG, CI, CM, GA, — with international search report
`GN, GQ, GW, ML, MR, NE, SN, TD, TG).
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`Declaration under Rule 4.17:
`— as to applicant’s entitlement to apply for and be granted a
`patent (Rule 4.1 7(ii))
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`For two—letter codes and other abbreviations, refer to the ”Guid—
`ance Notes on Codes and Abbreviations ” appearing at the begin—
`ning of each regular issue of the PCT Gazette.
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`WO 2006/070323
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`PCT/IB2005/054377
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`Scanning backlight for LCD
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`Technical field
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`The present invention relates to a method and a display, wherein backlight is
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`generated depending on contents of displayed images.
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`Background of the invention
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`LCD (Liquid Crystal Display) panels suffer from motion blur due to their
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`sample-and-hold nature, i.e. the LC (Liquid Crystal) remains in the same state after
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`addressing during a whole frame. When displayed objects move, as is the case in e. g. TV
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`images, this causes a blurred image of the objects on the retina of a Viewer. In US
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`2004/0012551 A, it is disclosed a means to drive the data for the value corresponding to a
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`present frame display data. By comparing with previous frame of display data, the display
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`data in the present frame that have changes are then over emphasized and written into the
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`LCD driver with more than the amount of change to the picture element data. Further, a
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`backlight control means to control the lighting delay time, the lighting time width, the
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`lighting time interval and the number of times of lighting within one frame of a LCD
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`backlighting is disclosed. However, there is a need for improved backlight control to avoid a
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`flickering image.
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`Summm of the invention
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`It is therefore an object of the present invention to provide an improved
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`method for displaying images on a display, and an improved display.
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`The above object is achieved according to a first aspect of the present
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`invention by a method for displaying images on a display having backlight, wherein the
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`images are updated periodically with a period. The method comprises the steps of: generating
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`a signal with a pulse pattern for each period depending on contents of an image to be
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`displayed in that period; and activating the backlight in accordance with said signal.
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`An advantage of this is that the backlighting is depending on the contents of
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`the displayed images for providing an image that is experienced as less flickering.
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`2
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`The backlight may comprise a plurality of lighting units and each lighting unit
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`is associated with a part of the display, wherein the steps of generating a signal and activating
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`backlight are separately adapted to each of the parts.
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`An advantage of this is that an image comprising contents with very different
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`contents in different parts is improved in each part.
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`The pulse pattern may comprise a plurality of pulses for each period when
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`contents of the displayed image comprise relatively high brightness.
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`An advantage of this is that a viewer often experiences a bright image as more
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`flickering, but this is compensated for by increasing the backlighting frequency for such
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`images.
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`The term “relatively high brightness” should in this context be construed to be
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`a brightness essentially higher than an average brightness of an average image.
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`The plurality of pulses may be symmetrical during said period when contents
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`of displayed images comprise low changes between subsequent images.
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`An advantage of this is optimal reduced flickering when an image is relatively
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`static, i.e. when a viewer would experience flickering the most, and the equal distribution
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`would not introduce any blurring.
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`The plurality of pulses may be asymmetrical during said period when contents
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`of displayed images comprise high changes between subsequent images.
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`An advantage of this is reduced flickering, and counteracting blurring by
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`distributing the pulses asymmetrically when there is a lot of motion in the image.
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`The pulse pattern may comprise one pulse for each period when contents of
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`displayed images comprise high changes between subsequent images and relatively low
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`brightness.
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`The term “relatively low brightness” should in this context be construed to be
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`a brightness essentially lower than an average brightness of an average image.
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`An advantage of this is optimized counteracting of blurring, while there is
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`little or no experienced flickering due to low brightness.
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`By symmetrical pulses, it is meant that the pulse in each half of the frame
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`period is symmetrical in effective brightness and position, and for higher multiples of
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`frequency, for each corresponding fraction of frame period. By asymmetrical pulses, it is
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`meant that the pulse in each half of the frame period is symmetric in effective brightness
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`and/or position, and for higher multiples of frequency, for each corresponding fraction of
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`frame period. Effective brightness depend on pulse amplitude and/or width.
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`3
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`Where contents change, the method may further comprise the steps of:
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`generating said signal with a first pattern; generating said signal with intermediate patterns;
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`and generating said signal with a second pattern, wherein said intermediate patterns are such
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`that an average value of said signal is kept constant upon a transition from said first pattern to
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`said second pattern.
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`An advantage of this is a seamless transition from one backlighting pattern to
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`another, without any brightness dips or peaks. This is particularly advantageous when
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`transition from one backlighting pattern to another is performed within a single image, i.e.
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`from one part to another.
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`Where the first pattern is a single pulse for each period, and the second pattern
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`is two symmetrical pulses, the intermediate patterns may be two pulses with different
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`effective pulse brightnesses. Where the first pattern is two symmetrical pulses, and the
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`second pattern is a single pulse for each period, the intermediate patterns may be two pulses
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`with different effective pulse brightnesses. An aggregated effective pulse brightness of said
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`pulses within each period may be constant.
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`An advantage of this is an efficient way to seamlessly transition from one
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`backlighting scheme to another.
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`The above object is achieved according to a second aspect of the present
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`invention by a display comprising a display panel and a backlight unit, wherein the backlight
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`unit comprises a controller and a lighting device, wherein the controller is arranged to
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`generate a control signal, and the lighting device is arranged to provide backlight to the
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`display panel according to the control signal, wherein the control signal comprises a pulse
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`pattern depending on contents of displayed images.
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`The backlight unit may comprise a plurality of lighting devices, and each
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`lighting device is associated with a part of the display, and the control signal is separately
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`adapted to each of the parts.
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`The advantages of the second aspect of the present invention are essentially
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`the same as those of the first aspect.
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`30
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`Brief description of the drawings
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`The above, as well as additional objects, features and advantages of the
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`present invention, will be better understood through the following illustrative and non-
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`lirniting detailed description of preferred embodiments of the present invention, with
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`reference to the appended drawings, wherein:
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`4
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`Fig. 1 illustrates a display according to an embodiment of the present
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`invention;
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`Fig. 2 is a mode transition diagram showing transition between two modes via
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`intermediate modes;
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`Fig. 3 is a mode transition diagram showing transition between modes related
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`to image contents;
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`Fig. 4 is a flow chart illustrating a method according to an embodiment of the
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`invention;
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`invention.
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`Fig. 5 is a flow chart illustrating a method for mode transition;
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`Figs. 6-20 are pulse diagrams; and
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`Fig. 21 illustrates a display according to another embodiment of the present
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`Detailed description of the preferred embodiments
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`Fig. 1 illustrates a display 100 comprising a display panel 102. The display
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`panel 102, which can be a LCD (Liquid Crystal Display) panel, is provided with backlighting
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`105. The backlighting 105 can for example comprise one or more light sources (not shown),
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`such as light emitting diodes (LEDs) or gas discharge lamps. The backlight is flashed, either
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`for the entire panel 102 or, preferably, by scanning backlight segments of the panel 102.
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`Thus, an LC cell is illuminated only for a certain fraction of the frame time. A backlight
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`controller 104, which is connected to the backlighting 105 of the panel 102, controls
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`backlight flashing. To avoid large area flicker, the backlight controller 104 provides a
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`backlight control signal which is dependent on an image displayed on the panel 102.
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`Therefore, the backlight controller 104 is connected to a display controller 106, which in turn
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`receives image data from an image data source 108. It should be noted that this description is
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`for illustrative purpose, and both the backlight controller 104 and the display controller 106
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`can be a common video controller, or divided between two or more units, which provide the
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`same fimction as the backlight and display controllers 104, 106. The data source 108 can be a
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`TV decoder, a DVD player, a computer, or any other means providing images to be viewed
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`on the display 100.
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`An effective way to reduce large area flicker and achieving motion blur
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`reduction would be to drive the panel at a higher refresh rate and use motion-compensated
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`video up-conversion to achieve a higher video rate with smooth motion. For an LCD it is
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`however not possible to increase refresh rate above 7 5-80 Hz. Moreover, it is very expensive
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`5
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`to up-convert video signals with motion compensation. The present invention provides a less
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`expensive way to achieve less flicker and less motion blurring.
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`To achieve this, the backlight is operated at double refresh frequency, or a
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`higher multiple. This introduces a higher frequency brightness modulation, which is far
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`above the flicker threshold, even for a white image.
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`To provide a clearer view in examples provided below, Figs 6-20 illustrate a
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`plurality of pulse patterns in pulse diagrams, which will be referred to in the description of
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`the embodiments. It should be noted that the pulse diagrams show principles, from which the
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`artisan is able to understand the spirit of the invention according to the embodiments
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`presented below, and pulse shapes, widths, amplitudes and positions, as well as ways of
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`transition from one pulse pattern to another via intermediate pulse patterns, are simplified to
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`avoid obscuring the basic ideas of the present invention.
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`Fig. 6 is a pulse diagram illustrating a single pulse per frame period, i.e. one
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`pulse is provided for each period of refresh of the display. The effective brightness produced
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`by the pulse, by controlling a light generating means, or regarding the pulse as an output of
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`the light generating means, is dependent on the pulse width and the amplitude of the pulse.
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`Fig. 7 is a pulse diagram illustrating a symmetrical double pulse, i.e. there is
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`provided two pulses for each frame period and the pulses in each half of the frame period is
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`symmetrical in effective brightness and position.
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`Fig. 8 is a pulse diagram illustrating an asymmetrical double pulse, which
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`pulses are symmetric in position, but asymmetric in effective brightness, i.e. there are two
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`pulses for each frame period that are symmetric in position, but the pulse in each half of the
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`frame period is asymmetric in effective brightness. Thus, the double pulse, considered as a
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`whole, is asymmetric.
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`Fig. 9 is a pulse diagram illustrating an asymmetrical single pulse, where the
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`pulse is asymmetrical in sense of position.
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`Fig. 10 is a pulse diagram illustrating an asymmetrical double pulse where the
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`pulse pattern is asymmetrical in sense of effective brightness, since the amplitudes of the
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`pulsed differ.
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`Fig. 11 is a pulse diagram illustrating a double pulse pattern, where the two
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`pulses are close together to achieve a lighting effect relatively similar to a single pulse pattern
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`as illustrated in Fig. 6, and are therefore referred to as a quasi-single pulse.
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`Fig. 12 is a pulse diagram illustrating a double pulse pattern, where the two
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`pulses provide very different effective brightness by having very different pulse widths. Also
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`6
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`with this pattern, a lighting effect relatively similar to a single pulse pattern as illustrated in
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`Fig. 9 is achieved, and is therefore also referred to as a quasi-single pulse. Fig. 13 illustrates
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`an even more extreme quasi- single pulse pattern, where two pulses are very different in both
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`pulse width and amplitude.
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`Fig. 14 is a pulse diagram illustrating a transition between two pulse patterns,
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`where a brightness peak occurs at the transition. During a period, here marked by a bracket,
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`the average pulse width and amplitude are higher than over other periods, and a brightness
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`peak can be experienced by a viewer.
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`Fig. 15 is a pulse diagram illustrating a transition between two pulse patterns,
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`where a brightness dip occurs at the transition. During a period, here marked by a bracket, the
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`average pulse width and amplitude are lower than over other periods, and a brightness dip
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`can be experienced by a viewer.
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`Fig. 16 is a pulse diagram illustrating a first pulse pattern with eight
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`symmetrical pulses, and a transition to another pulse pattern with three symmetrical pulses
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`via an intermediate pulse pattern, which is asymmetrical and comprises five pulses.
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`Fig. 17 is a pulse diagram illustrating a transition from a quasi-single pulse
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`pattern, similar to that illustrated in Fig. 12, to a symmetrical pulse pattern, similar to that
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`illustrated in Fig. 7, via an intermediate pulse pattern, here illustrated similar to the quasi-
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`single pulse pattern as illustrated in Fig. 11. It should be noted that a transition via
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`intermediate pulse patterns normally comprises more patterns to achieve a seamless
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`transition, and Fig. 17 illustrates the principle to avoid a brightness peak, which would occur
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`as illustrated in Fig. 14.
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`Fig. 18 illustrates the use of intermediate pulse patterns when a transition is to
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`be made to a more extreme pulse pattern.
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`Fig. 19 illustrates transition from a single pulse pattern to a double pulse
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`pattern via a quasi-single pulse pattern as intermediate pulse pattern.
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`Fig. 20 is a pulse diagram illustrating an instantaneous transition without
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`intermediate pulse patterns when a scene shift is occurring. This is possible, since brightness
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`dips or peaks would not be visible at a scene shift. Thus, no transition using intermediate
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`pulse patterns is needed.
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`The operation will be described with an example using double pulses in a
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`display refresh period, i.e. double frequency, but the same principle applies for three or more
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`pulses in a period, i.e. higher multiples of frequency.
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`7
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`For a perfect flicker reduction, these two pulses need to be spaced exactly half
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`a frame distance apart and to have the exactly the same brightness, i.e. symmetrical pulses as
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`illustrated in Fig. 7, resulting in a pure double frequency backlight pulsing. It is observed for
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`50 Hz display refresh and double flashing, flicker is already visible when the two pulses
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`differ 0.5 % in brightness at a total display brightness of 500 cd/m2 and for 60 Hz display
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`refresh and double flashing, flicker is visible at 3.5 % difference in brightness between the
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`pulses.
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`The lamps are preferably operated at a fixed current. Therefore, the backlight
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`brightness modulation is preferably done using pulse width modulation. The pulses can also
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`comprise a series of even higher frequency pulses, i.e. the modulation can be done by pulse
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`number modulation of pulse trains. Further, the amplitude of the pulses can be modulated,
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`and a combination of the above mentioned backlight modulation techniques can be applied.
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`Flicker is most visible in bright scenes with little or no motion, although
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`flicker also is visible in bright scenes with a lot of motion, but in the latter case, motion blur
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`problems increase. For example, when a bright scene with some or a lot of motion is paused,
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`flicker becomes more visible, but motion blur problems, of course, disappear. Therefore, the
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`backlight is operated in double pulse mode, with the two pulses in the frame exactly spaced
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`at half a frame distance, and with exactly the same brightness for the two pulses, when the
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`flicker problem is the most apparent.
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`When there is some or a lot of motion in the scene, it is only needed to
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`introduce a bit of higher frequency content in the brightness modulation. Therefore, backlight
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`is operated with two pulses spaced at half a frame distance, but with different brightness of
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`the pulses. A first pulse, half a frame period earlier than the second pulse takes care of
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`reducing the flicker to a large extent, while it is sufficiently low in brightness not to cause a
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`clear double image or to cause blur. The second pulse gives the main brightness.
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`Alternatively, two pulses of same brightness can be moved closer together, as
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`illustrated in Fig. 11, to improve moving image quality compared to distributing the pulses at
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`half frame period distance and at the same time having some higher frequencies in the
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`display brightness to reduce flicker. The reduction of motion blur is now due to that the two
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`illuminated images in this case of asymmetrically distributed pulses are closer in time.
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`By asymmetrically distributed pulses, it is meant that the pulse in each half of
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`the frame period is asymmetric in effective brightness and position, and for higher multiples
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`of frequency, for each corresponding fraction of frame period.
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`It is observed that for a total duty cycle of 40 %, the flicker of a 25 % to 75 %
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`pulse ratio is the same as of two pulses of 20 % duty cycle each separated by approximately
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`2/7 of a frame period, center to center. It is also observed that moving image quality is very
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`similar for these two cases for both natural scenes and edge quality.
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`When there is little or no motion and the scene is not too bright, it is preferable
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`to use the asymmetrical pulse distribution. However, in this case it is not critical, and the
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`backlight mode can be chosen arbitrarily, preferably in a way to avoid mode change.
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`When there is a lot of motion and the scene is not too bright, no flicker
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`reduction is needed, and a single or quasi-single pulse backlight operation can be used to
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`achieve best performance for scenes with a lot of motion.
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`Fig. 2 is a mode transition diagram showing transition between two modes
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`200, 202 via intermediate modes 206, 208. If a direct transition to another mode is performed
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`instantaneously, the effect could be that there is a larger gap between the last pulse of the first
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`mode and the first pulse of the second mode, causing a brightness dip due to that the average
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`value of the pulses temporarily dips, as illustrated in Fig. 15, or that there is a smaller gap
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`between the last pulse of the first mode and the first pulse of the second mode, causing a
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`brightness peak, as illustrated in Fig. 14. To avoid these backlight dips or peaks during
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`change of backlight mode, intermediate modes 206, 208 are formed to achieve a seamless
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`transition.
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`To illustrate this, the operation will be described for double pulse as in the
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`example above in relation to Fig. 1, i.e. double frequency, but as above, the same principle
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`applies for three or more pulses, i.e. higher multiples of frequency. For an illustrative
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`example, transition is to be performed between a single pulse mode 200 to a symmetrical
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`double pulse mode 202. This can for example be the case when a scene with low brightness
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`and a lot of motion changes to high brightness and little or no motion.
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`A first transition 210 is performed to a first intermediate mode 206. This mode
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`can be a double pulse mode with asymmetrical pulses, e. g. a pulse width ratio of 5 % to 95
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`%, and only a small distance between the pulses, i.e. a double pulse pattern that is relatively
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`similar to the single pulse pattern. A second transition 212 is then performed to a second
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`intermediate mode (not shown) with two pulses with less asymmetry, and then further
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`transitions to intermediate modes with more and more symmetry to a transition 214 to a last
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`intermediate mode 208 where the pulse width ratio between the pulses is almost 50 % to 50
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`% and the distance between the pulses is almost a half frame distance, center to center. A last
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`transition 216 is performed is performed to the symmetrical double pulse mode 202, where
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`the pulse width ratio is exactly 50 % to 50 %, and the distance between the pulses is exactly a
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`half frame distance, center to center. The transition between the modes 200, 202 is then
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`complete, and performed such that a viewer do not experience any dips or peaks in
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`brightness. The transitions 210, 212, 214, 216 can be performed between each frame, or
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`between each couple of frames.
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`Alternatively, the transition is performed, as illustrated in Fig. 19, by forming
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`a quasi-single pulse pattern with two pulses with equal effective brightness, and then
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`separating the pulses in one or more steps to get to the symmetrical pulse pattern.
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`The same applies with transition from symmetrical double pulse mode 202 to
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`single pulse mode 200 via intermediate modes 208, 206 and transitions 218, 220, 222, 224.
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`This example illustrated transition between single pulse mode and symmetrical
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`double pulse mode. The same principle applies between other modes, e. g. between single
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`pulse mode and asymmetrical double pulse mode, and between symmetrical and
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`asymmetrical double pulse modes. Further, the principle is also applicable to multi pulse
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`modes. The general principle of the transitions is to insert intermediate modes that gradually
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`change the pulse patterns from one mode to another to avoid brightness dips or peaks.
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`When there is a change of scene, a transition can be made directly from the
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`first mode 200 to the second mode 202 by a direct transition 226, and from the second mode
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`202 to the first mode 200 by a direct transition 228. A control signal, from e. g. the display
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`controller, would enable the backlight controller to do such direct transitions 226, 228.
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`Fig. 3 is a mode transition diagram showing transitions 300, 302, 304, 306,
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`308, 310 between modes 312, 314, 316 related to image contents. Each of the transitions 300,
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`302, 304, 306, 308, 310 can comprise intermediate modes, as illustrated in Fig. 2. Three
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`modes 312, 314, 316 are illustrated as an example, e.g. single pulse mode 312, asymmetrical
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`double pulse mode 314, and symmetrical double pulse mode 316. However, more modes can
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`be comprised, e.g. different quasi-single pulse modes, asymmetrical modes, and modes with
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`three or more pulses.
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`Fig. 4 is a flow chart illustrating a method according to an embodiment of the
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`invention. In a content determination step 400, the contents of the image is determined.
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`Contents can comprise brightness of the image or a part of the image, and presence of motion
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`in the image. A backlight control signal is generated in a backlight generation step 402 in
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`dependence on the determined contents. Examples of this dependence is described above.
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`Backlight is then activated based upon the backlight control signal in a backlight generation
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`step 404. The backlight is activated with a backlight driver driving lamps or LEDs.
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`Fig. 5 is a flow chart illustrating a method for mode transition. In a first pattern
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`signal generation step 500, a backlight control signal with a first pattern is generated. A
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`signal with an intermediate pattern relatively similar to the first pattern is generated in an
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`intermediate pattern signal generation step 502. In a determination step 504 it is determined if
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`more intermediate patterns should be inserted. This can be dynamically determined or
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`determined from a predefined transition procedure. If further patterns are to be inserted, the
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`method returns to the intermediate pattern signal generation step 502. Otherwise, the method
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`continues with a second pattern signal generation step 506 where the backlighting is operated
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`in the second mode, and the transition is ready.
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`Fig. 21 illustrates a display 2100 comprising a display panel 2102. The display
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`panel 2102, which can be a LCD (Liquid Crystal Display) panel, is provided with a plurality
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`of backlighting units 2105. Each of the backlighting units 2105 can for example comprise
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`one or more lighting units, such as light emitting diodes (LEDs) or gas discharge lamps. The
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`backlight is flashed, either for the entire panel 2102 or, preferably, by scanning backlight
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`units 2105. Thus, an LC cell is illuminated only for a certain fraction of the frame time.
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`Backlight controllers 2104, which are connected to the backlighting units 2105 of the panel
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`2102, controls backlight flashing. To avoid large area flicker, the backlight controllers 2104
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`provide backlight control signals which are dependent on an image displayed on an
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`associated part of the panel 2102. Therefore, the backlight controllers are connected to a
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`display controller 2106, which in turn receives image data from an image data source 2108. It
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`should be noted that this description is for illustrative purpose, and both the backlight
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`controllers 2104 and the display controller 2106 can be a common video controller, or
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`divided between two or more units, which provide the same fianction as the backlight and
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`display controllers 2104, 2106. The data source 2108 can be a TV decoder, a DVD player, a
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`computer, or any other means providing images to be viewed on the display 2100.
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`In some cases, image contents are segmented, e.g. a cloudy, bright sky at top
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`and at bottom a dark ground, with sharp letters in subtitles. Therefore, in some cases, it is
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`desirable to segment the driving of the backlight accordingly, i.e. by backlight units 2105
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`associated to the part of the image to be shown on the display 2100. The present invention is
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`also applicable to this. Thus, the backlighting is not only improved for each type of image,
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`the backlighting is also improved for each part of the image associated to backlight units
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`2105. To be able to implement this, there is a few things to consider.
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`Analysis of the image is performed for each part of the image, where the part
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`can be defined by a part illuminated by a certain lighting unit, or a part comprising a certain
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`type of image contents.
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`To avoid unwanted effects at borders between parts of the image, the transition
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`between a pulse pattern in one part to another part is treated similar to the transition between
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`a first and a second backlight pattern described above. If there is a moving object at a border
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`between two parts of the image, the different effects of the different pulse patterns are
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`reduced by crosstalk between the backlighting units associated with the pulse patterns.
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`It can be noted that driving the backlighting in double pulse modes, or multi
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`pulse modes, will in some cases produce more light than with single pulse, although the same
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`total pulse duration. An explanation to this is that a switch-off time for a backlight unit last
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`longer than a switch-on time. This is the case for some types of backlight units, and the
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`opposite effect can be observed for other types of backlight units. The difference in lighting
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`can, as described above, be prevented by using quasi-single pulse patterns. As an alternative
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`to quasi-single pulse patterns, single pulse patterns, which provides some additional time for
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`reactive components to settle and thus a somewhat sharper image, can be used, but with a
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`compensation factor added to the pulse to equalize to a quasi-single, double, or multi pulse
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`pattern. It is preferable to have a look-up table, with compensation factors for different pulse
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`patterns for the actual light source or sources, from which compensation factors are used to
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`enable seamless transitions between different pulse patterns, especially when used in
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`neighboring partitions of an image.
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`However, when a seamless transition is to be made between single and dual or
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`multi pulse patterns, the following