WO2009113036A1

WO2009113036A1 - Liquid crystal display device and method for controlling a liquid crystal dysplay device

Info

Publication number: WO2009113036A1
Application number: PCT/IB2009/051048
Authority: WO
Inventors: Willem Hendrikus Groot Hulze; Petrus Maria De Greef
Original assignee: Nxp B.V.
Priority date: 2008-03-14
Filing date: 2009-03-13
Publication date: 2009-09-17

Abstract

A liquid crystal display device is provided which comprises a display unit (DU) for displaying a video signal (V), a backlight unit (BL) having a plurality of locally controllable light sources for backlighting the display unit (DU), an analyzing unit (SDU) for locally analyzing the input video signal (V) and a backlight control unit (BCU) for controlling the operation of the backlight unit (BL) and for performing a locally dimming and/or scanning operation on the locally controllable backlight unit (BL) according to the analyzing results of the analyzing unit (SDU).

Description

Liquid crystal display device and method for controlling a liquid crystal display device

FIELD OF THE INVENTION

The present invention relates to a liquid crystal display device and a method for controlling a liquid crystal display device.

BACKGROUND OF THE INVENTION

In order to improve the performance of a liquid crystal display (LCD) device, LCD backlights are used in particular to enhance the brightness of the displayed images or video signals. Previously, side-lit backlights have been used but they have been recently replaced by direct-lit backlights as direct-lit backlights improve the brightness of larger size LCD and TV devices.

The continuous lit backlights can be replaced by scanning backlights to further improve the picture quality of a LCD device. By means of scanning backlights, motion artifacts which result from sample and hold effects of the LCD panel can be reduced. Back- lit LCD backlights may comprise fluorescence lamps like cold cathode fluorescence lamps CCFL or hot cathode fluorescence lamps HCFL. Typically, a number of fluorescence lamps are arranged horizontally, wherein each lamp illuminates the area in front of it. The light of such a lamp may also contribute to the lighting of more distance areas. If all lamps are on at the same time, a uniformly illuminated backlight can be obtained. However, a scanning backlight can be obtained by lighting the fluorescence in a time sequential manner. To enable a scanning backlight, the respective lamps have to be individually controlled, e.g. by means of separate drivers and by means of a brightness control of each of the lamps. To avoid the scattering of the lights emitted by the fluorescence lamps, optical barriers can be provided between the lamps.

The motion portrayal of LCD display panels can be performed by an overdrive correction which compensates for the slow response of the liquid crystal material, an alternate field insertion which reduces the sample and hold effect of LCD panels in the video domain and by use of scanning backlights for reducing the sample and hold effects of the LCD panels in the backlight domain. A blinking backlight is a simple and low-cost alternative to scanning backlights. However, a blinking backlight may introduce image flicker and may vary the phase relation between the backlight exposure time and the addressing time.

Fig. 1 shows a schematic representation of a backlight unit for a LCD display according to the prior art. The backlight comprises seven rows of colored light emitting diodes LED, i.e. red, green and blue LEDs RLED, GLED, BLED are provided in the rows of the backlight. A control means is provided for each of the rows or horizontal stripes of the backlight. The contrast ratio of the backlights may be increased by means of OD, ID, 2D dimming and boosting. Preferably, LEDs with saturated colors are used in order to support a wide color gamut. Furthermore, LEDs are mercury free products hence environmental friendly light sources. The light emitted by the light emitting devices arranged in rows or stripes can be mixed in order to obtain white light with a desired color temperature, e.g. 9000K. In order to achieve this, a driver needs to be provided for each of the colors R, G and B as each of these colors have different current settings and luminance efficiencies.

The advantage of a scanning backlight is that such a scanning backlight can enable a stroboscopic exposure of any moving images (in the video signal) on the LCD panel which is back- lit by the scanning backlight thus improving the motion portrayal of the LCD display. Here, the phase relation between the backlight exposure time and the addressing time of the backlight is fixed but can be adjusted to enable an optimal liquid crystal settling across the panel. In the case of moving images, the optimal exposure time is just before the liquid crystal is addressed due to the fact that the liquid crystal comprises the longest settling time. Accordingly, the backlight needs to be generated in a shorter period of time than the complete frame period. Therefore, the lamps of the backlight need to be operated with a higher intensity.

The perceived flicker of bright objects due to the nominal frame rates of 50 or 60 Hz for a scanning backlight, is mainly determined by the actual brightness and size of the bright objects. In order to remove these flicker or artifacts, the scanning frequency of the backlight can be doubled to 100 or 120 Hz, i.e. a scanning backlight based on double-pulses is used to enhance the motion portrayal. Any scanning backlights with a frame rate of 100 or 120 Hz is invisible to the human eye such that an image flicker will not be perceived anymore.

Fig. 2 shows a diagram for illustrating the light output of the R, G and B sub- pixel for a single pulse scanning backlight. In the upper diagram, the relationship of the light output L with respect to time t is depicted for green G, red R and blue B sub-pixels. The time period of the single pulse scanning is T. In the lower diagram, the response LCR of the liquid crystal cells depicted over time corresponding to the scanning of red, green and blue pixels in the upper diagram is shown. In particular, the light output L of the R, G and B sub-pixels are depicted with an individual duty cycle for each of the sub-pixels. The particular duty cycle ratio of the R, G and B sub-pixels are determined by means of the above-mentioned white- point setting. Therefore, the green G sub-pixel has the shortest duty cycle while the blue B sub-pixel has the longest duty cycle. Preferably, the pulses of the red R, green G and blue B sub-pixels are aligned such that the light output L of all three sub-pixels is active at ti just before the liquid crystal cells are addressed for an upcoming frame of the video data t₂, i.e. the red R, green G and blue B sub-pixels are aligned to the moment when the liquid crystal material is optimally settled (ti). In other words, the single pulse scanning backlight is optimized to provide an optimal exposure of the LCD panel at that point of time ti when the liquid crystal material has settled. On the other hand, such a driving scheme may be disadvantageous for moving objects as they may comprise different motion blurs on various edges of an object. In particular, the end of the light-pulses will be very sharp while the start of the light-pulses may have some kind of colored blur, as the primary light-sources may start at a slightly different moment in time.

In addition to providing a scanning backlight, the backlight can be dimmed. The dimming of the backlight can be performed by OD, ID and 2D dimming. Performing a OD dimming may create a temporal (dynamic) contrast depending on the brightest object of the image. In order to maintain an optimal picture quality, a corresponding gain may be applied to the video data to be displayed on the LCD display. Dimming the backlight will also reduce the power consumption of the backlight. The use of ID dimming may create a temporal and extra spatial contrast depending on the brightest object of a segment. As with the OD-dimming in order to achieve an optimal picture quality, a corresponding gain may be applied to the video data to be displayed on the display. By means of 2D-dimming, i.e. by means of adaptive temporal and spatial dimming of the backlight, the contrast of the display can be further improved. A corresponding gain can be applied to the video data to be displayed on the display in order to achieve an optimal picture quality. If a 2D-dimming is to be applied to the backlight, the backlight should be based on light emitting diodes LEDs. The dimming of the LEDs of the backlight can for example be performed by the driver circuits of the backlight.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a liquid crystal display device with a scanning backlight and method for controlling the liquid crystal display device with an improved motion portrayal and with a reduced flicker. This object is solved by a liquid crystal display device according to claim 1, and by a method for controlling a liquid crystal display device according to claim 5.

Therefore, a liquid crystal display device is provided which comprises a display unit for displaying a video signal, a backlight unit having a plurality of locally controllable light sources for backlighting the display unit, an analyzing unit for locally analyzing the input video signal and a backlight control unit for controlling the operation of the backlight unit and for performing a locally dimming and/or scanning operation on the locally controllable backlight unit according to the analyzing results of the analyzing unit.

According to an aspect of the invention, the analyzing unit comprises a luminescence detector for detecting local luminescence in the video signal, a motion detector for detecting local motion in the video signal and/or a sharpness detector for detecting local sharpness in the video signal. The backlight control unit is adapted to locally control the dimming and/or scanning operation of the locally controllable backlight unit in dependence on the detected luminescence, the detected motion and/or the detected sharpness.

According to a further aspect of the invention, the backlight control unit locally controls the operation of the backlight unit such that the local scanning operation is performed when a local dimming operation is active on a location by dimming at least a single light pulse synchronous to an addressing of the display unit.

According to a further aspect of the invention, the backlight unit comprises locally controllable light sources which can have a plurality of primary colors. The locally controllable light sources are controlled such that during a scanning operation luminescence waveforms are generated by the light sources which have the same period in phase of each of the plurality of primary colors of the light source, wherein these are synchronized to the addressing of the display unit. This can be performed to avoid a discoloring of the edges of moving objects in the video signal. The invention also relates to a method for controlling a liquid crystal display device which comprises a display unit for displaying a video signal and a backlight unit having a plurality of locally controlled light sources for backlighting the display unit. The input video signal is locally analyzed. The operation of the backlight unit is controlled. A locally dimming and/or scanning operation on the locally controllable backlight unit is performed according to the analyzing results.

The invention relates to the idea to provide a liquid crystal display with a backlight unit, wherein the addressing of the backlight is aligned with the addressing of the LCD display panel. The phase relation between the LCD addressing and the exposure of the backlight can be adjusted by controlling the backlight such that the panel is exposed at an optimal setting, i.e. just before the liquid crystal is addressed. The light output of the scanning backlight is controlled to create all the required light in the shortest possible time by driving the LEDs of the backlight temporarily to their maximum luminescence.

The backlighting operation according to the invention combines a scanning operation and a OD, ID or 2D dimming operation. In particular, local backlight off-time can be used to create the required scanning backlight effect. When the local luminescence of the backlight is 100%, no scanning effect can be produced. However, a darker location in a scene to be displayed will require less luminescence, therefore also a smaller period of time to create the required light. In such a situation, the scanning effect can be created to locally improve the motion portrayal. Accordingly, a scanning backlight can be implemented while reducing the requirements for more installed light capacitance. When the scanning operation is coupled to the dimming operation, no dimming will occur, if the backlight has a duty cycle of 100% to provide for bright scenes and no scanning backlight operation will be present as the backlight is required for the complete duty cycle. Accordingly, visual flicker which can be introduced by scanning backlight will be avoided. On the other hand, for scenes where, locally, less than the maximum light output is required, a shorter exposure time and therefore a smaller duty cycle may be required to provide for the needed luminescence. An image flicker in the darker scenes is less problematic, as the eye is less sensitive to image flicker in darker areas of a scene. If 0D-dimming is used, an average power reduction of approximately 20% can be achieved. If the 0D-dimming is combined with a blinking operation, the introduced scanning of the backlight is limited as the dimming factor is approx. 80%. Furthermore, artifacts may be introduced. If ID-dimming is used, an average power reduction of 35% can be achieved. If the ID-dimming operation is combined with a scanning operation of the backlight, the scanning sequence can be aligned to the addressing sequence such that less artifacts are introduced. However, it should be noted that due to the dimming factor of approx. 65% of the luminescence, the scanning benefit will be improved as compared to a OD-dimming operation. For 2D-dimming operation in combination of a scanning operation of the backlight, a significant improvement can be noted due to the fact that the dimming does not constitute the limiting factor as 2D-dimming will lead to a global average of approx. 50% of the luminescence of the backlight, at at some moments in time te backlight can dim more then 20%, . Furthermore, if 3D-dimming, i.e. individually dimming the R, G and B colors in combination with a scanning operation, the effects can be improved even more as the dimming factor will correspond to a global average of approx. 40% of the luminescence.

Accordingly, local scanning effects can be more pronounced, in particular for 2D and 3D-dimming, the temporal and spatial local brightness duty cycle can be below 50%. Moreover, when a motion blur reduction is introduced, the picture quality of still images should not be reduced. Therefore, the scanning and dimming operations according to the invention should also be able to be motion adapted. For example, if a motion detector detects that parts of an image are not moving, the luminescence reduction could be performed by dimming the backlight without using a frame based duty cycle but a duty cycle can be applied at a higher frequency. This is beneficial in these parts of the image as less flicker is created. For moving objects, the potential flicker will not be perceived as such due to the fact that the content of the scene is moving away with a perceived enhanced motion portrayal.

The advantages and embodiments of the present application are now described in more detail with respect to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows a diagram of a backlight for a LCD device according to the prior art,

Fig. 2 shows a diagram of a single pulse scanning backlight;

Fig. 3 shows a schematic representation of a LCD device according to a first embodiment,

Fig. 4 shows a diagram of a dimming operation of a backlight according to a first embodiment,

Fig. 5 shows a diagram of a dimming operation of a backlight according to a second embodiment, Fig. 6 shows a diagram of a dimming operation of a backlight according to a third embodiment,

Fig. 7 shows a diagram of a dimming operation of a backlight according to a fourth embodiment, Fig. 8 shows a diagram of a dimming operation of a backlight according to a fifth embodiment,

Fig. 9 shows a diagram of a dimming operation of a backlight according to a sixth embodiment,

Fig. 10 shows a block diagram of a display device according to a seventh embodiment, and

Fig. 11 shows a block diagram of a display device according to an eighth embodiment.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Fig. 3 shows a basic representation of a liquid crystal display device according to a first embodiment of the invention. The liquid crystal display device comprises a LCD display panel DU, optionally a video processing unit VP, a backlight means BL and a control means BLU for controlling the backlight means BL. The backlight means BL comprises a plurality of colored light-emitting device LED, i.e. green LEDs GLED, red LEDs RLED and blue LEDs BLED. The LEDs are arranged in rows or horizontal stripes. Although in Fig. 1 only three rows of LEDs are depicted, the backlight means BL may comprise more than merely three rows LEDs. Each row of LEDs or each horizontal stripe of LEDs are driven by a control unit for red RCU, a control unit for green GCU and control unit for blue BCU. In other words, the control units RCU, GCU and BCU are associated to each of the rows of

LEDs for controlling the colored LEDs in that row. The LCD panel DU is used to display the images and video signals from the internal or external video processing unit VP, while the LCD panel DU is backlit by the backlit means BL.

The color control units RCU, GCU and BCU for each row of LEDs control the settings of the colored LEDs in the row of LEDs to achieve a global white-point setting. This global white-point setting can be achieved by selecting the luminance ratio of the individual color LEDs RLED, BLED, GLED in a row of LEDs accordingly. By selecting the luminance ratio of the respective rows of LEDs, a global luminance level can be realized. To achieve a vertical homogeneity the LEDs in the rows of LEDs are addressed individually in order to tune the color as well as the brightness thereof. As the rows of LEDs can be addressed individually by the color control units a scanning backlight can be provided in the same direction as the LCD panel is addressed. In addition, the scanning backlight may be switched on and off. By the control of the color control units, a ID segmented double-pulse can be achieved if the LEDs in the rows of LEDs are addressed and driven individually at a higher rate than the corresponding address rate. If a ID dimming is required, the duty-cycle of the individual LEDs in the rows of LEDs can be shortened. Furthermore, by driving the LEDs in the rows of LEDs at a high power, a ID boosting can be achieved provided that the temperature of the backlight is within a predetermined range. Furthermore, as the LEDs in the rows of LEDs can be individually addressed and driven with respect to their duty cycle as well as their power, the optimal settings for the LEDs can be determined for each frame period.

The modulation of the video data in the video processing unit VP is interpolated from the vertical luminance profiles of the rows of LEDs and the calculated dimming factors of the rows of LEDs, if a ID dimming operation is required.

Fig. 4 shows a diagram of a dimming operation of a backlight according to a first embodiment. The control means BCU will perform a dimming operation on the LEDs of the backlight. The dimming operation can be performed adaptively based on the local brightness of the input video data. Optionally, the frequency of the pulse width modulation duty cycle is beyond 100Hz in order to avoid perceived image flicker.

In Fig. 4, the luminescence of a backlight tile LBT in a backlight unit and the luminescence of the video pixels LVP is depicted over time. During the time period Tl, no dimming ND is performed while during the time period T2 dimming D is performed. At the end of the time period Tl, the liquid crystal LC will be settled such that no 100% backlighting is required.

It should be noted that according to the first embodiment, the dimming operation can be performed based on the local brightness of the input video data or video signal, i.e. more dimming for less brighter areas require less dimming.

Fig. 5 shows a diagram of a dimming operation of a backlight according to a second embodiment. In Fig. 5, the luminescence of the backlight tile LBT and the luminescence of the video pixels LVP is depicted. As in the first embodiment, the liquid crystal will be settled at the end of the time period Tl. Accordingly, during the time period Tl, no dimming ND is performed while dimming D is performed during the time period T2. Furthermore, all segments of the backlight may perform the scanning operation. The scanning operation can be adaptively based on the detection of bright or dark objects in the segment. Accordingly, each segment can have its own and dedicated duty cycle, i.e. each LED in the backlight corresponding to a segment can be controlled based on its own duty cycle. Preferably, the pulse width modulation frequency of the backlight corresponds to the video frame rate and the phase of the backlight is locked to the addressing of the liquid crystal display.

If a scene comprises bright objects, the duty cycle can be larger such that the reduction of the motion artifacts is not as high as with OD-dimming but the scanning and dimming operation according to the second embodiment is less sensitive to flicker. Fig. 6 shows a diagram of a dimming operation of a backlight according to a third embodiment. Here, also the luminescence of the backlight tile or unit LBT and the luminescence of the video pixel LVP is depicted over time. After the end of the time period Tl, the liquid crystals LC have been settled. During the time period Tl, a dimming operation D is performed and during the time operation T2, more dimming operation MD is performed. The dimming operation according to the third embodiment will be applied to scenes without bright objects. Here, merely a small duty cycle of the backlight will enable a motion artifact reduction. Due to the fact that the brightness is low in the absence of bright objects, the scene will be less sensitive to flicker.

To improve the operations of the LEDs in the backlight, the LED should be driven at the lowest junction temperature. Furthermore, a color breakup of moving objects should be avoided. Therefore, the light output of the R, G and B LED should be normalized, i.e. 100% light corresponds to R = 100%, G = 100%, B = 100% duty cycle.

Fig. 7 shows a diagram of a dimming operation of a backlight according to a fourth embodiment. Here, the luminescence of the backlight tile LBT and the luminescence of the video pixels LVP are depicted. At the end of the time period Tl, the liquid crystal LC will be settled. Here, during the time period Tl, no dimming operation ND is performed while during the time period T2 a dimming operation D is performed. In the fourth embodiment, a scene with changing image content is described. The changing image content may relate to fading. Here, a continuous backlight operation will not introduce any motion artifact reduction and will not create any flicker. If the backlight is dimmed, the backlight luminescence will be modulated by a pulse with modulation duty cycle.

If moving images are present in the video data, a scanning backlight will reduce the motion artifact reduction which is advantageous if a motion is present, and will create some flicker. However, this flicker will not be perceptible due to the fact that at the same location motion is present. If the system does not comprise any moving images in the video data, a scanning backlight will not be required as no motion is present. On the other hand, a scanning backlight will introduce flicker. However, this flicker can be prevented locally by disabling the scanning operation. Fig. 8 shows a diagram of a dimming operation of a backlight operation according to a fifth embodiment. Here, the luminescence of the backlight tile LBT and a luminescence of the video pixels LVP is depicted. At the end of the time period Tl, the liquid crystal LC will be settled. During the time period Tl, no dimming operation ND is present while during the time period T2 a dimming operation D is present. It should be noted that the frequency of the pulse width modulation during the time period T2 is higher than the frequency of the pulse width modulation in the time period Tl . According to the fifth embodiment, a motion adaptive scanning and dimming backlight operation is performed. In particular, it is decided for each segment of the video display whether or not to perform a scanning backlight or whether merely a dimming backlight is required. The decision whether or not to activate the scanning operation is performed adaptive Iy based on the detection of moving objects in a segment. Optionally, the moving sharp details and the textures may also be detected and used to decide whether or not to activate the scanning backlight operation. According to the fifth embodiment, the scanning backlight operation will be activated in the spaces which are created by the dimming operation. If only the dimming operation is activated, a dimming will be performed at a high pulse width modulation frequency in order to avoid flicker.

Fig. 9 shows a diagram of a backlight operation according to a sixth embodiment. Here, the luminescence of the backlight tile LBT and the luminescence of the video pixels NVP is depicted. At the end of the time period Tl, the liquid crystal LC is settled. During the time period Tl, no dimming operation ND is performed while during the time period T2 a dimming operation D is performed. The sixth embodiment is in particular related to scenes with moving images like moving objects. The scanning backlight operation is activated to reduce motion artifacts but it may create some flicker. This flicker will not be perceptible as motion is present in the segment. Fig. 10 shows a block diagram of a display device according to a seventh embodiment. The display device comprises a display unit DU, a backlight unit BL for backlighting the display unit DU, a backlight control unit BCU, optionally an overdrive processing unit OPU, optionally a frame memory FM and optionally a scanning and dimming unit SDU. The scanning and dimming unit SDU, i.e. a video analyzing unit, will receive the input video data or the input video signal V, analyze the input video data V and will forward a dimming value DV to the backlight control unit BCU. The scanning and dimming unit SDU (video analyzing unit) will output a current video frame crt to the overdrive processing unit OPU and to the frame memory FM, where the video frame is stored. The frame memory FM will forward a previous frame pre to the overdrive processing unit OPU. The overdrive processing unit OPU will drive the display unit DU and will output a sync signal sync to the backlight control unit BCU such that the display and the backlight are in sync.

Fig. 11 shows a block diagram of a display device according to an eighth embodiment. The display device comprises a LCD display DU, at least one backlight unit BL, at least one backlight control unit BCU, a video gain unit VG, a gain and timing unit GTU, a histogram unit HU (which may correspond to the frame memory in Fig. 11), a luminescence detector LDU, a motion detection MDU, a sharpness detector SDU. The input video data V are inputted to the luminescence detector LDU, the motion detector MDU and the video gain unit VGU. The architecture according to the eighth embodiment can be used for ID scanning and ID dimming backlight systems as well as for 2D and 3D systems.

The principles of the present invention can be applied to LCD-TV systems as well as for personal or automotive multimedia displays. The panel can use a OD, ID, 2D or 3D controllable backlight with a motion adaptive scanning and an adaptive OD, ID, 2D and 3D dimming.

The invention relates to the idea to provide a liquid crystal display device with a display unit DU and a backlight unit BL for backlighting the display unit DU. The backlight unit is controlled by a backlight control unit BCU. The backlight control unit BCU is adapted to control the backlight BL to perform a scanning operation. The backlight control unit BCU is also adapted to perform a dimming operation on the backlight BL. In particular, the backlight control unit BCU is adapted to perform the scanning operation only if the dimming operation is activated. The backlight control unit is also adapted to perform the scanning and/or dimming operation based on a detection of bright and/or dark objects in the segment of the video data. The backlight control unit BCU is also adapted to control the dimming and scanning operation based on a detection of motion in the segments of the video data.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word "comprising" does not exclude the presence of elements or steps other than those listed in a claim. The word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. In the device claim enumerating several means, several of these means can be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Furthermore, any reference signs in the claims shall not be constrained as limiting the scope of the claims.

Claims

Liquid crystal display device and method for controlling a liquid crystal display device CLAIMS:

1. Liquid crystal display device, comprising: a display unit (DU) for displaying a video signal (V), a backlight unit (BL) having a plurality of locally controllable light sources for backlighting the display unit (DU), an analysing unit (SDU) for locally analysing the input video signal (V), and a backlight control unit (BCU) for controlling the operation of the backlight unit (BL) and for performing a locally dimming and/or scanning operation on the locally controllable backlight unit (BL) according to the analysing results of the analysing unit (SDU).

2. Liquid crystal display device according to claim 1 , wherein the analysing unit (SDU) comprises: a luminescence detector (DLU) for detecting local luminescence in the video signal (V), a motion detector (MDU) for detecting local motion in the video signal (V); and/or a sharpness detector (SDU) for detecting local sharpness in the video signal

(V), wherein the backlight control unit (BCU) is adapted to locally control the dimming and/or scanning operation of the locally controllable backlight unit (BL) in dependence on the detected luminescence, the detected motion and/or the detected sharpness in the video signal (V).

3. Liquid crystal display device according to claim 1 or 2, wherein - the backlight control unit (BCU) is adapted to locally control the operation of the backlight unit (BL) such that a local scanning operation is performed when a local dimming operation is active on a location, by dimming at least a single light-pulse, synchronous to an addressing of the video signal (V) on the display unit (DU).

4. Liquid crystal display device according to claim 1, 2 or 3, wherein the backlight unit (BL) comprises locally controllable light sources having a plurality of primary colours which are controlled such that during the scanning operation the light sources generate luminance wave-forms, having the same period and phase for each of the plurality of primary colors of the light sources, synchronised to the addressing of a display unit (DU), to avoid discolouring on the edges of moving objects in the video signal.

5. Method for controlling a liquid crystal display device having a display unit (DU) for displaying a video signal, and a backlight unit (BL) having a plurality of locally controllable light sources for backlighting the display unit (DU), comprising the steps of: locally analysing the input video signal (V), and controlling the operation of the backlight unit (BL) and performing a locally dimming and/or scanning operation on the locally controllable backlight unit (BL) according to the analysing results.

6. Method according to claim 5, wherein the analysing step comprises the steps of: detecting local luminescence in the video signal (V), - detecting local motion in the video signal (V), and/or detecting local sharpness in the video signal, and locally controlling the dimming and/or scanning operation of the locally controllable backlight unit (BL) in dependence on the detected luminescence, the detected motion and/or the detected sharpness in the video signal (V).