WO2007125451A1

WO2007125451A1 - Method for controlling a backlight of a display panel

Info

Publication number: WO2007125451A1
Application number: PCT/IB2007/051349
Authority: WO
Inventors: Hendrikus W. Groot Hulze; Petrus M. De Greef
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2006-04-28
Filing date: 2007-04-16
Publication date: 2007-11-08
Also published as: TW200802268A

Abstract

The present invention relates to a method for controlling a backlight (102) of a display panel (101), comprising generating a control signal (107) with a primary pulse arrangement (200) and a secondary pulse arrangement (201; 401; 402) per frame, and activating said backlight in accordance with said control signal. The control signal is chosen such that the first harmonic (301) of the secondary pulse arrangement essentially cancels out the first harmonic (300) of the primary pulse arrangement, and the primary and secondary pulse arrangements have different shapes. By thus activating the backlight two times per frame, there will be a reduction of flickering as compared to typical backlighting methods, while at the same time it is possible to reduce motion blur artifacts, such as for example in the form of ghost images. The present invention also relates to a display unit comprising means for performing such a method.

Description

Method for controlling a backlight of a display panel

The present invention relates to a method for controlling a backlight of a display panel. The present invention also relates to a corresponding display unit comprising means for performing such a method.

Display panels, such as LCD (Liquid Crystal Display) panels, have been widely utilized in recent years instead of CRT (Cathode Ray Tube) based ones. However, LCD panels are not of a self light-emitting type, and hence require some sort of external lighting. Large LCD panels used for computer displays and video screens are typically backlit with light from fluorescent tubes or arrays of light-emitting diodes (LEDs) that are built into the sides or back of the panel.

When displaying moving objects on an LCD panel having such a static backlight, they are blurred due to a "sample and hold effect". When the human eye watches moving images at typical frame rates (50 Hz or 60 Hz), it perceives these moving images as unsharp when each image is displayed and backlit during the full frame time (1/50 second or 1/60 second respectively). This effect is one of the causes of "motion blur", i.e. the blurred portrayal of (fast) moving images in LCD displays.

A known remedy to this sample and hold effect is the use of a so-called scanning backlight. A scanning backlight illuminates parts of the display in sequence, such that each pixel is only backlit during part of the frame time. Preferably, the backlight 'scans' from the top of the screen to the bottom, during one frame period first illuminating a first segment at the top of the display, then a second segment below the first segment, and so forth until a final segment at the bottom of the display is reached.

However, a scanning backlight causes flicker if a low refresh rate is used (50 - 60 Hz), similar to conventional CRT based displays operating at these refresh rates. Higher refresh rates are feasible, but require frame rate conversion algorithms for the video content, which is expensive especially for High Definition (HD) video content.

A solution to the flickering problem is presented in the document US 2005/0248553 Al, wherein the backlight of the LCD panel is flashed or modulated at a multiple of the frame rate. For example, the backlight or a segment of the backlight is flashed two times per frame time (the so-called 'double pulse' operation). In this manner, the temporal frequency of the flash is higher than the frequency of the frame rate and thus the flickering becomes less visible to the viewer. However, double pulse scanning introduces double edges of moving objects. This artifact occurs because every frame is projected twice on the retina in the eye, but generally not at the same position due to tracking of the eye in the time between the two projections. Especially, in a LCD panel with a double pulse scanning or flashing backlight, a luminance error is often visible as a pre- and/or post-ghost: a ghost image that precedes and/or trails a moving object on the screen. There is therefore a need for an efficient backlight scanning method for a display panel, which overcomes or at least alleviates problems with ghost images and/or motion blur in an LCD panel.

The above object is met by a novel method for controlling a backlight of a display panel as defined in claim 1 , and a corresponding display unit comprising means for performing such a method as defined in claim 6. The appended sub-claims define advantageous embodiments in accordance with the present invention.

According to an aspect of the invention, there is provided a method for controlling a backlight of a display panel, comprising generating a control signal with a primary pulse arrangement and a secondary pulse arrangement per frame, and activating the backlight in accordance with the control signal, wherein the secondary pulse arrangement is chosen such that its first harmonic essentially cancels out the first harmonic of the primary pulse arrangement, and wherein the primary pulse arrangement and the secondary pulse arrangement have different shapes.

According to the present invention, by activating the backlight two times per frame, or in other words "flashing" the backlight two times per frame, by means of a control signal with two pulse arrangements per frame, the first and second pulse arrangement having an equal or at least similar first harmonic in an opposite phase, there will be a reduction of flickering as compared to typical backlighting methods. Furthermore, by selecting the primary and the secondary pulse arrangements such that they are not identical, it is possible to further reduce motion blur artifacts, such as for example artifacts in the form of ghost images. Also, as a double pulse flashing method is used, it is possible to keep a high effective brightness of the display panel, in comparison to a display panel using a single pulse flashing method. The effective brightness of the display panel depends on the total amplitude and width of the pulse arrangements, as in a normal pulse width modulation (PWM) lighting system.

In the present invention, the double edges motion artifact will not be visible anymore, as a primary exposure, e.g. the primary pulse arrangement of the control signal for activating the backlight, of an image frame is projecting all objects as sharp shapes on the retina in the eye, and a secondary exposure, e.g. the secondary pulse arrangement, of the same frame is projecting moving objects as blurred shapes on the retina (due to eye-tracking). It has been found that the human eye integrates the sharp shape and the blurred shape. The eye will perceive moving objects with some sharpness modulation at the frame-rate, resulting in a rather smooth motion portrayal without flicker and without noticing double edges. For still images the eye will not be tracking, and as the two sharp shapes will integrate at one location on the retina, the eye will see a sharp static object without flicker.

The display panel can for example be an LCD (liquid crystal display) or an LCD-projector. Further, a frame, or an image frame, should in the context of this application be understood to mean the set of all panel elements, or pixels, that represent one complete image. A "normal" image is generally defined as two-dimensional, 2-D, having a large plurality of lines and columns, such as 768 lines by 1280 columns, or 1080 lines by 1920 columns. In relation to moving images, e.g. video content, the video content normally consists of a plurality of consecutive image frames. It is possible to either display a static image comprising only one image frame, or displaying images that are updated periodically, such as in the case of video content.

Preferably, the secondary pulse arrangement has lower amplitude than said primary pulse arrangement, which provides for the possibility to reduce the contrast of the ghost image. Also preferably, the primary pulse arrangement is shorter in time than the secondary pulse arrangement. By arranging the primary pulse arrangement such that it is relatively short in time and has a relatively high amplitude, in comparison to the secondary pulse arrangement, it is possible to firstly present a sharp image frame on the display panel, and secondly reduce the amplitude of the first harmonic frequency component. Furthermore, the secondary pulse arrangement preferably has a smoother shape than the primary pulse arrangement. Ideally, the secondary pulse arrangement contains a limited number of higher harmonics, to minimize the sharpness of the ghost image. As a result, the perceived sharpness of the image frames of the video content will be as high as possible. At least one of the primary and the secondary pulse arrangements can consist of one single pulse. For example, each of the pulse arrangements can consist of a single pulse. Alternatively, the secondary pulse arrangement can consist of a pulse-train, preferably modulated by a sine-shape like waveform, while the primary pulse train may consist of a single pulse. These different types of pulse arrangements can for example be used depending on the type of backlight that is used.

According to another aspect of the present invention, there is provided a display unit comprising a display panel, a backlight, means for generating a control signal with a primary pulse arrangement and a secondary pulse arrangement per frame, and means for activating the backlight in accordance with the control signal, wherein the secondary pulse arrangement is chosen such that its first harmonic essentially cancels out the first harmonic of the primary pulse arrangement, and wherein the primary pulse arrangement and the secondary pulse arrangement have different shapes. The advantages of the second aspect of the present invention are essentially the same as those of the first aspect. Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following description. Those skilled in the art realize that different features of the present invention can be combined to create embodiments other than those described in the following.

These and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing currently preferred embodiments of the invention, wherein:

Figure 1 is a block diagram showing a display unit according to a currently preferred embodiment of the present invention;

Figure 2 is a graph illustrating a first example of a control signal used in a preferred embodiment of the present invention;

Figure 3 is a graph illustrating said first example of the control signal in the frequency domain; and Figures 4a - b illustrate two alternative examples of a control signal used in preferred embodiments of the present invention. Referring now to the drawings and to Figure 1 in particular, there is depicted a block diagram showing a display unit 100, such as for example a flat screen LCD-unit, according to a currently preferred embodiment of the present invention. The display unit 100 comprises a display panel 101, a backlight 102, and a backlight control unit 103. The display panel 101 is adapted to receive a video signal 104 from a video source 105, such as for example a set-top box, a hard drive recorder, a DVD-unit, and/or a video signal from an analogue or digital television receiver. All these sources 105 can provide a video signal 104 to be used to display static and/or moving images on the display panel 101.

In Figure 1, the backlight 102 is illustrated as a single device. However, it is possible to backlight a display panel 101 in many different ways, and the backlighting technique according to the present invention can be used in at least a majority of them. For example, it is possible to provide a backlight 102 consisting of different kind of light members such as for example HCFLs (hot cathode florescent lights), CCFLs (cold cathode florescent lights), others types of fluorescent lamps, or LEDs (light emitting diodes). LED backlighting is the most popular backlighting for small and medium LCDs. LED backlighting provides low cost, long life, immunity to vibration, low DC operating voltage, and precise intensity control.

Furthermore, a normal display unit 100 generally comprise of a plurality of LC-elements arranged in lines and columns, wherein each LC element of the display unit 100 generally is matched to a pixel element, or pixel, of the frame, e.g. image or video content, to be displayed onto the display unit 100. Preferably, the backlighting is line based, and each of the lines of the display panel 101 can be illuminated by a separate light member. However, this is generally not a realistic solution as a normal display panel 101 at least comprises several hundreds of lines, going up to more than 1000 lines for high-density (HD) display panels. A typical implementation uses instead ten to fifteen light members, generally operated sequentially with some overlap so that two or more light members are in their fully activated states simultaneously. In a backlight using fluorescent lamps, each light member generally corresponds to a single fluorescent lamp, which for example provides backlighting to between 50 and 100 lines of the display panel. The backlight control unit 103 is adapted to generate a control signal 107 with a primary and a secondary pulse arrangement per frame time, and activate the backlight 102 in accordance with the control signal 107.

According to the present invention, the secondary pulse arrangement is chosen such that its first harmonic essentially cancels out the first harmonic of the primary pulse arrangement, and such that the primary and the secondary pulse arrangements have different shapes.

The synchronization between the first and the second pulse arrangements in relation to the timing of the frame is provided by a synchronization signal 106. This signal is essentially based on the video signal 104. It is furthermore preferable to activate the backlight 101 with an optimal timing in relation to the switching of the LC elements comprised in the display panel 101.

More preferably, the primary pulse arrangement should be effected as late as possible within the frame time. Ideally, the primary pulse arrangement is given, and a light member of the backlight is activated accordingly, just before the LC elements illuminated by that light member will be addressed for the next image frame. Hence, after addressing of a pixel the liquid crystal material in the pixel has the longest time possible to settle before the "sharp" exposure is executed (sampling of the valid display panel status in the eye). Thus, the backlight is activated using the primary pulse arrangement, and thus the "sharp" exposure is given, at a time when the transmission of the liquid crystal pixels is as close as possible to its intended value, minimizing the amount of motion blur due to inherent slowness of the liquid crystal material.

A first example of a control signal 107 used in a preferred embodiment of the invention is illustrated in Figure 2. In this figure, the control signal is shown in the time domain. As can be seen, the first example of the control signal comprises a primary pulse arrangement 200 and a secondary pulse arrangement 201, wherein the secondary pulse arrangement 201 has an amplitude that is lower then the amplitude of the primary pulse arrangement 200, a duration that is longer than that of the primary pulse arrangement 200, and a shape that is smoother then the shape of the primary pulse arrangement 200. By using such a double pulse arrangement according to the invention, i.e. by activating the backlight 102 two times per image frame time as described above, effectively the first harmonics of the primary pulse arrangement 200 and the secondary pulse arrangement 201 cancel out and therefore, the lowest harmonic of the backlight is twice the frame rate (100 Hz for a PAL television system, and 120 Hz for a NTSC television system). Such frequencies no longer can be detected as flicker by human perception.

A preferred shape of the secondary pulse arrangement 201 has a limited number of higher harmonics to minimize the sharpness of the ghost image. Further, the amplitude of the secondary pulse arrangement 201, and the total light output during the secondary pulse, should be as low as possible to reduce the contrast of the ghost while still being able to essentially eliminate the first harmonic of the primary pulse arrangement. At the same time, the primary pulse arrangement 200 should be as short and bright as possible. These measures ensure the sharpest picture possible.

The relation between the first harmonic of the primary pulse arrangement 200, and the first harmonic of the secondary pulse arrangement 201 in the first example of the control signal is illustrated in Figure 3. In Figure 3, the first 300, second 300' and the third 300" harmonics of the primary pulse arrangement 200 are illustrated, in the frequency domain. Furthermore, it can be seen that the secondary pulse arrangement 201 includes a negative first harmonic 301 that essentially cancels out the first harmonic 300 generated by the primary pulse arrangement 200.

In Figures 4a and b, two alternative examples of a control signal for use in currently preferred embodiments of the present invention are shown. Figure 4a illustrates an example of a control signal that can be generated by a "two level inverter" for controlling the backlight 102 of the display unit 100. In Figure 4b, a further example of a control signal is shown, wherein the secondary pulse arrangement 402 has been modulated with a sine- wave, generating a pulse-train. A pulse-train is a power effective implementation of a secondary pulse arrangement, which is very suitable for a LED-based backlight system, since LEDs are normally drive using a pulse width modulation (PWM) driving scheme.

The person skilled in the art realizes that the present invention by no means is limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims. For example, the present invention may also be combined with 0-, 1- or 2-dimensional dynamic backlighting algorithms to reduce LC-leakage (black level) and to save power. Dimming by amplitude control is straight forward as the primary and secondary pulses should be dimmed with the same factor. Dimming per duty cycle is more complex because the first harmonic of the pulse arrangement needs not to be linear proportional to its duty cycle. A preferred dimming strategy is a combination of duty and amplitude control, reducing the duty cycle of the primary pulse arrangement and lowering the amplitude of the secondary pulse arrangement equally to the reduction of the first harmonic. With this method the secondary pulse arrangement can stay in its most effective shape with a relative large duty cycle.

The method and device according to the invention may be combined with the adaptive double pulse technology according to our unpublished international patent application with filing number WO/IB/2005/054377. If the local picture content is dark and/or there is movement of sharp details, some flicker can be allowed to further improve on the sharpness. The amplitude of the secondary pulse is gradually reduced and at the same time the duty-cycle of the primary pulse is enlarged (to preserve the amount of light). For bright and/or still local picture content, the pulses should remain ideal flicker free. The combination of both technologies provides excellent performance with respect to perceived sharpness/flicker ratio.

Further, the method and device according to the invention may be combined with for example a high-pass/low-pass filtering of the video content. According to this technique, each image frame is split into two fields, one high-contrast field including high frequency components of the image signal and one low-contrast field including low frequency components.

Then, advantageously, the picture elements of the display panel are addressed using image data of the high-contrast field while the display panel is illuminated using the primary pulse arrangement, and subsequently the picture elements of the display panel are addressed using image data of the low-contrast field while the display panel is illuminated using the secondary pulse arrangement. As a consequence, the video sharpness modulation of the high-pass field can be reduced, further enhancing the image quality of static objects.

Claims

CLAIMS:

1. A method for controlling a backlight ( 102) of a display panel (101), comprising: generating a control signal (107) with a primary pulse arrangement (200) and a secondary pulse arrangement (201; 401; 402) per frame; and - activating said backlight (102) in accordance with said control signal

(107), wherein said secondary pulse arrangement is chosen such that its first harmonic (301) essentially cancels out the first harmonic (300) of said primary pulse arrangement, and wherein said primary pulse arrangement and said secondary pulse arrangement have different shapes.

2. A method according to claim 1, wherein said secondary pulse arrangement has a lower amplitude that said primary pulse arrangement.

3. A method according to claim 1 or 2, wherein said primary pulse arrangement is shorter in time than said secondary pulse arrangement.

4. A method according to claim 1 or 2, wherein said secondary pulse arrangement has a smoother shape than said primary pulse arrangement.

5. A method according to claim 1, wherein at least one of said pulse arrangements consists of a single pulse.

6. A method according to claim 1 or 5, wherein said secondary pulse arrangement consists of a pulse-train.

7. A method according to claim 6, wherein the pulse train of the secondary pulse arrangement is modulated by a sine-shape like waveform.

8. A method for displaying image frames on a display panel (101) including a backlight (102), comprising: providing the image frames to the display panel (101) activating picture elements of the display panel in accordance with the provided image frames, and selectively illuminating the picture elements using the backlight (102), wherein the backlight (102) is controlled using a method according to claim 1.

9. A display unit (100), comprising: - a display panel (101); a backlight (102); a backlight control unit (103) for generating a control signal (107) with a primary pulse arrangement (200) and a secondary (201; 401; 402) pulse arrangement per frame, and activating said backlight in accordance with said control signal (107), wherein the backlight control unit (103) is arranged for selecting said secondary pulse arrangement so that a shape of the secondary pulse arrangement is different from a shape of the primary pulse arrangement and a first harmonic (301) of the secondary pulse arrangement essentially cancels out the first harmonic (300) of the primary pulse arrangement.

10. A display unit according to claim 9, wherein said secondary pulse arrangement has a lower amplitude that said primary pulse arrangement.

11. A display unit according to claim 9 or 10, wherein said primary pulse arrangement is shorter in time than said secondary pulse arrangement.

12. A display unit according to claim 9 or 10, wherein said secondary pulse arrangement has a smoother shape than said primary pulse arrangement.

13. A display unit according to claim 9, wherein at least one of said pulse arrangements consists of a single pulse.

14. A display unit according claim 9 or 13, wherein said secondary pulse arrangement consists of a pulse-train, preferably modulated by a sine-shaped wave.