WO2004049292A1

WO2004049292A1 - Active matrix display

Info

Publication number: WO2004049292A1
Application number: PCT/IB2003/005097
Authority: WO
Inventors: Alan G. Knapp
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2002-11-27
Filing date: 2003-11-11
Publication date: 2004-06-10
Also published as: AU2003278494A1; GB0227632D0

Abstract

In an active matrix display, data derived from at least two sequential frames of image data is compared. A backlight output light intensity is varied to change in response to a change in image content. The maximum rate of change of output light intensity away from a default output light intensity is greater than the maximum rate of change of output light intensity towards the default output light intensity. In this arrangement, a rapid change in backlight output is provided in response to a change in image content, particularly brightness, and a more gradual return to the default output is provided. The change to light output intensity is temporary, and the brightness preferably returns to the default even if the image content does not subsequently change.

Description

DESCRIPTION

ACTIVE MATRIX DISPLAY

The invention relates to an active matrix display, and in particular to an active matrix display having illumination by means of a backlight.

Active matrix liquid crystal displays (AMLCDs) are one well known example of active matrix display. In such displays, an active plate and a passive plate sandwich a liquid crystal. The active plate includes a number of electrodes for applying electric fields to the liquid crystal and the electrodes are generally arranged in an array. Row and column electrodes extending along the rows and columns of pixel electrodes connect and drive thin film transistors which drive respective pixel electrodes. The row and column electrodes are driven to control the thin film transistors to control the charge stored on corresponding pixel electrodes. The charge stored on each pixel determines how the corresponding portion of the liquid crystal layer modulates light passing through the layer, from a backlight to a viewing surface. It is generally reported that the perceived quality for TV images on a cathode ray tube is better than for an AMLCD. This is in part due to the fact that the black parts of the CRT image are darker than the black parts of the AMLCD image and the small bright areas in the CRT image can attain a very high peak brightness. These effects combine to give an image with a high contrast and a high perceived quality.

It has been recognised that control of the backlight of an AMLCD can be used to change the characteristics of a displayed image. For example, a wide range of publications have appeared recently concerning flashing backlights. One proposed use of flashing backlights is to improve motion blur. The article "Image Synchronised Brightness Control" by Hwang et al. in

SID '01 digest, p.492 proposes system in which the brightness of a backlight is adjusted according to the content of the image. This aims to increase the peak luminance whilst maintaining a low power consumption and backlight lifetime.

According to the invention there is provided an active matrix display, comprising: an active matrix display module; a backlight having a default output light intensity; a processor for comparing data derived from at least two sequential frames of image data; and means for adjusting the backlight output light intensity such that the backlight output light intensity can change in response to a change in image content, and wherein the maximum rate of change of output light intensity away from the default output light intensity is greater than the maximum rate of change of output light intensity towards the default output light intensity. In this arrangement, a rapid change in backlight output is provided in response to a change in image content, particularly brightness, and a more gradual return to the default output is provided. The change to light output intensity is temporary, and the brightness preferably returns to the default even if the image content does not subsequently change. The means for adjusting the backlight output light intensity is preferably for increasing the light intensity from the default in response to an increase in image brightness. This has the greatest impact on perceived image quality. However, the means for adjusting the backlight output light intensity may also be for decreasing the light intensity from the default in response to a decrease in image brightness.

The backlight may comprise a plurality of lamps independently controllable by the adjusting means.

A memory is preferably provided for storing at least two frames of image data. This enables the comparison of sequential frames of date before display. For example, an average brightness can be determined for the stored frames of image data, and the backlight output light intensity for a given frame is set by comparing the average brightness for the given frame of data to be displayed with the average brightness for the preceding frame of data. Alternatively, a proportion of the image area having a brightness above a threshold can be determined for the stored frames of image data. There are other ways of analysing the image content to arrive at a measure indicative of a rapid change in brightness.

The backlight may be driveable to two possible output light intensities, the default output light intensity and one higher output light intensity, or to three possible output light intensities, the default output light intensity, one higher output light intensity and one lower output light intensity. However, intermediate levels may also be provided.

The rate of change of output light intensity towards the default output light intensity can also variable in dependence on image content.

The invention also provides a method of displaying data using an active matrix display having a backlight with a default output light intensity, the method comprising: displaying an image with the backlight at the default output light intensity; comparing the content of an image to be displayed and a preceding image; determining whether a brightness factor varies by more than a threshold between the image to be displayed and the preceding image; if so, adjusting the backlight output light intensity at a first rate of change of output light intensity away from the default output light intensity, and subsequently adjusting the backlight output light intensity towards the default output light intensity at a second rate of change of output light intensity which is lower than the first rate of change of output light intensity.

As explained above, the brightness factor may be an average brightness or a proportion of the image area having a brightness above a threshold.

For a better understanding of the invention, embodiments will now be described, purely by way of example, with reference to the accompanying drawings in which:

Figure 1 shows a known liquid crystal pixel circuit;

Figure 2 shows the general components of a liquid crystal display;

Figure 3 is used to explain the backlight control method of the invention; Figure 4 shows a display of the invention;

Figure 5 is used to explain in greater detail a first implementation of the method explained with reference to Figure 3; and

Figure 6 is used to explain in greater detail a second implementation of the method explained with reference to Figure 3. It should be noted that none of the Figures are to scale. Like or corresponding components are generally given the same reference numeral in different Figures.

Figure 1 shows a conventional pixel configuration for an active matrix liquid crystal display. The display is arranged as an array of pixels in rows and columns. Each row of pixels shares a common row conductor 10, and each column of pixels shares a common column conductor 12. Each pixel comprises a thin film transistor 14 and a liquid crystal cell 16 arranged in series between the column conductor 12 and a common electrode 18. The transistor 14 is switched on and off by a signal provided on the row conductor 10. The row conductor 10 is thus connected to the gate 14a of each transistor 14 of the associated row of pixels. Each pixel additionally comprises a storage capacitor 20 which is connected at one end 22 to the next row electrode, to the preceding row electrode, or to a separate capacitor electrode. This capacitor 20 stores a drive voltage so that a signal is maintained across the liquid crystal cell 16 even after the transistor 14 has been turned off.

In order to drive the liquid crystal cell 16 to a desired voltage to obtain a required grey level, an appropriate analogue signal is provided on the column conductor 12 in synchronism with a row address pulse on the row conductor 10. This row address pulse turns on the thin film transistor 14, thereby allowing the column conductor 12 to charge the liquid crystal cell 16 to the desired voltage, and also to charge the storage capacitor 20 to the same voltage. At the end of the row address pulse, the transistor 14 is turned off, and the storage capacitor 20 maintains a voltage across the cell 16 when other rows are being addressed. The storage capacitor 20 reduces the effect of liquid crystal leakage and reduces the percentage variation in the pixel capacitance caused by the voltage dependency of the liquid crystal cell capacitance.

The rows are addressed sequentially so that all rows are addressed in one frame period, and refreshed in subsequent frame periods.

As shown in Figure 2, the row address signals are provided by row driver circuitry 30, and the pixel drive signals are provided by column address circuitry 32, to the array 34 of display pixels. The display has a backlight 36, and each liquid crystal cell 16 modulates (i.e. variably attenuates) the light from the backlight 36 to change the pixel image brightness (represented by arrows 38) viewed from the opposite side of the array 34 of display pixels. The array 34 constitutes an active matrix display module. Colour filters are used to provide red, green and blue pixels, enabling a colour display device to be formed.

There are a number of possible backlight types, including electroluminescent sheets, fluorescent tubes and LED lamps. The invention concerns a method of giving the AMLCD image a greater perceived image quality, closer to that of a cathode ray tube. The basic idea is to dynamically alter the backlight level depending on changes in the luminance content of the image, in such a way that the equilibrium backlight level always remains the same but that rapid changes in the image produce a fast temporary change in backlight level (up or down). After the fast change, the backlight illumination returns more slowly to its equilibrium level.

Figure 3 is used to show the backlight control method of the invention. The backlight has a default output light intensity (luminance) BDEF, which is an equilibrium level. The backlight output light intensity is varied in response to a rapid change in image content. A rapid increase in scene brightness triggers the rapid step increase in backlight brightness 40 to brightness B AX- This has a first rate of change, which is sufficiently rapid for the brightness output to change from one frame of image date to the next. The time period t may typically be of the order of 1 field period (i.e. in the range 10 - 20ms). The light output then returns to the default with a lower rate of change of output light intensity during the period 42. The period of time over which the light output returns to the default level may be from a few field periods up to a few seconds. The period of time may for example be 40ms to 10 seconds, or more preferably 40ms to 2 seconds. During the time period 42, no further change to the light output may be allowed, or else it may be possible to apply a further step within this period. In one example, no further change to the brightness output level may be allowed for a period until the light level has changed by a fixed proportion of the difference between the peak and the equilibrium level. This proportion may be 25% or 50% for example.

There are other possible control schemes for the rise and decay of the backlight output. A rapid decrease in scene brightness triggers the rapid step decrease in backlight brightness 44 to brightness B_MIN, with similar rate of change to the step 40. Again, the light output then returns to the default with a lower rate of change of output light intensity during the period 46.

Figure 3 shows possible increases and decreases in brightness from the default, but in some versions of such a system, only increases in the lamp brightness may be implemented, as these will have the greatest impact on the perceived image quality.

This control of the backlight enhances the viewer's perception of a high image quality because for certain periods the viewer will see a high brightness (BMAX in Figure 3). If the equilibrium default brightness is BDEF then the typical black level of the display is BDEF/C, where C is the display contrast. If the display operated continuously with a brightness level BMAX then the black level would be at the higher level BMAX/C which would be perceived as less dark black areas by the viewer. Hence, the approach of the invention offers the advantage of a perceived high peak brightness (represented by BMAX) combined with a low dark brightness represented by B EF/C. Figure 4 shows the basic form of the system required to implement the method of the invention. The incoming video signals 50 are fed to a signal processing and memory system 52, 53 whose function is to perform, in addition to any processing functions related to other requirements of the display system, the additional functions required for image enhancement as described above. The key function is to detect significant changes in image brightness between the next field (frame period) and the current field and to generate control signals for the backlight which cause it to be switched in the manner shown in Figure 3. Thus, the system 52 comprises a processor, typically in the form of a hardware signal processing block, for comparing at least two sequential frames of image data, and for generating a control signal for driving the backlight 36. For best results, it is necessary to store the video data in a memory as the backlight needs to be switched after the information about the difference in brightness of the current and previous fields is available but before or at the same time as the data is driven onto the display. Thus, the system 52 includes a memory 53 for storing at least two sequential data frames.

The system 52 provides the display drive data 54 and backlight control signals 56. The backlight may consist of a number of lamps 37, and in this case the lamps may be controlled at different positions with independent control signals. Multiple control signals 56 are shown in Figure 4.

For example, because the data is fed to the display in a sequential manner with pixels at the top of the image being addressed before pixels lower down the display, the timing of the changes in the backlight intensity may be synchronised with the addressing so that the pixels have time to reach their required brightness before the appropriate lamp is switched on. This approach has the additional advantage of giving improved motion rendition.

The backlight may be on permanently, or else it may flash in synchronism with the addressing of the display. In the case of a flashed backlight, the peak intensity is controlled in the manner described above. The curves of light intensity then represent the envelope of the time-dependence of the light pulse amplitudes.

As well as controlling the backlight, the signal processing system 52 may also adjust the drive levels of the display driving signals in synchronism with the backlight switching, in order to optimise perceived image quality. The control of the backlight output brightness will be achieved with standard components within the system 52, to provide different output levels for driving the backlight or backlights.

Figure 5 is used to explain in greater detail a first implementation of the method of the invention, in which the backlight is controlled in dependence on mean brightness.

The mean brightness of each field is measured, for example by summing the data values over that field. This value is compared with the corresponding value in the next field and, if the change is more than a predetermined threshold value the backlight is switched up or down (depending on the direction of the mean change).

In Figure 5, in time period 60, the processor stores the first field of image data 61 and derives the average luminance L1. In time period 62, the processor stores the second field of image data 63 and derives the average luminance L2. The processor needs sufficient memory to store one full frame of data and to store the two luminance values for the most recent two fields of image data. This process continues, so that in period 64, the processor stores the third field of image data 63 and derives the average luminance L2.

At each frame boundary 65 (apart from the first) a comparison is made of the average luminance values. Thus, at frame boundary 65, L1 and L2 are compared, and the backlight is switched if the difference exceeds a preset threshold.

The display is driven in period 70 to display the first image 61 with the default backlight brightness, and the display is driven in period 72 to display the second image 63 with the backlight brightness which is dependent on the comparison at 65.

Figure 6 is used to explain in greater detail a second implementation of the method of the invention, in which the backlight is controlled in dependence on the fraction of the image above a critical brightness level.

The same reference numerals are used as in Figure 5, as essentially the same process is carried out. However, instead of obtaining an average luminance (L1 and L2 in Figure 5), the fraction of the image with a brightness above a defined threshold L0 is measured in successive fields. At the comparison points, such as 65, if the difference exceeds a certain level then the backlight is intensity is switched.

In the examples described in detail above, the backlight has only one level of increased light level to which it can be switched, together with possibly one lower level as required. Also the thresholding functions have only one threshold level.

The principles described above can also be applied to systems in which the backlight can be switched to one of several increased and, if appropriate, decreased, light levels. This can be combined with either a set of different thresholds or a combination of different types of threshold. For example, the decision on whether to switch the light to a higher level can be made based on one criteria, for example that of Figure 6, while the light level to which the lamp is switched can depend upon a different criteria, for example the peak light level in the second of the fields being compared. In the examples above, the backlight has a single default output brightness, to which the backlight output is returned after each step change. The control of the backlight in accordance with the invention can be combined with other methods for improving image quality, in which the long-term average of the backlight intensity is changed in response to image content or possibly external conditions such as illumination level.

In such cases, the default brightness may itself vary over time. Thus, the value of backlight brightness at the end of the slow decay period 42 may be different from the brightness before the rapid change 40. The variation in backlight brightness resulting from the invention in such cases is characterised by a rapid initial change followed by a slower decay to a new equilibrium value.

The term "default" thus does not necessarily indicate a single value, but rather a brightness value determined for that particular point in time by the normal image control algorithm. The control of the invention then overrides the normal image control algorithm.

Typically, the change of the backlight output in such an image control algorithm will be slower than the change implemented by the invention. Information about the image content can additionally be used to control how fast the light level decays following the initial increase in the light level (or increases following an initial drop). For example, an image in which there is a short period of high brightness followed by a rapid decrease would cause the backlight output to be switched back to the equilibrium value with a short time constant. An image where the brightness level increased and stayed high would produce a slower decay in brightness.

Several possible algorithms are possible for controlling the backlight and display drive based on the underlying approach described above. Only some examples are given above. Other examples will be apparent to those skilled in the art.

Claims

1. An active matrix display, comprising: an active matrix display module (34); a backlight (36) having a default output light intensity (B_DEF); a processor (52) for comparing data derived from at least two sequential frames of image data; and means (52) for adjusting the backlight output light intensity such that the backlight output light intensity can change in response to a change in image content, and wherein the maximum rate of change of output light intensity away from the default output light intensity is greater than the maximum rate of change of output light intensity towards the default output light intensity.

2. A display as claimed in claim 1 , wherein the means for adjusting the backlight output light intensity is for increasing the light intensity from the default (BDEF) in response to an increase in image brightness.

3. A display as claimed in claim 2, wherein the means for adjusting the backlight output light intensity is also for decreasing the light intensity from the default (BDEF) in response to a decrease in image brightness.

4. A display as claimed in any preceding claim, wherein the backlight (36) comprises a plurality of lamps (37).

5. A display as claimed in claim 4, wherein the plurality of lamps are controllable independently by the adjusting means (52).

6. A display as claimed in any preceding claim, further comprising a memory (53) for storing at least one frame of image data, and for storing image analysis data for at least two frames of image data.

7. A display as claimed in claim 6, wherein the processor comprises means for determining as the image analysis data an average brightness (L1 , L2) for the stored frames (61 , 63) of image data.

8. A display as claimed in claim 7, wherein the adjusting means determines the backlight output light intensity for a given frame in dependence on a comparison (65) of the average brightness for the given frame of data to be displayed and the average brightness for the preceding frame of data.

9. A display as claimed in claim 6, wherein the processor comprises means for determining as the image analysis data a proportion of the image area having a brightness above a threshold (L0) for the stored frames of image data.

10. A display as claimed in claim 9, wherein the adjusting means determines the backlight output light intensity for a given frame in dependence on a comparison (65) of the proportion for the given frame of data to be displayed and the proportion for the preceding frame of data.

11. A display as claimed in any preceding claim, wherein the backlight is driveable to two possible output light intensities, the default output light intensity (BDEF) and one higher output light intensity (BMAX)-

12. A display as claimed in any one of claims 1 to 10, wherein the backlight is driveable to three possible output light intensities, the default output light intensity (BDEF), one higher output light intensity (BMAX) and one lower output light intensity (BMIN)-

13. A display as claimed in any one of claims 1 to 10, wherein the default output light intensity (BDEF) is variable over time.

14. A display as claimed in claim 13, wherein the default output light intensity (BDEF) is variable in response to image content.

15. A display as claimed in claim 13 or 14, wherein the default output light intensity (BDEF) is variable in response to illumination levels.

16. A display as claimed in any preceding claim, wherein the rate of change of output light intensity towards the default output light intensity is variable in dependence on image content.

17. A display as claimed in any preceding claim, comprising a liquid crystal display.

18. A method of displaying data using an active matrix display having a backlight (36) with a default output light intensity (BDEF), the method comprising: displaying an image (61) with the backlight at the default output light intensity; comparing the content of an image to be displayed (63) and a preceding image (61); determining whether a brightness factor varies by more than a threshold between the image to be displayed (63) and the preceding image (61); if so, adjusting the backlight output light intensity at a first rate of change of output light intensity away from the default output light intensity (BDEF), and subsequently adjusting the backlight output light intensity towards the default output light intensity at a second rate of change of output light intensity which is lower than the first rate of change of output light intensity.

19. A method as claimed in claim 18, wherein the step of adjusting comprises increasing the light intensity from the default in response to an increase in brightness factor.

20. A method as claimed in claim 18, wherein the step of adjusting also comprises decreasing the light intensity from the default in response to a decrease in brightness factor.

21. A method as claimed in any one of claims 18 to 20, wherein the brightness factor comprises an average brightness (L1 , L2).

22. A method as claimed in any one of claims 18 to 20, wherein the brightness factor comprises the proportion of the image area having a brightness above a threshold (L0).

23. A method as claimed in any one of claims 18 to 22, wherein the backlight is driveable to two possible output light intensities, the default output light intensity (B_DEF) and one higher output light intensity (BMAX).

24. A method as claimed in any one of claims 18 to 22, wherein the backlight is driveable to three possible output light intensities, the default output light intensity (B_DEF), one higher output light intensity (B AX) and one lower output light intensity (BMIN)-

25. A method as claimed in any one of claims 18 to 22, wherein the default output light intensity (BDEF) is variable over time.

26. A method as claimed in claim 25, wherein the default output light intensity (BDEF) is variable in response to image content.

27. A method as claimed in claim 25 or 26, wherein the default output light intensity (BDEF) is variable in response to illumination levels.

28. A method as claimed in any one of claims 18 to 27, further comprising varying the rate of change of output light intensity towards the default output in dependence on image content.