This invention relates generally to displays, such as LCD panel displays for example. The invention relates to displays of the type that have backlights comprising an array of light emitting devices, such as light emitting diodes (LEDs), and to backlights suitable for application in such displays.
Some displays, such as liquid crystal displays (LCDs), comprise a spatial light modulator that is illuminated by a backlight. Light from the backlight interacts with the spatial light modulator which spatially modulates the light so as to present images to a viewer. The images may be still images or video images for example. The spatial light modulator may comprise an array of controllable pixels.
In some such displays, the backlight includes multiple light emitting devices, such as LEDs, for illuminating regions of the spatial light modulator. Such light emitting devices or groups of such light emitting devices may be separately controllable so that the intensity of light emitted by the backlight can be made to vary in a desired way over the spatial light modulator. Such displays are referred to herein as dual-modulation displays. Some examples of dual modulation displays are described in: U.S. Pat. No. 6,891,672 issued 10 May 2005 and entitled “High Dynamic Range Display Devices”, U.S. Pat. No. 7,403,332 issued 22 Jul. 2008 and entitled “High Dynamic Range Display Devices”, and United States Patent Application Publication No. 2008/0180466 published 31 Jul. 2008 and entitled “Rapid Image Rendering on Dual-Modulator Displays”, all of which are hereby incorporated herein by reference for all purposes.
The brightness of light emitters on a backlight may be controlled by a technique known as pulse width modulation (PWM). A light emitting device such as an LED may be switched between an ON state at 100% brightness and an OFF state at 0% brightness by switching on and off a suitable fixed electrical current through the device. PWM operates by pulsing each light emitter to its ON state for some percentage of a repeating time period. If the time period is sufficiently short (e.g. 1 millisecond) the human visual system does not detect the light emitter cycling between ON and OFF states. An observer merely perceives the average emitted light intensity, which is proportional to the percentage of the PWM period that the device is in the ON state. This percentage is referred to as the duty cycle of the PWM signal. For example, a light emitter driven by a PWM signal with a duty cycle of 75% is switched on for 75% of each PWM period and appears to an observer as if it were steadily emitting light having a brightness of 75% of its maximum brightness.
This invention has a number of aspects. One provides displays. The displays may comprise, for example, computer displays, televisions, video monitors, home cinema displays, stadium displays, specialized displays such as displays for medical images, displays in vehicle simulators or virtual reality systems, or the like. Another aspect of the invention provides backlights for displays. Another aspect of the invention comprises controllers and control devices useful for controlling backlights for displays. Other aspects of the invention provide methods for operating displays and methods for driving display backlights.
In addition to the exemplary aspects and embodiments described above, further aspects and embodiments will become apparent by reference to the drawings and by study of the following detailed descriptions.
Exemplary embodiments are illustrated in referenced figures of the drawings. It is intended that the embodiments and figures disclosed herein are to be considered illustrative rather than restrictive.
Throughout the following description specific details are set forth in order to provide a more thorough understanding to persons skilled in the art. However, well known elements may not have been shown or described in detail to avoid unnecessarily obscuring the disclosure. Accordingly, the description and drawings are to be regarded in an illustrative, rather than a restrictive, sense.
Display 30 comprises a controller 36 that generates control signals 37 that control light emitters 33 of backlight 32 to emit light having an intensity that varies spatially over the area of spatial light modulator 34. Controller 36 also generates control signals 38 that control the pixels 35 of spatial light modulator 34. Controller 36 receives image data at an input 39 and, based on the image data, generates control signals 37 and 38 to cause a viewer to see images according to the image data.
The waveform Ptotal in
A drawback to PWM when used with multiple light emitters is that the light emitters are all turned on simultaneously for some duration during the beginning of each PWM cycle (for any non-zero brightness setting). The result is that the power supply for the display must be able to deliver enough power to fully drive all of the light emitters for at least a short time and to provide this power almost instantaneously, regardless of the display's effective brightness level. This requirement increases the cost and complexity of the display's power supply, particularly for backlights having large numbers of light emitters. Some backlights may have dozens, hundreds or thousands of individual light emitters. This problem is particularly acute in the case that the display has the capability of displaying very bright images as is the case, for example, in some high dynamic range (HDR) displays. Such displays may be capable of displaying images having local light intensities of 2000cd/m2 or more. In such displays, light emitting elements may be of types that consume significant electrical power in their ON states. This invention may be applied to such displays as well as to other displays.
In some embodiments, such transient power requirements are reduced by dividing the light emitters of a backlight into several groups and staggering the start times of PWM cycles for different ones of the groups over time. The light emitters can be divided into groups in any convenient manner.
The waveform Ptotal of
For example, assuming for the sake of simplicity that each PWM signal I1′-I4′ drives a light emitter consuming power of (Ion) (Von) when in an ON state, Ptotal′ step ups incrementally by (Ion) (Von) at times t0, t1 and t2 to a maximum Pmax′ of 3(Ion)(Von). Thus the maximum power Pmax′ is 75% of the equivalent maximum power Pmax of 4(Ion)(Von) required when the PWM signals are in phase as illustrated in
This concept may be extended to provide embodiments having any number of groups of light emitters having any suitable relative phase shift between their PWM signals. For example, in some embodiments light emitters are divided into N groups wherein PWM signals of each group are phase-shifted by 360/N relative to one another. The power requirements of a backlight will vary depending on a number of factors including the number of light emitters and the duty cycles and phase offsets of the PWM signals applied to each light emitter. The duty cycles (and hence the brightness levels) of the light emitters may be independently controllable as mentioned above. In some embodiments, advantages obtained by phase-shifting PWM signals may include the advantage that total power ramps up more gradually, is distributed more evenly and is held to a lower maximum value than if the same PWM signals were applied in phase.
PWM signals for a given image may cycle without change for as long as that image is being displayed. When a new image is displayed, the PWM driving signals may be updated to reflect image data for the new image. During the first PWM cycle of each new image, the total power may be required to ramp up from zero to a maximum value determined by the updated PWM signals. As described above, this initial ramp-up time may be extended by configuring groups of PWM signals to be out of phase with one another. During subsequent PWM cycles of the same image the total power may remain constant at this maximum value (as in the example illustrated in
For video images, image data and corresponding PWM driving signals may be updated at the start of each video frame. The PWM period may be much shorter than the video frame period such that multiple PWM cycles occur within a single video frame. For example, in some embodiments video frame periods are in the range of 3 to 16.7 milliseconds while PWM periods are in the range of 0.1 to 2 milliseconds.
Example waveforms representing frame periods and PWM periods are illustrated in
According to another embodiment, the duration of the first PWM cycle after an image update is extended in time relative to subsequent PWM cycle periods of the same image. The image may be a video frame or a still image. Since power fluctuation or surges tend to be greatest during the first PWM cycle (as power ramps up from zero to a maximum value as illustrated in
Waveform 54 of
Decreasing the ramp-up rate, magnitude and frequency of power variations of a backlight as described above may in turn decrease the complexity and cost of the power supply needed to power the backlight. For example, various parameters of a power supply such as surge capacity, load regulation and transient response may be eased where PWM signals are offset as illustrated in
Furthermore, the efficiency and reliability of the power supply may be increased where PWM signals are offset as illustrated in
A display incorporating backlight 60 may also comprise a controller 66 that generates brightness signals 68 according to input image data 70. Brightness signals 68 may be analog or digital signals representing the desired brightness level for one or more light emitters 64. Backlight 60 may also comprise one or more PWM controllers 72 for converting brightness signals 68 into PWM driving signals 74, which may directly control the brightness of light emitters 64. In some embodiments, backlight 60 comprises multiple PWM controllers 72, each controlling multiple light emitters 64 such as LEDs. In some embodiments, each tile 62 comprises one or more PWM controllers 72 for controlling light emitters 64 on that tile. For example, tiles 62 comprise PCBs having PWM controllers 72 integrated therein for controlling light emitters 64 on that PCB. Controller 66 and PWM controller 72 may be separate physical devices or may be combined within the same physical device.
PWM driving signals 74 may be waveforms comprising a sequence of cycles having a given duration, duty cycle and phase offset. PWM driving signals 74 may operate to switch on and off a fixed electrical current through a light emitter 64. In some embodiments, PWM driving signals 74 of one tile are phase shifted relative to PWM driving signals 74 of another tile (as illustrated for example in
In the illustrated embodiment, PWM controller 72 outputs multiple PWM driving signals 74 that each control a separate tile 62. In some embodiments, all light emitters 64 on a tile 62 are controlled by a common PWM driving signal 74 generated for that tile. In other embodiments, duty cycles of PWM driving signals 74 for each light emitter 64 are independently controllable by one or more PWM controllers 72.
In some embodiments a controller chip or circuit individually controls multiple light emitters. In some embodiments the PWM controller chip or circuit is configured so that start times of the PWM signals generated for the light emitters are staggered relative to one another. In a backlight constructed using such PWM controller chip or circuits the times at which different groups of light emitters are turned ON are automatically staggered.
Backlight 60 also comprises a power supply 76 for providing electrical power to light emitters 64 on the backlight. Power supply 76 may be configured to satisfy particular power requirements necessary to generate the desired range of brightness of light emitters 64. Such power requirements may include load regulation, transient response and/or surge capacity for example. If the start time of groups of PWM signals are staggered as illustrated in
Similarly, in some embodiments power supply 76 is capable of a maximum output surge current (out-rush current) that is less than the total in-rush current that would be required by light emitters 64 if all light emitters 64 were switched on at the same time. For example, if backlight 60 comprises N light emitters and each light emitter requires an in-rush current of Irush when switched on, then power supply 76 may have a maximum out-rush current of less than N(Irush) while being capable of supplying the average current required. In some embodiments, power supply 76 has a maximum out-rush current less than 0.75(N)(Irush). In some embodiments, power supply 76 has a maximum out-rush current less than 0.5(N)(Irush).
Power supply 76 may be configured to have the capacity to supply a continuous output current sufficient to sustain a desired average brightness of backlight 60. In some embodiments, power supply 76 is capable of generating a maximum average light intensity over the entire backlight 60 that is less than localized light intensities it may generate over portions of backlight 60. For example, power supply 76 may be capable of generating localized light intensities of 2000 cd/m2 or more over portions of backlight 60 while only capable of generating a maximum average light intensity of 400 cd/m2 over the entire backlight 60.
Block 102 of method 100 involves determining brightness values for all light emitters on a backlight of a display based on image data representing an image to be displayed. In this method, the light emitters are divided into a plurality of groups. The brightness values may be determined independently for each separate light emitter or for each separate group so that the intensity of light emitted by the backlight and incident on a spatial light modulator can be made to vary in a desired way over the spatial light modulator. The brightness values may be represented by electronic analog or digital signals, for example.
At block 104 of method 100, PWM duty cycles are determined for the light emitters of each group based on the brightness values determined at block 102. The duty cycles may be expressed for example as the percentage or ratio of each PWM period that the light emitter should be in an ON state to produce the desired brightness level.
At block 106 of method 100, PWM driving signals having the duty cycle determined at block 104 and a phase offset predetermined for each group are generated and applied to each light emitter. The phase offsets applied for each group differ from one another so as to stagger the start times of PWM cycles of different groups (as illustrated in
At block 108, the duration of each PWM cycle is set such that a first PWM cycle of the image is longer than duration of subsequent PWM cycles for the given image (as is illustrated in
It is not necessary that a PWM cycle always comprise a contiguous on-time portion followed by a contiguous off-time portion. For a given duty cycle, the pattern of on-time and off-time may varied so long as the overall ratio of on-time to off-time within the cycle is maintained. For example, the order of on-time and off-time within a cycle may be reversed such that a light emitter remains off for some first portion of the cycle and then turns on for the remaining portion of the cycle. In this case, light emitters having different brightness levels may turn on at different times within the same PWM cycle (and switch off at the same time at the end of the cycle).
As another example, on-time may also be centered within a PWM cycle such that different power levels rise and fall at different times. On-time and off-time may be interspersed within a PWM cycle in any other chosen manner so long as the overall proportion of on-time to off-time within the cycle remains the same. Where a discrete number of brightness levels are defined for light emitters of the display (for example 2n brightness levels where n is a number of bits defining brightness), each cycle may be divided into that number of segments (for example 2n segments) during which a light emitter may be set ON or OFF. Each brightness level may correspond to a particular pattern of ON/OFF segments within a PWM cycle. Different groups of light emitters may employ different sets of ON/OFF patterns for each brightness level such that on-times between groups are staggered even if set to the same brightness level. The total power requirements may thus be distributed more evenly across PWM cycles.
Variations in the distribution of on-time and off-time within PWM cycles may be combined with variations in phase offsets for groups of PWM signals as described above. For example, the start times of individual light emitters within a group having a common phase offset may be staggered by measuring duty cycles from the end of each PWM cycle. If the duration of the first cycle of each new image is made longer than a default PWM period, the initial ramp-up time required may be correspondingly extended as well.
In some embodiments, each clock signal 124 may be switched between a first clock signal used for the first PWM cycle of a displayed image and a second clock signal used for subsequent PWM cycles of the same image. The first clock signal may have a longer period than the corresponding second clock signal (for example a period of 2T compared to T), but the same phase offset. The first clock signal may thus be used to extend the duration of the first PWM cycle of each displayed image relative to the duration of subsequent PWM cycles of the same image. In alternative embodiments the frequency of a clock signal may be changed such that a period of a first PWM cycle is longer than that of subsequent PWM cycles.
In some embodiments, driving signals of multiple PWM controllers are time multiplexed within a period T such that different groups of light emitters are driven for different non-overlapping time intervals within a period T. In this way, the on-times of light emitters driven by different PWM controllers never overlap and thus the power requirements of the backlight are reduced.
Certain implementations of the invention comprise computer processors which execute software instructions which cause the processors to perform a method of the invention. For example, one or more processors in a control system for a display may implement the method of
Where a component (e.g. a controller, processor, assembly, device, etc.) is referred to above, unless otherwise indicated, reference to that component should be interpreted as including as equivalents of that component any component which performs the function of the described component (i.e., that is functionally equivalent), including components which are not structurally equivalent to the disclosed structure which performs the function in the illustrated exemplary embodiments of the invention.
While a number of exemplary aspects and embodiments have been discussed above, those of skill in the art will recognize certain modifications, permutations, additions and sub-combinations thereof. It is therefore intended that the following appended claims and claims hereafter introduced are interpreted to include all such modifications, permutations, additions and sub-combinations as are within their true spirit and scope.
This application claims priority to U.S. Provisional Patent Application Nos. 61/228,156 filed 23 Jul. 2009 and 61/234,148 filed 14 Aug. 2009, hereby incorporated by reference in its entirety.
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/US2010/042798 | 7/21/2010 | WO | 00 | 1/19/2012 |
Publishing Document | Publishing Date | Country | Kind |
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WO2011/011548 | 1/27/2011 | WO | A |
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