Patent Publication Number: US-2023154422-A1

Title: Display device, method for generating offset current values and current offsetting system

Description:
RELATED APPLICATIONS 
     This application is a continuation of International application No. PCT/CN2021/131383, filed Nov. 18, 2021 which is herein incorporated by reference. 
    
    
     BACKGROUND 
     Field of Invention 
     The present disclosure relates to a method for generating offset values for a backlight module of a display device. 
     Description of Related Art 
     A display is one of the most common electronic devices in modern life and used in various scenarios and situations. Some displays include a backlight module to provide a light source through a plurality of light-emitting diodes. The brightness of the light-emitting diodes can be independently controlled based on the technology of local dimming, thereby improving the contrast ratio of the display. The light-emitting diodes are driven according to preset parameters in conventional technologies. However, due to factors such as process variation, the preset parameters may not necessarily provide preset brightness of light. Therefore, how to correct these parameters is a topic of concern to those skilled in the art. 
     SUMMARY 
     Embodiments of the present disclosure provide a method for generating offset current values for a display device. The display device includes a display panel and a backlight module. The display panel includes multiple regions, the backlight module includes multiple light emitting units, and each of the regions corresponds to at least one of the light emitting units. The light emitting units are driven by currents to serve as a backlight source of the regions of the display panel. The method includes: establishing a current setting sequence including multiple current setting values, driving a first light emitting unit of the light emitting units, and measuring a first current value of the first light emitting unit; establishing a recurrent neural network including input layer, a hidden layer, and an output layer; and inputting the first current value into the hidden layer and sequentially inputting the current setting values into the input layer so as to obtain multiple offset values from the output layer sequentially. The offset values correspond to the current setting values respectively. 
     In some embodiments, the method further includes: obtaining one of the offset values, and driving the first light emitting unit according to the obtained offset value. 
     In some embodiments, an operation of the recurrent neural network includes performing a following equation. 
         s ( d )= f   2 ( W×f   1 ( V×t ( d )+ U×m ( d+ 1))) 
     d is one of multiple dimming levels. t(d) is one of the current setting values. The dimming levels are arranged in descending order in the current setting sequence. s(d) is one of the offset values, and m(d+1) is an input of the hidden layer. When the dimming level d is equal to a maximum dimming level, m(d+1) is the first current value. W, V and U are weights, and f 1  and ƒ 2  are activation functions. 
     In some embodiments, the current setting values correspond to multiple driving values, and the method further includes: in a training stage, for each of the current setting values, driving one of the light emitting units according to the corresponding driving value to obtain a second current value, adjusting the corresponding driving value based on a negative feedback control such that the second current value meets the corresponding current setting value. The adjusted driving value and the second current value corresponding to the maximum dimming level constitute a training sample. 
     In some embodiments, the activation functions ƒ 1  and ƒ 2  are Sigmoid functions, rectified linear units, or hyperbolic tangent functions. The dimming levels are arranged as an arithmetic sequence. 
     From another aspect, embodiments of the present disclosure provide a display device including a display panel, a backlight module and at least one circuit. The display panel includes multiple regions. The backlight module includes multiple light emitting units. Each of the regions corresponds to at least one of the light emitting units. The light emitting units are driven by currents to serve as a backlight source of the regions of the display panel. The circuit includes an offset lookup table containing multiple offset values corresponding to multiple dimming levels respectively. The offset values are built by a recurrent neural network. The at least one circuit is configured to obtain one of the offset values, and generate a corrected current according to the obtained offset value to drive a first light emitting unit of the light emitting units. 
     In some embodiments, the at least one circuit includes a time controller and a microcontroller unit. The time controller is configured to calculate driving values corresponding to the first light emitting unit according to a local dimming algorithm. The microcontroller unit stores the offset lookup table. 
     From another aspect, a current offsetting system includes the display device and an electrical device. The recurrent neural network is performed by the electrical device and includes an input layer, a hidden layer, and an output layer. The electrical device is configured to generate offset current values based on a calibration procedure including: establishing a current setting sequence including multiple current setting values, driving a first light emitting unit of the light emitting units, and measuring a first current value of the first light emitting unit; inputting the first current value into the hidden layer and sequentially inputting the current setting values into the input layer so as to obtain multiple offset values from the output layer sequentially, in which the offset values correspond to the current setting values respectively; and building the offset lookup table corresponding to the first light emitting unit according to the offset values. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention can be more fully understood by reading the following detailed description of the embodiment, with reference made to the accompanying drawings as follows. 
         FIG.  1    is a schematic diagram of a current offsetting system in accordance with an embodiment. 
         FIG.  2    is a schematic diagram of regions of the display panel and the corresponding light emitting units in accordance with an embodiment. 
         FIG.  3    is a flow chart of generating offset current values of training samples in accordance with an embodiment. 
         FIG.  4    is a schematic diagram of a recurrent neural network in accordance with an embodiment. 
         FIG.  5    is a flow chart of a calibration procedure in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Specific embodiments of the present invention are further described in detail below with reference to the accompanying drawings, however, the embodiments described are not intended to limit the present invention and it is not intended for the description of operation to limit the order of implementation. Moreover, any device with equivalent functions that is produced from a structure formed by a recombination of elements shall fall within the scope of the present invention. Additionally, the drawings are only illustrative and are not drawn to actual size. 
     The using of “first”, “second”, “third”, etc. in the specification should be understood for identifying units or data described by the same terminology, but are not referred to particular order or sequence. 
       FIG.  1    is a schematic diagram of a current offsetting system in accordance with an embodiment. Referring to  FIG.  1   , a current offsetting system  100  includes an electrical device  110  and a display device  120 . The electrical device  110  may be a personal computer, a server or any electrical device with computation capability. The display device  120  includes a circuit  130 , a backlight module  140 , and a display panel  150 . The circuit  130  includes a time controller  131  and a microcontroller unit (MCU)  132 . The microcontroller unit  132  may be replaced with a field programmable gate array (FPGA) which is not limited in the disclosure. The backlight module  140  includes multiple light emitting units such as light emitting diodes which are driven by currents of the backlight module  140  to serve as a backlight source. The display panel  150  is, for example, a liquid crystal display panel.  FIG.  2    is a schematic diagram of regions of the display panel and the corresponding light emitting units in accordance with an embodiment. In the embodiment of  FIG.  2   , the display panel  150  includes 15 regions (e.g. regions  151 - 153 ), and each region corresponds to four light emitting units (e.g. light emitting units  141 - 142 ). The brightness of each light emitting unit can be controlled by the magnitude of the current flowing through the corresponding light emitting unit for increasing the contrast ratio of a frame. For example, if a portion of the frame in a particular region is relatively dark, the brightness of the corresponding light emitting units are decreased; and if a portion of the frame in that particular region is relatively bright, the brightness of the corresponding light emitting units are increased.  FIG.  2    is merely an example, and the number of the regions in the display panel  150  and the number of the light emitting units corresponding to one region are not limited in the disclosure. 
     Referring to  FIG.  1   , when a frame is to be rendered, the time controller  131  calculates a dimming level of each region of the display panel  150  according to a local dimming algorithm. The dimming level represents the brightness of the backlight source. The time controller  131  also calculates a driving value of each light emitting unit based on the dimming level. The driving value is used to generate a particular magnitude of current to drive the light emitting units. In some embodiments, the driving value is positively correlated to the dimming level. For example, the dimming level has 8 digits in a range of 0 to 255, and the driving value has 10 digits in a range of 0 to 1023. The mapping between the dimming level and the driving value may be linear or non-linear which is not limited in the disclosure. Due to factors such as process variation, a preset driving value may not necessarily drive the light emitting unit to produce required brightness, and hence the preset driving value needs to be corrected. The microcontroller unit  132  stores multiple offset lookup tables, and each offset lookup table corresponds to one light emitting unit. Each offset lookup table stores multiple offset values corresponding to the driving values. The time controller  131  can read the offset lookup table according to the calculated driving value so as to obtain the corresponding offset value, and drive the corresponding light emitting unit based on this offset value. Consequently, the light emitting unit will produce required brightness. A method for generating offset current values is provided. The method is executed by the electrical device  110  in which a recurrent neural network (RNN) is used to generate the said offset values. 
       FIG.  3    is a flow chart of generating offset values of training samples in accordance with an embodiment. Referring to  FIG.  3   , the light emitting unit  141  is taken as an example. In a training stage, the light emitting unit  141  is driven according to a driving value  301 , and then a current value of the light emitting unit  141  is measured by the measurement unit  302  such as a current meter or a power meter. In step  303 , it is determined if the measured current value meets a current setting value  304 . The current setting value  304  is a target current magnitude which may be an objective value calculated by a program of the electrical device  110 , or a subjective value set by the user, or a value of the specification asked by the manufacturer of the electrical device  110 . The current setting value  304  corresponds to the driving value  301 . For example, the driving value is equal to “995”, and the current setting value is equal to 64 milliampere (mA). That is, 64 mA is sufficient to drive the light emitting unit  141  to produce required brightness of the driving value “995”. It is determined if the measured current value is close to 64 mA enough. In some embodiments, whether the measured current value is equal to the current setting value  304  is determined in the step  303 . In some embodiments, whether the difference between the measured current value and the current setting value  304  is within a range is determined in the step  303 . If the result of the step  303  is “No”, then the driving value is adjusted in a step  305 . For example, if the measured current value is less than the current setting value  304 , then the original driving value  301  is increased a little as a new driving value; if the measured current value is greater than the current setting value  304 , then the original driving value  301  is decreased a little as a new driving value. The light emitting unit  141  is then driven by the new driving value, and the step  303  is repeated. When the measured current value meets the current setting value in the step  303 , the adjusted driving value is outputted in the step  306 . In other words, the driving value  301  is adjusted based on negative feedback control such that the measured current value meets the current setting value  304  corresponding to the driving value  301 . Note that the negative feedback control is performed for each driving value. If there are 256 dimming levels, there will be 256 corresponding driving values, and the negative feedback control is performed 256 times for each driving value. The adjusted driving values serve as a portion of a training sample. 
     The following Table 1 includes dimming levels, current setting values, original driving values, measured current values, and adjusted driving values of a light emitting unit. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 dimming 
                 current 
                 original 
                 Measured 
                 adjusted 
               
               
                 level 
                 setting value 
                 driving values 
                 current value 
                 driving values 
               
               
                 d 
                 t(d) 
                 si(d) 
                 m(d) 
                 s(d) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 255 
                 64 
                 995 
                 65.3 
                 993 
               
               
                 254 
                 63.75 
                 991 
                 65 
                 989 
               
               
                 253 
                 63.5 
                 988 
                 64.8 
                 985 
               
               
                 . . . 
                 . . . 
                 . . . 
                 . . . 
                 . . . 
               
               
                 1 
                 0.25 
                 3 
                 0.35 
                 3 
               
               
                 0 
                 0 
                 0 
                 0 
                 0 
               
               
                   
               
            
           
         
       
     
     For example, referring to the second row of the Table 1 (i.e. the dimming level is equal to 255), the current setting value is equal to 64 mA, the original driving values is equal to 995, the measure current value is 65.3 mA which is greater than the current setting value of 64 mA, and thus the driving value is adjusted into 993, and so on for the other dimming levels. In the following description, d denotes the dimming level, t(d) denotes the current setting value corresponding to the dimming level d, si(d) denotes the original (i.e. preset) driving value corresponding to the dimming level d, m(d) denotes the measured current value corresponding to the dimming level d, and s(d) denotes the adjusted driving value corresponding to the dimming level d. In the embodiments, the current value m(d) of one dimming level (e.g. d=255) is used to predict the adjusted driving values s(d) of all dimming levels. Since the measurement of the current value m(d) needs certain amount of time, it will take too much time for measuring the current values m(d) of all dimming levels. In the disclosure, the adjusted driving values s(d) are rapidly estimated by means of prediction. 
       FIG.  4    is a schematic diagram of a recurrent neural network in accordance with an embodiment. Referring to  FIG.  4   , a recurrent neural network  400  includes an input layer  410 , a hidden layer  420 , and an output layer  430 . The input of the input layer  410  is the current setting value t(d), the feature value m(d) of the hidden layer  420  is calculated as the following Equation 1 where V and U are weights to be trained. 
         m ( d )= V×t ( d )+ U×m ( d+ 1)  [Equation 1]
 
     The output of the output layer  430  is the driving value s(d) calculated as the following Equation 2 where W is a weight to be trained. Substituting the Equation 1 into the Equation 2 yields the following Equation 3. 
         s ( d )= W×m ( d )  [Equation 2]
 
         s ( d )= W ×( V×t ( d )+ U×m ( d+ 1))  [Equation 3]
 
     In addition, an activation function ƒ 1  is included between the input layer  410  and the hidden layer  420 , and an activation function ƒ 2  is included between the hidden layer  420  and the output layer  430 . These two activation functions may be Sigmoid functions, Rectified Linear Units (ReLU) or hyperbolic tangent functions which are not limited in the disclosure. The output of the two activation functions is within a limited range and the output curve is smooth without outputting positive infinity or negative infinity like the arithmetic sequence did, and thus a correction stability of the driving values s(d) is improved. Under this premise, the Equation 3 can be rewritten into the following Equation 4 by adding the activation functions. 
         s ( d )= f   2 ( W×f   1 ( V×t ( d )+ U×m ( d+ 1)))  [Equation4]
 
     The current setting values t(255)-t(0) are arranged in a sequence. The driving values s(d) outputted from the recurrent neural network  400  are also arranged in a sequence. In general, the recurrent neural network  400  can be expressed in an expanded way as shown in  FIG.  4   . For example, when the input is t(d), the output is s(d), and so on. Note that the dimming levels d are arranged as an arithmetic sequence in a descending order. That is, the current setting values t(255)-t(0) are sequentially inputted into the input layer  410 . The feature value m(d) is calculated based on the feature value m(d+1) of the previous iteration. When processing the greatest dimming level (i.e. d=255), the feature value m(256) is equal to the measured current value (e.g. 65.3 mA in the second row of Table 1) by driving the light emitting unit based on the driving value si(255). Therefore, only the current value m(256) needs to be measured, and the other values m(255)-(0) are calculated based on the Equation 1. A training sample at least includes the current value m(256) and all the adjusted driving values s(255)-s(0). 
     Each light emitting unit can provide a training sample. The trained recurrent neural network  400  is used to predict adjusted driving values. To be specific,  FIG.  5    is a flow chart of a calibration procedure in accordance with an embodiment. The calibration procedure is performed by the electrical device  110  for building an offset lookup table for a particular light emitting unit. Referring to  FIG.  5   , take the light emitting unit  142  as an example, a current setting sequence  501  is established to include current setting values t(255)-t(0) in which the dimming levels are arranges as an arithmetic sequence in descending order from “255” to “0”. Next, a driving value  502  corresponding to one of the current setting value is obtained. In the embodiment, the driving value si(255) corresponding to the maximum dimming level (i.e. d=255) is obtained. The light emitting unit  142  is driven according to the obtained driving value  502 , and then a current value  503  of the light emitting unit  142  is measured by the measurement unit  302 . The current value  503  is then inputted into the hidden layer of the recurrent neural network  400  as m(d+1) while the current setting values t(255)-t(0) are sequentially inputted into the input layer, and then offset values  504  are sequentially obtained from the output layer. The offset values  504  are used to build the offset lookup table. For example, the offset lookup table records the offset values (e.g. “999”, “989” . . . , “3”, “0”) corresponding to the dimming levels (e.g. “255” to “0”). The offset lookup table may be stored in the microcontroller unit  132  for further operations. For example, when the dimming level is equal to “254”, the offset values “989” is obtained by accessing the offset lookup table, and then the backlight module  140  generates a corrected current based on this offset value of “989” to drive the light emitting unit  142  as a backlight source of at least one region of the display panel. That is, the current for each region is corrected to provide uniform brightness to avoid uneven brightness across the regions. The embodiment may cooperate with a local dimming algorithm to ensure desired brightness of each region is achieved. 
     In the flow chart of  FIG.  5   , all offset values can be obtained by measuring the current value  503  only once. Conventional technology needs to measure the current value of each driving value to generate the offset values based on negative feedback control. The approach described in the embodiment can save a lot of time. 
     In the aforementioned embodiment, the driving value  502  is equal to the driving value si(255) corresponding to the maximum dimming level. However, the driving value corresponding to any dimming level may be adopted in other embodiments, and the corresponding current value should be adopted in the training stage. For example, if the measured current value m(128) is inputted into the hidden layer, then the current setting values for the input layer may be in the order of t(127)-t(0) and (255)-t(128). In other words, the current setting value t(127) has to be the first input. People in the art should be able to devise different current setting sequences based on the disclosure. The disclosure is not limited to the aforementioned embodiments. 
     Although the present invention has been described in considerable detail with reference to certain embodiments thereof, other embodiments are possible. Therefore, the spirit and scope of the appended claims should not be limited to the description of the embodiments contained herein. It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims.