Abstract:
A dynamic controller for a light emitting active-matrix display, the display being responsive to code values for producing a light output, including: photosensor located on the display for sensing the light output from the display and generating a feedback signal representative thereof; a feedback signal converter for converting the feedback signal to a converted feedback signal having the same form as the code value; a code-value corrector including a memory responsive to a code value for producing a corrected code value; and an update calculator responsive to the converted feedback signal, the code value and the corrected code value to update the memory to minimize the difference between the converted feedback signal and the code value.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an improved method for controlling active-matrix displays, in particular, a method employing feedback signals to correct input data and improve the display quality. 
     BACKGROUND OF THE INVENTION 
     Active-matrix light emitting displays include drive electronics associated with each light emitting pixel for controlling the light output of the pixels. Active-matrix emissive display devices suffer from a number of difficulties. For example, as the emissive materials in the light emitters age, the materials change so that the light output from the light emitters will also change. In addition, it is problematic to manufacture such display devices and maintain a consistent uniformity across the entire display due to process control difficulties. Moreover, the materials employed in active-matrix emissive display devices change from one generation to the next, and the cost of creating a custom controller for each generation of material add significantly to the cost of the display devices. 
     The use of smart controllers capable of controlling a variety of similar devices and incorporating programmable elements is known. For example, U.S. Pat. No. 6,100,879 issued Aug. 8, 2000 to DaCosta discloses a system for controlling an active-matrix display using a smart controller with a programmable register on board. The approach proposed by DaCosta does not compensate for changes in the light output of the display over time, thus the problems noted above still exist. 
     There is a need, therefore, for a controller that overcomes the problems noted above. 
     SUMMARY OF THE INVENTION 
     This need is met according to the present invention by providing a dynamic controller for a light emitting active-matrix display, the display being responsive to code values for producing a light output, that includes: photosensor located on the display for sensing the light output from the display and generating a feedback signal representative thereof; a feedback signal converter for converting the feedback signal to a converted feedback signal having the same form as the code value; a code-value corrector including a memory responsive to a code value for producing a corrected code value; and an update calculator responsive to the converted feedback signal, the code value and the corrected code value to update the memory to minimize the difference between the converted feedback signal and the code value. 
     ADVANTAGES 
     Because the present invention relies upon feedback and correction rather than a model of the active-matrix device behavior, it can be applied with few or no changes to a wide variety of devices. For example, if the light-emitting materials change or device-to-device variability is significant, no change to the design is necessary and the present invention will properly correct for any changes or variability. 
     The present invention provides a simple design for accommodating optical feedback from active-matrix display devices. It is suitable for feedback from individual pixels, sub-pixel elements, or from representative pixels or elements. The present invention is easy to implement and control and provides dynamic correction as each data value is written. Using conventional means, the converter device can be controlled from a computer, external memory, or programmable read-only-memory. The basic design can be either analog or digital and can readily accommodate a variety of feedback signal types. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of an active-matrix display having a dynamic controller according to the present invention; and 
         FIG. 2  is a diagram of an active-matrix display having a dynamic controller having additional intermediate storage device options. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is a system for the correction of image pixel output in an active-matrix, emissive display. The system relies upon a feedback signal generated by a sensor on the display device. This feedback signal is used to adjust the display output. The controller of the present invention is referred to as a dynamic controller because the adjustments applied by the controller change over time as the characteristics of the display device change. 
     A dynamic active-matrix controller  8  according to the present invention is shown in  FIG. 1 . Referring to  FIG. 1 , conventional address and data lines  10  and  12  are used to address the individual light emitting elements that make up the pixels in an active-matrix display  14  to specify the amount of light to be emitted by each pixel, respectively. For a color display, the address lines  10  can address color subpixels in each pixel separately or together. The data signals are encoded as code values which specify the level of light output desired from the pixels. According to the present invention, the code values are corrected to accommodate changes in the output characteristics of the display device using a code-value corrector  18 . The corrected code values  26  are presented to the active-matrix display device  14  which emits light in response. The light output from the display device is detected by a photosensor  15  to provide a feedback signal  42 . This feedback signal  42  is converted by a feedback signal converter  46  to a converted feedback signal  44  having the same form as the code value data signals  12 . An update calculator  48  combines the code value data signals  12 , the converted feedback signal  44  and the corrected code value  26  to create an updated corrected code value  49 . This updated corrected code value  49  is supplied to and stored in the code-value corrector  18 . 
     The controller  8  can include one or more photosensors  15  that can be associated with individual light emitting elements, with groups of elements, or with representative light emitting elements  17  that are provided on the display, but are not visible as a part of the display. The code-value corrector  18  includes a memory containing a lookup table  19  for each photosensor  15 . The lookup tables are selected according to the addresses of the pixels associated with the photosensors. Thus, if a single representative pixel is used, only one lookup table is required and all pixel addresses will be referred to the table. If the photosensors are associated with groups of pixels, the pixel addresses for the group will be referred to the corresponding lookup table. In the event that a photosensor  15  is provided for each pixel, there will be a corresponding lookup table for each pixel. Alternatively, the lookup tables  19  can contain one or more correction parameters and the corrected code values be computed using the correction parameters. This approach trades off speed and memory size for complexity. 
     If a photosensor  15  is provided for each light emitting element in the display, the present invention can be used to fully correct for any spatial nonuniformities in the display device. Where photosensors  15  are provided for groups of pixels, identical corrections are made for each light emitting element within the group, thereby limiting the amount of nonuniformity correction that can be performed. With the use of only one photosensor  15 , for example with a representative light emitting element  17 , nonuniformities across a display will not be addressed. Photosensors can be employed with representative pixels of each color in a color display, to compensate for color changes such as those resulting from aging. The controller  8  can include means for sending every code value to the representative pixel and producing a corrected code value for every code value. 
     The code values presented to the controller  8  are typically a digital value from zero to 256 and represent the amount of light to be emitted by the light emitting element at the associated address. The feedback signal  42 , in contrast, may be a current. This current represents the amount of light output by a light emitting element in the display. The conversion from the current measured and the light output is performed by applying calibration information initially obtained from a measurement of the light and related current in an ideal device. This current information is obtained for each light output level and used to calibrate the feedback signal converter  46 . 
     Once the converted feedback signal  44  is generated, it is used to update the code-value corrector  18 . The difference between the converted feedback signal  44  and the desired code value data signals  12  is calculated. This difference is then combined with the corrected code value  26  to create a new, updated corrected code value  49 . This updated corrected code value  49  is stored in the code-value corrector  18  and used to correct subsequent code values. The calculator and the code-value corrector may be integrated into a single integrated circuit or provided by discrete components. 
     Various arrangements for providing sensors on a display device are shown in more detail in copending U.S. patent application Ser. No. 09/577,241 filed May 24, 2000 by Cok et al.; U.S. Patent application Ser. No. 09/675,346 filed Sep. 29, 2000 by Cok et al.; and U.S. patent application Ser. No. 09/707,223 filed Nov. 6, 2000 by Cok et al., which are incorporated herein by reference. 
     In a practical embodiment of the present invention, additional timing, storage, and control signals may be used to increase signal availability, reliability, timeliness, and the like. For example, in the embodiment shown in  FIG. 2 , additional, intermediate storage devices  22  are provided for receiving and storing corrected data signals from the data signal corrector and supplying the corrected data signals to the display, for receiving converted feedback signals  44  and supplying them to the update calculator  48 , or for receiving update signals  49  and supplying them to the code-value corrector  18 . Any one or all of these storage devices may be used to facilitate system timing. 
     Once the code values have been corrected and the device has properly loaded the corrected factor into the code-value corrector  18 , the next time that the particular data signal occurs at that pixel location, the new, corrected code value will be applied and the display device will emit the desired amount of light in response to the corrected code value. When the comparison between the desired code value data signals  12  and the converted feedback value  44  goes to zero, the same value  26  is re-entered into the code-value corrector  18  and no change is made. Note that the code-value corrector  18  does not have to be pre-loaded and does not require a complex model of the behavior of the display device. The feedback circuit will adjust the contents of lookup table  19  over time to correct for changes in the display device. 
     In a preferred implementation, feedback from each pixel is obtained as the address and data values are applied. This avoids complex logic which would otherwise be necessary to intermingle the writing of corrected code value  49  into the code-value corrector  18  with the read-out of corrected signals  26  from the code-value corrector  18 . 
     It is also possible to implement the present invention as a start-up or initial calibration mechanism for a display. While this approach does not provide real-time feedback and correction, it may simplify the requirements for the system. In this embodiment, the various pixel elements from which feedback is obtained are completely exercised with all possible values, the feedback obtained, an update signal generated, and the code-value corrector updated for each value before the device enters normal operation. Once the code-value corrector is updated with the correct values, the device operates as normal but without any on-going feedback or correction. 
     In a preferred implementation, the code-value corrector  18  is made of conventional lookup tables. Likewise, the feedback converter is made of conventional lookup tables with an analog to digital voltage converter and/or current/voltage converters. The update calculator  48  can be implemented with conventional digital logic or analog operational amplifiers. 
     The code-value corrector  18  is capable of storing every possible output value for every possible pixel sub-element for which feedback is generated. In the ideal case, the feedback is generated from every sub-pixel element, thus requiring a separate value for each possible output level for each sub-pixel element which is readily implemented with modem integrated circuit technology. The size of the memory will scale with the size of the display and number of display elements. In the case that a single representative pixel is used for each of three colors, only three 8-bit tables are necessary. It may also be preferable to use a separate feedback signal for each color (particularly if representative pixels are used) together with separate conversion, calculation, and correction devices. This is a matter of circuit design structure and is well-known in the art. 
     The feedback signal converter  46  contains the information necessary to translate the feedback signal to the desired data value associated with that signal. Therefore a correspondence between each color value and a feedback value is maintained. For a representative pixel or for feedback that is only dependent on the color of the sub-pixel element, a three-color, 8-bit active-matrix display with a very small table containing only 768 bytes is used. If feedback is obtained from each pixel, the present invention can be used to correct for uniformity problems as well as aging of materials and ambient conditions. 
     Moreover, if global image corrections based on pixel positions are desired, the conversion calculation could include a dependency on pixel position, which is easily implemented by applying the address signals to the converter. This is useful, for example, if the active-matrix display is a part of a larger optical system for which pixel-position compensation is desired. In this case, a larger table like that of the code-value corrector  18  will be needed. It is also possible to provide a global correction to the display based on other attributes such as the ambient illumination by modifying the feedback signal to accommodate an ambient signal, for example by increasing or decreasing the feedback value for all pixels by an amount representative of the ambient. 
     If the frequency at which data is written to the active-matrix display device  14  exceeds the capability of the materials in the device to propagate signals, the display device is separated into separate, smaller sections driven in parallel, as is well known in the art. Each section then has a different feedback and correction circuit. If representative pixels are used, a separate representative pixel supplies the feedback from each section. If the device is separated into separate, smaller sections, the storage requirements for the code-value corrector  18  are reduced accordingly. If the number of feedback elements is reduced, the size of the feedback signal converter  46  will likewise be reduced. Hence the invention will scale reasonably well to large display sizes. 
     The present invention does not require a complex model of the pixel behavior under various conditions, simply a target or desired output matched to the code value data signals  12 , together with initial calibration data. Because the present invention relies upon feedback and correction rather than a model of the active-matrix device  14  behavior, it can be applied with few or no changes to a wide variety of devices. For example, if the light-emitting materials change or device-to-device variability is significant, no change to the design is necessary and the present invention will properly correct for any changes or variability. 
     The active-matrix address and data signals need not be digital. By supplying a digital to analog signal converter to convert the data and/or address control signals, an analog interface can be implemented. 
     Most active-matrix display devices require some color transformation to adjust the color and contrast ranges of the display. These transformations should generally be done before the signals reach the code-value corrector  18 . Although the code-value corrector  18  can be designed to implement these transformations as well, the code-value corrector becomes much more complex especially, for example, if color matrix transforms are required. 
     Although the Figures illustrate a design in which the feedback converter, comparator, corrections device, and data store are all separate from the display, it is possible to integrate any or all of these components on a common substrate with the display device itself. 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. 
     
       
         
               
             
               
               
               
             
           
               
                   
               
               
                 PARTS LIST 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                  8 
                 dynamic controller 
               
               
                   
                 10 
                 address signals 
               
               
                   
                 12 
                 code value data signals 
               
               
                   
                 14 
                 active-matrix display device 
               
               
                   
                 15 
                 photosensor 
               
               
                   
                 17 
                 representative light emitting element 
               
               
                   
                 18 
                 code-value corrector 
               
               
                   
                 19 
                 lookup table 
               
               
                   
                 22 
                 local storage device 
               
               
                   
                 26 
                 corrected code values 
               
               
                   
                 42 
                 feedback signal 
               
               
                   
                 44 
                 converted feedback signal 
               
               
                   
                 46 
                 feedback signal converter 
               
               
                   
                 48 
                 update calculator 
               
               
                   
                 49 
                 corrected code value