Abstract:
The disclosed embodiments relate to a video unit, comprising an illumination source. The video unit additionally comprises a circuit coupled to the illumination source, the circuit adapted to linearize the illumination source using characteristic parameters of the illumination source.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to display systems. More specifically, the present invention relates to a system and method for enhancing contrast ratio in certain display systems. 
       BACKGROUND OF THE INVENTION 
       [0002]    This section is intended to introduce the reader to various aspects of art, which may be related to various aspects of the present invention that are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art. 
         [0003]    Liquid Crystal Displays (LCD) panels are increasingly being used for television display applications mainly due to their light weight and thin profile, as compared to Cathode Ray Tubes (CRTs). However, the performance of LCD panels is still lagging behind CRTs in a number of key areas, one of which is contrast ratio. As an example, the contrast ratio of high-end LCD panels is generally about 500:1, while for a CRT, 10,000:1 is a common ratio. 
         [0004]    The contrast ratio may be defined as the ratio of the amount of light of the brightest white to the darkest black of a video frame. Unfortunately, due to their light transmitting properties, pixels of LCD panels transmit enough light, even when in their darkest state, such that a black colored pixel displayed on the LCD panel actually appears to be displayed as a dark gray pixel. Consequently, this significantly lowers the contrast ratio of the LCD panel, which may be more objectionable in low light viewing conditions. 
         [0005]    Furthermore, intensity modulation of an illumination source, such as backlight illumination, for improving the contrast ratio of the LCD panels may have an inherent nonlinear output. As one skilled in the art would appreciate, this nonlinear trait of the backlight illumination coupled with a well-known gamma characteristic of the LCD/CRT display may further complicate contrast ratio enhancement thereof. 
       SUMMARY OF THE INVENTION 
       [0006]    Certain aspects commensurate in scope with the disclosed embodiments are set forth below. It should be understood that these aspects are presented merely to provide the reader with a brief summary of certain forms the invention might take and that these aspects are not intended to limit the scope of the invention. Indeed, the invention may encompass a variety of aspects that may not be set forth below. 
         [0007]    The disclosed embodiments relate to a system and method for linearizing an illumination source of a display device, comprising determining a brightness level of a brightest object of a video frame, determining an illumination level for the video frame based on the brightness level of the brightest object, linearizing the illumination level, and providing an illumination of the display device based on the linearized illumination level. In addition to LCDs, the disclosed system and method may further apply to digital light displays (DLPs) and to liquid crystal on silicon (LCOS) display systems. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    Advantages of the invention may become apparent upon reading the following detailed description and upon reference to the drawings in which: 
           [0009]      FIG. 1  is a block diagram of an LCD panel in accordance with an exemplary embodiment of the present invention; 
           [0010]      FIG. 2  is a block diagram of a contrast ratio enhancing system in accordance with an exemplary embodiment of the present invention; 
           [0011]      FIG. 3A  is a block diagram of a backlight linearizer in accordance with an exemplary embodiment of the present invention; 
           [0012]      FIG. 3B  is a graph illustrating operation of the backlight linearizer in accordance with an exemplary embodiment of the present invention; 
           [0013]      FIG. 3C  is a graph illustrating operation of the backlight linearizer in accordance with another exemplary embodiment of the present technique. 
           [0014]      FIG. 3D  is a block diagram of a configuration of the backlight linearizer in accordance with an exemplary embodiment of the present invention; 
           [0015]      FIG. 4A  is a graph of a nonlinear curve of a backlight illumination in accordance with an exemplary embodiment of the present invention; 
           [0016]      FIG. 4B  is a graph of a family of curves for linearizing the backlight illumination in accordance with an exemplary embodiment of the present invention; 
           [0017]      FIG. 4C  is a graph of a linear curve of the backlight illumination in accordance with an exemplary embodiment of the present invention; and 
           [0018]      FIG. 5  is flow chart depicting a method for linearizing the backlight illumination in accordance with an exemplary embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    One or more specific embodiments of the present invention will be described below. In an effort to provide a concise description of these embodiments, not all features of an actual implementation are described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. 
         [0020]    Referring to  FIG. 1 , a configuration of an exemplary display system  10 , such as an LCD panel, in accordance with an exemplary embodiment of the present invention is shown. The figure depicts an LCD panel  20  and an illumination source  18 , such as a backlight, controlled by a control system  14 . The control system  14 , receives data  12 , which may include video backlight illumination and liquid crystal pixel data values. The control system  14  may use the data  12  to simultaneously adjust the backlight and the pixel values to enhance the contrast ratio of the LCD panel  20 . Accordingly, data  22  outputted by the control system  14  goes into the LCD panel  20  for adjusting the pixel values. Similarly, data  16  outputted by the control system  14  is transmitted into the backlight  18  for adjusting the backlight illumination of the video. 
         [0021]    Turning now to  FIG. 2 , a contrast ratio enhancement control system  40  in accordance with an exemplary embodiment of the present invention is shown. The description set forth of the control system  40  pertains to components controlling the video backlight illumination and the pixel values of the LCD panel  20 . Accordingly, a white horizon finder  44  and a black horizon finder  45  receive respective luminance component data  42 . The white horizon finder  44  and the black horizon finder  45  respectively determine statistical information relating to the brightness, dark and near-dark levels, and their distribution throughout a video frame. Information obtained by the white horizon finder  44  and the black horizon finder  45  is provided to a maximum white generator  46 . The maximum white generator  46  simultaneously controls the backlight illumination and the liquid crystal pixel values. In accordance with embodiments of the present invention, the two are adjusted in a complementary fashion to enhance the contrast ratio of the LCD panel  20 . 
         [0022]    The maximum white generator  46  adjusts the backlight illumination by determining the brightness of the brightest area of the video frame. This information is then utilized to determine the amount of backlight needed to illuminate the LCD panel  20 , for example, as applied by cold-cathode-fluorescent (CCF) lamps. Accordingly, to improve the contrast ratio, a reduced backlight illumination is desired. However, as one of ordinary skill in the art would appreciate, reducing the backlight illumination too much may cause an undesired “white reduction” of the video frame. In order to avoid this, brightness information obtained by the maximum white generator  46  is further utilized to modify the pixel values of the LCD panel to compensate for possible insufficient backlight illumination. 
         [0023]    The maximum white generator  46  produces output data  50  for modulating the backlight illumination, while adjusting red, green, and blue (RGB) input values of the LCD panel  20 . Hence, to compensate for backlight modulation, the maximum white data  50  is further processed for modifying the pixel values of the LCD panel  20  in a non-linear gamma-corrected domain. Accordingly, the data  50  is delivered to a contrast look-up table (CLUT)  60 , which stores adjustment values that are formatted as an RGB offset  62  and an RGB gain-value  64 . The RGB offset value  62  and the RGB gain-value  64  are delivered to an RGB contrast circuit  66 . Accordingly, input RGB pixel values  68 - 72  are combined with the RGB offset  62  and the RGB gain-value  64  to output gamma-corrected RGB pixel values  74 - 78 . 
         [0024]    In addition to modifying the color pixel values, the data  50  is also delivered to backlight control circuitry, which outputs backlight control data  58 . Such backlight control circuitry may include a backlight linearizer  54 , as described further below, for compensating nonlinearities in the light characteristic of the backlight. Also included is a rise/fall delay  52 , which compensates for time misalignments between the backlight and the raster scanning of the pixels. This may prevent viewer perceived white flashes from appearing on a screen, which are generally undesirable. The backlight control circuitry may further include a backlight pulse width modulator (PWM)  56 , which controls the illumination level of the backlight. 
         [0025]    Referring now to  FIG. 3A , an exemplary system for the backlight linearizer  54  ( FIG. 2 ) which compensates for non-linearities in the light characteristic of the backlight illumination is depicted. The backlight linearizer  54  may comprise a system generally referred to by the reference numeral  90 . The system  90  accepts linear backlight data  92  which is delivered to an I/O interface  112  and slope generating circuits  94 ,  98  and  102 . The slope generating circuits  94 ,  98 , and  102  generate slopes of an ensemble of linear curves that characterize the non-linear light characteristic of the backlight illumination and, thus, facilitate the linearization of the backlight illumination. In constructing such linear curves, the slope generating circuits  94 ,  98 , and  102 , are respectively complemented by offsets  96 ,  100  and  104 , which are delivered to adders  91 ,  93  and  97  for providing appropriate offsets for typifying the non-linear light characteristic of the backlight illumination. In doing so, it should be appreciated that the number of slopes and offsets may vary according to system characteristics and requirements. Accordingly, the slope generating circuits  94 ,  98 ,  102  and the offsets  96 ,  100 ,  104  are respectively combined to output linear curves data components  95 ,  99 , and  101 . 
         [0026]    Upon receiving the data of curves  95 ,  99 , and  101 , circuit  106  functions to identify points at which the curves  95 ,  99 , and  101  intersect. Accordingly, such intersection points define a collection of piecewise linear transfer characteristic functions utilized to linearize the backlight illumination. Depending on system specifications, block  106  may output a maximum, a minimum, or a combination thereof piecewise transfer characteristic function resulting from the intersections of the data  95 ,  99 , and  101 . Thus, the circuit  106  produces data  107 , delivered to limiter  108  to ensure the data  107  falls in a prescribed range. Thereafter, resulting data  109  is joined with the data  92  at the input/output (I/O) interface  112  which produces non-linearly compensated backlight data  114  for the backlight control. Also inputted into the I/O interface is a bypass signal  110 , which may be used for diagnostic purposes of the backlight. 
         [0027]      FIG. 3B  is a graph  130  illustrating the principle of operation of the system  90  in accordance with an exemplary embodiment of the present technique. The graph  130  has a horizontal axis  142  denoting light control input, and a vertical axis  140  denoting light control output. In an exemplary embodiment, two linear functions, as implemented by circuit  90 , intersect at point  131  to form four line segments. These four line segments are labeled by reference numerals  132 - 135 , and may comprise the ensemble of linear curves produced by the circuit  106  to form piecewise linear transfer characteristic functions of the backlight. In this exemplary embodiment, the circuit  106  may produce a curve corresponding to either a maximum or a minimum piecewise linear transfer characteristic function. Accordingly, a maximum curve may comprise the line segments  132  and  133 , while a minimum curve may comprise the line segments  134  and  135 . As illustrated by FIG.  3 B, dashed curves  136  and  138  respectively depict the general trend of the resulting maximum and minimum piecewise transfer characteristic functions. Further, these curves are distinguished by their positive and negative concavity, respectively. Thus, the curve  136  may have a concavity defining the maximum output, while the curve  138  may have a concavity defining the minimum output, as implemented by circuit  106  for linearizing the backlight. 
         [0028]      FIG. 3C  is a graph  150  illustrating the principle of operation of the system  90  in accordance with another exemplary embodiment of the present technique. The graph  150  depicts intersections of three linear functions forming nine line segments. The nine line segments comprise an ensemble of curves utilized by circuit  106  to form multiple piecewise linear transfer characteristic functions of the backlight. In this manner, increasing the number of linear functions, as implemented by circuit  90 , increases the number of intersection points, which increases the number piecewise linear transfer characteristic functions. This may be advantageous in typifying the backlight of the display device more accurately. Accordingly, the circuit  106  may produce a curve corresponding to a maximum, a minimum, or a combination thereof piecewise linear transfer characteristic function. In this exemplary embodiment, the minimum curve is identical to the minimum curve illustrated by  FIG. 3B . The same embodiment provides a maximum curve defined by intersection points  137  and  139 , forming line segments  172 ,  175 , and  176 . The general trend of the maximum piecewise linear transfer characteristic function formed by these line segments is shown by dashed curve  179 . 
         [0029]    Circuit  106  may further produce a combination of maximum and minimum line segments to form additional piecewise linear transfer characteristic functions for the backlight. For example, the intersection points  131 ,  137 , and  139  define four line segments  171 ,  173 ,  174 , and  177 . These later line segments form a distinct piecewise linear transfer characteristic function of the backlight. A general trend of a piecewise transfer characteristic curve resulting from the line segments  171 ,  173 ,  174 , and  177  is depicted by dashed curve  178 . The curve  178  is disposed between the maximum curve  179  and the minimum curve  138 . 
         [0030]      FIG. 3D  is another system in accordance with an exemplary embodiment of the present technique of a circuit  150  for linearizing the backlight illumination. The system  150  accepts linear backlight data  92 . The linear backlight data  92  is delivered to both a scale circuit  152  and a subtractor  162 . The scale circuit  152  subtracts the input data  92  from a value corresponding to a maximum brightness level, such as 255, a value corresponding to a maximum shade of gray. Circuit  154  multiplies the foregoing value by a slope or a gain coefficient, and delivers it to a minimum circuit  156 . Configurable offset data  158  inputted into the minimum circuit  156 , generates a family of curves for correcting the non-linear characteristic of the backlight. Accordingly, the minimum circuit  156  outputs a minimum value  160  from the data provided by gain block  154  and the offset data  158 . Thereafter, the minimum value  160  is subtracted from the linear backlight data  92  by subtractor  162 . The subtractor  162  produces data  163 . The data  163  is subsequently processed by the limiter  164  to ensure the data  163  falls in a prescribed range of values. The limiter  164  provides data  165  to circuit  166 , or more commonly known to those skilled in the art as a flip-flop. Lastly, circuit  166  produces linear backlight data for the backlight control  58  ( FIG. 2 ). 
         [0031]    The processing of linear backlight data  92  by the system  150  to output non-linear backlight data can be mathematically described by an equation of the form: 
         [0000]      OUTPUT=INPUT−MINIMUM(OFFSET,SLOPE(255−INPUT)) 
         [0032]    Referring now to  FIG. 4A , an exemplary graph  180  in accordance with an exemplary embodiment of the present invention is illustrated. The graph  180  characterizes a non-linear backlight output verses PWM control values of the backlight apparatus. Accordingly, a vertical axis  182  denotes percent of maximum white output, and a horizontal axis  184  denotes control values of the backlight illumination. Such an exemplary curve may comprise a maximum control value of 256 corresponding to a 100 percent light-output. As illustrated by the curve  186 , a non-linear component in the backlight illumination exists, particularly in the upper portion of the curve  186  corresponding to high brightness levels. 
         [0033]    Referring to  FIG. 4B , an exemplary graph  200  in accordance with embodiments of the present technique is depicted. The graph  200  illustrates a collection of curves  206 - 210  used by the backlight linearizer  54  to compensate for the nonlinear characteristic of the backlight. Accordingly, a vertical axis  202  and a horizontal axis  204  respectively denote control out and control in values of the backlight. Each of the curves  206 - 210  may correspond to different offset values. For example, curve  208  may provide a suitable offset curve for compensating a nonlinear characteristic of the backlight illumination, illustrated by curve  186 . 
         [0034]      FIG. 4C  illustrates a graph  220  in accordance with embodiments of the present invention. The graph  220  depicts a curve resulting from employing the exemplary curve  208  of  FIG. 4B . The curve  208  is best chosen out of the family of curves  200  of  FIG. 4B , for linearizing the nonlinear characteristic of the backlight, illustrated by the exemplary curve  186  of  FIG. 4A . Depending on the non-linear characteristic inherent in the backlight, different curves provided by the family of curves  200  may be used to produce a range of respective offset and slope parameters  96 ,  100 ,  104 , and  94 ,  98 ,  102  ( FIG. 3A-C ) for linearizing the backlight illumination. 
         [0035]    Referring to  FIG. 5 , a method for linearizing the backlight illumination is illustrated by a flow chart, generally referred to by reference numeral  240 . The method begins at block  242  where the data  42  is delivered into the white horizon finder  44 . At block  244 , the pixel brightness level of the brightest object is determined by the white horizontal finder  44 . Based on the brightness level of the brightest object, a desired backlight illumination is determined by the maximum white generator  46  as denoted by block  246 . Thereafter, at block  248  the backlight illumination is linearized, and at block  250  the linearized backlight illumination is provided for the display device. Lastly, the method ends at block  251 . 
         [0036]    While the invention may be susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the invention as defined by the following appended claims.