Abstract:
An adaptive gamma curve correction apparatus and method for an LCD with multiple display modes or various display states provides more than one gamma curves to the LCD to adapt the display performance of the LCD for various display mode or state. Each gamma curve is prepared corresponding to a suitable display mode or state to optimize the LCD. The gamma curves are switched to be selected a suitable one when a display mode is in operation.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to a liquid crystal display (LCD), and more particularly to, an adaptive gamma curve correction apparatus and method for an LCD with multiple display modes or various display states.  
         BACKGROUND OF THE INVENTION  
         [0002]    LCDs are conventionally identified with tow types, reflection type and transmission type. In a reflective LCD, as shown in FIG. 1A, the ambient light  10  in front of an LCD passes through the LCD  12  and is then reflected back through the LCD  12  by a reflector  14  so as to become the display light  16  to the viewer. When the ambient light  10  is weak or the environment is not bright enough for the viewer, a clear image display will not be obtained. In addition, the modulation efficiency in an LCD is poor, that requests more strictly to the ambient light intensity for a clear image display. As shown in FIG. 1B, a transmissive LCD can overcome the drawback of the low light intensity with a backlight  18  provided by a light source. In a transmissive LCD, the backlight  18  passes the LCD  12  and becomes the display light  16  to the viewer. However, the light source to provide the backlight  18  consumes electric power. That is also disadvantageous to reduce the scale and the weight of an LCD arrangement. Moreover, when the environment is bright, a transmissive LCD still cannot save the electric power for the backlight, as in a reflective LCD. A solution is proposed with a transflective LCD, which is a combination of the reflection type LCD and the transmission type LCD. In a transflective LCD, as shown in FIG. 1C, the ambient light  10  passing through the LCD  12  is partially reflected by a transflector  20  which also permits the backlight  18  to pass through. The transflective LCD has two display modes, reflection display mode and transmission display mode. When the ambient light  10  is intense enough, the electric power for the backlight  18  can be saved. Contrarily, the backlight source is turned on when the environment is dark.  
           [0003]    However, a problem is introduced into a transflective LCD since the reflective LCD and the transmissive LCD have different optical characteristics. A typical display system is illustrated in FIG. 2, in which a controller  20  receiving video signal generates a display data signal D transmitted to a data driver  26  and a scan signal S transmitted to a scan driver  24  to drive the LCD panel  28 . To correct the nonlinearity of the transmittance/reflectance to the driving voltage of the LCD panel  28 , the data driver  26  is provided with a gamma curve correction apparatus to supply a correction data called gamma curve  30  which is dependent on the LCD panel  28 . Unfortunately, the gamma curve for a reflective LCD is different from that for a transmissive LCD if the both types of LCDs are optimized. When the optical efficiency or performance is optimized in a transflective LCD, the gamma curve for the image gray level in the reflection display mode is different from the gamma curve for the image gray level in the transmission display mode. As a result, the display quality degrades in a transflective LCD.  
           [0004]    Further, the performance of an LCD with a prefixed gamma curve also degrades when environment temperature is changed, since the optical characteristics of liquid crystal are temperature dependent. The prior art gamma curve correction apparatus thus cannot be adaptive to environment conditions.  
           [0005]    Therefore, it is desired an adaptive gamma curve correction apparatus and method for an LCD with multiple display modes or various display states.  
         SUMMARY OF THE INVENTION  
         [0006]    One object of the present invention is an adaptive correction for the gamma cure provided to an LCD with multiple display modes or various display states.  
           [0007]    In an LCD with multiple display modes or various display states, according to the present invention, each display mode or state thereof is prepared with a respective gamma curve in advance, and when switched between the display modes or states, the LCD is provided with the corresponding gamma curve.  
           [0008]    In a hardware implemented embodiment, two voltage dividers are employed to generate an R-V curve for a reflection display mode and a T-V curve for a transmission display mode selected by a switch controlled by a mode signal to provide a suitable gamma curve. In a digital implementation embodiment, a memory is employed to store an R-V curve for a reflection display mode and a T-V curve for a transmission display mode in a digital form, and an addresser is controlled by a mode signal to read out a suitable gamma curve from the memory. In a software implemented embodiment, a memory stores parameters corresponding to an R-V curve for a reflection display mode and a T-V curve for a transmission display mode, and a gamma curve generator controlled by a mode signal calculates a gamma curve based on the suitable parameters stored in the memory.  
           [0009]    In a temperature adaptive embodiment, a thermal couple is employed to sense the environment temperature and thus to select a suitable gamma curve from a plurality of gamma curves prepared for various temperature conditions.  
           [0010]    With a plurality of gamma curves prepared for multiple display modes or states in advance, the present invention can adapt an LCD for its display performance by switching between the gamma curves. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    For a better understanding of the present invention, reference may be had to the following description of exemplary embodiments thereof, considered in conjunction with the accompanying drawings, in which:  
         [0012]    FIGS.  1 A- 1 C show three different types of LCDs;  
         [0013]    [0013]FIG. 2 shows the block diagram of a typical display system for an LCD;  
         [0014]    [0014]FIG. 3 shows the block diagram of a display system for an LCD with multiple display modes according to the present invention;  
         [0015]    [0015]FIG. 4 shows an R-V curve and a T-V curve for the gamma curve in a display system for an LCD according to the present invention;  
         [0016]    [0016]FIG. 5 shows the circuit diagram of the first embodiment gamma curve correction apparatus according to the present invention;  
         [0017]    [0017]FIG. 6 shows the block diagram of the second embodiment gamma curve correction apparatus according to the present invention;  
         [0018]    [0018]FIG. 7 shows the block diagram of the third embodiment gamma curve correction apparatus according to the present invention; and  
         [0019]    [0019]FIG. 8 shows the block diagram of a display system for an LCD with temperature adaptation according to the present invention; 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]    As for a display system of the present invention, FIG. 3 shows a block diagram for a transflective LCD similar to a conventional one shown in FIG. 2. A controller  32  receives video signal and generates display data signal D and scan signal S. A data driver  26  receives the display data signal D from the controller  32  and generates data voltages DV for the image gray level to drive the sources of the thin film transistors (TFT) array (not shown) in the LCD panel  28 . On the other hand, a scan driver  24  receives the scan signal S from the controller  32  and generates the scan voltages SV for the scan lines to drive the gates of the TFT array in the LCD panel  28 . A backlight  29  is prepared to be projected to the LCD panel  28  for the transmission display mode. To correct the nonlinearity of the LCD panel  28 , a gamma curve correction apparatus  34  supplies the data driver  26  with a gamma curve γ. However, there are two gamma curves prepared by the gamma curve correction apparatus  34 , they are an R-V curve  36  for the reflection display mode and a T-V curve  38  for the transmission display mode, and the output γ of the gamma curve correction apparatus  34  is switched between the R-V curve  36  and the T-V curve  38 . The selection of the R-V curve  36  and the T-V curve  38  is controlled by a mode signal M from the controller  32 . When the LCD panel  28  is operated in the reflection display mode, the controller  32  selects the R-V curve  36  in the gamma curve correction apparatus  34  by the mode signal M to supply to the data driver  26 . When the transmission display mode is desired, the controller  32  turns on the backlight  29  and switches the gamma curve γ to the T-V curve  38  in the gamma curve correction apparatus  34  by the mode signal M. In this embodiment, the mode signal M is also used to turn on or turn off the backlight  29 , the gamma curve correction apparatus  34  is therefore switched to the T-V curve  38  simultaneously to enable the backlight  29  when switched from the reflection display mode to the transmission display mode.  
         [0021]    To further enhance the automatic adaptation, an optical sensor  39  is used to monitor the environment light intensity in such a manner that the transmission display mode is enabled by the optical sensor  39  depending on one or more preset thresholds. When the environment is dim, the sensed signal by the sensor  39  reaches the preset threshold and thus issues an illumination notification signal I to switch to the transmission display mode, i.e., the backlight  29  is turned on and the gamma curve correction is switched to the T-V curve  38 . The optical sensor  39  can directly turn on the backlight  39  and then trigger the mode signal M by another signal, or the optical sensor  39  can active the cortroller  32  to trigger the mode signal M to simultaneously turn on the backlight  39  and switch to the T-V curve  38 . In addition, the threshold to switch the display mode can be planed in consideration of environment illumination and its fluctuation in time.  
         [0022]    For an exemplary practice, FIG. 4 provides an R-V curve  36  and a T-V curve  38  measured on a real transflective LCD to show their characteristics and distinguish between them. As well-known in the art, the reflectance is smaller than the transmittance for each specific voltage in the transition region between the black and white display voltage levels. Such a difference between the R-V curve  36  and the T-V curve  38  will degrade the display performance unless respective gamma curves are biased for the different display modes. Therefore, the display performance cannot be improved if only one gamma curve is used for both the reflection display mode and the transmission display mode.  
         [0023]    A hardware approach for the gamma curve correction apparatus  34  is illustrated in FIG. 5. In this apparatus two voltage dividers  40  and  42  respectively for the reflection display mode and the transmission display mode are connected in parallel and have their two end nodes connected respectively to a high voltage source Vh and a low voltage source V 1 . In other embodiments, the high voltage sources and the low voltage sources can be different for the voltage dividers corresponding to different display modes. In the present embodiment the voltage divider  40  for the reflection display mode is composed of five resistors Rr 1 , Rr 2 , Rr 3 , Rr 4 , and Rr 5  connected in series, and the voltage divider  42  for the transmission display mode is also composed of five resistors Rt 1 , Rt 2 , Rt 3 , Rt 4 , and Rt 5  connected in series. Both of the voltage dividers  40  and  42  are tapered to provide the gamma curve voltages to be coupled with switches SW 1 , SW 2 , SW 3 , and SW 4 . The switches SW 1 - 4  are controlled by a mode signal to simultaneously switch to the voltage divider  40  or to the voltage divider  42 . Thus, two respective gamma curves can be controlled to provide for the reflection display mode and the transmission display mode.  
         [0024]    [0024]FIG. 6 is another embodiment of the present invention to supply digital gamma curves. An R-V curve  46  and a T-V curve  48  are stored in a read only memory (ROM)  44  in a digital form, i.e., the values of both the gamma curves  36  and  38  are sampled and stored in the ROM  44  in advance. An addresser  50  is controlled by a mode signal M to generate a proper address ADDR to read out the data of the R-V curve  46  or the T-V curve  48  to be outputted. If needed, the digital gamma curve γ′ from the ROM  44  can be further transformed to be analog voltages by a digital to analog converter (DAC)  52 . In this manner, the gamma curve to be supplied is searched from a look-up table, which is suitable for a totally digital display system.  
         [0025]    A software implemented embodiment of the present invention is provided in FIG. 7. In this approach, only the basic parameters for an R-V curve  56  and a T-V curve  58  are stored in a ROM  54 , this way the storage volume can be reduced. A gamma curve generator  60  is controlled by a mode signal M to fetch the proper parameters P from the ROM  54 . In the gamma curve generator  60  a process is executed to calculate the gamma curve values based on the fetched parameters. The software in the gamma curve generator  60  can be updated for different systems. As in the prior art, the real gamma curve can be approached by a finite points method, thus only some points data and an equation are required for the parameters, which makes the calculations executed by the gamma curve generator  60  simplified. The approach curves can be directly derived from the real R-V curve and T-V curve, such as shown in FIG. 4.  
         [0026]    [0026]FIG. 8 shows the block diagram of a display system for an LCD with temperature adaptation. In this system, a reflective LCD panel  62  is optimized by a gamma curve correction apparatus  66  switched by a controller  64  with the help of a thermal couple  74 . There are prepared in the gamma curve correction apparatus  66  with three temperature dependent gamma curves, high temperature (HT) gamma curve  68 , middle temperature (MT) gamma curve  70 , and low temperature (LT) gamma curve  72 , for the LCD panel  62  to be operated in different temperature ranges. The thermal couple  74  monitors the environment temperature and issues a temperature notification signal T to the controller  64  to select a suitable gamma curve among the gamma curves  68 ,  70 , and  72  to be supplied to the data driver  26 . In other embodiments, the gamma curve correction apparatus  66  can be directly switched by the thermal couple  74  with the signal T served as the mode signal M.  
         [0027]    It is apparently for the multiple gamma curves to be applied to reflection type LCD, transmission type LCD, partial reflection type LCD, and transflection type LCD, twisted nematic (TN) LCD and super-twisted nematic (STN) LCD, thin film transistor (TFT) LCD, thin film diode (TFD) LCD, and metal insulator metal (MIM) LCD, and various working temperatures.  
         [0028]    From the above, it should be understood that the embodiments described, in regard to the drawings, are merely exemplary and that a person skilled in the art may make variations and modifications to the shown embodiments without departing from the spirit and scope of the present invention. All variations and modifications are intended to be included within the scope of the present invention as defined in the appended claims.