Patent Publication Number: US-11380250-B1

Title: Display apparatus having a self-luminous pixel module and a first non-self-luminous pixel module driven by a pulse width modulation driving circuit

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a display apparatus, and more specifically, the present invention relates to a display apparatus having a self-luminous pixel module and a first non-self-luminous pixel module driven by a pulse width modulation (PWM) driving circuit. 
     2. Description of the Prior Art 
     With the advancement of technology, users are getting more picky about visual displays and desire display devices with a high compactness, excellent display quality, large-sized panel, high color saturation, low cost and low power consumption. 
     Existing display devices may be categorized into self-luminous and non-self-luminous ones. Liquid crystal display (LCD) devices are one of the primary non-self-luminous flat panel display devices, wherein the amount of light passing through a liquid crystal medium is modulated by controlling the voltage of upper and lower electrodes of the liquid crystal medium. The effect of color display is achieved with further employment of a color filter layer, a polarizer and some optical films. 
     Self-luminous flat panel display devices may be categorized into field emissive display, plasma display, electroluminescent display, photoluminescent material, organic light-emitting diode display and so on. In an organic light-emitting diode display (OLED), light-emitting polymers are deposited between an upper electrode layer and a lower electrode layer. With further employment of a conductive layer of electrons and holes, light is generated by means of an external electric field which moves the carriers and causes the electrons and holes to re-combine. In comparison, an organic light-emitting diode display device is characterized by its wide viewing angle, fast responding speed, thin panel and flexibility; further, it requires neither backlight nor color filter and may be made large-sized. 
     SUMMARY OF THE INVENTION 
     An embodiment of the present invention provides a display apparatus. The display apparatus comprises a self-luminous pixel module, a first non-self-luminous pixel module, and a pulse width modulation (PWM) driving circuit. The self-luminous pixel module comprises a plurality of self-luminous pixels. The first non-self-luminous pixel module is disposed on the self-luminous pixel module and comprises a plurality of first non-self-luminous pixels. The pulse width modulation (PWM) driving circuit is configured to provide first PWM signals to the self-luminous pixel module to control values of brightness of the self-luminous pixels and provide second PWM signals to the first non-self-luminous pixel module to control values of transmittance of the first non-self-luminous pixels. 
     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a structure of a display apparatus according to an embodiment of the present invention. 
         FIG. 2  shows a structure of another display apparatus according to another embodiment of the present invention. 
         FIG. 3  shows a pixel group of the display apparatus in FIG. 
         FIG. 4  is a cross-section view of the pixel group in  FIG. 3 . 
         FIG. 5  is a timing diagram of the pixel group in  FIG. 3 . 
         FIG. 6  shows another pixel group composed of a self-luminous pixel and two non-self-luminous pixels aligned vertically. 
         FIG. 7  shows the PWM driving timing of the pixel group in  FIG. 6 . 
         FIG. 8  shows another pixel group composed of a self-luminous pixel and two non-self-luminous pixels. 
         FIG. 9  shows the PWM driving timing of the pixel group in  FIG. 8 . 
         FIG. 10  shows another pixel group composed of a self-luminous pixel and an array of four non-self-luminous pixels. 
         FIG. 11  shows the PWM driving timing of the pixel group in  FIG. 10 . 
         FIG. 12  shows another pixel group composed of a self-luminous pixel and an array of non-self-luminous pixels of a single non-self-luminous pixel module. 
         FIG. 13  shows the PWM driving timing of the pixel group in  FIG. 12 . 
         FIG. 14  shows a pixel group composed of a self-luminous pixel and two arrays of non-self-luminous pixels of two non-self-luminous pixel modules. 
         FIG. 15  shows the PWM driving timing of the pixel group in  FIG. 14 . 
     
    
    
     DETAILED DESCRIPTION 
     The implementation method of the present invention will be further illustrated by way of the following description of a plurality of embodiments. But it should be noted that the embodiments described below are illustrative and exemplary only rather than limiting the application of the present invention to the described environment, application, structure, procedure or steps. Elements that are not directly related to the present invention are ignored from the drawings. The scale relations among elements in the drawings are for illustration rather than limiting of the actual scales of the present invention. Unless noted otherwise, identical (or similar) reference symbols correspond to identical (or similar) elements. 
     The display apparatus of the present invention may be a free-form display apparatus, which provides customized shapes and novel design features, and its surfaces could be bent, folded, stretched or rolled up. This revolutionary advancement opens up a new world for designers of automotive vehicles, wearable devices, and other types of products. 
     The display apparatus of the present invention may be a freeform display with a multi-layer display structure. A self-luminous pixel module of the display apparatus may serve as a backlight for a plurality of non-self-luminous pixel modules of the display apparatus (only shown one or two non-self-luminous pixel modules). The self-luminous pixel module also offers local dimming to enhance the color contrast ratio. Active-matrix and/or Passive-matrix organic light-emitting diodes (OLED) are good candidates for the components of the self-luminous pixel module, also active-matrix and/or passive-matrix liquid crystal display (LCD) panels are good candidates for the components of the non-self-luminous pixel module of the freeform display. Furthermore, the self-luminous pixel module may comprise at least one member selected from a group consisting of an electroluminescent material, a photoluminescent material, a cathodoluminescent material, a field emissive luminescent material, a phosphorescent material, a fluorescent material, a vacuum fluorescent material, and a light-emitting diode material. Moreover, the non-self-luminous pixel module may comprise at least one member selected from a group consisting of an electrophoretic material, an electric fluid material, a liquid crystal material, a micro electromechanical reflective material, an electrowetting material, an electric ink material, a magnetic fluid material, an electrochromic material, an electromorphous material, and a thermochromic material. Moreover, the self-luminous and/or non-self-luminous pixel modules further include a light absorbing material, a light reflecting material, a light deflecting material, a light diffusing material, a light-filtering material, an electric conductive material, an insulating material, and a photo reflective material for enhancing the performance of display pixel modules. 
     The luminous of the self-luminous pixel module may be controlled by pulse-width modulation (PWM) scheme, and the non-self-luminous pixel module may be driven using a voltage and/or current driven modes. The transmittance of an LCD module (e.g., an LCD panel) depends on the root mean square (RMS) value of an applied voltage across the LC module, regardless of the polarity of the voltage. Therefore, the gray level of a pixel of the LC module may be controlled by the voltage level applied across the LC module. Alternatively, the gray level may be controlled with PWM scheme. 
     In the present invention, the PWM scheme for the freeform display with a self-luminous pixel module and a plurality of non-self-luminous pixel modules (only shown one or two non-self-luminous pixel modules). The resolution of self-luminous pixel module may be equal, less or greater than the resolution of the non-self-luminous pixel modules, simplistically, to select the resolution of self-luminous pixel module less than the resolution of the non-self-luminous pixel modules illustrated. The gray level is controlled by adjusting the brightness of the self-luminous pixel module and the transmittance of the non-self-luminous pixel module (s) with PWM driving circuit schemes. 
       FIG. 1  shows a structure of a display apparatus  10 A according to an embodiment of the present invention, and  FIG. 2  shows a structure of another display apparatus  10 B according to another embodiment of the present invention. The display apparatus  10 A has a two-layer display structure, and the display apparatus  10 B has a three-layer display structure. In other words, each of the display apparatuses  10 A and  10 B is a multi-layer display, which combines a self-luminous pixel module  20  (e.g., an electroluminescent panel, OLED) and one or two non-self-luminous pixel modules  30  and  40  (e.g., liquid crystal panels, LCD). The self-luminous pixel module  20  is used as the backlight of the non-self-luminous pixel modules  30  and  40 , and provides local dimming to achieve a wider color gamut and a higher contrast ratio of the display apparatuses  10 A and  10 B. The gray level of each pixel of the display apparatuses  10 A may be adjusted by controlling the brightness of the self-luminous pixel module  20  and by controlling the transmittance of the non-self-luminous pixel module  30 . An extra non-self-luminous pixel module  40  may be included to form the three-layer display  10 B as shown in  FIG. 2  to achieve a much wider color gamut and a much higher contrast ratio. The display apparatus  10 A further comprises a timing controller (Tcon)  50 , a scan driver  60 , and a pulse width modulation (PWM) driving circuit  70 . The PWM driving circuit  70  is used as a column driver. The timing controller  50  controls the scan driver  60  to scan the self-luminous pixel module  20  and the non-self-luminous pixel module  30 , and sends the image data to the PWM driving circuit  70 . 
       FIG. 3  shows a pixel group  100  of the display apparatus  10 A in  FIG. 1 . The display apparatus  10 A comprises a plurality of the pixel groups  100 , and each pixel group  100  comprises a self-luminous pixel  200  and a non-self-luminous pixel  300 . In other words, the self-luminous pixel module  20  comprises a plurality of self-luminous pixels  200  and a plurality of non-self-luminous pixels  300 , and the self-luminous pixels  200  and the non-self-luminous pixels  300  form the plurality of the pixel groups  100  of the display apparatus  10 A. In the embodiment, each pixel group  100  serves as a display pixel  110  of the display apparatus  10 A. 
     In the embodiment, the PWM driving circuit  70  is a pulse width modulation (PWM) driving circuit to drive the self-luminous pixel module  20  and the non-self-luminous pixel module  30  to control the gray levels of the pixels of the display apparatus  10 A according to the image data received from the timing controller  50 . In detail, the PWM driving circuit  70  provides first PWM signals C 1  to the self-luminous pixel module  20  to control brightness of self-luminous pixels  200  and provides second PWM signals C 2  to the non-self-luminous pixel module  30  to control transmittance of the non-self-luminous pixels  300 . 
     In the embodiment shown in  FIG. 2 , the scan driver  60  further scans the non-self-luminous pixel module  40 , and the PWM driving circuit  70  further drives the non-self-luminous pixel module  40  to control the gray levels of the pixels of the display apparatus  10 B according to the image data received from the timing controller  50 . The PWM driving circuit  70  provides third PWM signals C 3  to the non-self-luminous pixel module  40  to control transmittance of non-self-luminous pixels of the non-self-luminous pixel module  40 . 
       FIG. 4  is a cross-section view of the pixel group  100  in  FIG. 3 . The self-luminous pixel  200  comprises two electrodes  210 , and a self-luminous medium  220  is disposed between the two electrodes  210 . The self-luminous medium  220  may comprise at least one member selected from a group consisting of an electroluminescent material, a photoluminescent material, a cathodoluminescent material, a field emissive luminescent material, a vacuum fluorescent material, and a light-emitting diode material. The non-self-luminous pixel  300  comprises two electrodes  310 , and a non-self-luminous medium  320  (e.g., a liquid crystal (LC) layer) is disposed between the two electrodes  310 . The non-self-luminous medium  320  may comprise at least one member selected from a group consisting of an electrophoretic material, an electric fluid material, a liquid crystal material, a micro electromechanical reflective material, an electrowetting material, an electric ink material, a magnetic fluid material, an electrochromic material, an electromorphous material, and a thermochromic material. Moreover, the non-self-luminous pixel module  30  may further comprise a first substrate  120  and/or a second substrate  130 , one of the two electrodes  310  is disposed on the first substrate  120 , and the other electrode  310  is disposed on the first and/or second substrate  130  (e.g., the electrodes  310  may be disposed on the same substrate if the non-self-luminous medium  320  is in-plane-switching (IPS) liquid crystal). The first substrate  120  and/or the second substrate  130  may comprise at least one member selected from a group consisting of a transparent material, an opaque material, a flexible material, a rigid material, a metallic material, a ceramic material, an insulating material, a metal compound material, a metal alloy material, an organic material, an inorganic material, a composite material, and a semiconductor material. 
     The PWM driving circuit  70  is electrically connected to the electrodes  210  of the self-luminous pixel module  20  and the electrodes  310  of the non-self-luminous pixel module  30  via conductors  118  to control the electrodes  210  and  310  in a synchronized and/or non-synchronized manner to allow the electrodes  210  to change the state of the self-luminous medium  220  and allow the electrodes  310  to change the state of the non-self-luminous medium  320 . 
     The self-luminous pixel module  20  and/or the non-self-luminous pixel module  30  may further comprise at least one member selected from a group consisting of a light absorbing material, a light reflecting material, a light deflecting material, a light diffusing material, alight-filtering material, an electric conductive material, an insulating material, and a photo reflective material for enhancing the performance of display pixel modules. 
     The shape of the self-luminous pixel module  20  and/or the shape of the non-self-luminous pixel module  30  may be selected from a group consisting of a square shape, a rectangle shape, a fan shape, a triangle shape, a trapezoid shape, a circle shape, an oval shape, a diamond shape, an irregular polygon shape, a polygon shape, and an irregular shape. 
       FIG. 5  is a timing diagram of the pixel group  100  in  FIG. 3 . The driving voltage V S  is the voltage applied to the self-luminous pixels  200  when the self-luminous pixel module  20  is turned on. The brightness of the whole display apparatus  10 A can be adjusted by varying the driving voltage V S . The driving voltage V S  is one of the first PWM signals C 1  provided by the PWM driving circuit  70 . A common voltage V COM  is applied to a common electrode of the non-self-luminous pixel module  30  (e.g., a liquid crystal panel). The PWM driving circuit  70  further applies the driving voltages (V COM +ΔVn) and (V COM −ΔVn) to the driving electrode of the non-self-luminous pixel  300  for positive polarity driving and negative polarity driving respectively. The driving voltages (V COM +ΔVn) and (V COM −ΔVn) are determined by one of the second PWM signal C 2 . The transmittance of the non-self-luminous pixel  300  depends on the root mean square (RMS) value of the applied voltages (V COM +ΔVn) and (V COM −ΔVn) regardless of the polarity of the voltage across the non-self-luminous medium  320  of the non-self-luminous pixel  300 . For the PWM scheme, the periods of the voltage driving voltage (V COM +ΔVn) and/or (V COM −ΔVn) controls the transmittance of the non-self-luminous pixel  300 . The transmittance of the non-self-luminous pixel  300  is T N  when the voltage driving voltage is (V COM +ΔVn) or (V COM −ΔVn). Since the non-self-luminous medium  320  cannot completely block the light, the transmittance of the non-self-luminous pixel  300  is a non-zero value, T COM , when a voltage difference between the two electrodes  310  of the non-self-luminous pixel  300  is zero. The gray level of the display pixel  110  can be changed by adjusting the brightness of the self-luminous pixel  200  and the transmittance of the non-self-luminous pixel  300 . In the embodiment, the PWM driving circuit  70  provides the first PWM signal C 1  (i.e, V S ) to the self-luminous pixel  200  to adjust the brightness of the self-luminous pixel  200  and provides the second PWM signal C 2  (i.e., (V COM +ΔVn) and (V COM −ΔVn)) to the non-self-luminous pixel  300  to adjust the transmittance of the non-self-luminous pixel  300 . The period τ S  is the period when the self-luminous pixel  200  is turned on by the driving voltage V S . The period τ N  is the period when the driving voltage (V COM +ΔVn) or (V COM −ΔVn) is applied across the non-self-luminous medium  320  of the non-self-luminous pixel  300 . The period τ S  is greater than the period τ N . The gray level G p  of display pixel  110  could be expressed as: 
                     G   p     =       B   S     ⁢       [         τ   N     ·     T   N       +       (       τ   S     -     τ   N       )     ·     T   COM         ]       T   scan                 (   1   )               
where T scan  is the scan time while the scan driver  60  applies a scan voltage to the pixel group  100 . B S  is the brightness of the self-luminous pixel  200  when the voltage V S  is applied to the self-luminous pixel  200 . The ratio D S =(τ S /T scan ) is the duty cycle when the voltage V S  is applied to the self-luminous pixel  200 , and the ratio D N =τ N /T scan ) is the duty cycle when the voltage driving voltage (V COM +ΔVn) or (V COM −ΔVn) is applied across the non-self-luminous medium  320  of the non-self-luminous pixel  300 . Therefore, the gray level G p  of the pixel group  100  could be expressed as:
 
 G   p   =B   S [ D   N   ·T   N +( D   S   −D   N )· T   COM ]  (2)
 
     According to the equation (2), the gray level G p  of the display pixel  110  can be controlled by adjusting the duty cycles D S  and D N . For given image data for the gray level G p , the duty cycles D S  and D N  could be determined based on the equation (2). 
     To reduce the cost of the self-luminous pixel module  20 , each self-luminous pixel  200  may supply the backlight and offer local dimming to multi non-self-luminous pixels  300  of the non-self-luminous pixel module  30 .  FIG. 6  shows a pixel group  100 B composed of a self-luminous pixel  200  and two non-self-luminous pixels  300  aligned vertically. The two non-self-luminous pixels  300  are placed in the same row and are driven by two second PWM signals C 2  output from the PWM driving circuit  70 . In the embodiment, each pixel group  100 B serves as two display pixels  110 A and  110 B of the display apparatus  10 A.  FIG. 7  shows the PWM driving timing of the pixel group  100 B. For simplicity, only positive polarity driving of the two non-self-luminous pixels  300  is illustrated in  FIG. 7 . D N1  is the duty cycle when the voltage driving voltage (V COM +ΔVn) or (V COM −ΔVn) is applied across the left non-self-luminous pixel  300 , and D N2  is the duty cycle when the voltage driving voltage (V COM +ΔVn) or (V COM −ΔVn) is applied across the right non-self-luminous pixel  300 . The gray level G p1  of the display pixel  110 A could be expressed as:
 
 G   p1   =B   S [ D   N1   ·T   N +( D   S   −D   N1 )· T   COM ]  (3)
 
     The gray level G p1  of the display pixel  110 A could be controlled by adjusting the duty cycles D S  and D N1 . 
     The gray level G p2  of the display pixel  110 B could be expressed as:
 
 G   p2   =B   S [( D   N2   ·T   N +( D   S   −D   N2 )· T   COM ]  (4)
 
     B S  is brightness of the self-luminous pixel  200  when the voltage Vs is applied to the self-luminous pixel  200 , D 1  is the duty cycle of the voltage (V COM +ΔVn) or (V COM −ΔVn) applied across the non-self-luminous pixel  300  of the display pixel  110 A, D 2  is the duty cycle of the voltage (V COM +ΔVn) or (V COM −ΔVn) applied across the non-self-luminous pixel  300  of the display pixel  110 B, T N  is the transmittance of the non-self-luminous pixel  300  when the voltage (V COM +ΔVn) or (V COM −ΔVn) is applied across the non-self-luminous pixel  300 , D S  is a duty cycle of the voltage Vs, and T COM  is the transmittance of the first non-self-luminous pixel of the first display pixel when the voltage difference between the two electrodes  310  of the non-self-luminous pixel  300  is zero. 
     The gray level G p2  of the display pixel  100 B could be controlled by adjusting the duty cycles D S  and D N2 . The duty cycles of the self-luminous pixel  200  for the two non-self-luminous pixels  300  may be the same. For given image data for the gray levels G p1  and G p2 , the values of D S , D N1  and D N2  can be determined according to the equations (3) and (4). 
     Alternative, the two non-self-luminous pixels  300  could be placed in the same column and are driven by the same output channel of the PWM driving circuit  70  as shown in  FIG. 8 .  FIG. 8  shows a pixel group  100 C composed of a self-luminous pixel  200  and two non-self-luminous pixels  300 .  FIG. 9  shows the PWM driving timing of the pixel group  100 C within two scanning periods Tscan. In the embodiment, each pixel group  100 C serves as two display pixels  110 A and  110 B of the display apparatus  10 A. Moreover, because the self-luminous pixel  200  supplies the backlight for two non-self-luminous pixels  300  at different scanning periods Tscan, the average pixel gray level during the two scanning periods Tscan would be calculated. The gray level G p1  of the display pixel  110 A could be expressed as: 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           G 
                           
                             p 
                             ⁢ 
                             1 
                           
                         
                         = 
                         
                           
                             B 
                             S 
                           
                           ⁢ 
                           
                             
                               [ 
                               
                                 
                                   
                                     τ 
                                     
                                       N 
                                       ⁢ 
                                       1 
                                     
                                   
                                   · 
                                   
                                     T 
                                     N 
                                   
                                 
                                 + 
                                 
                                   
                                     ( 
                                     
                                       
                                         τ 
                                         
                                           S 
                                           ⁢ 
                                           1 
                                         
                                       
                                       - 
                                       
                                         τ 
                                         
                                           N 
                                           ⁢ 
                                           1 
                                         
                                       
                                     
                                     ) 
                                   
                                   · 
                                   
                                     T 
                                     COM 
                                   
                                 
                                 + 
                                 
                                   
                                     τ 
                                     
                                       N 
                                       ⁢ 
                                       1 
                                     
                                   
                                   · 
                                   
                                     T 
                                     COM 
                                   
                                 
                               
                               ] 
                             
                             
                               2 
                               ⁢ 
                               
                                 T 
                                 scan 
                               
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                         
                           
                             
                               B 
                               S 
                             
                             2 
                           
                           ⁡ 
                           
                             [ 
                             
                               
                                 
                                   D 
                                   
                                     N 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     1 
                                   
                                 
                                 · 
                                 
                                   T 
                                   N 
                                 
                               
                               + 
                               
                                 
                                   ( 
                                   
                                     
                                       D 
                                       
                                         S 
                                         ⁢ 
                                         1 
                                       
                                     
                                     + 
                                     
                                       D 
                                       
                                         S 
                                         ⁢ 
                                         2 
                                       
                                     
                                     - 
                                     
                                       D 
                                       
                                         N 
                                         ⁢ 
                                         1 
                                       
                                     
                                   
                                   ) 
                                 
                                 · 
                                 
                                   T 
                                   COM 
                                 
                               
                             
                             ] 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     The gray level G p2  of the display pixel  110 B could be expressed as: 
                     G     p   ⁢   2       =         B   S     2     ⁡     [         D     N   ⁢           ⁢   2       ·     T   N       +       (       D     S   ⁢   1       +     D     S   ⁢   2       -     D     N   ⁢   2         )     ·     T   COM         ]               (   6   )               
where B S  is brightness of the self-luminous pixel  200  of the pixel group  100 C when the voltage Vs is applied to the self-luminous pixel  200  of the pixel group  100 C, D N1  is a duty cycle of the voltage (V COM +ΔVn) or (V COM −ΔVn) applied across the non-self-luminous pixel  300  of the display pixel  110 A of the pixel group  100 C, D N2  is a duty cycle of the voltage (V COM +ΔVn) or (V COM −ΔVn) applied across the non-self-luminous pixel  300  of the display pixel  110 B of the pixel group  100 C, T N  is transmittance of the two non-self-luminous pixels  300  of the pixel group  100 C when the voltage (V COM +ΔVn) or (V COM −ΔVn) is applied across the two non-self-luminous pixels  300  of the pixel group  100 C, D S1  is a duty cycle of the voltage Vs in a first scanning period Tscan, D S2  is a duty cycle of the voltage Vs in a second scanning period Tscan, and T COM  is the transmittance of the two non-self-luminous pixels  300  of the pixel group  100 C when the voltage difference between the electrodes  310  of the non-self-luminous pixels  300  is zero.
 
     For given image data for the gray levels G p1  and G p2 , the values of D S , D N1 , and D N2  can be determined according to the equations (5) and (6). 
     In an embodiment of the present invention, a self-luminous pixel  200  would supply the backlight and offer local dimming to an array of the non-self-luminous pixels  300 .  FIG. 10  shows a pixel group  100 D composed of a self-luminous pixel  200  and an array of four non-self-luminous pixels  300 .  FIG. 11  shows the PWM driving timing of the pixel group  100 D within two scanning periods Tscan. In the embodiment, each pixel group  100 D serves as four display pixels  110  of the display apparatus  10 A. The gray level G p(i,j)  of the display pixel  110  in the i th  row and the j th  column can be expressed as: 
                     G     p   ⁡     (     i   ,   j     )         =         B   S     2     ⁡     [         D     N   ⁡     (     i   ,   j     )         ·     T   N       +       (       D     S   ⁢   1       +     D     S   ⁢   2       -     D     N   ⁡     (     i   ,   j     )           )     ·     T   COM         ]               (   7   )               
where i is equal to 1 or 2, and j is equal to 1 or 2. B S  is brightness of a self-luminous pixel of the display pixel  110  when the voltage Vs is applied to the self-luminous pixel  200 , D N(i,j)  is a duty cycle of the voltage (V COM +ΔVn) or (V COM −ΔVn) applied across the non-self-luminous pixel  300 , T N  is transmittance of the non-self-luminous pixel  300  when the voltage (V COM +ΔVn) or (V COM −ΔVn) is applied across the non-self-luminous pixel  300 , D S1  is a duty cycle of the voltage Vs in a first scanning period Tscan, D S2  is a duty cycle of the voltage Vs in a second scanning period Tscan, and T COM  is the transmittance of the non-self-luminous pixel  300  when the voltage difference between the two electrodes  310  of the non-self-luminous pixel  300  is zero.
 
     In an embodiment of the present invention, a self-luminous pixel  200  would supply backlight and offer local dimming to more non-self-luminous pixels  300 .  FIG. 12  shows a pixel group  100 E composed of a self-luminous pixel  200  and an array of m×n non-self-luminous pixels  300 , where m and n are integers greater than 2.  FIG. 13  shows the PWM driving timing of the pixel group  100 E within m scanning periods Tscan. In the embodiment, each pixel group  100 E serves as (m×n) display pixels  110  of the display apparatus  10 A. The gray level G p(i,j)  of the display pixel  110  in the i th  row and the j th  column can be expressed as: 
                     G     p   ⁡     (     i   ,   j     )         =         B   S     m     ⁡     [         D     N   ⁡     (     i   ,   j     )         ·     T   N       +       (         ∑     k   =   1     m     ⁢     D     S   ⁡     (   k   )           -     D     N   ⁡     (     i   ,   j     )           )     ·     T   COM         ]               (   8   )               
where 1≤i≤m, and 1≤j=n. B S  is brightness of a self-luminous pixel of the display pixel  110  when the voltage Vs is applied to the self-luminous pixel  200 , D N(i,j)  is a duty cycle of the voltage (V COM +ΔVn) or (V COM −ΔVn) applied across the non-self-luminous pixel  300 , T N  is transmittance of the non-self-luminous pixel  300  when the voltage (V COM +ΔVn) or (V COM −ΔVn) is applied across the non-self-luminous pixel  300 , D S(k)  is a duty cycle of the voltage Vs in a k th  scanning period Tscan, and T COM  is the transmittance of the non-self-luminous pixel  300  when the voltage difference between the two electrodes  310  of the non-self-luminous pixel  300  is zero. For given image data for the gray level G p(i,j) , the values of D S(k)  and D N(i,j)  can be determined according to the equation (8).
 
     When a single self-luminous pixel  200  supplies backlight and offers local dimming to more non-self-luminous pixels  300 , the contrast ratio of the display apparatus may be reduced. To increase the contrast ratio of the display apparatus, another non-self-luminous pixel module  40 , as shown in  FIG. 4 , might be added to the display apparatus.  FIG. 14  shows a pixel group  100 F composed of a self-luminous pixel  200 , an array of m×n non-self-luminous pixels  300 , and an array of m×n non-self-luminous pixels  400  of the non-self-luminous pixel module  40 .  FIG. 15  shows the PWM driving timing of the pixel group  100 F within m scanning periods Tscan. In the embodiment, each pixel group  100 F serves as (m×n) display pixels  110  of the display apparatus  10 B. In addition, the two non-self-luminous pixel modules  30  and  40  of the display apparatus  10 B may comprise two LC layers, and the gray levels of the display pixels  110  of the pixel group  100 F can be adjusted by adjusting the transmittances of the two LC layers and the brightness of the self-luminous pixel  200 . The gray level G p(i,j)  of the display pixel  110  in the i th  row and the j th  column can be expressed as: 
     
       
         
           
             
               
                 
                   
                     G 
                     
                       p 
                       ⁡ 
                       
                         ( 
                         
                           i 
                           , 
                           j 
                         
                         ) 
                       
                     
                   
                   = 
                   
                     
                       
                         B 
                         S 
                       
                       m 
                     
                     [ 
                     
                       
                         
                           D 
                           
                             NA 
                             ⁡ 
                             
                               ( 
                               
                                 i 
                                 , 
                                 j 
                               
                               ) 
                             
                           
                         
                         · 
                         
                           T 
                           NA 
                         
                         · 
                         
                           T 
                           NB 
                         
                       
                       + 
                       
                         
                           ( 
                           
                             
                               D 
                               
                                 NB 
                                 ⁡ 
                                 
                                   ( 
                                   
                                     i 
                                     , 
                                     j 
                                   
                                   ) 
                                 
                               
                             
                             - 
                             
                               D 
                               
                                 NA 
                                 ⁡ 
                                 
                                   ( 
                                   
                                     i 
                                     , 
                                     j 
                                   
                                   ) 
                                 
                               
                             
                           
                           ) 
                         
                         · 
                         
                           T 
                           COMA 
                         
                         · 
                         
                           T 
                           NB 
                         
                       
                       + 
                       
                         
                           ( 
                           
                             
                               
                                 ∑ 
                                 
                                   k 
                                   = 
                                   1 
                                 
                                 m 
                               
                               ⁢ 
                               
                                 D 
                                 
                                   S 
                                   ⁡ 
                                   
                                     ( 
                                     k 
                                     ) 
                                   
                                 
                               
                             
                             - 
                             
                               D 
                               
                                 NB 
                                 ⁡ 
                                 
                                   ( 
                                   
                                     i 
                                     , 
                                     j 
                                   
                                   ) 
                                 
                               
                             
                           
                           ) 
                         
                         · 
                         
                           T 
                           COMA 
                         
                         · 
                         
                           T 
                           COMB 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
           
         
       
     
     where 1≤i≤m, and 1≤j≤n. B S  is brightness of the self-luminous pixel  200  when the voltage Vs is applied to the self-luminous pixel  200 , D NA(i,j)  is a duty cycle of a voltage (V COM +ΔVa) or (V COM −ΔVa) applied across the non-self-luminous pixel  300 , D NB(i,j)  is a duty cycle of a voltage (V COM +ΔVb) or (V COM −ΔVb) applied across the non-self-luminous pixel  400 , T NA  is transmittance of the non-self-luminous pixel  300  when the voltage (V COM +ΔVa) or (V COM −ΔVa) is applied across the non-self-luminous pixel  300 , T NB  is transmittance of the non-self-luminous pixel  400  when the voltage (V COM +ΔVb) or (V COM −ΔVb) is applied across the non-self-luminous pixel  400 , D S(k)  is a duty cycle of the voltage V COM +ΔVa) or (V COM −ΔVa) in a k th  scanning period of the pixel group  100 F, T COMA  is the transmittance of the non-self-luminous pixel  300  when a voltage of zero volts is applied across the non-self-luminous pixel  300 , and T COMB  is the transmittance of the non-self-luminous pixel  400  when the voltage of zero volts is applied across the non-self-luminous pixel  400 . For given image data for the gray level G p(i,j) , the values of D S(k) , D NA(i,j) , and D NB(i,j)  can be determined according to the equation (9). 
     In summary, the present invention provides a display apparatus with a multi-layer display structure. A self-luminous pixel module of the display apparatus may serve as a backlight for one or two non-self-luminous pixel modules of the display apparatus. The self-luminous pixel module also offers local dimming to enhance the color contrast ratio. A PWM driving circuit of the display apparatus provides first PWM signals to the self-luminous pixel module to control values of brightness of self-luminous pixels of the self-luminous pixel module. The PWM driving circuit further provides second PWM signals to the non-self-luminous pixel module to control values of transmittance of non-self-luminous pixels of the one or two non-self-luminous pixel modules. The gray level of each display pixel could be adjusted by adjusting the duty cycles of the first PWM signals and the second PWM signals. 
     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.