Patent Publication Number: US-6906691-B2

Title: LCD device and a method for reducing flickers

Description:
FIELD OF THE INVENTION 
   The present invention relates to a liquid crystal display (LCD) device, and more particularly to an active matrix type LCD device of using thin film transistors (TFT) as switching elements and a method for reducing flickers. 
   BACKGROUND OF THE INVENTION 
   In an active matrix type LCD device, all pixels are driven by a plurality of active elements having non-liner characteristics, each of which is disposed in each of pixels arranged in matrix shape. As the active elements, TFT elements are generally used. 
   In the LCD device, optical display characteristics are dependent on TFT elements, liquid crystal material, cell gaps, color filters and the like. Accordingly, as the TFT LCD device is used for a long time, flickers or image stickings deteriorating display characteristic may occur. 
   To reduce flickers, various methods have been proposed. Examples of the conventional methods of reducing flickers are disclosed in U.S. Pat. No. 5,253,091 issued to Kimura et al., on October, 1993 and entitled Liquid Crystal Display Having Reduced Flickers,” and U.S. Pat. No. 5,436,747 issued to Suzuki et al., on July, 1995 and entitled “Reduced Flicker Liquid Crystal Display.” 
   A general LCD device includes a TFT substrate having a plurality of pixel electrodes and TFT elements, an opposite color filter substrate having common electrodes and color filters, and a liquid crystal material therebetween. 
   In the LCD device, a plurality of TFT elements, each of which is disposed in a pixel, supply voltage to the common electrodes formed on the color filter substrate and the pixel electrodes formed on the TFT substrate to control electric fields which are to be applied to the liquid crystal. When the pixel electrodes and the common electrodes are applied with voltages by the operation of the corresponding TFT elements, the molecules of the liquid crystal material change their orientations in response to the electric fields due to the potential difference between the pixel electrodes and the common electrodes. At this time, the electric field between two electrodes is generally controlled to periodically change its direction. 
   For example, signal voltage supplied to the pixel electrodes through the TFT elements is periodically inverted with respect to common electrode voltage supplied to the common electrodes. At this time, if actual values of inverted signal voltages of positive and negative against the common electrode voltage are same, flickers and afterimages or image stickings do not occur. However, if the virtual values of the positive and negative voltages are different from each other, electric fields having elements of direct current may be applied between two electrodes to generate image stickings. Also, if positive and negative voltages to same gray scale are not symmetrical to each other with respect to the common electrode voltage, brightness of each pixel may come to be different according to each of the positive and negative voltages and thereby flickers occur. Even though at first the common electrode voltage has been correctly modulated not to impose the elements of direct current and the like, components of the LCD device such as the TFT elements, color filters, and a protecting sheet physically change as it is used. Accordingly, the common electrode voltage may be deviated from optimum condition and thereby increase flickers. Particularly, a large size LCD of a high definition may exponentially increase an amount of visually recognized flickers. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide an improved LCD device and a method for reducing flickers. 
   This and other objects are provided, according to the present invention, by an LCD device comprising a liquid crystal panel having a plurality of pixels, a backlight for supplying light having an uniform brightness to the liquid crystal panel, a timing control circuit for generating gate clock signal and a plurality of control signals, a gray scale voltage generating circuit for generating a plurality of gray scale voltages corresponding to data to be displayed in the liquid crystal panel in response to the gate clock signal, a gate driving circuit for scanning the pixels of the liquid crystal panel row by row in response to the gate clock signal, and a source driving circuit for outputting liquid crystal driving voltage to the liquid crystal panel every scanning. The timing control circuit senses whether or not data to be displayed in the liquid crystal panel is toggled, and generates control signals for controlling brightness of the backlight according to sensed results. 
   According to another aspect of the present invention, there is provided a method for reducing flickers comprising the steps of inputting data in a timing controller of an LCD device, detecting whether or not inputted data is toggled, counting the number of toggled data among data in one line to be displayed in the LCD device, counting the number of toggled line among data of one frame to be displayed in the LCD device, and controlling a brightness of the LCD device in response to the number of toggled line. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of a LCD device in accordance with the present invention. 
       FIG. 2  is a block diagram of a timing control circuit of the LCD device shown in FIG.  1 . 
       FIG. 3  is a circuit diagram of a flicker reducing portion of the LCD device shown in FIG.  2 . 
       FIG. 4  is a flow chart showing the process steps of a method for reducing flickers in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiment of the invention is shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiment set forth herein. Like numbers refer to like elements throughout. 
   An LCD device in accordance with the present invention includes a timing control circuit having a flicker reducing portion for detecting an amount of flickers included in one entire frame to be displayed in a liquid crystal panel and controlling a brightness of the backlight according to an amount of detected flickers. The timing control circuit can evidently reduce an amount of visually recognized flickers by generating control signal to dim the backlight brightness when the detected flickers are more than allowed. 
     FIG. 1  is a block diagram of a LCD device  100  in accordance of the present invention. 
   Referring now to  FIG. 1 , the LCD device  100  includes a liquid crystal panel  10 , a gate driving circuit  20  coupled to the liquid crystal panel  10 , a source driving circuit  30 , a timing control circuit  40  having a flicker reducing portion  60 , a gray scale voltage generating circuit  50 , and a backlight  70 . 
   The liquid crystal panel  10  comprises a plurality of gate lines G 0 -Gn, and a plurality of data lines D 1 -Dm crossing the gate lines G 0 -Gn, respectively. Each gate line is coupled to a gate driving circuit  20  and each data line coupled to a source driving circuit  30 . The liquid crystal panel  10  displays pure red, pure green, pure blue and gray levels or scales as well as color pictures by combining three kinds of color filters of red R, green G, and blue B. The backlight  70  is coupled to the liquid crystal panel  10  to provide plane light having an uniform brightness. The gray scale voltage generating circuit  50  is coupled to the source driving circuit  30  to generate standard voltages Vgray for providing standards in generating liquid crystal driving voltages. The gate driving circuit  20  scans pixels of the liquid crystal panel  10  one row at a time in order. When the gate driving circuit  20  scans the pixels of the liquid crystal panel  10 , the source driving circuit  30  generates the liquid crystal driving voltages according to color signals RGB inputted through the timing control circuit  40  in response to the standard voltages Vgray outputted from the gray scale voltage generating circuit  50 , and outputs generated liquid crystal driving voltages to the liquid crystal panel  10  every scanning. 
   The timing control circuit  40  generates control signals necessary to the gate driving circuit  20  and the source driving circuit  30  in response to color signals RGB, line distinction signals H_Sync, frame distinction signals V_Sync, and clock signals MCLK. Also, the timing control circuit  40  detects flickers included in the color signals RGB through the flicker reducing portion  60 , and reduces the visually recognized flickers displayed on the liquid crystal panel  10  by controlling the panel brightness depending on the amount of detected flickers. 
   Generally, recognizing flickers is different according to individuals and their existing condition. Accordingly, in some LCD technique fields, attempts have been made to measure flickers by a psychophysiology or psychological method. For example, the older has the less sensitivity on flickers. Also, the sensitivity on flickers is reduced according to fatigue degree. Thus, flickers are differently felt according to visual sensitivity, i.e., they are easily sensed when illumination is high, but scarcely sensed when illumination is low. By using this characteristic of flickers, the timing control circuit  40  of the present invention controls the picture brightness to make flickers scarcely sensed when flickers are more than given level, whereas to return the normal brightness level when flickers is below the level. 
   Here will be explained the structure or composition of the timing control circuit  40  operated as described above with reference to FIG.  2 .  FIG. 2  is a block diagram of the timing control circuit  40  of the LCD device  100  shown in FIG.  1 . Referring to  FIG. 2 , the timing control circuit  40  greatly comprises an input processor  41 , a data processor  42 , a clock processor  43  and a signal processor  44 . The signal processor  44  includes a flicker reducing portion  60  for controlling the brightness of the liquid crystal panel  10  according to the amount of flickers. 
   The data processor  42  and the clock processor  43  control timings of the color signals RGB and clock signals MCLK, respectively. The signal processor  44  generates control signals necessary to the gate driving circuit  2  and the source driving circuit  3 , for example start horizontal signal STH, start vertical signal STV, load signal TP, gate clock signal Gate Clock, gate on enabling signal OE and the like in response to the frame distinction signal V_Sync and the line distinction signal H_Sync inputted from a graphic controller (not shown), DE signal showing high level only in outputting of the color signals RGB, and the clock signal MCLK. The input processor  41  transforms variable signals coming from the graphic controller into given signals, and thereby to operate the data processor  42  and the signal processor  44 . The flicker reducing portion  60  disposed in the timing control circuit  40  senses an amount of flickers in the whole of one frame displayed in the liquid crystal panel  10 , and generates control signals Dim for controlling a brightness of the liquid crystal panel  10  according to an amount of sensed flickers. The backlight  70  includes a dimming circuit (not shown) to control a brightness of the backlight  70  in response to the control signals Dim generated from the timing control circuit  40 . This kind of dimming circuit of the backlight  70  is disclosed in the U.S. Pat. No. 5,939,830 issued to Praiswater on August, 1999 and entitled “Method And Apparatus For Dimming A Lamp In A Backlight Of A Liquid Crystal Display.” 
     FIG. 3  is a circuit diagram of the flicker reducing portion  60  of the LCD device  100  in accordance with a preferred embodiment of the present invention. Referring to  FIG. 3 , the flicker reducing portion  60  of the present invention comprises a flicker sensing portion  64  having a toggling detector  61 , an adder  62  and a first comparator  63 , a first counter  65 , a second comparator  66 , a second counter  67 , and a third comparator  68 . It should be noted that a first reference value Ref 1 , a second reference value Ref 2  and a third reference value Ref 3 , inputted in the first comparator  63 , the second comparator  66  and the third comparator  68  can vary according to the resolution and driving method of the LCD device  100 . In the present invention, they are explained as applied to a super extended graphics array (SXGA) having a resolution of 1280*1024 and a dual port driving method that can concurrently input odd pixel data and even pixel data. 
   In order to sense whether or not inputted color signal RGB has a flicker, the toggling detector  61  of the flicker sensing portion  64  detects whether or not each of bits forming the color signal RGB is toggled. For this, the toggling detector  61  receives the color signal RGB with dividing them into each of bits D 0 -D 47 , delays received bits D 1 -D 47  through delays Delay 0 -Delay 47  for a given time and then performs XOR operation to each of delayed bits and non-delayed original bits. When the received bit is toggled, a result of the XOR operation comes to “1”, whereas when the received bit is not toggled, a result of the XOR operation comes to “0”. These results of the XOR operation are inputted to the adder  62  to be added. The adder  62  calculates the number of toggled bits among inputted color signal RGB. The first comparator  63  compares the number of toggled bits calculated through the adder  62  with a first standard value Ref 1 . According to the results of the first comparator  63 , a result having value of “0” or “1” is inputted to the first counter  65 . At this time, the first standard value Ref 1  is set based on the number of bits in data inputted at a time. For example, when a color signal RGB composed of 8 bits is inputted, each of signals composed of red R, green G and blue B needs data of 8 bits, so that the number of bits necessary to show one pixel comes to 8*3, i.e., 24. Particularly, in case of the dual port driving method which at the present time, is widely used, since odd pixel data and even pixel data are concurrently inputted, the number of bits in data inputted at a time comes to 8*3*2, i.e., 48. 
   The flicker sensing portion  64  senses whether or not inputted data have flickersby detecting whether or not each of bits D 0 -D 47  forming the inputted data is toggled. The reason is that flickers are generated in a shape of toggled data. Also, the flicker sensing portion  64  outputs a value of “1” showing that the inputted data have flickers when all the bits D 0 -D 47  forming the inputted data are toggled, and otherwise a value of “0”. 
   Once the flicker sensing portion  64  senses whether or not the inputted data have a flicker, a sensed result having a value of “0” or “1” is inputted to the first counter  65 . The first counter  65  as a pixel toggler composed of 10 bit counter receives reset signal Reset and line distinction signal H_Sync as well as the sensed result through AND gate. Namely, the first counter  65  counts the number of inputted data generating flickers, in pixel unit, in response to the sensed result received from the flicker sensing portion  64 . When the line distinction signal H_Sync is input the first counter  65  outputs only the number of flickers generated in one line and is reset. Thus, the number of pixels with a flicker in one line is detected. 
   When the number of flickers generated in one line is counted by the first counter  65 , a counted result, i.e., a first count value is compared with a second standard value Ref 2  through the second comparator  66 . It should be noted that the second standard value Ref 2  means the number of pixels in one line. For example, in case the LCD device  100  employs the dual port driving method and has a resolution of 1280*1024 such as SXGA, the second standard value Ref 2  becomes 640. The second comparator  66  detects whether or not the counted result, i.e., the first count value is the same as the second standard value Ref 2 . As a result, if the counted result is the same as the second standard value Ref 2 , the second comparator  66  outputs “1” and otherwise, outputs “0”. Thus, the second comparator  66  detects whether or not flickers are generated in the entire one line. 
   Once the first counter  65  and the second comparator  66  detect whether or not flickers are generated in the entire one line, a detected result of each line having a value of “1” or “0” is inputted to the second counter  67 . The second counter  67 , a pixel toggler composed of 10 bit counter, receives reset signal Reset and line distinction signal V_Sync as well as the detected result, i.e., the number of detected flickers through AND gate. Namely, the second counter  67  counts the number of detected flickers on each line outputted from the second comparator  66 . Upon inputting of the frame distinction signal V_Sync, the second counter  67  outputs the number of flickered lines in one frame and is reset. Thus, the number of flickered lines in one frame is obtained. 
   When the number of lines having a flicker in one frame is counted by the second counter  67 , a counted result, i.e., a second count value is compared with a third standard value Ref 3  through the third comparator  68 . Here should be noted that the third standard value Ref 3  means a value which the number of lines forming one frame is multiplied by a given rate. For example, in SXGA having a resolution of 1280*1024, the third standard value Ref 3  comes to a value corresponding to about 90% of 1024, i.e., 921. The reason of setting like this is to reduce the amount of visually recognized flickers by dimming the brightness of the backlight  70  when flickers are generated at more than 90% of pixels in one frame. For this, the third comparator  68  compares the counted result, i.e., the second count value with the third standard value Ref 3 . As a result, if the counted result is the same as or larger than the third standard value Ref 3 , the third comparator  68  outputs a control signal Dim having a value of “1” to the backlight  70  to control the brightness thereof to be dimmed and otherwise, generates a control signal Dim having a value of “0” to the backlight  70 . 
   The flicker reducing portion  60  of the present invention operating as described above is characterized to have simple circuit components such as counters and comparators without separate memories. Thus, it occupies only a small amount of circuit area and thereby reduces its fabrication cost. 
   As described above, the timing control circuit  40  of the present invention detects whether or not a flicker is generated in each pixel of one frame, and generates the control signal Dim for controlling the brightness of the backlight  70  by dimming when flickers are generated above a given level. On the other hand, when the backlight  70  is dimmed by the timing control circuit  40 , the timing control circuit  40  generates a control signal Dim that restores the brightness of the backlight  70 , if the flicker level falls below the given level. This can reduce the visually recognized flickers. At this time, brightness control standard for the backlight  70  can be obtained by modulating the standard values Ref 1 -Ref 3  properly. Also, brightness control levels for the backlight  70  can be controlled by modulating the standard values Ref 1 -Ref 3  properly. 
     FIG. 4  is a flow chart showing steps of the method for reducing flicker level in an LCD device in accordance with a preferred embodiment of the present invention. Particularly,  FIG. 4  shows the operation steps of the flicker reducing portion  60  shown in FIG.  3 . 
   Referring to  FIG. 4 , first, color signal RGB corresponding to each pixel of the LCD device is inputted (S 10 ). Then, each of bits forming inputted color signal RGB is checked whether or not it is toggled (S 12 ). And then, the number of toggled bits is counted (S 14 ). Thereafter, the number of counted bits is checked whether or not it is the same as a first standard value Ref 1  (S 16 ). Here should be noted that the first standard value Ref 1  means the number of the entire bits of the color signal RGB inputted at a time. In case of a LCD device complying with a dual port driving method and having a resolution of 1280*1024 such as SXGA, the first standard value Ref 1  is 48. 
   At the step S 16 , when the number of counted bits is the same as the first standard value Ref 1 , a first count value is increased (S 18 ), and otherwise the operation step is returned to the first step S 10  to repeat the operations as described above. Here, it should be noted that the fact that the number of counted bits is the same as the first standard value Ref 1  means that all the bits of inputted color signal RGB are toggled to generate flickers, and the first count value means the number of toggled pixels, i.e., flickered pixels in one line. 
   Next, the first count value is compared with a second standard value Ref 2  to detect whether or not they are same each other (S 20 ). Here should be noted that the second standard value Ref 2  means the number of pixels forming one line. In case of the LCD device complying with the dual port driving method and having a resolution of 1280*1024 such as SXGA, the second standard value Ref 2  is 640. At the step S 20 , when the first count value is the same as the second standard value Ref 2 , i.e., when flickers are generated in the entire one line, a second count value is increased (S 22 ), and otherwise the operation step is returned to the first step S 10  to repeat the operations as described above. It should be noted that the second count value means the number of toggled lines, i.e., flickered lines in one frame. 
   Then, the second count value is checked whether or not it is the same as a third standard value Ref 3  (S 24 ). Here, it should be noted that the third standard value Ref 3  means a value which the number of lines forming one frame is multiplied by a given rate, for example about 90%. In case of the LCD device complying with the dual port driving method and having a resolution of 1280*1024 such as SXGA and the given rate being 90%, the third standard value Ref 3  is 90% of 1024, or 921. At the step S 24 , when the second count value is the same as or larger than the third standard value Ref 3 , i.e., when flickers are generated above a given rate, for example about 90% in the entire one frame, a control signal Dim that dims a backlight  70  is generated (S 26 ). Otherwise, the operation step returns back to the first step S 10  to repeat the operations as described above. 
     FIG. 4  shows that when flickers are generated above the given rate in the whole of one frame, the control signal Dim is generated to dim the brightness of the backlight  70 . However, on the other hand, when flickers are generated below the given rate in the entire frame but the brightness of the backlight  70  is dimmed, the timing control circuit  40  of the present invention can also restore the brightness of the backlight  70 . Also, in the method of the present invention, brightness control standard for controlling the brightness of the backlight  70  can be changed by modulating the standard values Ref 1 -Ref 3  properly. Also, it is possible to control brightness levels of the backlight  70  at more than one level. 
   As apparent from the foregoing description, it can be appreciated that the present invention provides an LCD device and a method for reducing flickers, which can evidently reduce the visually recognized flickers by a simple circuit composition. 
   In the drawings and specification, there has been disclosed typical preferred embodiment of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purpose of limitation, the scope of the invention being set forth in the following claims.