Patent Publication Number: US-6903732-B2

Title: Image display device

Description:
TECHNICAL FIELD 
   The present invention relates to an image display device, particularly an image display device which can switch between the display of dynamic images and static images. 
   BACKGROUND ART 
   Development of communication technology, etc. in recent years makes it possible to provide information display devices for use in personal computers, portable information terminals and the like which can switch between the display of dynamic images and static images in accordance with the signal received This makes it possible to provide various kinds of information depending upon the user&#39;s needs and preference. However, in such an image display device, electricity consumption is increased because an on-off action is repeated whenever a dynamic image is displayed. A portable information terminal uses a battery as a driving power source thereof, and therefore reducing the amount of energy used is a particularly important object because an increase in energy consumption results in a shortened usage duration per charge. 
   A method for reducing the number of bits of image data has been proposed for reducing energy consumption in image display devices. As an example of an image display device provided with an image processing device which performs such processing,  FIG. 4  shows a structure disclosed in Japanese Unexamined Patent Publication No. 1997-101771. 
   As shown in the figure, an image display device  50  comprises a video controller  51  for controlling the image display device  50 , a VRAM  52  for storing RGB image data, a liquid crystal display  53  for displaying the RGB image data and a liquid crystal controller  54  for controlling the liquid crystal display  53 . The image display device  50  is connected to a personal computer  56  and an image processing device  58 . When RGB image data is input from a hard disk drive (not shown) of the personal computer  56 . the image processing device  58  reduces the size of the image data by masking the lowest three bits thereof which are nearly uninfluential on the quality of the display image, conducts scaling and subtractive color processing, and then outputs the image data to the image display device  50 . As the subtractive color process, as disclosed in the above publication, a dither method, an error diffusion method and the like are known. The RGB image data input into the image display device  50  is transferred to the VRAM  52  and displayed on the liquid crystal display  53  controlled by the liquid crystal controller  54 . 
   The above-described image display device saves energy by reducing the number of bits of the RGB image data in the image processing device  58 ; however, the image processing device  58  continues to operate while an image is displayed on the liquid crystal display  53  regardless of whether the image is dynamic or static. Therefore. there is room for further improvement in energy reduction. 
   DISCLOSURE OF THE INVENTION 
   The present invention provides an image display device which has reduced energy consumption while maintaining a high quality display image. 
   To achieve the object. the image display device of the invention comprises: a first storage device for storing image data: an image processing device for reducing the number of bits of the image data; a second storage device for storing the image data after being processed; a display device for displaying the image data after being processed; a display drive device for driving the display device; and a control device for controlling the display drive device. Upon determining whether the image data stored in the first storage device is dynamic or static, and, in the case of a static image, after storing the signals corresponding to one frame of the image data in the second storage device, the control device operates the control device operates only the second storage device, the display drive device and the display device. 
   In the image display device, it is preferable that the memory capacity of the second storage device be smaller than that of the first storage device. Thereby, the second storage device and the display drive device can be disposed on the same chip and united into one body achieving further power savings. 
   In the image processing device, it is preferable that the image be processed by a dither method or an error diffusion method. It is also preferable that the image processing device reduce the total number of bits of the three elements (RGB) of the image data in such a manner that the number of G bits becomes the largest and that of B bits becomes the smallest. Thereby, a high duality display image can be obtained. 
   The image processing device can also be structured so as to comprise a dynamic image processing device for reducing the number of bits of a dynamic image, a static image processing device for reducing the number of bits of a static image and a switching device for switching between the dynamic image processing device and the static image processing device. In this case, the control device determines whether the image data stored in the first storage device is that of a dynamic image or a static image, and by operating the switching device according to that determination, if the image data is that of a dynamic image, the dynamic image processing device is made to process the image data, and if the image data is that of a static image, the static image processing device is made to process the image data. It is preferable that the dynamic image processing device process the image by an FRC method and the static image processing device process the image by a dither method or an error diffusion method. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a schematic block diagram showing an image display device in accordance with one embodiment of the present invention. 
       FIG. 2  is a schematic block diagram showing an image display device in accordance with another embodiment of the present invention. 
       FIG. 3  is a schematic block diagram showing an image display device in accordance with still another embodiment of the present invention. 
       FIG. 4  is a schematic block diagram showing a known image display device. 
   

   BEST MODE FOR CARRYING OUT THE INVENTION 
   Embodiments of the invention will be described below with reference to the accompanying drawings.  FIG. 1  is a schematic block diagram showing an image display device in accordance with one embodiment of the invention. This image display device can be mounted on a portable information terminal, etc. 
   As shown in FIG.  1 . the image display device comprises a signal processor  2  for outputting a digital image data based on the signals input thereinto, a VRAM  4  serving as a first storage device for storing the image data, a gamma processor  6  (γ processor) for correcting the γ property of the image data, an image processor  8  for reducing the size of the image data by reducing the number of bits thereof. a panel correspondence RAM  10  serving as a second storage device for storing the reduced image data, a display panel  12  comprising a liquid crystal panel for displaying the image data and a driver  14  for driving the display panel  12 . The operations of the signal processor  2 , the VRAM  4 , the γ processor  6 , the image processor  8 , the panel correspondence RAM  10 . the panel display  12  and the driver  14  are controlled by the controller  16 . The signal processor  2  comprises a DSP (Digital Signal Processor) and has a function for decompressing compressed data based on the MPEG (Moving Picture Experts Group) standards, etc. 
   The operation of the image display device will be explained below. The signals demodulated after having been input through an antenna (not shown) are input into the signal processor  2  and subjected to digital signal processing, and then stored in the VRAM  4 . Based on the signals input into the signal processor  2 , the controller  16  determines whether the image data is dynamic or static and controls the image display device according to that determination. Here, a dynamic image data comprising three elements, i.e., R (red), G (green) and B (blue). each having 6 bits is stored in the VRAM  4 . When the controller  16  determines that the image data is dynamic, it operates the signal processor  2 . the VRAM  4 , the γ processor  6 . the image processor  8 , the panel correspondence RAM  10 , the display panel  12  and the driver  14 . 
   The image data stored in the VRAM  4  is subjected to γ correction in the γ processor  6  in accordance with the prescribed γ correction data, and then is input into the image processor  8 . The image processor  8  reduces the number of bits of the image data in accordance with the prescribed image processing method. As the image processing method, widely known methods can be used in which the number of gradations of the image data is reduced. For example, it is possible to simply cut off the lower bits. However, a contour line may appear caused by the reduced number of gradations, therefore it is desirable that the subtractive color process be conducted by a dither method or an error diffusion method. The dither method is a technique whereby the number of gradations of the image data is reduced while being compared with the threshold value of a prescribed dither matrix. The error diffusion method is a technique whereby the errors arising when reducing the number of gradations are dispersed among the adjacent pixels. These methods can prevent contour lines caused by the reduced number of gradations. According to the present embodiment of the invention, each channel of the RGB image data is reduced from 6 bits to 4 bits by the image processor  8  and stored in the panel correspondence RAM  10 . 
   The image data stored in the panel correspondence RAM  10  is displayed on the display panel  12  driven by the driver  14 . The controller  16  controls the image display device in such a manner that the above operation is repeated when the image data is determined to be dynamic based on the signals input into the signal processor  2 . 
   On the other hand, based on the signals input into the signal processor  2 , when the controller  16  determines that the image data is static, it processes the signals corresponding to the first screen of the image (one frame) in the same manner as dynamic images as described above. Then, it halts the operation of the signal processor  2 , the VRAM  4 , the γ processor  6  and the image processor  8 , and operates only the panel correspondence RAM  10 , the display panel  12  and the driver  14 . Thereby, on the screen of the display panel  12 , the static image first displayed is maintained. The image display device is kept controlled in this manner until the controller  16  recognizes dynamic image data based on the input signals. 
   As described above, when the image display device according to the present embodiment of the invention displays a static image, it operates only the minimum constituent components required to maintain the display image. Therefore, it exhibits reduced power consumption compared to the heretofore known devices, which operate the same way regardless of whether the image is dynamic or static. 
   Particularly, according to the present embodiment of the invention, since the panel correspondence RAM  10  stores the image data after the image processor  8  has reduced the number of bits, the memory capacity of the panel correspondence RAM  10  can be smaller than that of the VRAM  4 . Therefore, when a static image is displayed while operating the panel correspondence RAM  10  without operating the VRAM  4 , further power consumption savings are realized. 
   As described above, the memory capacity of the panel correspondence RAM  10  can be smaller, and this enables the panel correspondence RAM  10  and the driver  14  to be readily united into one body on a single IC chip. Thereby, the stray capacitance between the panel correspondence RAM  10  and the driver  14  can be lowered, and this arrangement achieves further power consumption savings. Note that, the γ processor  6  and the image processor  8  can also be united into one body on a single IC chip, and the versatility thereof is improved by internally storing its interface. 
   One embodiment of the invention is described above; however, embodiments of the invention are not limited to this explanation. For example, according to the present embodiment of the invention, the γ correction is conducted in the γ processor  6  before processing the image in the image processor  8 . However, instead of having the γ processor  6 , a structure where the driver  14  functions as a γ corrector and the image data of the VRAM  4  is directly input into the image processor  8  is also possible. Furthermore, the present embodiment is structured so that the signal processor  2  and the VRAM  4  are separate; however, it is also possible that the signal processor  2  contain the VRAM  4 . 
   The processing method employed in the image processor  8  is not limited to that used in the present embodiment. A method is acceptable as long as it reduces the number of bits of the image data and the number of reduced bits can be suitably selected depending on the capacity of the display panel  12 . For example, in the present embodiment, the number of bits of the processed image is the same in each of the three elements (RGB). However, it is also possible to make the number of bits of G, to which the human eye has a high sensitivity, the largest and that of B, to which the human eye has a low sensitivity, the smallest. 
   For example, as shown in  FIG. 2 , it is preferable that the number of bits after being processed in the image processor  8  be G: 5 bits, R: 4 bits and B: 3 bits. Such an image processing technique is particularly effective for the dither method and the error diffusion method where the gradations of the image are spatially dispersed. Roughness of the image surface will be prevented even if the total number of bits of the three elements (RGB) is the same as the present embodiment. Therefore, a high quality image can be maintained even when the number of bits is reduced for reducing power consumption. 
   In the present embodiment, the image processor  8  processes dynamic and static image data in the sane manner; however, as shown in  FIG. 3 , it is also possible that the image processor  18  have a dynamic image processor  18   a , a static image processor  18   b  and a switch  18   c.    
   In this structure, the controller  16  determines whether the image data is that of a dynamic or static image based on the signals input into the signal processor  2 . Then, the controller  16  operates the switch  18   c  according to that determination and selects either the dynamic image processor  18   a  or the static image processor  18   b . Thereby, the image data processed in the γ processor  6  is, processed by the dynamic image processor  18   a  if it is dynamic image data, and processed by the static image processor  18   b , if it is static image data. When a static image is displayed. the controller  16  operates the entire image display device until the signals corresponding to one screen (one frame) of the image data are stored in the panel correspondence RAM  10  and displayed on the display panel  12 , and then operates only the panel correspondence RAM  10 , the display panel  12  and the driver  14 . 
   Thereby, the image processing method can be altered depending on whether the image is dynamic or static. This makes it possible to process both kinds of image in a suitable manner, resulting in reduced power consumption while maintaining high image quality. In addition, because the frame rates for displaying dynamic images and static images can be differentiated, by lowering the frame rate used for static images compared to that of dynamic images, reduction of power consumption can be achieved while maintaining high image quality. As an example of the image processing method, a spatial subtractive color process such as the dither method or the error diffusion method is effective in the static image processor  18   b . In the dynamic image processor  18   a , a temporal subtractive color process such as the FRC (Frame Rate Control) method is effective. The FRC method is a technique wherein the average voltage applied is controlled for displaying multiple gradations by inserting OFF-frames between ON-frames at a fixed rate when an arbitrary pixel of the display panel  12  is to be turned on. 
   According to the present embodiment, a liquid crystal panel is used as the display panel  12 . However, reduction of power consumption as in the present embodiment can be also achieved by using light emitting displays such as organic EL and the like as the display panel  12 . 
   Further, other kinds of rewritable semiconductor memory can also be used as the VRAM  4  and the panel correspondence RAM  10 .