Abstract:
A drive circuit for a CRT locally enhances the drive signal range in a window. Circuitry is further included for monitoring the average luminosity in the window, and the drive signal range is reduced if there is a danger of doming in the window.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an image display device comprising an input for receiving information representing an image; drive means for generating a drive signal from the information; a cathode ray tube for displaying the image driven by the drive signal; and input means coupled to the drive means for receiving further information identifying a window of pixels in the image, the drive means being arranged to change a relation between the information and the drive signal in the window as compared to a further relation between the information and the drive signal in an area outside the window, so as to enhance a drive signal range in the window. 
     2. Description of the Related Art 
     Such a device is known from PCT Patent Application No. WO 96/17338, corresponding to U.S. Pat. No. 5,638,090. This device makes it possible to show an image which contains video and text and to enhance the luminosity range of the image in windows which contain video beyond the luminosity range of areas outside the windows which contain text. The increase in the luminosity range makes the video richer in contrast, but, on a CRT, it also increases the spot-size, which would be undesirable for text areas. By increasing the range of luminosity only locally where a window with video is shown, video with a richer contrast can be combined with text. 
     However, it has been found that by extending the luminosity range, one increases the risk of doming (deformation of the shadow mask due to heating by the electron beam) and overloading of the high voltage (EHT) power supply. 
     SUMMARY OF THE INVENTION 
     Among others, it is an object of the invention to combine the display of high luminosity video, photographs or graphics with high resolution text on a CRT, without problems due to EHT overloading. A further object is to avoid problems due to doming in such a device. 
     The device according to the invention is characterized in that the drive means is arranged to change said relation between the information and the drive signal in the window so as to reduce the drive signal range when an average luminosity on the cathode ray tube exceeds a threshold value. By locally reducing the range of drive signal values in the windows where it has been enhanced in case the luminosity (i.e., beam energy) range becomes too high, overloading problems can be prevented. 
     A further embodiment of the device according to the invention is characterized in that said drive means is arranged to determine said average luminosity averaged over said window from the information received for said window, the drive means controlling said relation so that said average luminosity remains below the threshold value. By measuring the average luminosity in the window, or in each one of a number of separate windows where the drive signal range is increased, and reducing the drive signal range selectively if the average luminosity is too high, local doming effects are prevented. At the same time, text windows retain their original drive signal range and, therefore, they retain the same level of readability. 
     In an embodiment of the device according to the invention, the drive signal range is adjusted by varying an amplification gain of an amplification circuit for the image signal (e.g., for each of the R, G and B components). This can be readily implemented by inserting an amplifier with adjustable gain in the video signal path of known monitors, or using gain control inputs of available video amplifier IC&#39;s. In another embodiment, the window or several windows are identified by coordinates of corner points of the window(s) and/or values of the width and height of the window. Thus, the further information can be received with very little bandwidth using existing bus protocols, e.g., between monitors and PC&#39;s. 
     In a further embodiment of the device according to the invention, the average luminosity in a window is averaged over time, and the average over time is used to control the drive signal range for the window. By averaging the average luminosity over time, sudden artificial changes in visible luminosity due to the invention are prevented. Doming effects occur only with a long time constant of typically several minutes. Short excesses in the luminosity can be tolerated without visible effect. As a result, the time range over which the average is taken may be typically one minute or more. This also means that the circuit which selects the drive signal range may be relatively slow. 
     In another embodiment of the device according to the invention, the window is selected on the basis of a spatial frequency content of the image in the window. This makes it possible to discriminate between text and video, photo or graphics information. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     These and other advantageous aspects of the invention will be described using the attached drawing, in which 
     FIG. 1 shows an image display system according to the invention, 
     FIG. 2 shows an image containing several windows; and 
     FIG. 3 shows a graph of amplification gains of the amplifier in the image display system. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows an image display system. The system contains a processing apparatus  10  (which is, for example, a PC) and a monitor  20 . The processing apparatus  10  contains several image sources  104   a - 104   c  connected to a device driver  100 . Although the image sources  104   a - 104   c  and the device driver  100  are shown as individual units, they may actually be implemented as programs stored in the processing apparatus  10  and executed by a processing unit (not shown) of the processing apparatus  10 . The device driver  100  has an output connected to a video card  102 . The video card  102  is connected to an RGB output and a sync output of the processing apparatus  10 . 
     The monitor  20  has an RGB input  208  and a sync input  209 . The monitor  20  contains a video amplification circuit  202  which is coupled to the RGB input  208 . An output of the video amplification circuit  202  is coupled to a CRT (Cathode Ray Tube)  200 . The monitor contains a deflection control circuit  201  having an input coupled to the sync input  209  and an output coupled to the CRT  200 . 
     The monitor  20  also contains a processing element  204  and a gain control unit  206 . The RGB input  208  is coupled to the processing element  204  and the gain control unit  206 , which is also connected to the sync input  209 . An output of the processing element  204  is coupled to the gain control unit  206  and an output of the gain control unit  206  is connected to a gain control input of the amplification circuit  202 . Of course, instead of an RGB representation of the image, other representations of the image may be used, for example, like YUV representation, but, for the sake of clarity, the invention will be explained using RGB signals. 
     In operation the image display system will cause the display of an image on the CRT  200 . 
     FIG. 2 shows an example of a displayed image  20 . The image contains two rectangular windows  22 ,  24 . Different window sources  104   a - 104   c  may produce image information that defines the image inside respective windows  22 ,  24 , but it is also possible that an image source  104   a - 104   c  define the image inside more than one window, or in one or more sub-windows contained in a window. 
     The image sources  104   a-c  supply the image information to the device driver  100 , which converts the image information into a form that is adapted to control the video card  102 . The video card  102  processes the information to form an RGB signal and horizontal and vertical sync signals to control the operation of the CRT  200 . The RGB signals are supplied to the amplification circuit  202 , which amplifies each of the R (Red), G (Green) and B (Blue) signals and supplies the amplified signals to the CRT  200 . The deflection control circuit  201  processes the sync signals and supplies deflection signals to the CRT to ensure that the RGB information is written at the right location on the screen of the CRT  200 . 
     In some of the windows  22 ,  24  the image is shown on the CRT  200  with a greater luminosity range than in other windows. This is used, for example, for windows in which video information or (photo)graphic information is displayed. On the other hand, in other windows, for example windows where letters are displayed, the luminosity range is not extended. Increasing the luminosity range so that a higher luminosity can be produced increases the CRT beam spot size, so that the maximum possible resolution is decreased. By increasing the luminosity range in certain windows and not in others, the other windows (e.g., text windows) retain high resolution but, at the same time, the perceptual quality of video/(photo)graphic information is improved by the greater luminosity range. 
     The video card  102  sends a signal which identifies the windows in which the luminosity range should be greater, for example, in the form of coordinates (x 0 ,y 0 ) of the lower left corner, and width and height values (xw,yw) of the window. This signal is sent, for example, in an otherwise unused video line, or using a separate bus, using, for example, one of the known MVL (Moving Video Labs), USB or DDC2B+ protocols. The processing element  204  in the monitor  20  receives this signal, decodes it and sends information to the gain control unit  206  to identify these windows, for example in the form of coordinates (x 0 ,y 0 ) of the lower left corner, and width and height values (xw,yw) of the window. 
     The gain control unit  206  processes the sync signals and determines when RGB values concerning pixels inside identified windows are transmitted. When this is the case, the gain control unit  206  supplies a signal to the amplification circuit  202  to increase the amplification gain. 
     FIG. 3 illustrates the increased gain for two raster lines  26 ,  28  shown in FIG. 2. A first trace  30  shows an amplification gain along a first raster line  26  which crosses a first window  24  for which information is transmitted indicating that it should have higher gain. Outside the windows, the gain has a lower value. Similarly, a second trace  32  shows an amplification gain along a second line, which crosses two such windows  22 ,  24 . 
     Preferably a monitor  20  is used that is capable of displaying an image in which each pixel has maximum luminosity in case the amplification gain is not increased anywhere. In such a monitor  20 , the high voltage (EHT) does not drop below a minimum acceptable level in this case and no unacceptable doming effect will occur due to heating of the CRT  200 . 
     With such a monitor, the increased amplification gain in some windows may cause EHT or doming problems because too much power is used in the CRT. Often, the increased amplification gain will not lead to any problems with the EHT or doming because the average luminosity level inside a window is usually much less than the maximum possible luminosity value. For example, in a video/photographic window, the average luminosity is typically one third of the maximum possible luminosity. So if the amplification gain is increased by a factor three or less no problems will occur on average. 
     However, without further measures, these problems are avoided only on average. When the image has a near maximum luminosity over most of a window in which the amplification gain is increased, local doming may still occur and the EHT supply may fall too low. 
     To prevent such problems, the gain control unit  206  preferably computes, for each window in which the amplification gain is increased, a respective sum of the luminosity values defined by the RGB values. Such a sum is indicative of the per pixel average luminosity in the corresponding window and if the sum is too high, problems may be expected if the amplification is applied to that window. To prevent such problems, the gain control unit  206  compares the average for each window with a threshold value and if the average exceeds the threshold, the gain control unit  206  reduces the amplification gain for the corresponding window, so that the average luminosity in the window on the screen of the CRT does not cause doming problems. To illustrate this, in the second trace  32 , the first window  24  is seen to have a lower increased gain than the second window  22 , because the average luminosity in the first window  24  has a higher average luminosity. 
     For example, suppose the monitor  20  is capable of handling an average luminosity LM without problems. (LM is typically the luminosity which occurs when maximum RGB values are used without increased amplification gain). If the average luminosity of the RGB values in a window is A and the increased amplification gain is G then G*A should be less than LA and if A is greater than LM/G, then the gain control unit  206  will reduce G to LM/A at the highest. 
     The size of doming effects depends on the location on the screen. For example, for most CRT&#39;s doming effects are smaller close to the edges of the screen. Therefore, one may select the maximum luminosity LM as a function of the position of the window, or the positions covered by the window, dependent on the size of the acceptable doming effect for that position or those positions. 
     If the contents of the window change as a function of time, the average A is preferably computed as a temporal average averaged over a certain amount of time, typically one or more minutes, for typical CRT&#39;s  200  in which doming effects occur in a time-scale of one or more minutes. This avoids visible sudden changes in gain and prevents unnecessary changes if the average luminosity in the window is temporarily too high. Temporary excess luminosity is no problem because it takes a certain amount of time before a high luminosity leads to doming problems. 
     Thus, the areas of the image where the gain is not increased (e.g., areas with text containing letters) retain the same luminosity and remain readable, but an enhanced windows is made less luminous in its entirety, i.e., without artificial local gain variations inside that window. 
     The temporal average may be determined, for example, by averaging the average computed for one window in one video frame over one or more minutes or by low-pass filtering the average computed for one window in one video frame with a filter that has a bandwidth of one inverse minute or less. 
     Of course, the gain control unit  206  may implement the prevention of doming in one of several ways. For example, instead of adjusting the amplification gain continuously or in small steps, the gain control unit may simply switch-back the amplification gain for a window to the normal amplification gain in one step if the average luminosity is too high. This results in a very simple digital control. 
     For the prevention of problems with the high voltage supply (EHT), it is not necessary to average the luminosity of the windows separately. Instead, one may average the luminosity of the entire image with increased gain in individual windows, to determine  20  whether the average luminosity in the image is too high. If so, the gain can be reduced for individual windows in which the gain was increased. To prevent EHT problems, time-averaging of the average luminosity is performed in a time-scale that is typical for the response time of the EHT supply, typically, in the order of seconds. 
     The gain control unit  206  may combine two control mechanisms, one for preventing problems with the EHT supply which controls the average luminosity over the entire image, and one for preventing problems with doming which controls the averages for individual windows. 
     The windows in which the amplification gain is increased can be selected at will. In one version, this is done by the image sources  104   a - 104   c , for example, by application software in a PC. One such image source  104   a - 104   c  is, for example, a WEB browser which shows a WEB page containing text and sub-windows with photos or video. In this case, the browser may indicate one or more of the sub-windows with photos or video in order to increase the amplification gain. 
     Alternatively, the device driver  100  may select windows in which the gain must be increased. This can be done, for example, by determining the high spatial frequency energy-content in a window that would be lost by increasing the gain due to increased spot size in the CRT, and comparing it with the low spatial frequency energy-content (excepting zero-spatial frequency). 
     Only if the high frequency content is sufficiently small relative to the low frequency content, the image may be taken to be a photo, video or graphics instead of text and the increased amplification may be switched on. Preferably, the device driver switches on increased amplification only for windows that are larger than a certain size. This reduces the number of windows for which averages must be computed and prevents increased gain for secondary information such as logo&#39;s. 
     In this way, image sources  104   a-c  can be used that are independent of the possibility to increase the gain in selected windows. 
     The shape of the windows is not necessarily rectangular. However, for rectangular windows, the position and size of the windows can be easily communicated in the form of coordinate values of corners and/or height width values. For most other window shapes, more parameters need to be communicated, or a signal can be supplied for each pixel to identify the window to which the pixel belongs. 
     Preferably, gain control is implemented in the monitor  20 , as shown. However, it is also possible to implement it in the processing apparatus  10 , e.g., in the device driver  100 . In this case, a conventional monitor  20  is preferably augmented with a switch, to increase the gain of the RGB amplifiers. This switch is operable by the processing apparatus, for example using the MVL protocol. 
     In this version, the processing apparatus  10  decides whether increased gain is needed and if so the processing apparatus causes RGB amplifiers in the monitor  20  to switch to high gain for the entire image. The processing apparatus then reduces the RGB output strength correspondingly everywhere except in the windows where higher gain is desired. When the average luminosity in such a window is too high, the RGB output is reduced for such a window too. 
     Thus the same effect can be achieved as with the gain control unit  206 , except that at least one bit of RGB quantization resolution is lost.