Abstract:
A cathode current limiting circuit and method for use with projection color television display systems to prevent overheating and possible cracking of the blue cathode ray tube (CRT) when a continuous blue background is presented. The circuit monitors differences between the respective low level red, green, blue (RGB) signals sent from the video signal processor to the CRT drive which produces the respective RGB cathode or beam currents. When the low level blue signal exceeds both of the red and the green signals by a predetermined differential, the circuit sends a feedback signal to the video signal processor to limit the blue signal and thus the cathode current from the CRT drive.

Description:
FIELD OF INVENTION  
         [0001]    The present invention is directed generally towards cathode current limiting circuits for video displays, and more particularly towards a cathode current limiting circuit for projection television display systems which monitors differences between low level red, green and blue signals to provide cathode current limiting feedback to the video signal processor.  
         BACKGROUND OF THE INVENTION  
         [0002]    A projection television display system includes three individual cathode ray tubes (CRTs), each having a different screen phosphor corresponding to one of red, green, and blue (RGB) colors. Each CRT emits a respective RGB light image responsive to a received television signal of the system, which light images are reflected from a mirror and merged onto the rear of an opaque screen for viewing as a single full color image. High level RGB video drive signals are coupled to respective cathodes of the electron guns of the CRTs from respective drive amplifiers or “drivers” which amplify the respective low level RGB video signals produced by a video signal processor. In response to the respective video drive signals, the CRTs generate electron beams at a cathode or beam current, which beams strike the respective screen phosphors within the CRT causing them to emit light outputs of particular wavelengths corresponding to the respective color emitted by the CRT. The light outputs produced by the CRTs are transmitted by an optical system to a screen where a composite color image is reproduced.  
           [0003]    In such projection television display systems, having the three separate cathode CRTs for producing RGB light, certain signals such as a high level flat field in an individual primary color can result in that respective CRT providing much more than its nominal share of light Unfortunately, many video accessory devices such as video games, cable television, etc. provide a blue flat field when no program content is being provided. While it is common practice to sense and limit the total beam current under such flat field conditions, one CRT may draw most if not all the beam current ordinarily allocated for all three CRTs. The CRT may overheat and fracture if allowed to remain displayed continuously.  
           [0004]    Typically, the share of the total cathode current drawn by all of the CRTs relative to that drawn by each individual CRT is determined by directly measuring the cathode ray current of each individual CRT. The overall cathode current is then controlled by feedback to control the gains of the RGB video signals thus limiting the maximum average cathode current in an individual electron gun of the respective CRT to a desired level. For example, the Philips/Magnavox Model GR-9D projection television display system directly measures the cathode current of the blue CRT. The gains of the RGB video signals are controlled in a feedback loop to limit the average blue cathode current to a safe level. However, the system requires the use of costly high voltage transistors as current sampling devices to directly sample the individual cathode currents, and furthermore introduces additional circuit complexity if the current sampling devices must also serve in an AKB stabilization loop.  
         SUMMARY  
         [0005]    In accordance with the present invention there is provided a cathode current limiting circuit and method of use with a projection color television display system. The display system is generally of the type having a video signal processor which feeds respective low level red, green, blue (RGB) signals to a cathode ray tube (CRT) drive which amplifies the low level RGB signals to high level RGB signals which drive separate RGB cathode ray tubes producing respective RGB beam currents. The circuit comprises a sensing, i.e., switching, device having inputs for receiving the respective low level RGB signals being fed from the video signal processor to the CRT drive. The sensing device is adapted to monitor differences between the low level RGB signals and to produce an output signal at an output thereof when one of the RGB signals exceeds each of the other individual RGB signals. The output signal provides feedback to the video signal processor to adjust the low level RGB signals to limit the cathode beam current of the CRT having the highest level RGB signal.  
           [0006]    The method of limiting the cathode current comprises the steps of 1) providing a cathode current limiting circuit of the type described above; 2) monitoring differences between the respective low level RGB signals being fed from the video signal processor to the CRT drive using the cathode current limiting circuit; and 3) producing an output signal using the cathode current limiting circuit when one of the RGB signals exceeds each of the other individual RGB signals, which signal provides feedback to the video processor to adjust the low level RGB signals to limit the cathode beam current of the CRT having the highest level RGB signal. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    A more complete understanding of the present invention may be obtained from consideration of the following description in conjunction with the drawings in which:  
         [0008]    [0008]FIG. 1 shows an exemplary embodiment of a cathode current limiting circuit according to the present invention for providing feedback in a projection television display system. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0009]    Referring to FIG. 1, therein is shown an exemplary embodiment of a cathode current limiting circuit  20  according to the present invention in a projection television color video system  23 . The cathode current limiting circuit  20  monitors differences between the low level red, green, and blue signals Sr, Sg, and Sb from a video signal processor  26  to respective red, green, and blue drivers  29 ,  32 , and  35  of a CRT drive  38  of the video system  23 . The limiting circuit  23  provides cathode current limiting feedback to the video signal processor  26  which limits the RGB cathode currents of the respective RGB drivers  29 ,  32 , and  35  delivered to respective red, green, and blue CRTs  41 ,  44 , and  47  of the video system  23 .  
         [0010]    In the exemplary embodiment shown, the separate RGB low level video signals, measured as respective RGB voltages Vr, Vg and Vb, are connected from the video signal processor  26  to the individual CRT drivers  29 ,  32 , and  35  of the CRT drive  38  for amplification to a cathode current sufficient to drive the respective CRTs  41 ,  44 , and  47 . The green and red video signals are also fed through respective diodes  50  and  53  to be applied to the base of a PNP transistor  56 . The base of the transistor  56  is also connected to a signal ground  59  through a resistor  62 . The blue video signal is fed to the emitter of the transistor  56  through a parallel diode  65  and resistor  68 , which are arranged in series with a resistor  71 . The collector of the transistor  56  is connected to the base of an NPN transistor  74 , and to a resistor  77  and  80  arranged in parallel connected to ground  59 . The emitter of the transistor  74  is connected intermediate a pair of resistors  83  and  86  arranged in series interconnecting a respective positive (+) and negative (−) bias voltage in the form of a battery  89 . The positive (+) output of the battery  89  is connected through the resistor  83  to the emitter of the transistor  56 , which is connected through the resistor  86  along with the negative output (−) of the battery  89  to ground  59 . The collector of the transistor  74  is connected to the video signal processor  26  as a feedback signal Sfb, which can also be input to one or more other feedback signal receiving devices  92 .  
         [0011]    The cathode current limiting circuit  20  monitors the differences between the low level RGB video signals as RGB voltages Vr, Vg and Vb, and predicts when the cathode current of one of the three CRTs, the blue CRT  47  in the illustrated embodiment, can be expected to significantly exceed more than its nominal one-third share of the total cathode current A feedback signal Sfb in the form of a feedback voltage Vfb generates a feedback current to the video processor  26  to signal reduce the voltage drive of the low level RGB video signals Vb, Vg and Vb that the appropriate high level RGB cathode current is delivered to the respective cathode guns of the RGB CRTs  41 ,  44 , and  47 . This limits the respective RGB beam currents to an acceptable level so as to not overheat the cathode gun or cause emission of x-rays by exceeding the x-ray threshold beam current level.  
         [0012]    The cathode current limiting circuit  20  operates as follows. The low level RGB input voltages Vr, Vg and Vb each have nominally the same direct current (DC) and black/white (B-W) levels. The red and green input voltages Vr and Vg are applied to the base of the transistor  56  through the diodes  50  and  53 , which prevent the reverse flow of current to the lesser of Vr and Vg. The blue input voltage Vb is applied to the emitter of the transistor  56 , which is initially nonconductive, therefore there being no appreciable voltage drop across the diode  65 , and the resistors  68  and  71  since there is no appreciable current flow therethrough. The non-linear resistor-diode network  65 ,  68  and  71  coupled to the emitter of the PNP transistor  56  renders the collector current of transistor  56  more accurately representative of the blue cathode current than does use of a single resistor. The final cathode current is exponentially related to the cathode drive voltage by the equation I cathode =k(V drive ) 2.5 .  
         [0013]    Transistor  56  does not conduct until the level of the blue input voltage Vb is greater than each of the red and the green input voltages Vr and Vg, at which time the transistor  56  conducts to supply an output voltage Vo at the collector thereof (i.e. Vo=voltage across the resistor  77  and the capacitor  80 , the opposite connections of each being grounded). The resistor  77  and the capacitor  80  function as an integrator which renders the output voltage Vo at the collector proportional to the average level by which the blue input voltage Vb exceeds the greater of the green and red input voltage Vg or Vr. Therefore with the input voltages Vr, Vg and Vb representing the average low level input signals or average voltage drive levels for the red, green, and blue colors: 
           Vo=k ( Vb−Vg )  for Vb&gt;Vg&gt;Vr   
           Vo=k ( Vb−Vr )  for Vb&gt;Vr&gt;Vg   
           Vo= 0  for Vb≦Vg and Vb≦Vr   
         [0014]    The constant k is the proportion to the average level by which the blue input voltage Vb exceeds the greater of the green or red input voltage Vg or Vr, and is primarily controlled by the resistance value of the resistor  77 , with the averaging function being primarily controlled by the capacitance of the capacitor  80 . The output voltage Vo is maximum when the video is displaying a one-hundred-percent blue field. The emitter of the transistor  56  is negative-biased by the battery  89  through a voltage splitter formed by the resistors  83  and  86 . When the output voltage Vo at the base of the transistor  74  versus the voltage at its emitter begins to exceed the turn-on for the transistor  74 , the feedback signal Sfb in the form of a feedback current driven by a feedback voltage Vfb at the collector of the transistor  74  is fed back to the video processor  26  to limit the levels of the low level RGB input voltage Vr, Vb and Vb to the respective RGB drivers  29 ,  32 , and  35  of the CRT drive  38  to have an acceptable high level output signal Sor, Sog and Sob to the respective red, green, and blue CRTs  41 ,  44 , and  47 . The feedback signal Sfb may be shared with additional drive limiting devices responsive to the total average beam current, such as the feedback receiving device  92 . It is noteworthy that the cathode current limiting circuit  20  is insensitive to any common mode voltage level associated with the RGB voltages Vr, Vg and Vb, being sensitive only to differential voltage levels therebetween. Therefore, the cathode current limiting circuit  20  is readily adaptable to various projection television color video systems  23  wherein the respective red, green, and blue voltages Vr, Vg and Vb are different since only the differential voltage levels are measured. Further adaptability and flexibility can be attained such as by making resistor  77  a variable resistor to change the constant k, and by replacing the source of a fixed biasing voltage such as the battery  89  with a variable source of biasing voltage to permit changing the point at which the feedback signal Sfb is sent from the collector of the transistor  74  back to the video signal processor  26 .  
         [0015]    Numerous modifications to and alternative embodiments of the present invention will be apparent to those skilled in the art in view of the foregoing description. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the best mode of carrying out the invention. Details of the structure may be varied substantially without departing from the spirit of the invention and the exclusive use of all modifications, which come within the scope of the appended claims, is reserved.