Abstract:
Video signal control circuitry for use in a video display system in which a variation in a brightness level of a video display signal causes a corresponding variation in a beam current signal, wherein such video signal control circuitry maintains a controllable video display signal brightness level at a substantially constant average value notwithstanding a variation in the incoming video signal brightness level.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to control circuits for video display signals, and in particular, to circuits for controlling the brightness level of a video display signal. 
   2. Description of the Related Art 
   Circuits and techniques for controlling, e.g., limiting, the scanning beam current for a video display are well known in the art. These are particularly important for cathode ray tube (CRT) displays which typically require automatic brightness (or beam) limiter (ABL) circuits to ensure that the average CRT anode current does not exceed a predetermined value. This is important to not only maintain a desired brightness level for the display, but also to not damage or significantly reduce the useful life of the display. 
   SUMMARY OF THE INVENTION 
   In accordance with the presently claimed invention, video signal control circuitry is provided for use in a video display system in which a variation in a brightness level of a video display signal causes a corresponding variation in a beam current signal, wherein such video signal control circuitry maintains a controllable video display signal brightness level at a substantially constant average value notwithstanding a variation in the incoming video signal brightness level. 
   In accordance with one embodiment of the presently claimed invention, video signal control circuitry for use in a video display system in which a variation in a brightness level of a video display signal causes a corresponding variation in a beam current signal includes a control electrode to convey the beam current signal, control circuitry and amplifier circuitry. The control circuitry is coupled to the control electrode and responsive to the beam current signal and a brightness control signal by providing a gain control signal, wherein the gain control signal has a nominal value determined substantially by the brightness control signal and is variable about the nominal value in substantial relation to the beam current signal. The amplifier circuitry is coupled to the control circuitry and responsive to the gain control signal and an input video signal by providing the video display signal, wherein a ratio of the video display signal and the input video signal has a value determined substantially by the gain control signal. A substantially constant average video display signal brightness level is maintained notwithstanding a variation in a brightness level of the input video signal. 
   In accordance with one embodiment of the presently claimed invention, video signal control circuitry for use in a video display system in which a variation in a brightness level of a video display signal causes a corresponding variation in a beam current signal includes a control electrode to convey the beam current signal, control circuitry and amplifier circuitry. The control circuitry is coupled to the control electrode and responsive to the beam current signal by providing a gain control signal, wherein the gain control signal is variable in substantial relation to the beam current signal. The amplifier circuitry is coupled to the control circuitry and responsive to a brightness control signal, the gain control signal and an input video signal by providing the video display signal, wherein a ratio of the video display signal and the input video signal has a value determined substantially by the brightness and gain control signals. A substantially constant average video display signal brightness level is maintained notwithstanding a variation in a brightness level of the input video signal. 
   In accordance with still another embodiment of the presently claimed invention, video signal control circuitry for use in a video display system in which a variation in a brightness level of a video display signal causes a corresponding variation in a beam current signal includes controller means and amplifier means. The controller means is for receiving the beam current signal and a brightness control signal and in response thereto providing a gain control signal, wherein the gain control signal has a nominal value determined substantially by the brightness control signal and is variable about the nominal value in substantial relation to the beam current signal. The amplifier means is for receiving the gain control signal and an input video signal and in response thereto providing the video display signal, wherein a ratio of the video display signal and the input video signal has a value determined substantially by the gain control signal. A substantially constant average video display signal brightness level is maintained notwithstanding a variation in a brightness level of the input video signal. 
   In accordance with still another embodiment of the presently claimed invention, video signal control circuitry for use in a video display system in which a variation in a brightness level of a video display signal causes a corresponding variation in a beam current signal includes controller means and amplifier means. The controller means is for receiving the beam current signal and in response thereto providing a gain control signal, wherein the gain control signal is variable in substantial relation to the beam current signal. The amplifier means is for receiving a brightness control signal, the gain control signal and an input video signal and in response thereto providing the video display signal, wherein a ratio of the video display signal and the input video signal has a value determined substantially by the brightness and gain control signals A substantially constant average video display signal brightness level is maintained notwithstanding a variation in a brightness level of the input video signal. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a functional block diagram depicting a video display system in which video signal control circuitry in accordance with one embodiment of the presently claimed invention is used for brightness control. 
       FIG. 2  depicts how video signal control circuitry in accordance with the presently claimed invention can be used to dynamically control brightness within a video display image. 
       FIG. 3  illustrates a typical video signal including the horizontal and vertical blanking intervals. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The following detailed description is of example embodiments of the presently claimed invention with references to the accompanying drawings. Such description is intended to be illustrative and not limiting with respect to the scope of the present invention. Such embodiments are described in sufficient detail to enable one of ordinary skill in the art to practice the subject invention, and it will be understood that other embodiments may be practiced with some variations without departing from the spirit or scope of the subject invention. 
   Throughout the present disclosure, absent a clear indication to the contrary from the context, it will be understood that individual circuit elements as described may be singular or plural in number. For example, the terms “circuit” and “circuitry” may include either a single component or a plurality of components, which are either active and/or passive and are connected or otherwise coupled together (e.g., as one or more integrated circuit chips) to provide the described function. Additionally, the term “signal” may refer to one or more currents, one or more voltages, or a data signal. Within the drawings, like or related elements will have like or related alpha, numeric or alphanumeric designators. Further, while the present invention has been discussed in the context of implementations using discrete electronic circuitry (preferably in the form of one or more integrated circuit chips), the functions of any part of such circuitry may alternatively be implemented using one or more appropriately programmed processors, depending upon the signal frequencies or data rates to be processed. 
   Referring to  FIG. 1 , a portion of a video display system implementing brightness control in accordance with the presently claimed invention includes video signal control circuitry  10  coupled to and interacting with a video display (e.g., CRT)  40  and its EHT (electrical high tension) power supply  30 , interconnected substantially as shown. The video signal control circuitry  10  includes control circuitry and amplifier circuitry as follows. Connected between the power supply Vs and the circuit reference (e.g., circuit ground) is a lowpass filter in the form of a serially connected resistor R 1  and capacitor C 1 . These components R 1 , C 1  lowpass filter the current Ir 1  through the resistor R 1 , thereby producing a lowpass filtered voltage Vc 1  across the capacitor C 1 . Also connected to the node common to the resistor R 1  and capacitor C 1  is the EHT supply  30 , a current source  14 , and the gain control terminal for a video amplifier circuit  12 . A beam current Iab 1  flows into the EHT supply  30 , and has a magnitude dependent upon the amount of supply current Ia (e.g., anode current) flowing to the display  40 . In other words, an increase in the brightness of the video image being displayed by the display  40  causes increased supply current Ia to flow, thereby causing a corresponding increase in the beam current Iab 1 . 
   The current source  14  is controllable by a control signal  17 , and is preferably a current DAC, i.e., a digital-to-analog converter that converts a digital control signal  17  to an analog current Idac. This digital signal  17  is provided by one or more data registers  16 . The original control data  11   b  stored in such registers  16  can be provided in a static manner for a constant brightness level, or can alternatively be provided in a dynamic manner for dynamic brightness control (discussed in more detail below). Such a current source circuit  14  is well-known in the art and can be implemented in any of a number of well-known ways. 
   The incoming video signal  11   v , with its constituent average as well as instantaneous brightness levels, is buffered or amplified by the video amplifier circuit  12  to provide the video display signal  13  for the display  40 . The gain A of the video amplifier circuit  12  is controlled with a gain control signal in the form of a control voltage Vab 1  which appears across the current source circuit  14  and would be expected to be equal to the voltage Vc 1  across the capacitor C 1 . In one embodiment, increases and decreases in this voltage Vab 1  cause the amplifier gain A to increase and decrease, respectively, thereby producing corresponding increased and decreased brightness levels in the video display signal  13 . 
   Operation of the control circuit  10  in conjunction with the EHT supply  30  and display  40  is in conformance with convention negative feedback circuit principles and can be described as follows. As the peak-to-peak amplitude of the incoming video signal  11   v  increases, i.e., as the brightness level of the video signal Vin  11   v  increases, so does the amplitude and brightness level of the video display signal Vout  13 . Accordingly, the image displayed by the display  40  becomes brighter, thereby causing an increased supply current Ia to flow and, in turn, an increased beam current Iab 1 . This increased beam current Iab 1  causes the current Ir 1  through the resistor R 1  to increase, thereby increasing the voltage drop Vr 1  across the resistor R 1 . As a result, the control voltage Vab 1  decreases, thereby causing the signal gain A of the video amplifier  12  to decrease. As a result, the video display signal  13  also decreases in amplitude, as does its brightness level. This, in turn, reduces the supply current Ia and beam current Iab 1  until all signals converge to a final value. (It will be readily understood by one of ordinary skill in the art that the resistance and capacitance values of the resistor R 1  and capacitor C 1  can be appropriately selected so as to cause this feedback operation to operate as an appropriate automatic gain control (AGC) for the average brightness level of the video display signal  13 .) Operation of this control circuit  10  when the incoming video signal  11   v  decreases in amplitude and brightness level is similar, but with decreases and increases in the respective signal levels instead of increases and decreases, respectively. 
   Referring to  FIG. 2 , controlling the amount of current Idac being sunk by the current source circuit  14 , in accordance with its control signal data  17 , the nominal (e.g., average) brightness of the video display signal  13 , about which any AGC-related variations occur, can be selectively controlled. For example, it may be desirable to “highlight” a portion  44  of the overall video image  42  being displayed by the display  40 , i.e., display such image portion  44  at an increased or decreased brightness level. With the control circuit  10  of  FIG. 1 , such brightness control is easily achieved. For example, if it is desired to highlight a rectangular region as defined by vertices A, B, C and D, the appropriate control data  17  can be provided to the current source  14  during the time intervals corresponding to the scanning of such region  44 . By increasing or decreasing the shunted current Idac, the average brightness level of the image being displayed will be decreased or increased, respectively. Alternatively, irregularly shaped regions can also be highlighted, such as the polygon defined by vertices A, B, E, F, G and D. 
   As noted above, the control data  17  (e.g., initially stored in the registers according to input control data  11   b ) can be provided a priori based upon the desired region to be highlighted. Alternatively, such control data  11   b  can be retrieved dynamically from the incoming video signal  11   v . For example, referring to  FIG. 3 , if the incoming video signal  11   v  has been encoded with the appropriate brightness control data, e.g., within a vertical blanking interval  54  or a horizontal blanking interval  52  (between the active video intervals  50 , as well known in the art), such brightness control data can be retrieved and applied to the current source  14  as the corresponding active video regions  50  are processed by the amplifier circuitry  12 . 
   In accordance with one implementation of one embodiment of the presently claimed invention, the resistance value of the resistor R 1  was selected such that the beam current Iab 1  is approximate 2 mA when the shunted current Idac is zero, i.e., for maximum brightness level of the displayed image, thereby resulting in the maximum beam current Iab 1 . During normal modes of operation, i.e., less than maximum brightness levels, the shunted current Idac can be adjusted to vary the onset of beam limiting as discussed above. For example, with reference to the following table, for a 4-bit control signal  17 , example values of the shunted current Idac can be as shown: 
                                           C[3]   C[2]   C[1]   C[0]   Current Sunk to GND                   0   0   0   0   0 μA (highlight mode)       0   0   0   1     80 μA       0   0   1   0    160 μA       0   0   1   1    240 μA       0   1   0   0    320 μA       0   1   0   1    400 μA       0   1   1   0    480 μA       0   1   1   1    560 μA       1   0   0   0    640 μA       1   0   0   1    720 μA       1   0   1   0    800 μA       1   0   1   1    880 μA       1   1   0   0    960 μA       1   1   0   1   1.04 mA       1   1   1   0   1.12 mA       1   1   1   1   1.20 mA                    
Various other modifications and alternations in the structure and method of operation of this invention will be apparent to those skilled in the art without departing from the scope and the spirit of the invention. Although the invention has been described in connection with specific preferred embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments. It is intended that the following claims define the scope of the present invention and that structures and methods within the scope of these claims and their equivalents be covered thereby.