Abstract:
A multi-channel video signal processing circuit with a bias control and respective video channel gain controls. The bias signal, which corresponds to a nominal video signal brightness level, is shared among the video channels which, in accordance with respective video gain control signals, process incoming component video signals to provide corresponding outgoing component video signals such that each one of the outgoing component video signals has a nominal outgoing video signal brightness level related to the bias signal and a corresponding one of the gain control signals.

Description:
This is a continuation of U.S. patent application Ser. No. 10/830,338, filed on Apr. 22, 2004 now U.S. Pat. No. 7,236,203, and entitled “Video Circuitry For Controlling Signal Gain And Reference Black Level”. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to video amplifier circuitry for controlling a signal to be displayed on a display device such as a cathode ray tube (CRT), and in particular, to video circuitry for controlling signal gain and reference block level of a video signal. 
   2. Description of the Related Art 
   Video display devices are used for many purposes, including video monitors for displaying video images generated by computers and televisions for displaying animated or live action video images such as those received over cable or broadcast systems. One of the more common types of video display devices uses a cathode ray tube (CRT) to display the video image information. As is well known, the CRT includes three primary color cathode ray guns which are manipulated to converge on a screen and produce a color image. The three ray guns produce converged scanning rasters having red, green and blue fields which combine to produce all colors from black through white. For manufacturers of such display devices, one important requirement is that of establishing and maintaining color balance by appropriately balancing the signals driving the red, green and blue (RGB) cathodes of the CRT. This is generally quite difficult since adjusting independent video gain stages for each channel (red, green and blue) often conflicts with establishing and maintaining proper reference black level, or brightness, adjustment for each channel. 
   SUMMARY OF THE INVENTION 
   In accordance with the presently claimed invention, a multi-channel video signal processing circuit is provided with a bias control and respective video channel gain controls. The bias signal, which corresponds to a nominal video signal brightness level, is shared among the video channels which, in accordance with respective video gain control signals, process incoming component video signals to provide corresponding outgoing component video signals such that each one of the outgoing component video signals has a nominal outgoing video signal brightness level related to the bias signal and a corresponding one of the gain control signals. 
   In accordance with one embodiment of the presently claimed invention, a multi-channel video signal processing circuit with a bias control and respective video channel gain controls includes bias control circuitry and video signal amplification circuitry. The bias control circuitry is responsive to a bias control signal by providing a bias signal corresponding to a nominal video signal brightness level. The video signal amplification circuitry is coupled to the bias control circuitry and responsive to the bias signal, a plurality of gain control signals and a plurality of incoming component video signals by providing a corresponding plurality of outgoing component video signals. Ratios of corresponding ones of the pluralities of outgoing and incoming component video signals have respective values related to corresponding ones of the plurality of gain control signals, and each one of the plurality of outgoing component video signals has a nominal outgoing video signal brightness level related to the bias signal and a corresponding one of the plurality of gain control signals. 
   In accordance with another embodiment of the presently claimed invention, a multi-channel video signal processing circuit with a bias control and respective video channel gain controls includes bias controller means and video signal amplifier means. The bias controller means is for receiving a bias control signal and in response thereto generating a bias signal corresponding to a nominal video signal brightness level. The video signal amplifier means is for receiving the bias signal, a plurality of gain control signals and a plurality of incoming component video signals and in response thereto generating a corresponding plurality of outgoing component video signals. Ratios of corresponding ones of the pluralities of outgoing and incoming component video signals have respective values related to corresponding ones of the plurality of gain control signals, and each one of the plurality of outgoing component video signals has a nominal outgoing video signal brightness level related to the bias signal and a corresponding one of the plurality of gain control signals. 
   In accordance with still another embodiment of the presently claimed invention, a method for processing a plurality of video signals with a bias control and respective video channel gain controls includes: 
   receiving a bias control signal and in response thereto generating a bias signal corresponding to a nominal video signal brightness level; and 
   receiving the bias signal, a plurality of gain control signals and a plurality of incoming component video signals and in response thereto generating a corresponding plurality of outgoing component video signals; 
   wherein
         ratios of corresponding ones of the pluralities of outgoing and incoming component video signals have respective values related to corresponding ones of the plurality of gain control signals, and   each one of the plurality of outgoing component video signals has a nominal outgoing video signal brightness level related to the bias signal and a corresponding one of the plurality of gain control signals.       

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a functional block diagram of a portion of a video system for driving a video display device containing video circuitry in accordance with one embodiment of the presently claimed invention. 
       FIG. 2  is a functional block diagram of a voltage reference generator in accordance with one embodiment of the presently claimed invention. 
       FIG. 3  illustrates the video gain and brightness control adjustment capabilities of the circuit of  FIG. 1 . 
       FIG. 4  further illustrates such video gain and brightness control adjustment capabilities. 
       FIG. 5  illustrates the effects of such video gain and brightness control adjustments for the white and black levels of a video signal. 
   

   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. 
   Referring to  FIG. 1 , a portion  100  of a video system including video circuitry for controlling signal gain and reference black level of a video signal in accordance with one embodiment of the presently claimed invention can be described as follows. (It should be understood that this portion  100  represents one video channel of a typical system, with three such channels being used for RGB control.) This subsystem  100  includes a clamp circuit  710 , contrast control circuit  712 , auto beam limit circuit  714 , an on-screen-display (OSD) data source  706 , a switch or multiplexor  716 , a video gain circuit  718 , another switch or multiplexor  728 , output offset circuitry  732 , and a voltage reference source  600 , all connected substantially as shown. As indicated, the clamp circuit  710 , contrast control circuit  712 , auto beam limit circuit  714 , OSD data source  706 , video gain circuit  718  and output offset circuitry  732  are biased by a reference voltage  601  provided by the voltage reference generator circuit  600  (discussed in more detail below). 
   An incoming video signal Vin is AC-coupled with a coupling capacitor Cin to provide an AC-coupled signal to the clamp circuit  710 . The DC-clamped signal  711  is then processed by the contrast control circuit  712  in accordance with contrast control signals (not shown) and the reference voltage  601  to establish the contrast for the incoming video signal. The contrast-controlled signal  713  is then processed by the auto beam limit circuit  714  (various forms of which are well known in the art) in accordance with an ABL control signal (not shown) and the reference voltage  601 . 
   The resulting video signal  715  is then selectively combined in the switch or multiplexor  716  with OSD data  707 . The resulting signal  717  is controlled with respect to signal gain by the video gain circuit  718  in accordance with the reference voltage  601  and a control signal  501  (discussed in more detail below). The resulting gain-controlled signal  719 , which has now been DC-clamped, controlled for video contrast, controlled for beam signal strength, selectively combined with OSD data, and controlled for video gain, is then selectively combined with a blanking signal in the switch or multiplexor  728 . The resulting signal  729  then has a DC offset voltage added to it within the offset circuitry  732  to produce the final video output signal  731 . 
   The output offset circuitry  732  selectively introduces a DC offset voltage by establishing a reference offset voltage  709  at one input of the amplifier  734 . This voltage is produced as a combination of the reference voltage  601  summing with the voltage generated in the voltage divider circuit composed of resistors R 1  and R 2  depending upon the value of the current  705  provided by the current source circuit  704  (e.g., a current digital-to-analog converter) as either a source current  705   a  or sinking current  705   b.    
   A more detailed discussion of this video signal path can be found in commonly assigned, co-pending U.S. patent application Ser. No. 09/698,739, entitled “Multiplexor Video Signal Interface Signal System and Method”, the disclosure of which is incorporated herein by reference. 
   As can be seen in  FIG. 1 , the signal gain of the video signal is controlled by a control signal  501 . This same control signal  501  also controls the voltage reference generator  600  which provides the common, or shared, reference voltage  601  used to establish and maintain the reference black level of the video signal as it is processed by the various stages. Accordingly, when the video signal gain or the reference black level for the subject video signal is adjusted, a corresponding adjustment is made for the reference black level or video signal gain, respectively, thereby providing mutual tracking for such signal characteristics. 
   Referring to  FIG. 2 , a preferred embodiment of the voltage reference generator  600  for use in the circuit of  FIG. 1  includes voltage source circuitry and voltage conversion circuitry. One embodiment of the voltage source circuitry includes a current DAC  602  and a current-to-voltage converter  606 . In the current DAC  602 , a source current I 1  provided by a current source  604  is converted to an output current  603  in accordance with a digital control signal  503 . This output current  603 , variable (e.g., proportional) in accordance with the control signal  503 , is summed with a sinking current I 2  provided by another current source  606  to produce a net current for conversion by the current-to-voltage converter  606 . 
   The resulting voltage  607  is used as an input voltage for the voltage conversion circuitry, which in this example embodiment, is implemented using a voltage magnitude control circuit  608  which provides the analog reference voltage Vref  601  based upon the input voltage  607  in accordance with the value of the digital control signal  501  (e.g., proportional) which is also used to control the video gain, as discussed above. The reference voltage  505 , preferably established by a stable voltage source, such as a bandgap voltage source (many types of which are well known in the art), also provides the voltage reference for the current-to-voltage converter  606 . The net result is that the reference voltage  601  has a nominal voltage level equal to the reference voltage  505  which can be adjusted upward (more positive) or downward (more negative) in accordance with the binary value of the digital control signal  501  (discussed in more detail below). 
   This magnitude control circuit  608  used to establish the reference voltage  601  is preferably the same type of circuitry as that used for the video gain controller  718 . A more detailed discussion of this type of magnitude control circuit can be found in U.S. Pat. No. 6,166,579, entitled “Digitally Controlled Signal Magnitude Control Circuit”, the disclosure of which is incorporated herein by reference. 
   Referring to  FIG. 3 , the adjustment and tracking of the video signal gain and reference black level can be better understood. As discussed above, the nominal reference black level is equal to the internal reference voltage  505 , e.g., 1.2 volts for a bandgap reference voltage. For one embodiment of the circuitry used for the magnitude control circuit  608  and video gain controller  718 , the control signal  501  is a 7-bit signal and equal voltage adjustments of 400 millivolts more positive and more negative are available, thereby establishing a reference black level within the range of 0.8 volts through 1.6 volts over the full range of the digital control signal  501 . Box  301  identifies the full range of adjustment for the video gain controller  718 , which for this example of a 7-bit control signal  501 , provides a 10 dB gain adjustment. Box  302  identifies the full range of the 7-bit control signal  501  for controlling the reference black level over the range of 0.8 volts through 1.6 volts. This 800 millivolt adjustment range translates to a 42-volt (+/−21 volts) adjustment range at the cathodes of a typical CRT (not shown). 
   Referring to  FIG. 4 , the effect of the output offset circuitry  732  can be better understood. As illustrated, additional output offset voltages in the range of −0.7 volt through +0.2 volt is available to shift the nominal center of the output video signal Vout  731 . 
   Referring to  FIG. 5 , the net effect for the white and black levels of the output video signal Vout is illustrated. 
   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.