Abstract:
A method and system are provided for balance control for component video signals. The system and method of the present disclosure allow for adjusting or compensating for possible color encoding errors, as well as allowing individual viewer preferences to be accommodated, in consumer electronics devices, such as high definition monitors and other imaging devices and appliances. The system includes circuitry for receiving the U and V component video signals and outputting balance adjusted component color difference signals Uout and Vout.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present disclosure relates generally to balance control, and more particularly, to a method and system for balance control, as opposed to tint control, for component video signals. 
     2. Background of the Related Art 
     Common practice when decoding NTSC signals is for a “tint” or “hue” adjustment to compensate for possible differential phase errors between a QAM chroma signal and its accompanying burst reference. Tint adjustment is also useful in compensating for other possible system errors. 
     Additionally, tint adjustment together with saturation adjustment allows for variations in personal preference to be accommodated, permitting any color to be shifted in saturation and hue without affecting the gray scale. In order to retain this capability for video signals that do not experience NTSC decoding, such as component video signals, e.g., from a DVD player or set-top box, it is generally necessary to apply tint adjustment to the baseband color difference signals. 
     The three main component video signals as known in the art are Y, B−Y and R−Y. Derived from the Y, B−Y and R−Y component video signals, Y/U/V and Y/Pb/Pr are defined as follows: U=(B−Y)/2.03, V=(R−Y)/1.14, Pb=(B−Y)/1.772 and Pr=(R−Y)/1.402. U and V amplitude scaling is applied in encoding an NTSC signal to prevent RF overmodulation as known in the art. Pb and Pr amplitude scaling is applied to parallel component video signals to make each signal approximately equal to 0.7 Volt p-p. 
     Prior art tint control circuitry, as shown by  FIG. 1 , derive the bi-directional crosstalk components, i.e., +/−kU for coupling into V and −/+kV for coupling into U, necessary for tint control. The opposite polarities associated with the bi-directional crosstalk components are significant, since they are equivalent to the effect of a tint control in NTSC decoding which is derived as follows: 
     As known in the art, an NTSC chroma signal may be represented as follows: C(t)=[V cos ωt+U sin ωt], ω=2πfsc. Therefore, demodulating V and U with 2 cos(ωt±φ) and 2 sin(ωt±φ), respectively, and tint range being ±φ, the bi-directional crosstalk components can be derived.
 
 V   demodulation   =[V  cos(ωt)+ U  sin(ωt)]2 cos(ωt±φ)=2 V  cos(ωτ)cos(ωt±φ)+2 U  sin(ωt)cos(ωt±φ)
 
= V [cos(2ωt±φ)+cos(±φ)]+ U [sin(2ωt±φ)−sin(ωφ)]
 
= V  cos(±φ)− U  sin(ωφ), disregarding 2ωt terms
 
= V−U  sin(ωφ), for small values of φ
 
= V−kU, k= sin φ
 
 U   demodulation   =[V  cos(ωt)+ U  sin(ωt)]2 sin(ωt±φ)=2 V  cos(ωτ)sin(ωt±φ)+2 U  sin(ωt)sin(ωt±φ)
 
= V [sin(2ωt±φ)+sin(±φ)]− U [cos(2ωt±φ)−cos(±φ)]
 
= U  cos(±φ)+ V  sin(±φ), disregarding 2ωt terms
 
= U+V  sin(±φ), for small values of φ
 
= U+kV, k= sin φ
 
     As evident from  FIG. 1 , prior art tint control circuitry for consumer electronics devices, such as high definition monitors and other imaging devices and appliances, in order to obtain the bi-directional crosstalk components +/−kU and +/−kV is generally complex. Further, the prior art tint control circuitry as shown by  FIG. 1  employs two modulator/demodulator ICs, i.e., the MC1496 balanced modulator/demodulator ICs which contains eight transistors, and peripheral circuitry, which add to the cost of the high definition monitors and other imaging devices and appliances. Further still, the prior art tint control circuitry as shown by FIG.  1  and most other prior art tint control circuits introduce a variable amount of bi-directional, opposite polarity crosstalk between the component video channels. 
     A need therefore exists for an alternative method and system which provide a less complex and lower cost approach for adjusting or compensating for possible color encoding errors, as well as allowing individual viewer preferences to be accommodated. A method and system are also needed which do not introduce a variable amount of bi-directional, opposite polarity crosstalk between the component video channels. 
     SUMMARY OF THE INVENTION 
     The present disclosure provides a method and system for balance control, as opposed to tint control, for component video signals. The system and method of the present disclosure provide significant less complex circuitry and a less complex approach, respectively, as compared to prior art systems and methods, for adjusting or compensating for possible color encoding errors, as well as allowing individual viewer preferences to be accommodated, in consumer electronics devices, such as high definition monitors and other imaging devices and appliances. Additionally, the balance control system of the present disclosure is less expensive than typical prior art tint control circuits and systems. 
     The system of the present invention includes circuitry for receiving the Uin and Vin component video signals and outputting balance adjusted component color difference signals Uout and Vout. Specifically, the balance control system of the present invention for color balancing component video signals includes a first input for receiving a first component video signal; a second input for receiving a second component video signal; circuitry including a first differential amplifier and a second differential amplifier for receiving the first and second component video signals from the first and second inputs, respectively, and a variable voltage source; a first output connected to the circuitry for outputting a first color balanced signal for the first component video signal; and a second output connected to the circuitry for outputting a second color balanced signal for the second component video signal. The gain of the first and second color balanced output signals is dependent on the voltage of the variable voltage source. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is further explained by way of example and with reference to the accompanying drawings, wherein: 
         FIG. 1  is a schematic diagram of prior art circuitry for deriving the bi-directional crosstalk components necessary for tint control; and 
         FIG. 2  is a schematic diagram of circuitry for receiving the Uin and Vin component video signals and outputting balance adjusted component color difference signals Uout and Vout according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 2  is a schematic diagram of a system having circuitry for receiving the U and V component video signals and outputting balance adjusted component color difference signals Uout and Vout according to the present invention. The system is designated generally by reference numeral  100  and includes circuitry  110  for receiving video component signals Uin and Vin. The circuitry  110  outputs balance adjusted component color difference signals Uout and Vout, where Uout equals 2 kUin and Vout equal 2(1−k)Vin, and where k is a constant greater than or equal to zero and less than or equal to one. 
     The circuitry  110  includes resistors R 1 -R 14 , capacitors C 1 -C 4  and transistors Q 1 -Q 4 . Video component signal Uin is received at input “A” connected to a first terminal of resistor R 9  and video component signal Vin is received at input “B” connected to a first terminal of resistor R 10 . 
     A second terminal of resistor R 9  is connected to a first terminal of resistor R 12  and a first terminal of capacitor C 1 . A second terminal of capacitor C 1  is connected to a first terminal of resistor R 6 . A second terminal of resistor R 6  is connected to a first terminal of resistor R 14  and the emitters of transistors Q 1  and Q 2 . The collector of transistor Q 1  is connected to a positive terminal of a voltage source V 1 . The collector of transistor Q 2  is connected to a first terminal of resistor R 1  and an output  120  for outputting the balance adjusted component color difference signal Uout. A second terminal of resistor R 1  is connected to the positive terminal of the voltage source V 1 . 
     The base of transistor Q 1  is connected to a first terminal of resistor R 4 , a first terminal of resistor R 5 , a first terminal of capacitor C 4 , and the base of transistor Q 4 . The base of transistor Q 2  is connected to a first terminal of resistor R 2 , a first terminal of resistor R 7 , a first terminal of capacitor C 3 , and the base of transistor Q 3 . The collector of transistor Q 3  is connected to a second terminal of resistor R 2  and a first terminal of resistor R 3 . The second terminal of resistor R 5  is connected to a positive terminal of variable voltage source V 2 . 
     A second terminal of resistor R 10  is connected to a first terminal of resistor R 13  and a first terminal of capacitor C 2 . A second terminal of capacitor C 1  is connected to a first terminal of resistor R 8 . A second terminal of resistor R 8  is connected to a first terminal of resistor R 11  and the emitters of transistors Q 3  and Q 4 . The collector of transistor Q 4  is connected to a second terminal of resistor R 3  and an output  130  for outputting the balance adjusted component color difference signal Vout. 
     A second terminal of resistor R 7 , resistor R 11 , resistor R 12 , resistor R 13 , and resistor R 14  are connected to ground. A second terminal of capacitor C 3  and capacitor C 4  are connected to ground. A negative terminal of voltage sources V 1  and V 2  is also connected to ground. 
     As an example, the following values may be used for suitable operation of the circuitry  110 : resistors R 1  and R 3 , 680 ohms; resistor R 2 , 750 ohms; resistor R 4 , 220 ohms; resistor R 5 , 2.2 kilo-ohms; resistors R 6  and R 8 , 330 ohms; resistor R 7 , 250 ohms; resistors R 9  and R 10 , 75 ohms; resistors R 11  and R 14 , 470 ohms; resistors R 12  and R 13 , 100 ohms; capacitors C 1  and C 2 , 1 micro-farad; capacitors C 3  and C 4 , 10 nano-farad; voltage source V 1 , 12 volts; and variable voltage source V 2 , variable. 
     Since the differential amplifiers  140  and  150  are commonly connected via the respective bases of the transistors Q 1 , Q 4  and Q 2 , Q 3 , then using the values noted above, during operation of the circuitry  110  the input current through resistor R 6  is split equally between transistors Q 1  and Q 2  when the base voltages of transistors Q 1  and Q 2  are equal. Similarly, the input current through resistor R 8  is split equally between transistors Q 3  and Q 4  when the base voltage of transistors Q 3  and Q 4  are equal. 
     The ratio of resistance between resistors R 14  and R 4  is approximately 2:1. Therefore, the signal output at output  120  is two times the signal which is input at node “A”, i.e., Uin, multiplied by the constant k, and the signal output at output  130  is two times the signal which is input at node “B”, i.e., Vin, multiplied by (1−k). That is, Uout equals 2 kUin and Vout equals 2(1−k)Vin as indicated above. 
     The balance adjustment provided by the circuitry  110  allows for the relative levels of the component color difference signals to be adjusted by the viewer. As can be appreciated by one ordinarily skilled in the art, if the above values for the resistors, capacitors and voltage sources are used, at a nominal, mid-setting of the variable voltage source V 2 , i.e., approximately 3 volts, such that the voltage at node “C” is also approximately 3 volts due to the one-quarter resistor divider formed by resistors R 2  and R 7  (¼ of 12 volts is 3 volts), the relative gains of the component video signals Uin and Vin are unaltered (k is approximately equal to one-half). Varying the variable voltage source V 2  below the nominal, mid-setting, increases the gain of Uout and decreases the gain of Vout (k is equal or approximately equal to one). Varying the variable voltage source V 2  above the nominal, mid-setting, increases the gain of Vout and decreases the gain of Uout k is equal or approximately equal to zero). 
     Accordingly, during operation of the circuitry  110 , the Uin and Vin component video signals are color balanced. High accuracy and stability is ensured, due to the two balanced differential amplifiers  140 ,  150 . The first differential amplifier  140  is mainly formed by transistors Q 1  and Q 2  and resistor R 1 , and the second differential amplifier  150  is mainly formed by transistors Q 3  and Q 4  and resistor R 3 . 
     Further, during operation of the circuitry  110 , all of the transistors Q 1 -Q 4  are active at all times. During a variation of the variable voltage source V 2 , i.e., a balance control voltage range, the current is split between transistors Q 1  and Q 2  and transistors Q 3  and Q 4 . The system  100  is designed such that the current is split by having transistor Q 1  conduct less than transistor Q 2  if the voltage output by the variable voltage source V 2  is lower than 3 volts, such that the gain of transistor Q 2  is increased and the gain of transistor Q 4  is decreased. Likewise, the current is split by having transistor Q 3  conduct less than transistor Q 4  if the voltage output by the variable voltage source V 2  is higher than 3 volts, such that the gain of transistor Q 4  is increased and the gain of transistor Q 2  is decreased. Hence, the gain of the transistor Q 2  is approximately inversely proportional to the gain of transistor Q 4 . Accordingly, the gain of the color balanced output signals Uout and Vout are dependent on the voltage of the variable voltage source V 2  and are approximately inversely proportional. 
     It will be understood that various modifications may be made to the embodiments disclosed herein and that the above description should not be construed as limiting, but merely as exemplifications of preferred embodiments. Accordingly, those skilled in the art will envision other modifications within the scope and spirit of the claims appended hereto.