Abstract:
A method and apparatus for gamma-correcting digital video data and for adjusting the video signal to a desired pedestal (black) level, gain (white) level, and a gamma correction value includes an apparatus for calculating digital values corresponding to a video signal having the desired gamma correction, black level and white level and a memory for storing the digital values. Uncorrected input digital video signals address the memory and cause the corresponding digital values stored in the memory, which represent the corrected video signal, to be output. The invention can be implemented in both software or hardware.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a method and apparatus for gamma correction and independent modification of black and white (pedestal and gain, respectively) levels. 
     Typically, gamma correction is accomplished by implementing a piecewise (step by-step) linear transfer function utilizing a load resistor network for a transistor. The network is interconnected with diodes to provide a plurality of gain break points at particular pre-determined voltage values. A gain/voltage characteristic is generated by the gamma corrector and is selected to compensate for the non linearities of the transducer (e.g. camera or monitor). While this yields an acceptable gamma curve, it does not operate effectively when used in systems requiring the matching of several channels, for example, in a color television channel having red, green and blue channels. The result of gamma correction mismatch is a color shift for various signal amplitudes. This is most noticeable on monochrome signals which appear in color if the gain of all three channels is not the same. 
     Additional problems occur when it is desired to have a variable gamma value. Gamma values can be made variable using a plurality of resistors of different values, but the resolution of such systems is limited by the number of resistors available in the circuit. In addition, while circuits do exist that digitally select the resistor values to allow control of the gamma value, the gamma correction circuit is still an analog circuit and therefore includes the inherent stability problems of such analog circuits. Analog gamma correctors are made with transistor amplifiers and use resistor-diode combinations to set the various gain break points. Temperature and component age affect these components such that their characteristics do not remain constant. 
     Digital gamma correctors are also known in the art. It is known, for example, to input data relating to a desired transfer characteristic into a random access memory (RAM) and then, using the data stored in the RAM, to make adjustments to the video signal. See, for example, U.S. Pat. No. 4,394,688 to Iida et al. (hereinafter, &#34;Iida&#34;) and U.S. Pat. No. 4,786,968 to Kutner (hereinafter, &#34;Kutner&#34;). U.S. Pat. No. 4,568,978 to Cosh teaches &#34;real time&#34; gamma correction without the use of RAMs. While these prior art systems eliminate the need of the above-mentioned resistor network/diode configuration for gamma correction, they do not allow independent adjustment of black and white levels. In addition, the time required to compute the data to be loaded to the RAM in the Iida Patent and the Kutner Patent slows down the correction process. 
     The gamma value level is not the only value that is of concern to users of television systems. The black level, or pedestal, and the white level, or gain, are also signals that affect the signal and which are routinely adjusted in the generation of a television signal. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a method and apparatus enabling gamma correction of digital video data and adjustment of a desired pedestal level, gain level and gamma. 
     A method and apparatus for gamma-correcting digital video data and for adjusting a desired pedestal (black) level, gain (white) level, and gamma is disclosed, comprising means for calculating digital values representing desired values for gamma correction, black level and white level and storage means, such as a random access memory (RAM), loaded with said digital values, for receiving input digital video signals and outputting corresponding digital video signals modified by said digital values stored in the storage means. 
     While there are a number of ways in which the data to be stored in the storage means can be calculated, in the preferred embodiments of the invention, the digital values representing the desired values for gamma correction, black level and white level, are calculated according to one of the following equations: 
     
         RAM(i)=0 for i&lt;BL; 
    
     
         RAM(i)=2.sup.(N-1) for i&gt;WL; 
    
     
         and 
    
     
         RAM(i)=F2[(F1(i-BL)-F1((WL-BL))*gamma] for BL≦i≦WL;(1) 
    
     
         OR 
    
     
         RAM(i)=[(i-BL)/WL-BL)].sup.gamma *2.sup.N                  ( 2) 
    
     where i equals the particular address being loaded; N is the number of bits of address e.g. N=256 for an eight bit system); BL is a value representing a desired black level; WL is a value representing a desired white level; F1 is the LOG of the result of the equation following in parentheses; F2 is the ANTI-LOG of the result of the equation following in brackets; and gamma is the desired gamma value. 
     The present invention may be embodied in either hardware or software. A hardware embodiment of the present invention includes a digital gamma correction circuit which also allows independent modification of black and white levels (pedestal and gain). The value of gamma is continuously variable within the resolution of the number of bits used in the circuitry. A single RAM look up table (LUT) is loaded with data representing the calculated values for desired gamma, black level and white level. If the black level of an incoming video signal is too high (high gain), it can be reduced along the curve of the gamma characteristic. Likewise, if the white level of the incoming video is too low (low gain), it may be increased along the curve of the gamma characteristic while leaving the black level fixed, thus raising the video gain. The transfer characteristic can be down-loaded from a controller, for example, a microprocessor or discrete control logic. The controller calculates the values to be loaded into the RAM using, for example, either equation (1) or (2), based upon desired values for black level, white level and gamma input by a system operator. The black level adjustment, white level adjustment and gamma correction are performed independently of each other, i.e., they do not interact with each other during camera set-up. 
     The present invention takes advantage of the above-mentioned formulas to quickly compute the values for loading to the RAM with values at each address in the RAM corresponding to a particular desired gamma characteristic, black level and white level. The foregoing and other objectives, features and advantages of the invention will be more readily understood upon consideration of the following detailed description of certain preferred embodiments of the invention, taken in conjunction with the accompanying drawings and claims. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of an apparatus for gamma correction and black and white level modification according to the present invention. 
     FIG. 2 is a block diagram of an embodiment of the calculation unit of the controller of FIG. 1, which operates to compute values to be loaded into RAM in accordance with Equation (1). 
     FIG. 3 is a block diagram of an embodiment of the calculation unit of the controller of FIG. 1, which operates to compute values to be loaded in RAM in accordance with Equation (2). 
     FIG. 4 is a flow chart of a software implementation of the present invention using either Equation (1) or Equation (2). 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, an incoming digital video signal is applied via input line 2 to input A f a multiplexer 4. Output 6 of multiplexer 4 is coupled via intermediate line 8 to ADD input of RAM 10. RAM 10 has a defined number of addresses, for example, 256 for an eight bit system. Data Out of RAM 10 is the output video signal which is gamma corrected and white and black level adjusted and is coupled to the output of the system for transmission to the television monitoring equipment via output video line 12. 
     For purposes of the present explanation, it is assumed that RAM 10 has been loaded with the desired parameters of gamma correction and white and black levels in accordance with either Equation (1) or Equation (2) at each of its addresses. The input video signal is applied to multiplexer 4 via input line 2, and is output directly to RAM 10 via intermediate line 8. In RAM 10, the data therein acts upon the signal in a known manner to change the video signal and outputs the changed video signal to the output of the system via line 12. Since the present invention loads the addresses in RAM 10 with data relating to black level, white level and gamma, all three of those values of the input video signal are affected by the data in RAM 10. In other words, RAM 10 adjusts the values of black level, white level and gamma of the input video signals to the levels defined by the stored data. 
     Still referring to FIG. 1, controller 20 is coupled to input B of multiplexer 4 via controller output line 22. Controller output 24 is coupled to the Data In terminal of RAM 10 via controller output line 26. Data select control 28 of controller 20 is manipulatable, either electronically or manually, by an operator of the system as desired to change the values of gamma, white level and/or black level for the system. Data select control 28 can comprise any selection means which allows a parameter to be adjusted and which actually adjust the parameter, for example, a selector switch in conjunction with a potentiometer, or an optical tach wheel which outputs pulses to increment or decrement a counter. When data select controller 28 is manipulated by the user, multiplexer 4 is switched from input A to input B via A/B Select line 30 upon receipt of a signal from the data select controller 28, which indicates that a parameter adjustment is to be made. 
     If changes in any of the gamma, white level or black level is desired, the system operator manipulates data select controller 28 which cause controller 20 to output a signal on A/B Select line 30, which switches the input of multiplexer 4 from input A to input B. Controller 20 senses the desired data change indicated by the degree of manipulation of data select control 28 and causes calculating unit 32 to calculate new values for the data to be changed for the addresses of RAM 10 in accordance with Equation (1), Equation (2), or other suitable equations. Controller 20 causes the new data to be loaded into RAM 10 via input B of multiplexer 4, reflecting the desired changes in the video input signal, as described more fully below. 
     Once manipulation of the data select control 28 is terminated and the new data is loaded to the RAM 10, controller 20 switches multiplexer 4 from input B back to input A via a signal on A/B select line 30. RAM 10 then receives the input video signal and the new data in RAM 10 is used to process the incoming video signal so that the output video signal is corrected and adjusted for gamma, black and white levels, in accordance with the desires of the operator based on the new data in RAM 10. 
     FIG. 2 is a block diagram of the preferred embodiment of a calculation unit 32 of controller 20 which performs the calculations set forth in Equation 1, above, Calculation unit 32 responds to manipulation by the operator data select control 28 and performs the actual calculation of the data to load into the RAM 10 to achieve the desired black level, white level, and gamma values. An address output from counter 100 is applied to RAM 10 via multiplexer 4 so that the inputs to the Data In terminal of RAM 10 are associated with the proper address. Counter 100 can comprise, for example, an eight bit address counter. A subtractor 102 also receives the output from counter 100, at input A. Input B of subtractor 102 receives a value BL indicative of the desired black level value set by the system operator via data select control 28. Subtractor 102 subtract the value input to input B from the value input to input A (A-B) and outputs that value to log converter 104, which is &#34;burned in&#34; with values satisfying an equation which calculates the log of the output of subtractor 102. For example, the following equation will calculate the log of X in a form readable by an eight bit digital system: 
     
         INT[256*log.sub.2 (X)+1/2]                                 (3) 
    
     where X is the output of subtractor 102 and where &#34;INT&#34; rounds off the result of the equation within the brackets to the nearest integer. The output of log convertor 104 is applied to input E of subtractor 106. 
     The value BL indicative of the desired black level is also applied to input D of subtractor 110. A value WL indicative of the desired white level set by the system operator via data select control 28 is applied to input C of subtractor 110. Subtractor 10 subtracts the value BL from the value WL (C-D) and applies the result of the subtraction to log convertor 112, which is also &#34;burned in&#34; with values satisfying the same equation as log convertor 104, thereby outputting the log of (BL-WL). The output of log convertor 112 is applied to input F of subtractor 106. 
     Subtractor 106 subtracts the value applied to input F from the value applied to input E (E-F). The result is multiplied by the desired gamma value set by the system operator via data select control 28 in multiplier 114. The signal resulting from the multiplication by multiplier 114 is applied to a clipper circuit 116. Clipper circuit 116 generates data to clip the video signal at values below the desired black level or above the desired white level. The values between the desired black and white level are unaffected by clipper circuit 116. 
     The output from clipper circuit 116 is applied to anti-log converter 118, which is &#34;burned in&#34; with values satisfying an equation which calculates the anti-log of the value input thereto in a form readable by the digital system. For example, the following equation will calculate the anti-log of X in a form readable by an eight bit digital system: 
     
         INT[256*2.sup.-X/256 +1/2]                                 (4) 
    
     where X is the output of clipper circuit 116 and where &#34;INT&#34; rounds off the value of the result of the equation within the brackets to the nearest integer. The output of anti-log converter 118 is then applied to the Data In terminal of RAM 10. 
     In this manner, each address location of RAM 10 is loaded with data corresponding to the desired black level, white level and gamma. Once all the data has been loaded into RAM 10, the operation of the system is returned to its processing state, whereby the input video signals applied to RAM 10 are modified by the data stored therein for gamma correction and desired black and white levels. In the preferred embodiment, the changing of the values and loading of the new data to RAM 10 is performed during blanking intervals in a known manner so as not to interrupt the processing of the picture. 
     FIG. 3 is block diagram of an alternative embodiment of the calculating unit 32 of controller 20, which performs the calculations set forth in Equation 2. An address output from counter 200 is applied to RAM 10 via multiplexer 4 so that the inputs to the Data In terminal of RAM 10 are associated with the proper address. The address output from 200 is also input to input A of subtractor 202. Input B of subtractor 202 receives a value BL indicative of the desired black level value set by the system operator via data select control 28. Subtractor 202 subtracts the value BL from the address value input to input A and outputs the resultant value to input F of divider 204. 
     The value BL indicative of the desired black level is also applied to input D of subtractor 206. A value WL indicative of the desired white level set by the system operates via data select control 28 and is applied to input C of subtractor 206. Subtractor 206 subtracts BL from WL (C-D) and applies the result of the subtraction to input E of divider 204. Divider 204 divides the output of subtractor 202 by the output of subtractor 206 and provides the resultant value to log converter 208. Log converter 208 calculates the log of the output from divider 204 in a digitally readable form. This log value is multiplied by the desired gamma value set by the system operator via data select control 28 in multiplier 210, the result of the multiplication being applied to anti-log converter 212. Anti log converter 212 calculates the anti-log of the output of the multiplier 210 in a digitally readable form, and multiplier 214 scales the output of anti-log converter 212 by multiplying the output by 2 N , bringing the resultant value back into a value consistent with the number of bits specified by the system hardware (for example, 0 to 255 for an eight bit system). The output of multiplexer 214 is applied for appropriate addresses of RAM 10. 
     The present invention can also be embodied in software. FIG. 4 is a flow-chart of a program for performing the operation of the video signal processor of the present invention. Referring to FIG. 4, in block 300, the desired values of BL, WL, and gamma level are read. In step 302, the address, i, is set to 0. Proceeding to block 304, the data in RAM 10 corresponding to the address in block 302 is set to a digital value representing black, e.g. 00 H . At block 306, the data corresponding to the address is compared with the address of the desired black level BL. If the address i does not equal BL, the address i is incremented by 1 (block 308) and the process is carried out again. This continues until BL equals the address. 
     Upon reaching the point where the address equals the black level BL, the gamma characteristic is calculated at block 310 using, for example, Equation 1 or Equation 2, depending on which process or peripheral hardware is used. This calculates the exponential gamma curve. Each iteration through the gamma calculating step sets the value for an additional address location (block 312) in the RAM. When the address i equals the desired white level, WL, value as determined in block 314, the data in the remaining address locations are set at the digital value representative of peak white, for example, FF H  (blocks 316, 318 and 320), thereby forming a white clipper. 
     The black level and white level are established with the gamma curve being formed between these two values. Using the above-described approach, the three parameters (BL, WL and gamma) are independently controlled and do not interact with each other during camera set-up. 
     The following example illustrates the operation of an eight-bit system according to the present invention. Eight bits were chosen for example only, and the present invention is not limited to an eight-bit system. For an eight-bit system there are 256 input addresses to the RAM 10. Assuming that the address of the desired black level is 36 and the address of the desired white level is 226, loop A of FIG. 4 will repeat 37 times, loop B will repeat 189 times, and loop C will repeat 30 times, for a total of 256 iterations. 
     Using the above values of 36 for the address of the desired black level and 226 for the address of the desired white level, and further assuming a desired gamma of 0.45, the following table illustrates the arithmetic results performed by the controller illustrated in FIG. 2 of the present invention: 
     
         ______________________________________ADDRESS          0        36       37   255OUTPUT OF        -36      0        1    219SUBTRACTOR 102OUTPUT OF        190      190      190  190SUBTRACTOR 110OUTPUT OF LOG    -32768   -32768   0    1990CONVERTER 104OUTPUT OF LOG    1938     1938     1938 1938CONVERTER 112OUTPUT OF        34706    34706    1938 -52SUBTRACTOR 106OUTPUT OF MULTIPLIER            15646    15646    872  -23114OUTPUT OF CLIPPER 116            4095     4095     872  0OUTPUT OF ANTI-LOG            0        0        25   255CONVERTER 118______________________________________ 
    
     As is evident from the above example, changing the black level, gamma level, and/or white level will change the value in each address location of RAM 10. Thus, changing any one or all of these values will change the output video signal. For the above example illustrating an eight bit system, note that clipper 116 will output a zero if the input thereto is less than zero, output 4095 if the input thereto is greater than or equal to 4095, and output the input value without modification if the input value is greater than or equal to zero but less than 4095. 
     Although the present invention has been shown and described with respect to preferred embodiments, various changes and modifications will be apparent to a person skilled in the art to which the invention pertains and are deemed to come within the spirit and scope of the invention which is set forth in the appended claims.