Abstract:
A process for the gamma correction of video signals, in which the luminance signal in the signal transmission path and at least one signal containing an item of color information are multiplicatively corrected, provides that, in addition to the luminance signal, at least two color difference signals are corrected by way of individual, controlled correcting stages whose control signals are obtained from the luminance signal. The gamma correction is carried out in a single tube color television camera and provides that an additional color difference signal may be corrected in a separate correcting stage.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to techniques for providing a gamma correction of video signals in which, in the signal transmission path, a luminance signal and at least one signal containing an item of color information are multiplicatively corrected. 
     2. Description of the Prior Art 
     The picture tube of a television receiver possesses a nonlinear relationship between the electric input signal supplied thereto and the emitted luminance. The camera tube of a television camera, in many cases, also exhibits a nonlinear relationship between the quantity of light supplied thereto and its electric output signal. Normally, one endeavors to achieve a linear transmission characteristic, which is obtained by means of the so-called gamma correction, also referred to as gradation correction. 
     In color television, gamma correction is also required for the color information, where the correction factors for the individual color component signals (RGB) differ from each other. Generally, the gamma correction is carried out in the television camera, either in that special camera tubes are provided for the individual color component signals, and both the luminance signal and color component signals can be produced and corrected discretely, or in that, in the case of a single tube color television camera, the color component signals are derived from a chrominance signal obtained from the camera tube by way of a matrix specified for gamma correction, and are then reassembled by way of a matrix to form the corrected chrominance signal. 
     The latter of the two methods mentioned above is expensive because of the utilization of a matrix. It is less expensive to carry out the gamma correction directly for the chrominance signal in dependence upon the luminance signal. That is to say that the individual color component signals are no longer separately corrected, but only the overall chrominance signal in which they are contained is corrected. However, because of the differing transmission characteristics of the individual colors, this technique results in the fact that a linear transmission characteristic for the entire transmission is no longer possible. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a process for gamma correction of video signals, in which, in order to achieve a linear overall transmission characteristic for a color television signal, the gamma correction can be carried out individually for the individual colors, without the necessity of specially producing the color component signals in the color television camera for this purpose. 
     In order to achieve the foregoing objective, in a process type briefly described in the introduction, it is proposed, in accordance with the present invention, that in addition to the luminance signal, at least two color difference signals are each corrected by way of separate controlled correcting stages, whose control signals are obtained from the luminance signal. 
     Preferably, this correction, according to the present invention, is carried out in a single tube color television camera. Advantageously, the multipliers of the individual correcting stages can be individually set. This is of particular advantage when the picture source already exhibits color distortions which are not to be reproduced. This is possible, for example, in the case of film scanning. 
     An advantageous development of the present invention consists in the correction of at least one further color difference signal. Modern color television scanners contain extensive matrix circuits for color value correction, including the so-called &#34;color-comp,&#34; a device which, by forming color difference signals, facilitates the separate setting of color saturation and color tone for various color zones and, thus, considerable freedom of color correction without influencing the white balance. 
     Making reference to an example of utilization of the process in accordance with the present invention for the case of a single tube color television camera, in which in addition to the luminance signal two color difference signals are also corrected, the invention and the mathematical basis thereof will be explained in further detail below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The objects, features and advantages of the invention, its organization, construction and mode of operation will be best understood from the following detailed description taken in conjunction with the accompanying drawing on which there is a single FIGURE which is a block circuit diagram of apparatus for practicing the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The color component signals are, G and B, provided for the gamma correction, and representing the colors red, green and blue, can be represented as consisting of a luminance component Y and a color difference component: 
     
         R = Y + (R - Y); 
    
     
         g = y + (g - y); 
    
     and 
     
         B = Y + (B - Y) 
    
     and are governed by the equation 
     
         x = x.sub.o + Δ x. 
    
     For the purpose of gamma correction, the above equation is involved by the power  , resulting in the equation 
     
         y = x.sup.  = (x.sub.o + Δ x).sup. . 
    
     An example can now be considered which expresses the gamma-corrected color component signals by the sum of the gamma-corrected luminance signal and, in each case, one corrected color difference signal which is yet to be established: 
     
         R.sup.  = Y.sup.  + (R - Y)*; 
    
     
         g.sup.  = y.sup.  + (g - y)*; 
    
     and 
     
         B.sup.  = Y.sup.  + (B - Y)*. 
    
     in general terms, this is described by the equation 
     
         y = x.sub.o.sup.  + Δ y. 
    
     If this equation is not developed at the point x = x o  (Δ x = 0) into a series, ceasing after the first term, one arrives at the equation ##EQU1## and the following representation of the gamma-corrected color component signals: ##EQU2## This series development is sufficiently accurate as the color difference components are small in comparison to the luminance components. It is therefore possible to represent the gamma-corrected color component signals by the sum of a gamma-corrected luminance signal and a linear color difference signal which is multiplied by a function of the luminance signal. As all three color difference signals are multiplied by the same function, and one of the three color difference signals is, of course, a linear combination of the two others, it is sufficient to carry out the multiplication for two color difference signals. Thus, it is possible to correct the three color component signals by means of a discrete correction of the luminance signal and of two color difference signals, the formation of the multipliers, the luminance multiplier M y  and the color difference multiplier M c , to be explained in the following. In general terms: 
     
         Δ y =   · f (x.sub.o) · Δ x, 
    
     and 
     
         x.sub.o.sup.  = x.sub.o · x.sub.o.sup. -1 = x.sub.o · f (x.sub.o), 
    
     which results in ##EQU3## 
     It will thus be seen that the gamma correction of the luminance signal Y can likewise be effected by multiplying the linear luminance signal Y with the multiplier f(Y) already required for the color difference signals (R-Y and (B-Y), respectively. The common component f(Y) of the luminance multiplier M y  and the color difference multiplier M c  can be represented with sufficient accuracy in accordance with the rules of the Taylor&#39;s Series as a linear function of the luminance signal: 
     
         f (Y) ≈ 1 - (1 -  ) · (Y - 1), 
    
     where 0 ≦ Y ≦ 1. 
     The block circuit diagram illustrated on the drawing shows an input 1 for receiving the luminance signal Y, an input 2 for receiving the color difference signal (R - Y) and an input 3 for receiving the color difference signal (B - Y). The input 1 is connected by way of a correcting stage 4 to an output 5 for a corrected luminance signal Y*. The input 2 is connected by way of a correcting stage 6 to an output 7 for a corrected color difference signal (R - Y)*. Likewise, the input 3 is connected by way of a correcting stage 8 to an output 9 for a corrected color difference signal (B - Y)*. 
     The input 1 is also connected to a block 10 in which a signal f(Y) is formed in accordance with the function, f(Y). This is effected by way of analog calculating amplifiers which carry out the operations required in accordance with the last-mentioned equation, subtraction and multiplication, in the conventional manner. The correcting stages 4, 6 and 8 are conventional amplifiers of the type which exhibit controlled amplification and whose control inputs are each connected to the tapping of respective ones of three potentiometers 11, 12 and 13. In each case, one terminal of the potentiometers 11-13 is connected to a reference potential by way of a d.c. voltage source 17. The other terminal of the two potentiometers 12 and 13 is connected to the output of the block 10, and carries the color difference multiplier M c . In addition, the output of the block 10 is connected by way of an ohmic voltage divider composed of two resistors 14 and 15 and, in series with the voltage divider, a d.c. voltage source 16, to the reference potential. The point of connection of the two resistors 14 and 15 is connected to the other terminal of the potentiometer 11 and conducts the luminance multiplier M y . 
     According to this block diagram, a process in accordance with the present invention can be carried out, in particular, in a single tube color television camera in which the color component signals are present in electrically non-discrete form. With this concept, no errors can occur in the white balance of the grey scale. In addition, by virtue of the individual setting of the multipliers which control the correcting stages 4-8, employing the potentiometers 11-13, it is possible to effect gradation corrections for different colors, which is extremely advantageous in the case of film scanning. 
     For a precise definition of the multipliers, it should be borne in mind that, although generally a value of 0.6 is currently selected for  , the differential amplification for small signals is limited to approximately 2 in order to avoid too great an impairment of the signal-to-noise ratio. In the case of particularly high contrast sources, a value of 0.4 is also used for  . The value for   is to be continuously adjustable between a maximum value of 1 and a minimum value of 0.4. In accordance with the last-mentioned equation, the luminance multiplier M y  which is supplied from the block 10 by way of the voltage divider 14, 15 to the potentiometer 11, obtains a value of 1 - (1 -  ) · (Y - 1). The color difference multiplier M c  which is fed directly from the block 10 to the potentiometers 12 and 13 obtains a value of 1 - 2.6 · (1 -  ) · (Y - 0.625) with an approximation analysis of optimum gamma curves. An individual setting is effected with potentiometers 11-13. At the one end setting of the potentiometers, the correcting stages 4, 6 and 8 are supplied with a d.c. voltage which corresponds to an amplification of 1 (corresponding to   = 1). At the other end setting, the negative luminance signal Y is supplied with maximum amplitude and in a d.c. voltage state which is such that the desired multiplication curves are formed. Advantageously, field effect transistors can be used as controlled amplifiers in the correcting stages 4, 6 and 8. 
     The above-mentioned advantageous development in accordance with which a further color difference signal is gamma corrected for the purpose of a more deliberate and more free correction without influencing the white balance, is represented in broken lines on the drawing. A potentiometer 18 is connected in parallel with the potentiometers 12 and 13. The tap of the potentiometer 18 is connected to a control input of a further correcting stage 19, which stage is connected to an input 20 for receiving a color difference signal (G - Y), and which has an output 21 for a gamma-corrected color difference signal (G - Y)*. The multiplier which controls the correcting state 19 is individually set by way of the potentiometer 18 by the color difference multiplier M c . 
     In spite of the fact that it provides extensive correction possibilities, the techniques involved in the present invention, both apparatus and method, for the gamma correction of the luminance signal and of the difference signals produces a substantial simplification of a television transmission system, in particular when the correction is carried out in a single tube color television camera. 
     Although I have described my invention by reference to particular illustrative embodiments thereof, many changes and modifications of the invention may become apparent to those skilled in the art without departing from the spirit and scope of the invention. I therefore intend to include within the patent warranted hereon all such changes and modifications as may reasonably and properly be included within the scope of my contribution to the art.