| 摘要 |
PURPOSE:To make the contrast of a reproduced picture suitable automatically even under the image pickup conditions easily causing excess or deficiency of contrast, such as rear light, home illumination, and field, by controlling gamma of converting function of X<gamma> or an equivalent value so that the frequency of generation of data in a prescribed amplitude of a color video signal is a prescribed value. CONSTITUTION:A data conversion circuit 3 includes a data conversion table 4 of a conversion function (X<gamma>) for contrast correction and a data conversion table 5 of conversion function (Y<gamma>0) (generally, gamma0=0.45) for the substantial gamma correction of nonlinearity of a color picture tube, and an output signal V is obtained. In case of two-gradation equally, the most significant bit of the prescribed sample of an output signal being 0 (1st gradation) is counted at a counter 6 and the count value h0 is applied to a detection control circuit 7. To the circuit 7, the 1st gradation value of the amplitude distribution to be an object and its corresponding value d0 are applied from a terminal 8 for the comparison of h0 and d0. The value Y of the data conversion table 4 is changed gradually so that the both are coincident. Through the negative feedback control system like this, the convergence of h0=d0 is attained. |
