A gamma-control circuit using crystal diodes

First, the requirements of gamma-control circuits are discussed in relation to a variety of television processes and it is noted that it is desirable to have a close approximation to a simple power law under all conditions, a continuously-variable index over a wide range with the amplitude of the output signal maintained constant, and an adequate bandwidth following gamma distortion. A basic bridge-type circuit designed to meet these requirements is then described. The errors in departure from a pure power law are shown to be determined by the characteristic of the non-linear element and the method of compounding an arbitrary index. The magnitude of these errors is calculated for some typical cases and shown to be small for most television purposes. An elaboration of the basic circuit is described, including a device for obtaining the necessary high degree of d.c. stabilization. The paper concludes with a brief discussion of the relationship between bandwidth requirements before and after gamma distortion.