The application of negative feedback to flying spot scanners

Abstract
The paper analyses the performance of a television transmitting system using the flying spot principle, in which the signal is applied as feedback to the control electrode of the scanning cathode ray tube. A system using such an overall feedback in negative polarity possesses several advantages over the normal method of operation of these devices.The main advantage is a convenient control of the “tone rendition” (gamma) of the transmitted picture when working in conjunction with normal gamma increasing receiving equipment. Gamma control may also be necessary in film scanning to compress the wide range of film transparency into the limited tone scale available in television pictures. Problems introduced by the time delay existing in the feedback loop are discussed. The influence of feedback on the appearance of screen afterglow is analysed and finally the effect on the signal-to-noise ratio is considered.