GB652060A - Improvements relating to apparatus for generating picture signals suitable for television transmission, from cinematograph film - Google Patents

Abstract

652,060. Television transmitters ; photoelectric control systems. ELECTRIC & MUSICAL INDUSTRIES, Ltd. June 3, 1948, No. 14962. [Class 40 (iii)] Apparatus for generating television-picture signals from cinematograph film comprises a light source for projecting images of successive frames of the film on to the target of a television pick-up tube, means for generating a control signal in dependence upon the mean transparency of the film, and means for causing the control signal to control the intensity of the light image projected on to the screen so that as the transparency increases the intensity of the image is reduced relatively to the transparency and as the transparency decreases the intensity is relatively increased. The television pick-up tube 1 is of the cathode-potential-stabilized type wherein instability is liable to occur if the light-intensity at the target 2 becomes too great. Light from a source 15 is projected through a film 13 in the film-gate 10 on to the target 2 at which television signals are generated and applied to transmission circuits 9. Light from a further source 1@ is projected through the film 13 in a film-gate 18 on to a photo-cell 23 the output from which is proportional to the mean transparency of the frame in the gate 18. The photo-cell output is amplified at 25, integrated at 26 and applied to control circuit 27, which controls the intensity of the source 15. The gates 10 and 18 are spaced apart an integral number of film frames, the film passing through the gate 18 before the gate 10. The integrating circuit has a relatively long time constant so that rapid fluctuations of transparency are smoothed out and also so that the control circuit 27 does not respond to changes in transparency until the frame wherein the change occurs has reached the gate 10. Alternative methods of effecting control may be used; for example the dimensions of an aperture in front of the source 15 or the position of a shutter of varying transparency may be the controlled factor. The tendency to instability in the tube 1 may be further reduced in the manner described in Specification 652,059.