GB711268A - Improvements in or relating to apparatus for televising from cinematograph film and the converse - Google Patents

Abstract

711,268. Television; photo-electric control systems. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Oct. 22, 1951 [Aug. 2, 1950], No. 19341/50. Class 40 (3). [Also in Group XXXV] In a film scanning arrangement, either for the recording of received television pictures upon, or for the development of television picture signals from, kinema film, the scanning raster is maintained in its correct position with respect to the film frames despite the motion of the film through the gate by means of a control system involving the projection of a beam of light through the sprocket holes in the film on to a photo-cell. The invention is applicable both to continuously-advanced and intermittently-advanced systems ; in the latter type of system the invention allows following or " chasing to occur during the pull-down intervals. In one embodiment of the invention described as applied to an intermittently advanced system, a wedge-shaped "light slot" 1, Fig. 1, is focused on to the edge of the film which is blackened to prevent the passage of light except by the sprocket holes 4. The film when stationary is in the position shown and the amplitude of the corresponding current developed by the photocell is such as to effect no control on the position of the raster. During movement of the film, however, the photo-cell current is varied and exercises a varying control on the position of the raster so as to maintain it stationary with respect to the film. In a modification, Fig. 3 (not shown), the position of the slot with respect to the sprocket holes is changed so that normally the narrow tip of the slot is focused on the holes 4. In an alternative embodiment, Fig. 2, the image of a cathode-ray tube scanning spot is focused on the film and scans with respect to the sprocket holes 4 as indicated by the scanning path 6. The phase of the pulse developed as the spot passes the hole 4 is compared with that of the scanning waveform in a suitable comparator circuit and the output is used to effect both a retro-active control on the position of the auxiliary scan and a control on the position of the main raster scanning the film. A double-beam cathode-ray tube may be used to provide the main raster and auxiliary scan or alternatively two separate cathode-ray tubes together with a suitable optical system may be used. In this arrangement the edges of the film need not be blackened since by the use of limiter circuits the amplitudes of the pulses developed in the photocell may be made unimportant. The repetition rate of the auxiliary scan is preferably arranged to be an integral multiple of the line scanning frequency. By means of suitable correcting arrangements in the control circuits, the main raster may be made to follow the movement of the sprocket holes 4 accurately despite the fact that the finite gain and the presence of time constants in the loop circuit prevents the accurate following of the holes 4 by the scanning path 6.