GB743402A - Improvements in and relating to television - Google Patents

Abstract

743,402. Television. BRITISH BROADCASTING CORPORATION. Oct. 1, 1951 [Sept. 29, 1950], No. 23949/50. Class 40 (3). In a method of producing a composite tele. vision signal whereby, at a receiver, a picture is reproduced in which part of a background image provided by a first -television camera is replaced by a further image provided by a second television camera, control pulses are generated for rendering the two cameras alternately operative by means of a photo-electric cell viewing an unmodulated light raster through a mask having a profile of the same shape as a silhouette of the further image. In the Figure, two television cameras 1, 2 are connected respectively to camera control units 3, 4 and the outputs of the two cameras are fed through black level clamps 6 and 7 to an electronic switch 8 which may be of any suitable form, for example as described in Specification 743,403, and operates to suppress the outputs of the cameras alternately. A waveform generator 9 feeding the camera control units also provides suppression, synchronizing and scanning signals which are applied through a mixer 10 to the control unit 11 of a short afterglow cathode-ray tube 12 so as to produce an unmodulated raster in synchronism with those in the cameras. A mask 13 is placed between the screen of the tube and a photo-electric cell 15 the output from which passes through a black-line clamp 16, equalizer 17 and amplifies clipper 18 to control the electronic switch 8. A clamp pulse generator '19 controls the clamps 6, 7 and 16. Assuming for example it is desired to insert an actor televised by camera 1 within the window of a house a photograph of which is televised by camera 2, the output of camera 2 is applied initially to modulate the raster of tube 12 and transparent material for forming mask 13 is placed over the screen and blackened over the area corresponding to the window. The arrangement is then restored to normal and during scanning the light to cell 15 is interrupted as the spot passes the blackened area, the resulting cell pulses controlling switch 8 to suppress the output from camera 2 and render camera 1 effective whereby the combined signal represents the actor within the window of the house. The black levels of the two camera outputs are equalized by adjusting the bias voltages of clamps 6 and 7. In an alternative arrangement the cameras may be rendered alternately operative by applying the control pulses to suppress the electron beams in the cameras. Specification 489,717 also is referred to.