Patent GB-773725-A (GB)
Title: Methods and apparatus for television montage
IPC: H04N5/272
Filed: 1955-06-30
Abstract: 773,725. Television. COLUMBIA BROAD. CASTING SYSTEM, Inc. June 30, 1955 [June 30, 1954], No. 18978/55. Class 40 (3). In an arrangement for combining separately generated " foreground " and " background " signals the " foreground subject, which is normally illuminated, is " viewed " by a first television camera against a screen which is illuminated by polarized light, the first camera being unresponsive, or rendered unresponsive during alternate field periods, to polarized light, and the signals generated by this camera, which are representative of the " foreground " subject, are supplied to a circuit together with signals representative of a desired " background" scene, generated by a second camera, and with signals generated either by the first camera during field periods when it is responsive to the polarized light or by means of a further camera so responsive, these latter signals acting as keying signals which produce a " gap " in the " background " signals into which the signals representative of the " foreground " subject are inverted. Fig. 1 shows one embodiment in which a normally illuminated " foreground " subject 10 is positioned in front of a screen 11a which is illuminated by a source 19 provided with a polarizing filter 20 and which has the property of reflecting polarized light in the direction of the incident light without change of the angle of polarization. In this manner the keying signals for producing the " gap " in the " background" signals (which are produced by means of a camera 21 " viewing" a suitable scene 22 and supplied to a combining amplifier 26) are generated by a camera 16 which is provided with a polarizing filter 17 (orientated similarly to filter 20) and the " foreground " object signals generated by means of a camera 12 provided with a polarizing filter 13 which is " crossed " with respect to filters 17 and 20. In order to ensure the production of clearly defined keying signals (e.g. when the brightness of the subject is close to that of the screen 11a) the sensitivity of camera 16 is reduced (by optical or electrical means) so that this camera " sees " subject 10 as a dark silhouette against a brightly illuminated background. The keying signals so produced are then employed to control amplifier 26 to block the signals from camera 21 during those instants when signals representative of the subject are supplied by camera 12 and thus produce a composite picture signal at the output 30. In another embodiment (Fig. 3, not shown) a separate keying camera is not employed, the " subject" camera 12 being utilized for the production of the keying signals by including a rotating polarizing filter in the optical path, the angle of polarization being arranged to change through 90 degrees during alternate field periods so that the composite signal produced consists of signals representative of the " gapped " background scene and of the " foreground" subject occurring during alternate field periods. Suitable switching and combining arrangements are described with reference to Figs. 2 and 5 (not shown). Specification 647,714 is referred to.