Patent GB-833248-A (GB)
Title: Improvements in apparatus for mixing television signals
IPC: H04N5/262
Filed: 1956-09-28
Abstract: 833,248. Television. FERNSEH G.m.b.H. Sept. 28, 1956 [Sept. 29, 1955], No. 29702/56. Class 40 (3). Relates to apparatus for -mixing television signals and is concerned with the provision of the type of cross-fade known as a " rolling cut " in a system in which a picture from one television source is inlaid in part of area of the picture from a second television source. The mixing of the two television signals is effected in the customary manner by applying the signals to two amplifiers feeding a common output, which are rendered operative alternately by identical but oppositely poled control signals at their grids. In accordance with the invention the control signal is produced by combining three signal trains, the first of which consists of a train of pulses at the line and/or frame frequency which are varied in duration to effect the rolling cut, the second of which consists of signals generated by a television scanner viewing a black- and-white pattern corresponding to the inlay area, and the third of which consists of a sequence of pairs of alternate positive and negative pulses which occur wholly within the synchronizing interval and are provided to ensure well-defined grid potentials for the amplifiers when the " rolling cut " signals are set at the extreme condition where only one amplifier is operative. The combined signal train is fed to a limiter which conducts to produce the control signal only when the combined amplitude exceeds a limiting level. The effect is that the second or inlay signal becomes effective only when supplemented by the first signal and may thus be introduced progressively in accordance with the " rolling cut." The arrangement is illustrated in Figs. 1 and 2. The " rolling cut " signal is produced by selecting line- and/or frame-frequency signals, such as triangular frame frequency signal C, from sources 2-7, combining them in a mixer 8 with pulse train A from source 1 consisting of pairs of alternate positive and negative pulses occurring wholly within the synchronizing interval t<1>, and then applying the combined signal H to a double limiter stage 12 which selects only between the narrow amplitude range indicated by lines S to produce finally a signal K. To make the rolling cut, the limiting level is progressively changed, either manually or automatically, to increase the interception of the signal. In the case illustrated the cut will be effected upwards and downwards from a horizontal centre line. Other forms of cut, such as left-to-right or top-to-bottom, may be provided by choice of appropriate waveforms from sources 2-7. Reference is made to the use of parabolic, half sinewave and saw-tooth waveforms, and also waveforms at multiples of line and frame frequency. The inlay signal M is derived from a television camera 17 which is arranged to view a pattern, e.g. a white circle on a black background, corresponding to the desired inlay pattern. The signal is passed through a top limiter 19 and then combined in mixer 14 with a pulse train A<1> corresponding to pulse train A, and signal K. The resulting signal, which takes the form shown at O, is applied to a double-limiter stage 23 which selects between the limits T to produce signal P. This signal constitutes the control signal and is applied to phase splitter 25 to develop oppositely-poled versions Q, Ql. These signals are then applied to render operative alternately amplifiers 32 and 33 whereby the video 'signal V from a first television camera 26 is replaced in part by a signal W from a second camera 27 to produce in a common output 34 a mixed signal representing one picture inlaid in another over an area as determined by camera 17. The " rolling cut " signal determines the portion of the inlay signal which is effective to produce an output from limiter 23. Thus by changing the limiting levels S in limiter 12 as described above, the area of the second picture, a circle as described, is inserted in the first picture progressively, upwards and downwards from a horizontal centre line for the particular form of cut considered. As a further feature the inlay signal may be derived from an impulse generator 22 arranged to produce a predetermined shape, for example a heart. A further generator 21 may be included to produce a triangular black or white mark in the picture to act as a pointer. In a modification, Fig. 3 (not shown), limiter 12 is dispensed with and waveforms H and N are combined directly in mixer 14. In this case the " rolling cut " is effected by varying the level of limiting in limiter 23.