Patent GB-1260568-A (GB)
Title: Mix-effects system for television video signals
IPC: H04N9/76
Filed: 1969-08-20
Abstract: 1,260,568. Television. CENTRAL DYNAMICS Ltd. 20 Aug., 1969 [28 May, 1969], No. 41643/69. Heading H4F. A mix-effects system for television video signals comprises a plurality of signal sources connected to a switching crosspoint matrix so that any one of the sources may be switched to any one of a plurality of output buses, switching circuitry having two input terminals connected to two of the matrix output buses and having two output terminals connected to a signal mixer incorporating a fader, and means for controlling the switching circuitry in such a way as to produce the required mix-effects output. Two video signal sources 65, 67 selected by means of a crosspoint matrix (not shown) are connected to black level clamps 70, 72 and then to the inputs 74, 48 of a mixer amplifier 76, provided with a fader control, over selector switches S a1 , S a2 , S b1 , S b2 . A colour matte source 84 may also be connected to the mixer inputs over switches S c1  and S c2 . The configuration of the six switches is determined by a control system, part of which is shown in Fig. 4B, which includes a pattern generator 102, a key processor slicer 110, a blanking signal source 114 and two phase splitters 90, 92 arranged so that when both are energized over switches 118 and 119 the outputs on lines 94 and 98 are in phase. The control circuitry is arranged so that for each mode of operation some switches are continuously closed, some are continuously open, and the rest are intermittently opened and closed under control of the phase splitters 90, 92. In a Mix mode S a1  and S b1  are closed and the output signal is a mixture of signals A and B, the proportion depending on the position of the fader control. In a Split-Screen mode S a1 , S a2 , S b1 , S b2  are opened and closed under control of the pattern generator 102. When S a1  and S a2  are closed the A signal is fed to both inputs of the mixer and when S b1  and S b2  are closed the B signal is fed to both inputs. The pattern generator provides a signal such that the output signal comprises signal A for the first part of each scanning line and signal B for the second part of the line, the fader exerting no control. Wipes may be achieved by adjusting the pattern generator to vary the proportion of the line period during which each signal provides the output. In a Key-into-Mixer mode S b1  is closed and S a1  and S b2  are closed alternately by phase splitter 90 under control of the key processor slicer 110 fed from signal source A. When S a1  is closed signal A, which forms a key such as a title, is applied to input 74 and when S b2  is closed signal B is applied to input 74. When the fader control is at the A bus (input 74) the output signal comprises signal A inserted as a key into a background comprising signal B. When the fader control is at the B bus (input 78) the output comprises signal B alone so that fades from background signal B to the background with inserted key may be effected. A Split-screen-into-Mixer mode is achieved by the same arrangement as that of the Key-into-Mixer mode except that the switching is under control of the pattern generator. In a Fade-to-Black mode S a1  is closed and S a2  is controlled by blanking signal source 114 so that it is closed during the blanking interval and open during the picture interval. With the fader control at bus B the output signal comprises a black-level during the picture interval but the sync. pulses (and colour burst if present) of signal A occurring during the blanking interval are preserved. A similar arrangement for signal B may be provided. In a Coloured-Key mode S b1  is closed and S c1  and S b2  are closed alternately under control of key processor slicer 110 fed with signal A. When the fader control is at the A bus the output comprises a key, the colour of which is determined by source 84, inserted into a background signal B. When the fader is at the B bus the output comprises signal B alone. When the signals A and B are Non-Synchronous they cannot be mixed since picture break-up and/or colour shift may occur so both switches S a1  and S a2  are closed until the fader control is moved completely to the B bus when they open and both switches S b1  and S b2  are closed. This removes the necessity for providing separate by-pass lines for non-synchronous signals.  In any of the modes of operation the roles of signal sources A and B may be reversed by suitable modifications of the switching configuration.