GB1286806A - Improvements in or relating to video mixers - Google Patents

Abstract

1286806 Video mixers and faders EMI Ltd 10 Sept 1969 [10 Sept 1968] 42897/68 Heading H4F In a video mixer, combined video and synchronizing signals 1-3 are added in variable absolute and relative amounts using faders 5-7 and an amplifier 11, and a control signal responsive to the settings of faders 5-7 determines the phase and amplitude of further sync. signals, derived from a constant amplitude sync. input 4, which are added to the composite signal to maintain the amplitude of its sync. component substantially constant. As described, the faders may be of the type described in Specification 1,126,685, where a manually set potentiometer controls the fader D.C. control current ahd hence the output signal level. The D.C. control currents are derived from a source 9 so that the potential across a resistor 10 depends on the summated currents. The potential is fed to amplifiers 21, 22 giving outputs of opposite polarity, of which one or the other is applied, via respective diodes 27, 28, to an input 15 of a further fader 8, i.e. to give a D.C. control current dependent on the potential at 10. Fader 8 is similar to units 5-7, but the manually set potentiometer is omitted. The output level of amplifier 22 controls gates 25, 26 via a switch 23 to determine which of the constant amplitude sync. signal outputs from a phase splitter 24 is fed to the fader 8. Excessive picture signal levels at the output 12 are prevented by limiting the summated currents through resistor 10 to a predetermined level determined by the design of source 9.