GB1416140A - Circuit arrangements for the non-additive mixing of electrical signals - Google Patents

Abstract

1416140 Television EMI Ltd 28 Nov 1972 [30 Dec 1971] 60813/71 Heading H4F A video signal A at input 11, e.g. a colour signal, is non-additively mixed with a video signal B or caption signal at input 18, the composite output signal from mixer 17 comprising at any instant the larger of the two input signals, and the arrangement including a selectively operable switch 20 which when a caption signal is being mixed with signal A causes the caption signal's amplitude to be increased, e.g. doubled by multiplier 21, so that it is greater than the maximum level of signal A. This prevents the mixed signal containing chroma information instead of the required peak white caption signal. In addition the caption signal is preferably clamped 22 below the minimum level of the signal A during blanking intervals so that the negative portions of the burst signals in signal A are not removed by the caption signal, i.e. the complete burst signal from signal A is present in the mixed signal. With ganged switches positioned as shown, i.e. for a caption signal input, the inversely related (inverter 26) clamping levels of circuits 13 and 22 are determined by potential divider 25, but when the video signal A is mixed with the video signal B the switches 20 and 24 are caused to assume their V positions and the relative clamping levels of the two signals are controlled by fader 15.