Patent GB-1173341-A (GB)
Title: Improvements relating to apparatus for combining video signals
Inventor: STROVER MICHAEL CHRISTOPHER
IPC: H04N5/272
Filed: 1967-03-28
Abstract: 1,173,341. Television. FERRANTI Ltd. 26 March, 1968 [28 March, 1967], No; 13904/67. Heading H4F. An arrangement for selectively connecting individual ones of a plurality of input video channels to a single output terminal comprises gating means which are so primed by a control means that when two or more co-synchronous video signals above a predetermined threshold level are connected to respective input channels only one of the video signals, selected in accordance with a predetermined order of priority by the control means, is connected to the output terminal at any instant. In this manner one video signal may be utilized to inhibit and replace a second video signal over that portion of the picture frame occupied by the first signal, i.e. " overlay " may be produced. If the level of the selected signal falls below the predetermined threshold it is automatically replaced by the signal having the next highest order of priority and so on. As shown in Fig. 1, three input channels A, B, C are connected to respective threshold gates T the threshold levels of which are below that signal level which produces the darkest tones required in the final display and the outputs of which consist of an " 0 " when the inputs are above the threshold level and of a " 1 " when they are below this level. The gates T are connected as shown to three groups of three two-input NOR input gates IG which have as second inputs different ones of nine control inputs A1 . . . A3, B1 ...  B3, C1. . . C3. These may consist of a voltage V signifying an " 0 " or of zero voltage signifying a " 1 " and may be derived from the switch arrangement shown in Fig. 2. The outputs of each group of the two-input gates are then supplied to respective inputs of threeinput gates IG4, IG14, IG24 and then via inverting gates P to three further groups of three two-input NOR gates OG which have as second inputs the control inputs A, B, C, but in a different order to that employed for the IG gates. The outputs from the groups of OG gates are then supplied to respective linear gates L1, L2, L3 having as second inputs the video signal input to channels A, B and C respectively and the outputs of the gates L are combined at the output terminal D. The control means (Fig. 2) is such that only one of the three outputs of each of the groups A1 . . . A3, B1 ... B3, C1 ... C3 may be an " 0 " and thus the priority order of each input A, B, C is determined by a three-digit " word " one digit of which is " 0 " and the position of this " 0 " in the " word " indicates the priority order of the particular channel. Thus with the arrangement shown the " words " for the channels A, B and C are 110, 011 and 101 respectively so that channel B which has " 0 " in the first position takes first priority and channel C which has " 0 " in the second position takes second priority &c. Reference is made to the use of the apparatus in aircraft and shiptraining simulators in which the " overlay " effect is employed, e.g. to block that portion of a first target which is " obscured " by a second target between the observer and the first target.