Abstract: In a video mixer each input video signal is modified by only a single multiplier before its contribution is added to the contributions of other input video signals to produce the output video signal. The sums of the contributions of the video signals which participate at any time in the output signal corresponds to a predetermined value for the overall amplitude range. The mixing factor signals supplied to each of the mixers which are in line with the inputs are generated by two priority cascades of subtracters, multipliers and adders, each of which provides for the composition of an output picture, up to six video input signals and a switching system permits allocating priorities between the inputs in any desired way on a permanent or variable basis.

Abstract: The invention concerns an alarm system for detecting a movement or change in the field of view supervised by a television camera. A discriminator, for example an edge discriminator or an amplitude discriminator, produces a signal upon the occurrence of a predetermined characteristic in the picture content of the video signal from the camera. The number of such signals occurring during each of a plurality of successive television fields is compared with a reference value which is periodically updated. The reference value is derived by counting the total number of signals occurring during a plurality of field periods preceding that for which the comparison is effected and dividing the total by the number of fields thereby generating an arithmetical mean value. Each comparison generates a difference signal which is further evaluated to determine whether a relevant variation in the field of view of the camera has occurred, whereupon an alarm is released.

Abstract: A television-based alarm system comprises a plurality of television cameras connected in time-multiplex mode to a set of discriminators. The discriminators are sequentially connected to receive the video signals from the cameras, the arrangement being such that each discriminator receives the video signal from a predetermined camera. Each discriminator is adapted to generate a pulse upon the occurrence of a predetermined characteristic in the picture content of the video signal corresponding to a particular type of event in the field of view of the camera. The discriminating functions of the discriminators differ so that different types of event may be detected by the various cameras. The pulses generated by the discriminator at any time connected to a camera are logically combined with a masking signal which defines a portion or portions of the field of view from which pulses are not required. A plurality of different masking signals are provided and these are provided in sequence to the discriminators.