Abstract: The invention concerns a video alarm system for discriminating a video signal for detecting a movement or change in a scene under supervision by a television camera, the television picture being sub-divided into a plurality of areas which are individually evaluated to determine whether or not an alarm is to be raised.In order to avoid the release of a spurious alarm by fluctuations in basic brightness of the scene, caused for example by intermittent cloud cover, a particular area of the picture is investigated for variations in average brightness. If a brightness change greater than a predetermined threshold is detected in the selected area, any alarm signal initiated by the alarm system is suppressed.

Abstract: The invention concerns an apparatus for the amplitude discrimination of a video signal of the kind wherein the video signal is applied to a window discriminator which is adapted to provide a binary signal during periods when the amplitude of the video signal lies in a predetermined sub-range of the total amplitude range of the video signal.In order to avoid the presence in the binary signal of transient pulses which result from the slow rise time of the video signal when the latter effects a transition across an edge between an area of picture with the video signal amplitude below the said sub-range and an area of picture with a video signal amplitude above the sub-range, second and third binary signals are produced when the amplitude of the video signal lies above and below the sub-range respectively.

Abstract: In a video alarm system with digital evaluation the analog video signal generated by a television camera is converted into a binary signal by comparison with a threshold-value signal in an amplitude discriminator, and the binary signal is then evaluated, for example by summing. The threshold-value signal is in the form of an alternating voltage which sweeps over the black-to-white amplitude range of the picture information containing portion of the video signal a plurality of times during each line period. Compensation is provided for fluctuations of overall brightness in the scene from which the video signal is derived by inverting the binary signal to be evaluated during alternate sections of each line period.