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 .