Surveillance systems

A surveillance system has an image pick-up sensor (1), an image processing unit (30) connected to the image pick-up sensor (1) for detecting a scene change of a video signal obtained from the image pick-up sensor, an alarm generator (13,24) connected to the image processing unit (30) for generating an alarm signal upon detection of a scene change, and a self-tester including a noise signal generator (27) and means (23) for superimposing a noise signal from the generator (27) on the video signal to be supplied to the image processing unit (30) as quasi-scene change information.