Abstract: In a extraneous of signal method of preventing detection of signal information from raster scan signals in a plurality of consecutive line signals intended to form an image or partial images there is generated a random signal sequence, correlated with the information-carrying raster scan signals, but without information content, which is transmitted as protection for the raster scan signals. An apparatus for this purpose includes a generator (10,12) adapted for generating a random signal sequence synchronous with the raster scan signals, and a switching element (5) adapted to transmit the random signal sequence round the equipment containing the raster scan signals that are to be protected.