An audio or other programme signal is modified to provide protection against copying on a magnetic tape recorder. The signal from a source (10) is amplified (16) and then subjected to Fourier analysis (11) to determine the signal amplitude in a plurality of subbands across the audible range. The subband containing the component of greatest magnitude is selected on an adaptive basis and applied to a stored psycho-sensorial model (12), which determines the maximum amplitude of interference which can be masked by the selected subband. An anti-copy signal is generated (14) by complex division of the desired interference by a stored representation of the response of a tape recorder (13).