Unauthorized duplication of recorded audible program material upon magnetic tape, commonly known as tape piracy, is prevented and rendered detectable by the present method and apparatus for preparing the program material. The method and apparatus prepare recorded audible program material by generating a high frequency modulated signal which varies in a predetermined manner over a selected spectrum of frequencies, and by recording the modulated signal together with the audible program material so that subsequent copying of the program material and modulated signal onto magnetic tape with conventional high frequency bias will produce on the copied tape detectable and identifiable interference signals arising between the modulated signal and the conventional recording bias. The modulated signal may sweep a range of frequencies at a rate which will produce an audible playback noise spoiling the copied tape, and the modulated signal can be coded to identify the origin of the program material. In one embodiment, the method and apparatus record the modulated signal in superposition upon recorded program material, applying the modulated signal in square wave form to reduce modulation noise and rocking the recording head to intercept any azimuth deviation of subsequent playback means. In another embodiment, a frequency modulated signal is arranged to form the bias signal during the recording of the program material. The method and apparatus do not interfere with normal playback of the recorded program material since the modulated signal is beyond the audible range, but when copying onto magnetic tape is attempted, unacceptable interference signals, such as beat signals and cross-modulation signals, arise either to spoil the copy or to mark it in a detectable fashion as pirated copy.