The present invention relates to a method of copy protecting optical discs, such as CD-ROMs. Instead of encoding data onto the CD-ROMs in a manner which ensures balanced dc content, for copy protection the digital data is encoded and recorded onto the optical disc such that it is arranged to give some of the recorded digital data unbalanced dc content. Thus, for example, the DSV value of the encoded data may be changed rapidly in less than (100) frames to (FIG. 9). For an area of digital data on the optical disc in which the DSV has been caused to rise steeply (FIG. 10a), heavy distortions in audio arise (FIG. 10c) during an attempt to copy an audio track including the single segment of unbalanced dc content.