Abstract: A copy protection system for computer discs in which a key sequence of 1101 in a copy protection section of the program is deliberately rendered ambiguous by repeatedly recording a word containing that sequence while gradually shifting the phase of the transition forming the central "1" of the sequence until the sequence becomes 1011. When this series of words is repetitively read, successive reads will be inconsistent with one another. The gradual phase shift cannot be reproduced if the disc is copied on a standard personal computer, so that the inconsistency on successive readouts will not be present in a copied disc. The program is so designed that it cannot run unless inconsistencies are detected in the copy protection section of the program. The provision of "1"s on each side of the central, variable two-bit sequence prevents spurious loss of synchronization on the copied disc which could be interpreted as an inconsistency.