Management of use of information that is recorded on an optical disk

PHNL031053 An apparatus for processing data from an optical disk (20) generates a data signal by decoding information from the track (21) and a track position signal that is indicative of the radial position and/or depth of the track (21), and/or jitter in the position of edges of bit signals from the track (21). From the track position signal, values of plurality of characteristic measures are computed so that the computed values are substantially invariant under a phase of disk rotation, for example by determining the absolute values of Fourier transform components at multiples of the revolution frequency. Conditional use of the data signal is controlled dependent on the values computed for the characteristic measures. In one embodiment access is granted when the computed values match predetermined values for the disk. In another embodiment it is ensured that the disk is not removed from the apparatus during a session, by comparing a values that is sensitive to eccentricity with a value determined at the start of the session.