A digital recording medium such as an optical disc is recorded using an
encoding process this is modified to cause a peak or rapid change in the
digital sum value in the encoded data for part of the recording data.
This provides the corresponding part of the recorded medium with a
modified DC characteristic that affects an operational characteristic of
the recording medium. This modified DC characteristic will not be present
in unauthorized copies of the recording medium because the normal
encoding process will encode the recording data so as to minimize DC
imbalance. Profile data is obtained from operational data acquired during
operation of the recording medium. This profile data is compared with
expected profile data that is the profile data that would be expected to
be obtained if the modified DC characteristic is present. The results of
this comparison are used to determine whether or not the recording medium
is authentic.