When digital data is to be copy protected at a processing station, for example, prior to its encoding and recording onto an optical disc, it is necessary to control a compliant encoder to utilise copy protection files to perform the copy protection routine. To ensure that the digital data is reliably copy protected, predetermined noise signals are incorporated into the digital data and the processing steps which add the copy protection are additionally caused to remove the noise signals. By this means, if the digital data is not copy protected, the noise signals remain. This enables non-copy protected digital data to be readily identified and ensures that it is effectively unusable to a consumer.