1. A controller for controlling displaying on a display means of received video or image data which has been encoded with invisible digital watermarks integrated within the received video or image data and representing a content classification for the video or image data, the controller having connections for connecting to a display means and for connecting to a data receiver or data reading apparatus, the controller comprising: means for detecting digital watermarks integrated within the received video or image data, said received video or image data representing content for display; means for decoding the digital watermarks to obtain content classification information; and means for controlling the displaying of said video or image data in response to the content classification information obtained from the watermarks.