A playback unit decrypts data contents of electronic audio and video media that are supplied in entirely or partially encrypted or enciphered form by means of one or more "melody" keys for encrypting the data contents. This key is transmitted via a secure channel from an authentic source into the playback unit, and then the playback unit transfers these data contents from the digital domain into the analog domain in such a way that the data contents of the electronic audio and video media are not present at any time in unencrypted form as a digital data stream that can be copied.