An identification code signal is impressed on a carrier to be identified (such as an electronic data signal or a physical medium) in a manner that permits the identification signal later to be discerned and the carrier thereby identified. The method and apparatus are characterized by robustness despite...http://www.google.ca/patents/US5636292?utm_source=gb-gplus-sharePatent US5636292 - Steganography methods employing embedded calibration data
