A system for preventing the unauthorized copying of audio or video recording by (1) processing the recorded material so as to identify the protected material in a manner that does not audibly distort the program material, and (2) processing the recorded material by a second method that produces audible...http://www.google.com.tw/patents/US5394274?utm_source=gb-gplus-share專利 US5394274 - Anti-copy system utilizing audible and inaudible protection signals
