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5 213 337 A 5/1993 Sherman raudulent activation of a time gate, the time gate device can 5: 270: 42 “) A 12/1993 Hikawa be con? gured to react only to Watermarks commg from live 5,319,735 A 6/1994 Preuss et a1_ broadcasts, and not from replays from tapes or other storage 5,581,800 A 12/1996 Fardeau et a1_ devices. In another feature, robustness of the Watermark is 5,719,619 A* 2/1998 Hattori et al................. 725/24 improved, eg, for acoustic propagation channels, by shift 5,752,880 A 5/1998 Gabai et a1. ing the detection time of the Watermark based on a measured 5,822,360 A 10/1998 Lee et a1~ bit error count of the Watermark. Furthermore, the Water 5’822’432 A* 10/1998 Moskowltz et al'""""" 380/28 mark may be inserted before the desired action along With

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