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To prevent digital films being pirated, Hollywood wants to embed 鈥渨atermarks鈥�
in DVDs. These watermarks could be used to control copying or to identify the
source of illegal copies. But in GB 2 335 816, IBM admits it is difficult to
embed a watermark that can be read without spoiling the picture. So it has taken
a new tack: using a very weak signal that can be identified only gradually as
successive frames are played. The idea is that the DVD recorders soon to be
launched will be able to store strings of pictures, so they can recognise these
repeated sequences of faint watermarks.