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Practical jokers and disgruntled staff have been known to insert an offensive
message or image into just one or two frames of a TV programme. No one will
notice a message this short until the clip is slowed or viewed as a
freeze-frame. So Philips in Eindhoven has devised an analyser that monitors the
brightness and colour content of each frame (WO 01/50730). Any scene change
registers as an abrupt switch. If there are two abrupt changes within a few
frames, an alarm warns the video editor to rerun that section in slow motion so
they can check what’s there.