METHOD AND ARRANGEMENT FOR DETECTING

Recently developed methods for copy protection rely on a watermark detector to judge whether multimedia content can be copied or not. In such copy protection schemes, a watermark detector examines the multimedia content and outputs a signal (D) indicating whether a watermark is present or not. Known watermark detectors determine a decision variable (y) indicating to which extent the watermark is present, for example, the amount of correlation between input signal and a reference copy of the watermark to be detected. The watermark is detected if the decision variable exceeds a predetermined threshold (y.sub.thr) In accordance with the invention, the threshold value (y.sub.thr) is adaptively controlled in dependence upon statistical characteristics of the information signal and a desired probability of false alarms (watermark detected whereas the signal is not watermarked). In an embodiment, the watermark detector determines the standard deviation (.sigma.) of the pixel values threshold level and calculates the threshold value in accordance with the relation ##EQU1## in which erfc is the error function and .sigma. is said standard deviation of the information signal values.