METHOD AND ARRANGEMENT FOR DETECTING A WATERMARK

摘要

Recently developed methods for copy protection rely on a watermark detector to judge wheter 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 (ythr). In accordance with the invention, the threshold value (ythr) 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 P(falsealarm) = erfc(ythr/ sigma ) in which erfc is the error function and sigma is said standard deviation of the information signal values.