How to Embed a Watermark

January 1997 / International Features / Look, It's Not There / How to Embed a Watermark

Embedding a watermark requires three functional components: carrier selector, watermark generator, and carrier modifier. A watermark carrier is a list of data elements selected from the original unwatermarked signal and used for encoding a watermark (i.e., a sequence of noise-like signals). The noise signals must be generated p seudorandomly, depending on secret keys, but independent of the carrier. Ideally, the signal should be at maximum amplitude but below the levels of perceptibility.
The carrier modifier adds the generated noise signals to the selected carrier. To balance the tolerance level of the additional noise and the robustness of the wate rmark, the noise must be scaled and modulated according to the strength of the carrier.
To find the right embedding parameters, psychovisual or psychoaudio analysis and predistortion are effective means of generating the appropriate amounts of noise and modulation. The idea of predistortion is to optimally adjust noise and scaling parameters by continuously controlling the watermark's resistance to image processing during the embedding procedure. This requires an iterative embedding and distortion sequence that continuously evaluates and adjusts the noise level.

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