The application of nonlinear bistable detectors to DCT-domain watermarking schemes

Abstract

A DCT-domain watermarking scheme, based on nonlinear bistable detectors, is presented. A binary copyright character, i.e. watermark, is firstly reordered into a binary zig-zag sequence, and then mapped into pulse amplitude modulated waveforms. A certain desynchronization time delay can be arbitrarily placed into one code of the modulated signal, and is tolerated due to the superior robustness of nonlinear detectors over matched filters. The watermark signal is then embedded in a selected set of DCT coefficients of an image in a medium frequency range. The selected set of DCT coefficients is shuffled via the Arnold transform and looks more like background noise with respect to the watermark signal. The copyright character can be extracted by the nonlinear bistable detector without prior knowledge of the original image, i.e. blind watermark detection. Interestingly, a higher match between the original watermark character and the extracted one can be further achieved using a parallel array of bistable detectors via the mechanism of array stochastic resonance. Robustness of the proposed watermarking scheme is shown in the presence of noise, filtering, cropping and compression.