Analysis for Robustness of Spread spectrum watermarking

Robustness is one of the most important requirements when digital watermarking is applied. Different from StirMark test and various simulation tests， a mutual information function is defined as a criterion measuring the robustness of spread spectrum watermarking algorithm. As an example of the additive spread spectrum watermarking scheme， the calculation formula of mutual information function is derived to evaluate the robustness of algorithm. Numerical computation of mutual information is performed with change of watermark noise rate (WNR) under blind detection and nonblind detection. Error probability of watermarking scheme can be calculated to validate the measurement of mutual information function. In the experiment， middle and low frequency AC coefficients are selected as the host， and the statistic bit error rate is derived under the additive spread spectrum watermarking scheme and additive noise. Experiment results show evaluation conclusion of mutual information method is matched with that of empiric bit error rate (BER) against the WNR. Mutual information function can be selected as a cost function to evaluate the robustness of watermarking algorithm， and predict the BER.