Improved embedding efficiency and AWGN robustness for SS watermarks via pre-coding

Spread spectrum (SS) modulation is utilized in many watermarking applications because it offers exceptional robustness against several attacks. The embedding rate-distortion performance of SS embedding however, is relatively weak compared to quantization index modulation (QIM). This limits the relative embedding rate of SS watermarks. In this paper, we illustrate that both the embedding efficiency, i.e. bits embedded per unit distortion and robustness against additive white gaussian noise (AWGN) can be improved by pre-coding of message followed by constellation adjustment on the SS detector to minimize the distortion on the cover image introduced by coded data. Our pre-coding method encodes p bits as a 2 × 1 binary vector with a single nonzero entry whose index indicates the value of the embedded bits. Our analysis show that the method improves embedding rate by approximately p/4 without increasing embedding distortion or sacrificing robustness to AWGN attacks. Experimental evaluation of the method using a set theoretic embedding framework for the watermark insertion validates our analysis.