STEGANOGRAPHY AND DIGITAL WATERMARKING Improving the performance of RDM watermarking by means of trellis coded quantisation

We propose a method to improve the performance of the recently introduced rational dither modulation (RDM) watermarking scheme. The improvement is obtained by modifying the essentially scalar nature of RDM with the introduction of a vector quantiser. The vector quantiser is based on a properly modified version of classical trellis coded quantisation, thus leading to the new trellis-coded RDM scheme (TC-RDM). Due to the infinite memory of the trellis TC-RDM relies on, the standard Viterbi algorithm is no longer optimum, hence we introduce a new suboptimal encoding algorithm that keeps the computational complexity reasonably low while ensuring a minimal loss with respect to the optimal scheme. The simulations we carried out on synthetic sequences show that TC-RDM permits improvement of the performance of RDM by 2–4 dB in terms of robustness against Gaussian noise addition.