| As an effective method of multimedia copyright protection, digital watermarking has been a hot research area in information hiding field. Though the research and application about digital watermarking have had a significant advance in recent years, desynchronization attack is still one of the major challenges in digital watermarking domain. After desynchronization attack, the detector loses synchronization with the embedder, so that the watermark can not be detected correctly. The desynchronization attack is deemed to the bottleneck of digital watermarking application, so designing watermarking methods against desynchronization attack has large theoretical and practical significances.In this thesis we mainly study watermarking schemes with the ability to resist desynchronization attacks. Two major attacks that are translation and scaling are considered. The related techniques are first studied including the phase-locked loop, rational dither modulation, factor graph. Based on these techniques, watermark synchronization schemes are presented. The primary work included in this paper can be described as followed.Based on the conventional watermarking synchronization algorithm with phase-locked loop, we further investigate the effects of different interpolation algorithms and different kinds of watermarking hosts on the performance of timing error detector and watermark decoding. In addition, experimental test are utilized to evaluate the performance of the watermarking system in different data models (data-aided model and decision-directed model).To resist desynchronization attacks, a novel watermarking scheme using rational dither (RDM) modulation based on phase-locked loop (PLL) was proposed. Watermark is embedded by RDM and detected by joint synchronization and decoding. The detector adjusts the estimated attack parameters in PLL according to the results of RDM decoder. Then we use the new estimated parameters to aid watermark decoding, so as to improve the decoding performance. The simulation results show that this algorithm can resist translation and scaling attacks effectively.A new watermarking algorithm with the ability to estimate desynchronization parameters is presented based on factor graph. First considering Addition Gaussian White Noise (AWGN) channel, we detect the watermark by factor graph in the repetition code dither modulation watermarking system, and compare with the results of direct maximum a posteriori (MAP) decoder. Then in DA model, we estimate the scale parameters by factor graph and decode DM watermark. The experimental results show that the proposed scheme can estimate desynchronization parameters correctly and almost get watermark decoding performance as well as watermark with known scaling parameter. |
