Spread Spectrum Technology Is Applied Research In Digital Watermarking

With the development of multimedia technology the protection of digital products become more and more important. Digital watermarking is a new technology as a tool of copyright protection and develops fast recently. An important technology of the watermarking is robust watermarking. In this thesis a watermarking algorithm based on the parallel combinatorial spread spectrum (PC/SS) is presented.The parallel combinatorial spread spectrum provides high spectral efficiency and the data rate of the parallel combinatorial spread spectrum is higher than that of the direct sequence spread spectrum. This thesis studies on the mapping between data and spread sequence in the parallel combinatorial spread spectrum system and presents a mapping method based on the lexicographic order. The mapping method resolves the many to one mapping problem of the past method.Spread spectrum technique can improve the robustness of watermarking. Most present algorithm of watermarking based on spread spectrum compare the extracted watermarks with the origin watermarks and it is a non-blind watermarking. In this thesis PC/SS is used in digital watermarking and a watermarking algorithm in the discrete wavelet transform domain is presented in this thesis. The watermarking algorithm embeds watermarks into the local mean value of the discrete wavelet transform coefficients and original images and watermarking copies are not needed in the extraction of watermarking. So, the watermarking presented in this thesis is a blind watermarking.The thesis also gives the optimal threshold of bits resume in the watermark extraction in details.Experimental results show that the watermarking algorithm presented in this thesis can survive general lossy compression, such as JPEG, noise and lowpass filter manipulation. The robustness of watermarking is related with the length of spread spectrum sequences. The longer the spread spectrum sequences are, the more robust the watermarking is.