Research On Digital Watermarking Using CDMA

Digitized Multimedia (image, video, audio, text, etc) greatly provides convenience for information access. Along with the development of computer network and communication technology, information exchanging and transferring becomes a respectively simple process. As the side-effect of distributing in Internet environment there exists the possibility that some unauthorized person or organization will illegally copy, edit or distribute the multimedia works with copyright. These cases will depress the digital multimedia originator and halt the marching of on-line service. Therefore, how to take effective measures to protect copyrights and guard the security of information exchanging emerges recently. So as an effective way to protect the multimedia copyrights, digital watermarking has brought much attention of researchers.Robustness and Imperceptibility are two important requirement of digital watermarking. In order to improve the robustness and imperceptibility, the watermarking system is often regarded as communication system. And many advanced techniques in communication system have been brought into watermarking system, such as channel encoding & decoding, especially the spread-spectrum technology. Due to numbers of advantages, such as high resistant to noise and multi-path interference, high security, and high utilization of spectrum, the spread-spectrum technology is widely applied in watermarking system to improve its robustness and imperceptibility at the price of decreasing in capacity for using spread-spectrum encoding.Recently, in many papers, the CDMA (Coding Division Multiple Access), based on spread-spectrum technology, has also been introduced into watermarking system. But most researchers so simply regard it as one sort of spread-spectrum technique, as to ignore its characteristic of CDMA (Code Division Multiple Access) using orthogonal code. So it is necessary to make further research about how to take the full advantages of CDMA in watermarking system.In this paper, CDMA technique is exploited to enhance the robustness of watermarking, improve its capacity and realize a multiple-user digital watermarking in DCT (Discrete Cosine Transform) domain. Normally, in order to keep the balance of robustness and imperceptibility, we need to do experiments repeatedly to get the relevant value of coefficient for embedding. With the aim of resolving this inconvenience, a general model for applying the CDMA technique in watermarking system is proposed at the end of paper. With the model, we can estimate the value of coefficient for embedding in theory.