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Digital Watermarking Based on Spread Spectrum Technique
在我們的研究上，我們是使用通訊系統中的展頻通訊原理來實現數位浮水印。本文將數位浮水印系統比擬成通訊系統，原始媒體資訊(影像)之頻率域為傳輸通道，而浮水印資料為傳輸訊號，可能遇到的攻擊或破壞則視為通道雜訊。展頻通訊已區分為直接序列展頻(Direct Sequence Spread Spectrum DSSS)與跳頻展頻(Frequency Hopping Spread Spectrum FHSS)二種，因此，我們針對這二種展頻技術而設計出跳頻展頻浮水印(FHSS Watermarking)與直接序列展頻浮水印(DSSS Watermarking)。
本論文最後提出融合跳頻展頻與直序展頻的Combined SS Watermarking技術。Combined SS Watermarking可同時擁有跳頻展頻浮水印與直序展頻浮水印的優點，如此，Combined SS Watermarking技術便可以抵抗較多種的破壞，達到提高數位浮水印強韌度之目的。
The advances of computers and communication networks make digital content service such as digital document exchange, E-commerce, video on demand, and digital library very popular. Since digital data can be easily reproduced without any loss, copyright protection becomes an imperative requirement to prevent piracy. Digital watermarking has been proposed as one of the techniques to prevent data piracy and plagiarism. Digital watermarking is a novel and emerging technology. It has developed very quickly for the past few years. A digital watermark is a set of information that is robustly and imperceptibly embedded in the data to be protected. Applications include copyright protection, and authentication.
In the thesis, we developed two watermarking techniques using spread-spectrum technique. We interpreted the watermarking technology based on communication theory. The watermark was the signal to be transmitted. The frequency domain of the original image was the transmission channel. The attacks were regarded as channel noise. There were two methods to implement the spread-spectrum system: direct- sequence spread- spectrum (DSSS) and frequency-hopping spread-spectrum (FHSS). Based on these tow methods, we develop FHSS Watermarking and DSSS Watermarking.
JPEG compression is the most widely used image because compression technique of its high compression ratio and reasonable image quality. Here, we develop the watermark embedding method especially for JPEG encoding process since block Discrete Cosine Transform (DCT) was used for JPEG. In our approach, a block DCT-based algorithm was used to embed the image watermarking. In order to survive the JPEG compression, the watermark must be embedded in the bands that can with stand the JPEG quantization.
Experiments showed that FHSS Watermarking is more resistant to the attack of JPEG compression. The PhotoImpact 5 digital image processing software was used for other attacks. We found that FHSS Watermarking is more robust than DSSS Watermarking in geometric distortions. But in other signal processing modifications, DSSS Watermarking is more robust than FHSS Watermarking. Therefore, we developed a new method that combined with FHSS Watermarking and DSSS Watermarking, we call the new method “combined SS Watermarking”. The experimental results prove that combined SS Watermarking has both the advantages of FHSS Watermarking and DSSS Watermarking. Therefore, combined SS Watermarking is more robust and can with stand many different attacks.
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