A Second Order Spread Spectrum Modulation Scheme for Wavelet Based Low Error Probability Digital Image Watermarking

 

D. A. Karras

Chalkis Institute of Technology, Automation Dept. and Hellenic Open University, Rodu 2, Ano Iliupolis, Athens 16342, Greece,

 

Abstract:

The goal of this paper is to introduce an integrated second order approach in using spread spectrum modulation techniques and the wavelet methodology in modelling low error probability digital image watermarking. The combination of FEC, Modulation Schemes, Multiplexing and Multiple Access techniques constitute the stage of channel coding. The approach adopted here for embedding the watermark is similar to the NEC scheme with regards to considering direct sequence spread spectrum modulation on the wavelet domain. However, this modulation occurs after modulating the digital watermark signal with an analogue one, which consists of the summation of a properly selected set of sinusoids. This further modulation is imposed to the watermark in order to enhance its resistance in noisy signal/image transmission environments. The proposed approach is, therefore, a second order watermark modulation/ embedding scheme. It is imposed after application of the Forward Error Correction (FEC-RS) methodology. The paper evaluates the merits of this second order embedding technique in different SNR ratios concerning binary sequences based watermarks embedded in 2-D images, using the probability of error in the watermark detection as a criterion. While modelling spread spectrum watermarking usually involves embedding a watermark in the spread spectrum of a signal/image, it is herein attempted to improve the robustness and fault tolerance of the transform domain watermarking approach by properly modulating the watermark, using a set of predefined sinusoids, so that even severe destruction of signal/ image blocks will result in the graceful degradation of the extracted watermark quality.