Speech watermarking for analog flat-fading bandpass channels
IEEE Transactions on Audio, Speech, and Language Processing
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Watermarking technology can be beneficial in digital rights protection. However, the industry's acceptance of the technology has been lukewarm as experts have been able to hear audible artifacts introduced during the watermarking process. In this paper, we present what we believe to be a truly inaudible solution to this problem. Our proposed watermarking technique embeds the watermark signal in the phase of an audio signal, with secrecy as to which frequency components carry the watermark bits, achieved via a pseudorandom generator. Inaudibility is realized by exploiting the human auditory system's insensitivity to absolute phase. Further, our algorithm includes a novel mechanism for segmenting an audio signal into variable frame-lengths to provide robustness against de-synchronization attacks such as jitter and time-scaling. It uses a short-time Fourier transform to first characterize local changes in the frequency content of an audio signal, from which, pairs of frequencies satisfying specified conditions are identified, to mark the start and end of a segment. The insertion of synchronization marks adds further robustness against such attacks. Robustness against other common attacks may be further enhanced through the use of concatenated error-control codes which enable the correction of random and/or burst errors, which may be introduced during an attack.