Multiple watermarks for stereo audio signals using phase-modulation techniques

Audio watermarking techniques are used to embed extra information, typically relating to copyrights, into audio signals. In some applications, embedding of multiple watermarks in a single multimedia content is required. Considering an actual situation, it might be necessary to embed up to three kinds of information as watermarks, i.e., "copyright management information," "copy control information," and "fingerprints." For this purpose, a new watermarking technique capable of embedding multiple watermarks based on phase modulation is proposed in this paper. The idea utilizes the insensitivity of the human auditory system to phase changes with relatively long transition period. In the proposed technique, the phase modulation of the original signal is realized by means of a time-varying all-pass filter. To accomplish the blind detection which is required in detecting the copy control information, this watermark is assigned to the inter-channel phase difference between a stereo audio signal by using frequency shift keying. Meanwhile, the copyright management information and the fingerprint are embedded into both channels by using phase shift keying of different frequency components. Consequently, these three kinds of information are simultaneously embedded into a single time frame. Imperceptibility of the watermark was confirmed through a subjective listening test. Robustness against several kinds of signal processing were evaluated by computer simulations. The proposed method provided good performance in both those subjective and objective tests.