In echo watermarking, effort to improve robustness often conflicts with the requirement of imperceptibility. They have been inherent trade-offs in general audio watermarking techniques. In this paper, we challenge to the development of imperceptible but detectable echo kernels being directly embedded into the high-quality audio signal. Mathematical and perceptual characteristics of echo kernels are analyzed in frequency domain. Finally, we can obtain much flat frequency response in perceptually significant bands by combining closely located positive and negative echoes. The proposed echo makes it possible to improve the robustness of echo watermark without breaking the imperceptibility.
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