In spread-spectrum watermarking, the watermarked document is obtained from the addition of an attenuated watermark signal to a cover multimedia document. A traditional strategy consists of optimising the detector for a given embedding function. In general, this leads to sub-optimal detection and much improvement can be obtained by exploiting side-information available at the embedder. In some prior art, the authors showed that for blind detection of low-power signals, maximum detection power is obtained to first order by setting the watermark signal to the gradient of the detector. In the first part of the paper, we develop this idea further and extend Costa's decoding theory to the problem of watermarking detection. In the second part, we propose a practical implementation of this work using non-linear detectors based on our family of polynomial functions which demonstrate some
improved performance of the technique. Finally, the robustness of our side-informed scheme is assessed by computer simulations on real audio signals in the presence of additive, multiplicative and coloured noise resulting from perceptual coding for a low watermark to content power ratio.