Method and apparatus for decoding watermark information items of a watermarked audio or video signal using correlation

Watermark information consists of several data symbols which are embedded continuously in an audio or a video signal using modulation with a pseudo-random sequence if spread spectrum technology is used. At decoder site the watermark information is regained using correlation of the received signal with a candidate pseudo-random sequence. Such correlation can be performed by one-dimensional real-symmetric fast Fourier transform of the two input signal vectors, which each consist of a section of N values of the input signal, to which section N-1 zeroes are attached, by multiplying one of the transformed vectors by the conjugated version of the other transformed vector, followed by inverse RFFT transform of that frequency domain product vector. However, if the level of a spread-spectrum watermark is too low or if too much noise had been added during the transmission of the audio or video signals, the correlation does not show a clear peak, which means that the watermark information bits cannot be recovered. According to the invention, the correlation result is improved by setting to zero in the frequency domain frequency bins of the vector which do not have a positive impact on the correlation peak at location zero, i.e. which frequency bins do not contribute to that peak. Thereby the noise level in the correlation or matched filter output is reduced and the watermark information items can be recovered or decoded correctly from the received watermarked audio or video signal.