A local phase filter (315) for pre-processing data (310) including a watermark (320) is disclosed. The data (310) is pre-processed before detecting the watermark by correlation of the data (310) with the watermark (320). The local phase filter (315) starts by dividing the data (310) into a set of smaller sub-spaces. The frequency coefficients of the data contained in each sub-space are then computed, and the frequency amplitudes of the frequency coefficients are then spectrally shaped. Preferably, the amplitudes of the frequency coefficients of the sub-space are set to a predetermined function. Finally, filtered watermarked data is formed, having the same size as that of the data (310), by adding the filtered data of each sub-space together in the sub-space's original position.