The invention relates to a system for detecting a watermark using informed detection. A first signal potentially having a watermark embedded is received (601) as is a second signal corresponding to the original signal. The signals are segmented (605) into symbol segments. For each symbol segment a first characteristic is determined (607) for a first section and a second characteristic is determined (609) for a second section in response to the first and second signals in those sections. Specifically, ratios between average envelopes are determined. Thus, the first and second characteristic is indicative of the variations of the envelope during a watermark symbol. A watermark symbol estimate is determined (611) from the first and second characteristic. A sequence of estimated watermark symbols is compared to reference watermark symbols and the presence of a watermark symbol is determined (615) depending on the comparison. The invention is particularly suitable for improved detection of a multiplicative watermark.