Method of detecting a watermark embedded in a signal (S), in which a plurality of frames of the signal (S) is combined to a detection set (Dj) for one detection event. According to the invention, the reliability of watermark detection is enhanced by using non-consecutive frames to form the detection set (Dj). The invention also relates to an apparatus (2) for recording and/or playback of a signal, and to a system for broadcast monitoring, comprising such a watermark detector (24).