The method comprises: —generating a marked video signal by extracting an image fragment (V) from a region (R) of an original frame (F), and arranging a watermark (M) thereon; and —generating a data signal, including therein the extracted image fragment extracted (V) and/or reconstruction information; —performing an additional synchronisation step, which comprises: —obtaining, from the data signal, the image fragment (V) and/or the reconstruction information, —obtaining, from the marked video signal, an incomplete frame (Fr), and —attempting to reconstruct the original frame (F) from the information obtained from the data signal and the marked video signal, and, if the reconstruction is successful, determining that the synchronisation is correct. The system is adapted to implement the method of the invention.