Abstract: It is difficult to detect a watermark (40) in a video image sequence (20) if the image has been subjected (possibly by a hacker) to affine transforms such as scaling, rotation, flipping, etc. The transform carried out is generally unknown. Therefore, one or more inverse transforms (60) are performed to the image prior to detection (90) until a reliable decision can be made. The inverse transforms are performed with small stepsize variations of adequate parameters. In a preferred embodiment, an initial search for correlation is done between the inverse transformed image an a blurred version of the reference watermark, the blurred reference watermark being obtained by combining a number of e.g. rotated versions of the reference watermark. If some correlation has been found, the amount of blur and/or the stepsize is decreased. This requires fewer steps to detect the watermark.

Abstract: A method of caching teletext page data in a memory unit (21) of a teletext receiver system, comprises receiving and storing in the memory unit (21) data representing a sequence of teletext pages embedded in a signal belonging to a first one of a plurality of channels; and subsequently receiving and storing in the memory unit (21) data representing a sequence of teletext pages embedded in a signal belonging to a further one of the plurality of channels, whilst keeping at least some of the previously stored teletext page data stored in the memory unit (21), wherein for each received teletext page, it is determined whether the memory unit (21) has capacity available for storing the received teletext page data representing the received teletext page, and for each stored teletext page, information is stored associating the stored teletext page data representing the page with the channel to which the signal in which that teletext page data was embedded belongs.