A biststream of encoded video data captured by a camera (4) of a user device (1, 2, 3) is received and decoded into a stream of decoded video frames. A light pattern representing a VLC signal captured by the camera (4) is identified in at least one decoded video frame and decoded into a DRM identifier. Distribution of the bitstream is controlled based on a comparison of the DRM identifier and a defined DRM identifier. Hence, VCL is used to add watermarks to recorded video data and where such watermarks can be used to control distribution of copyright or digital rights protected video content.