Nowadays, robust watermarking clearly identified its functionality within the multimedia production chain, from the content creation to the end-user consumption: property right identification and copy-maker tracking. In the quest for the speed required by today's real-time applications, compressed-domain watermarking becomes a hot research topic. This study evaluates the watermarking capacity in the MPEG-4 AVC domain in order to establish whether and to what extent compressed domain watermarking is viable. In this respect, the additive watermarking techniques are modelled by discrete noisy channels with non-causal side information at the transmitter. The study considers several attacks (linear and non-linear filtering, geometric) and computes the capacity of the corresponding channels. The experimental results are obtained out of processing a natural video corpus of 10 video sequences belonging to different movies, each of them about 25 minutes long (35000 frames in each video sequence)