The explosion of VoD and HDTV services opened a new direction in watermarking applications: compressed domain watermarking, promising at least tenfold speed increase. While sound technical approaches to this emerging field are already available in the literature, at our best knowledge the present paper is the first related theoretical study. It considers the ISO/IEC 14496-10:2005 standard (also known as MPEG-4 AVC) and objectively describes with information theory concepts (noisy channel, noise matrices) the effects of the real-life watermarking attacks (like rotations, linear and non-linear filtering, StirMark). All the results are obtained on a heterogeneous corpus of 7 video sequences summing up to about 3 hours