The present paper is devoted to the MPEG-4 AVC (a.k.a. H.264) video stream protection by means of watermarking techniques. The embedding process is carried out on quantized index domain and relies on the m-QIM (m-arry Quantisation Index Modulation) principles. In order to cope with the MPEG-4 AVC peculiarities, the Watson's perceptual model is reconsidered and discussed. The experimental results correspond to the MEDIEVALS (a French National Project) corpus of 4 video sequences of about 15 minutes each, encoded at 512kbps. The transparency is assessed by both subjective and objective measures. The transcoding (down to 64kbps) and geometric (StirMark) attacks result in BER of 6.75% and 11.25%, respectively. In order to improve robustness, an MPEG-4 AVC syntax-driven counterattack is considered: this way, the two above mentioned attacks lead to BER of 2% and 10%, respectively. Finally, the overall theoretical relevance of these results is discussed by estimating the related channel capacities.