The present paper introduces a new insertion/detection technique for MPEG-4 AVC stream protection by means of watermarking techniques. The embedding process relies on the ST-DM (Spread Transform - Dither Modulation) principles and adapts/extends the Watson's perceptual model so as to cope with the compressed domain peculiarities. In contrast to the state-of-the art techniques considering only binary insertion techniques, our paper demonstrates the first m-ary DM technique; in such a way, for a prescribed transparency and robustness, the data payload is increased by a factor of log2m. The experimental results correspond to the MEDIEVALS (French National Project) corpus of 4 video sequences of about 15 minutes each. The robustness against noise addition, transcoding and geometric (StirMark) attacks is proved. The transparency is assessed by both subjective and objective measures. Published in: