Watermarking low bit-rate Advanced Simple Profile MPEG-4 bitstreams

This paper presents a novel watermarking method for low bit-rate video that is compressed according to the Advanced Simple Profile of MPEG-4. A spatial spread spectrum watermark was embedded directly to the MPEG-4 bit-streams by adopting Hartung´s approach of watermarking MPEG-2 compressed bitstreams. A synchronization template was employed to combat cropping, scaling, and rotation. A gain control algorithm adjusts the local strength of the watermark depending on local image characteristics, in order to maximize watermark robustness and to minimize the impact on the quality of the video. A drift compensator prevents the accumulation of watermark distortion and reduces inter-frame interference of watermark signals due to motion compensated prediction in inter-coded frames. The developed watermarking algorithm was tested at bit-rates ranging from 128-768 Kbit/s. The watermark´s impact on visual quality as well as its robustness after decompression, scaling, rotation, sharpening, and noise reduction was evaluated.