摘要
Transmitting video data over wireless networks can be very unreliable due to packet-loss, leading to serious video quality degradation which is annoying to human perception. The lost packets not only affect the quality of current frame, but also lead to error propagation to subsequent frames due to motion-compensated prediction techniques used in standard video codecs. Adding error-resilience to a video bitstream for robust video delivery to users thus becomes a very important issue. In this work, we propose a two-pass intra-refresh transcoding scheme for inserting error-resilience features to a compressed video at the media gateway of a three-tier streaming system. The proposed transcoder can adaptively vary the intra-refresh rate according to the video content and the channel's packet-loss rate to protect the most important macroblocks (MBs) against packet loss. In the first-pass encoding, the encoder estimates the amount of error propagation at MB level, and then generates side information as transcoding hints for use at the transcoder. In the second-pass transcoding, the error-resilient transcoder adaptively determines the intra-refresh rate and the locations of MBs to perform intra-refresh according to the side information. Experimental results show that the proposed method can effectively mitigate the error propagation due to packet loss so as to improve the visual quality significantly. The proposed transcoder can also meet the real-time requirement. © 2005 Elsevier Inc. All rights reserved.