A drift-constrained frequency-domain ultra-low-delay H.264/SVC to H.264/AVC transcoder with medium-grain quality scalability for videoconferencing

Scalable Video Coding (SVC) is an extension of H.264/ AVC, aiming to provide the ability to adapt to heterogeneous networks or requirements. It offers great flexibility for bitstream adaptation in multipoint applications such as videoconferencing. However, transcoding between SVC and AVC is necessary due to the existence of legacy AVC-based systems. The straightforward re-encoding method requires great computational cost, and delay-sensitive applications like videoconferencing require much faster transcoding scheme. This paper proposes an ultra-lowdelay SVC-to-AVC MGS (Medium-Grain quality Scalability) transcoder for videoconferencing applications. Transcoding is performed in pure frequency domain with partial decoding/encoding in order to achieve significant speed-up. Three fast transcoding methods in frequency domain are proposed for macroblocks with different coding modes in non-KEY pictures. KEY pictures are transcoded by reusing the base layer motion data, and error propagation is constrained between KEY pictures. Simulation results show that proposed transcoder achieves averagely 38.5 times speed-up compared with the re-encoding method, while introducing merely 0.71 dB BDPSNR coding quality loss for videoconferencing sequences as compared with the re-encoding algorithm.