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The delivery of high quality video program existing in a video server through heterogeneous networks is a fast emerging service and has attracted much attention recently. Transcoding technique that converts a pre-encoded video sequence from a high bitrate to a low bitrate is a key component in such a system. Based on our previously proposed VBR coding for fixed storage application, we propose in this paper a VBR transcoding architecture to accomplish video streaming over heterogeneous networks. First, we assume that the pre-encoded bitstream is generated using the proposed VBR encoding scheme at a relatively high bitrate. In addition, the relationship between the amount of bits for each frame and all possible quantization factors, i.e. rate (R) and quantization (Q) function, is also generated at the video server. Once the users provide their access requirements, the video server will optimally compute the appropriate quantization factors for each frame according to the constraints of desired bitrate and finite buffer size. Then the compressed bitstream will be decoded to DCT domain and requantized with the updated quantization factors. Because the generation of R-Q function is based on the original frame and is not related to the quantization factors, it is possible to reuse the motion information embedded in the compressed bitstream and implement the transcoding in the DCT domain. The computational expense required by this proposed transcoder is much lower than those required by schemes which decode the compressed bitstream to the pixel domain and transcode to the desired bitrate using a complete encoder. Furthermore, this VBR transcoding scheme is able to generate a CBR-like output while retaining all advantages of VBR encoding. This facilitates the delivery of VBR bitstream through various existing CBR channels. Experimental results demonstrate that our proposed VBR transcoding not only is capable of achieving higher mean PSNR and more consistent decoded visual quality, but also requires much less computational expense compared with conventional pixel domain CBR transcoder.
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