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Video transcoding is proposed for the bitrate adaption, spatial and/or temporal resolutions adaption, and video format conversion. In video streaming application, it converts videos at server to the compatible versions demanded by networks or clients' devices, so that the videos can be delivered over networks and displayed in the clients' devices successfully. This paper provides a universal rate control scheme for various video transcoding purposes. First, a new rate-distortion (R-D) model is established theoretically for better representing the real R-D feature of transcoding. Second, a window-level rate control algorithm is proposed for providing smooth visual quality with compliant buffer constraint by utilizing the two-pass R-D model and a new proposed sliding window buffer control strategy. Finally, a universal rate control scheme for transcoding is developed based on the established R-D model and the proposed window-level rate control algorithm. The extensive experimental results demonstrate that as compared to other state-of-the-art rate control algorithms for transcoding, the proposed scheme can achieve more bit control accuracy with the average mismatch below 0.2%, and much more consistent visual quality with 0.1 dB-0.3 dB peak-to-signal noise ratio improvement in average, while with low computational complexity.