因為網路頻寬會隨時變動，所以我們必須對壓縮過的影像做rate transcoding以符合我們估測到的網路頻寬。在本論文中，我們以位元控制的機制和頻寬估測的方法以達到上述的要求。因此，我們只需要一份壓縮過後的影像存放在server端。這樣也可以大大地節省在server端的儲存空間。我們實際實作出可重新編碼的伺服器，傳送經過重新編碼的串流資料，透過與Quick-Time播放器連接，觀賞影像資料。

　　本論文的第二部分是決定MPEG4 Advance Simple Profile中I、P、B框影像壓縮型態，大部分決定I、P、B框影像型態方法的論文中，通常會對一整個GOP影像作運算，並收集一整個GOP的可用資訊後，再判斷此GOP中I、P、B框影像壓縮型態的分布；我們提出的方法是動態地決定GOP架構，是我們只需運用幾張未知壓縮型態影像與前一個參考影像的相關性，而不須收集整個GOP的可用資訊，因此我們可以在較少的運算量與儲存資訊下，判斷出影像型態，且能即時地壓縮此張影像。

　　Due to dynamically changing bandwidth of network environment, rate transcoding of a compressed video is necessary to fit the estimated network bandwidth. In this thesis, a rate control algorithm and a real-time network bandwidth estimator are proposed for the above purpose. Therefore, only one copy of a compressed video source is necessary. This will greatly save the storage space on the server. We also implement the corresponding transcoding server and the bitstream can be accepted and viewed by using the popular Quick-Time player. The result is quite satisfactory. Real-time demo station is available.

　　The second part of the thesis is to deal with the determination of the I, P, B compression type of video frames for MPEG4 advance simple profile encoder. Most papers collect useful information of a whole GOP and then determine the distributions of I, P, B compression type in a GOP. We proposed a dynamic scheme to determine compression type. Much less computation and required memory space are needed in our scheme instead at the expected expense of reduced coding efficiency.

[1] Tihao Chiang and Ya-Qin Zhang, “A New Rate Control Scheme Using Quadratic Rate Distortion Model,” IEEE Transaction on Circuits and Systems for Video Technology, vol. 7, pp.246-250, Feb. 1997.

[2] S. C. Chang, J. F. Yang, C. F. Lee, and J. N. Hwang, ”A Novel Rate Predictor Based on Quantized DCT Indices and Its Rate Control Mechanism,” Signal Processing: Image Communication, vol. 18, pp.427-432, 2003.

[3] Zhijun Lei and Nicolas D. Georganas, “Rate Adaptation Transcoding for Precoded Video Streams,” Proceedings of the tenth ACM international conference on Multimedia, pp.127-134, December 2002.

[4] Limin Wang, Ajay Luthra, and Bob Eifrig, “Rate Control for MPEG Transcoders,” IEEE Transactions on Circuits and Systems for Video Technology, vol.11, NO. 2, pp.222-234, February 2001.

[5] Wei-Yuan Lin, “Design and Implementation for A PC Cluster-Based QuickTime Streaming Server,” NCKU Master Thesis, July 2003.

[6] Jungwoo Lee, and Bradley W. Dickinson, “Rate-Distortion Optimized Frame Type Selection for MPEG Encoding,” IEEE Transactions on Circuits and Systems for Video Technology, vol. 7, NO. 3, pp.501-510, June 1997.

[7] Xiaodong Gu, and Hongjian Zhang, “Implementing dynamic GOP in video encoding,” Proceedings of IEEE International Conference on Multimedia and Expo, vol. 1, pp.349-352, July 2003.

[8] Akio Yoneyama, Yasuyuki Nakajima, Hirornasa Yanagihara, and Masaru Sugano, “MPEG Encoding Algorithm with Scene Adaptive Dynamic GOP Structure,” IEEE 3rd Workshop on Multimedia Signal Processing, pp 297-302, Sept. 1999.

[9] Yutaka Yokoyama, “Adaptive GOP Structure Selection for Real-Time MPEG-2 Video Encoding,” Proceeding of IEEE International Conference on Image Processing, vol. 2, pp 832-835, Sept. 2000.

[10] “MPEG-4 Video Verification Model version 18.0”, ISO/IEC JTC1/SC29/WG11 N4350, July 2001

[11] RFC 3016, “RTP Payload Format for MPEG-4 Audio/Visual Streams”.

[12] RFC 1889, “RTP: A Transprot Protocol for Real-Time Applications”.