在資訊網路蓬勃發展的今天，視訊會議系統提供給我們一個透過網路與人面對面交談的環境。再多點視訊會議中，由於各用戶端的媒體及網路特性均不同，必須要有一個中央處理單元負責將資料流轉換成適合的格式及位元率，而視訊轉換編碼器便是實現這種轉換最好的一種方法。
傳統的視訊轉換編碼是採用固定畫框率的方式來實現。但是實際上一個影像序列中物件的動態特性是其實變化多端的，若是用固定畫框率的編碼方法將會導致編碼後視訊中動作的不流暢感，造成視訊品質的降低。因此若能夠根據一個視訊的特性而給予適當的畫框率，將可以達到較佳的影像品質與動態視覺效果。故藉由可變畫框率的機制，則可以同時對靜態影像品質與動態視覺品質進行最佳化，達到最佳的視訊品質。

本篇論文中，我們提出了一個畫框率的動態調整的方法。利用畫框率與動作程度等資訊來評估視訊的動態視覺品質。在靜態影像品質達一定水準的前提下，若是影像序列中物件的動作程度增加時，則提供較高的畫框率以提高其動態視覺品質。反之，若是在影像序列中物件的動作程度減緩時，則相對降低其畫框率以獲得較高的位元數提高其靜態影像品質。此外，加入了局部動作偵測(Local Motion Detection)以輔助偵測嘴、眼部等局部動作變化並加以補償，此有助於增進唇形同步(Lip Synchronization)。

本論文中所提出的方法，由於其少量的運算及適應性控制(Adaptive Control) 的方法，亦適合於真時實現(Real-time Implement)。

Video conference is an Internet application which provides an environment for people to see and talk to other persons who are far away.
A video transcoder is required in a multipoint video conference to convert input streams into different format, especially different bit-rate. Ordinary transcoder degrades temporal quality by setting a fixed frame-rate, but the requirement of motion smoothness changes with video properties. It’s necessary to adjust the frame-rate to a proper value to achieve better spatial and temporal quality.

We propose a dynamic frame rate control scheme to adjust frame-rate to fit the property of video. By using frame-rate and motion level of object as measurement of temporal quality, spatial and temporal quality can be considered at the same time. When bit-rate decreases, frame-rate will be decreased to improve frame quality. When motion level increases, frame-rate will be increased to provide better continuity.

Since the adaptive control method is used and all parameters are extracted from original transcoder processing, our proposed scheme is suitable for real-time applications.

[1] ITU-T Draft Recommendation H.263, “Video Coding for Low Bitrate Communication,” May. 1997.
[2] ITU-T/SG15, “Video codec test model, TMN8,” Portland, June 1997
[3] J. Ribas-Corbera and Shawmin Lei, “Rate Control in DCT Video Coding for Low-Delay Communications,” IEEE Trans. Circuits Syst. Video Technol., vol 9, No. 1, pp. 172-185, Feb. 1999.
[4] Jeongnam Youn, Ming-Ting. Sun and Chia-Wen Lin, “Adaptive Motion Vector Refinement for High Performance Transcoding,” IEEE Trans. On Multimedia, vol 1, No. 1, pp. 30-40 Mar. 1999.
[5] Tzong-Der Wu, Jenq-Neng Hwang and Ming-Ting Sun, "Video Transcoding," invited book chapter for Multimedia Image and Video Processing, L. Guan, S.Y. Kung, and J. larsen ed., CRC press.
[6] Ming-Ting Sun and I-Ming Pao, “Multipoint Video Conferencing,” Visual Communication and Image Processing, Marcel Dekker, C.W. Chen and Y.Q. Zhang ed., 1997.
[7] J. Youn, M.-T. Sun, and C.-W. Lin “Motion Estimation for High-Performance Transcoders,” IEEE Trans. Consumer Electronics, vol. 44, pp. 649-658, Aug. 1998.
[8] Chia-Wen Lin, Jeongnam Youn, Yung-Chang Chen, and Ming-Ting Sun, "A Study on Video Transcoder Architectures," IEEE Int. Symp. Consumer Electronics, Oct. 1998, Taipei, Taiwan.
[9] A. Eleftheriadis and D. Anastassiou, “Constrained and General Dynamic Rate Shaping of Compressed Digital Video,” ICIP ’95, 1995.
[10] P. A. A. Assuncao and M. Ghanbari, “Optimal Transcoding of Compressed Video,” in IEEE International Conference on Image Processing, Chicago, Oct. 1997.
[11] P. A. A. Assuncao and M. Ghanbari, “A Frequency Domain Video Transcoder for Dynamic Bit Rate Reduction of MPEG-2 Bit Streams,” IEEE Trans. Circuits Syst. Video Technol., Vol. 8, No. 8, pp. 953-567, Dec. 1998.
[12] J. Song and B.-L. Yeo, “A Fast DCT Domain Inverse Motion Compensation Algorithm Based on Shard Information in a Macroblock,” IBM Research Report RC 10880, June 1997.
[13] Jae-Beom Lee and Eleftheriadis, A. “Spatio-Temporal Model-Assisted Compatible Coding For Low And Very Low Bitrate Videotelephony,” Image Processing, 1996. Proceedings., International Conference on, Volume: 1 , 1996 , pp. 429 -432 vol.2
[14] Hwangiun Song; Jongwon Kim; Jay Kuo, C.-C. “Real-time H.263+ Frame Rate Control Algorithm for Low Vbr Video,” Circuits and Systems, 1999. ISCAS '99. Proceedings of the 1999 IEEE International Symposium on, Volume: 4 , 1999 , pp. 307 -310 vol.4
[15] Hwangjun Song; Jongwon kim; Jay Kuo, C.-C. “A Sliding Windows Approch To Real-Time H.263+ Frame Rate Adjustment,” Signals, Systems & Computers, 1998. Conference Record of the Thirty-Second Asilomar Conference on, Volume: 1 , 1998 , pp. 858 -862 vol.1
[16] Song, H.; Kim, J.; Jay Kuo, C.-C. “Improved H.263+ Rate Control Via Variable Frame Rate Adjustment And Hybrid I-Frame Coding,” Image Processing, 1998. ICIP 98. Proceedings. 1998 International Conference on, Volume: 2 , 1998 , pp. 375 -378 vol.2
[17] Chia-Wen Lin, Yung-Chang Chen, Ming-Ting Sun. “Video Transcoding Techniques For Multipoint Video Conferencing,” PHD Thesis, Dept. Electro. Eng., National Tsing Hua Univ., Hsinchu, Taiwan, Jan. 2000