在即時、雙向的視訊傳輸環境中，如何減少傳輸時所造成的延遲是一個非常重要的課題。因為資料流在編碼器的緩衝區中所停留的時間必須非常小，一個可維持高畫質並可避免緩衝區溢位的流量控制演算法是必要的。其中一個可降低緩衝區溢位方法便是略過某幾張影像而不做編碼的動作，以降低緩衝區的使用率，但此舉會造成視覺上的不連續。在此論文中，我們將討論低延遲對於H.264流量控制演算法的影響以及改進的方案。我們發現H.264流量控制演算法對於跳張(frame-skipping)的情形無法處理的很好。為了改進此缺點，我們調整每張I-frame的量化參數來避免緩衝區溢位與跳張的問題。對於一畫面群(GOP)的同一張I-frame而言，使用不同的量化參數(quantization parameter)會對此畫面群產生不同的編碼量(rate)及失真(distortion)大小。針對此一特性，我們使用Lagrangian最佳化來找出可以平衡編碼量和失真大小的最佳解。藉由可預估畫面群之編碼量以及失真的數學模型，針對每張I-frame所有可能的量化參數來計算所產生的Lagrangian成本。對於此張I-frame，具有最小Lagrange成本的量化參數便是最佳的選擇。實驗結果證明，在低延遲環境的限制下，無論在跳張的數目或是畫質方面，我們的方法都比標準的H.264流量控制演算法有更好的效能。

In real-time, two way video communications, how to minimize the end-to-end delay for transmitting video data is very important. Since the delay produced by bits accumulated in the encoder buffer must be very small, we need an improved rate control to encode the video with high quality and maintain low buffer fullness. One approach to reduce the buffer fullness is to skip the encoding frames, but the frame-skipping will produce undesirable motion discontinuity in the encoded video sequence. In this thesis, we study the impact of low delay constraint in H.264 rate control and its improvements. The drawback of the H.264 rate control is it cannot handle the frame-skipping mechanism well. To modify this, we control the quantization parameter of each I-frame to avoid the buffer overflow and frame-skipping. Since encoding the I-frame by different quantization parameter will generate different rate and distortion for a group of pictures (GOP), we use Lagrangian optimization to find the tradeoff between rate and distortion for a GOP. By the estimation models of rate and distortion for a GOP, calculate the Lagrangian cost for each possible quantization parameter of I-frame, the quantization parameter with minimum Lagrangian cost will be our choice for I-frame. Simulation results show that our proposed rate control encode the video sequence with less skipped frames and with higher PSNR compared to H.264 rate control under low delay constraint.

Abstract I
中文摘要 II
Contents III
List of Figures IV
Chapter 1 Introduction and Motivation 1
1.1 Introduction to Rate Control 1
1.2 Introduction to Low Delay Rate Control 3
1.3 Motivation 4
Chapter 2 Background and Relative Work 7
2.1 H.264/MPEG-4 Part 10 7
2.1.1 New Features 7
2.1.2 Introduction to H.264 Encoder 8
2.2 Rate Control for MPEG-4 9
2.2.1 Quadratic Rate Distortion Model 9
2.2.2 Framework of Scalable Rate Control 10
2.2.3 Other Improved Rate Control Schemes for MPEG-4 14
2.3 Rate Control for H.264 16
2.3.1 Terminology 18
2.3.2 Overview to the Rate Control Scheme 19
2.3.3 GOP Layer Rate Control 20
2.3.4 Frame Layer Rate Control 21
2.3.5 Basic Unit Rate Control 23
2.4 Lagrangian Optimization 24
Chapter 3 Rate control for Low Delay Communication 25
3.1 The Proposed Rate Control for Low Buffer Delay 25
3.2 Rate Estimation for I-frame 27
3.3 Distortion Estimation for I-frame and P-frame 33
3.4 QP Estimation for I-frame 35
Chapter 4 Simulation Results 39
Chapter 5 Conclusion and Future Work 42
Reference 43

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