H.263視訊無線傳輸抗誤技術之研究

Abstract

在這篇論文中，我們對於視訊影像在無線通道上傳輸時，因為雜訊干擾所造成的效應作了一些討論。同時我們也對過去曾提出的各種錯誤修正方法做了一次簡短的回顧與描述。除此之外，我們藉由簡單的分類來考慮誤差控制的方法，也就是由訊源編碼和多工(multiplexing)兩個層級來考慮。 首先，在多工級方面，我們對因為使用了可變長度編碼所造成的錯誤做討論。由對H.263編碼排列方式與其誤差敏感度的分析，我們提出了一個不同於標準所制訂的傳輸順序，用以降低因為失去同步碼所造成的錯誤蔓延。接下來，我們使用了分槽多工(SM)，利用把可變長度區塊放置到固定長度空位內，來縮短解碼失去同步時再取回同步碼的時間延遲。 在訊源編碼層級，我們於編碼端將各個區塊的運動向量作匹配，而在解碼端利用這匹配的運動向量做正確性檢查，並在錯誤產生時加以更正。經由這個方法，可避免因錯誤蔓延而導致一整帶的區塊同時錯誤位移。最後，對於取得的影像我們進行後處理。利用大部分自然影像所具有的最大平滑特性，我們找出錯誤發生的區塊，並藉著使相鄰影像區塊變異度最小來使得修復過後的影像變化能盡可能地平緩。 我們利用電腦模擬來驗證這些方法的效能。有些方法結果相當不錯，也有些方法不如預期。而這些原因都會在每個章節中加以討論。

In this thesis, we discuss the error effects when transmitting video sequences over a wireless channel. On the other hand, we review some methods proposed in recent years to combat the errors. Besides, we also implement some error concealment techniques by considering them as two levels, namely, the multiplexing level and the source codec level. At the multiplexing level, first, we discuss the error effects that result from the variable length coding. By analysing the error sensitivity of the H.263 syntax arrangement, we use a new transmitting order to alleviate the effects of synchrnoization loss. Then we consider a technique called slotted multiplexing (SM) which places the variable length blocks into fixed length slots to shorten the duration before regaining the synchronization codeword. At the source codec level, we do the motion vector paring in the encodr and a corresponding motion vector checking and recovery in the decoder. By these methods, corrupted strips in the images caused by error propagation from loss of synchronization are avoided. Finally, we do some postprocessing in the decoder. By using the maximally smooth property, which is the natural property of most images, we find out the erroneous blocks. Then we conceal the erroneous blocks by minimizing the variation between them and the adjacent blocks so that the repaired images are as smooth as possible. The effectiveness of these techniques we implemented are examined by computer simulations. Some of them look well, while some are not as good as we expected. The reasons are also discussed. 2 A Brief Introduction to H.263 2.1 Source Coding Algorithm 2.2 Source Format 2.3 Encoding Process 2.3.1 Prediction 2.3.2 Motion Compensation 2.3.3 Transform and Qunantization 2.4 Syntax and Semantics 2.4.1 Picture Layer 2.4.2 Group-of-Blocks Layer 2.4.3 Macroblock Layer 2.4.4 Block Layer 2.5 Decoding Process 2.6 H.263 Optional Modes 2.6.1 Unrestricted Motion Vector Mode 2.6.2 Syntax-Based Arithmetic Coding Mode 2.6.3 Advanced Prediction Mode 2.6.4 PB-Frames Mode 2.6.5 Other Optional Modes 3 The Mobile Transmission Environment, Channel Noise Effects and Concealment 3.1 Mobile Transmission Environment 3.1.1 Effects of Wireless Channel Noise 3.1.2 Jakes' Model 3.1.3 Simulated Environment 3.2 Effects of Transmission Errors 3.2.1 Error Effects in Source Coding Level 3.2.2 Error Effects in Multiplexing Level 3.3 Brief Introduction to Error Control and Concealment 3.3.1 Error Detection 3.3.2 Error Concealment 4 Combat of Error Effects at the Multiplexing Level 4.1 H.263 Syntax Rearrangement 4.1.1 Syntax Dependency in Different Layers 4.1.2 Rearrangement of Syntax Transmission Scheme 4.1.3 Recovery of DC Coefficients in Intra Pictures 4.1.4 Concealment of COD in Inter Pictures 4.1.5 Simulation and Summary 4.2 Slotted Multiplexing (SM) 4.2.1 The SM Algorithm 4.2.2 Error Propagation in SM 4.2.3 SM with Interleaving 4.2.4 Simulation and Summary 5 Combat of Error Effects at the Source Codec Level 5.1 Motion Vector Pairing 5.1.1 Proposed Scheme for Motion Vector Pairing 5.1.2 Motion Vector Pairing in the Source Encoder 5.1.3 Error Concealment in the Source Decoder 5.1.4 Simulation and Summary 5.2 Postprocessing at the Decoder 5.2.1 Error Detection Using Maximally Smooth Property 5.2.2 Adaptive Error Detection 5.2.3 Coefficients Recovery with DC Compensation 5.2.4 Best Neighborhood Matching 5.2.5 Simulation and Summary 6 Conclusions and Future Work Bibliography