本論文主要是在H.263視訊編碼系統的架構下，分析探討雜訊通道中傳輸錯誤對於壓縮視訊所造成的影響，並進而研究發展出一套完整的防錯機制。在H.263的標準本身就有提供起始碼，使其於錯誤發生時可以重新達到同步的功能。但不幸的是，如果錯誤就剛好發生在起始碼時，便可能會造成更嚴重的解碼錯誤，以致影像品質大受影響，甚至連解碼所得的畫面張數亦不正確。因此，我們發展出一套名為「視訊區段重整」的技術，在解碼端以前置處理的方式，利用區塊組的前後相連性偵測出起始碼與區塊組位址的錯誤，進而更正這些錯誤。除此之外，我們在編碼端亦利用一種資料隱藏的技巧，俾使解碼器得以偵測出編碼區塊錯誤真正發生之處。在編碼端先將前一張編碼畫面中每一區塊的同位檢測碼，隱藏在目前編碼畫面的移動預估向量或殘餘區塊量化值之中，解碼器便可經由取出這些同位檢測碼，來進一步偵測出原本可變長度碼解碼後所無法找到的錯誤。最後，我們在編碼端發展出一套名為「精確錯誤追蹤」的技術，以進一步防止錯誤蔓延效應的發生。將經過視訊區段重整與同位隱藏錯誤偵測過後的錯誤發生區塊位址，利用一回饋通道送回編碼端，編碼器即可依據這些資訊，利用畫面間像點反向移動的相關性，精確地計算出目前編碼畫面中受這些錯誤影響的程度，並將預計會受錯誤污染的區塊改以INTRA的方式編碼，如此一來便可防止錯誤再繼續蔓延下去。我們將所提出的技術在DECT系統環境下作模擬實驗，結果證實在32kbps下，比起單純使用移動補償之錯誤隱藏技術可大幅提高整體視訊品質的訊雜比達4至6dB之多。更值得一提的是，我們所發展的技術架構均與H.263標準相容，並且具備低運算複雜度與低記憶體需求等優點，尤其適合實際應用與硬體實作。 This dissertation presents an error resilient H.263 video compression scheme over noisy channels. The start codes in the H.263 bit stream syntax, which inherently provide the resynchronization functionality for error handling, may cause significant error damage if they are incorrectly decoded. Therefore, we develop a video segment regulation algorithm at the decoder to efficiently identify and correct erroneous start codes and block addresses. In addition, a parity-embedded error detection technique is also implemented. At the encoder, the requisite parity-check codes for all macroblocks in the previous frame are embedded into the motion vectors and the quantized residual DCT coefficients of the current encoding frame. Then the decoder can effectively manipulate these embedded bits to enhance the error detection capability at the macroblock-layer. Finally, the precise error tracking technique is used to further reduce the error propagation effects. After performing the video segment regulation and the parity-embedded error detection, the decoder can report the accurate addresses of detected corrupt blocks back to the encoder via a feedback channel. With these negative acknowledgments, the encoder can precisely calculate and trace the propagated errors by examining the backward motion dependency for each pixel in the current encoding frame. With this precise tracking strategy, the error propagation effects can be terminated completely by INTRA refreshing the affected blocks. Simulation results show that the proposed scheme yields significant video quality improvements over the motion compensated concealment by gains of 4 to 6 dB PSNRs at bit rate around 32 kbps in the error-prone DECT environments. In particular, this scheme complies with the H.263 standard and has the advantages of low memory requirement and computation complexity that are suitable for practical real-time implementation.
