最新發展出的H.264視訊標準，不管在時間域或空間域都使用較複雜的預測方式，使其壓縮效率較以往的標準還要好，但在網際網路上傳送時一旦發生錯誤則可能會造成嚴重的錯誤漫延效應。因此本論文的目標便在於發展一套強健性抗誤性機制來完全制止錯誤漫延，此機制稱為同步式逆向錯誤追蹤演算法(SBET)。 若編碼端的狀態能和解碼端的狀態同步，則錯誤蔓延的效應就能完全被制止，因此我們假設有回授通道存在且編碼端可由某些方式知道解碼端所使用的錯誤隱藏機制。再藉由以像素為單位的精確性逆向錯誤追蹤方式(PBET)來追蹤錯誤位置所在，並且將錯誤畫面的隱藏誤差帶往之後受到該錯誤畫面汙染的區域，進而重建解碼端的狀態，使得編碼端和解碼端的參考畫面暫存器之狀態可達同步。所提方式的計算複雜度只包含記憶體存取、加法和乘法運算，相較於對H.264重新編碼的過程來說複雜度較低許多。而從模擬結果來看，所提方式的碼率失真表現都較以往的錯誤追蹤方式為佳。譬如，就QCIF的Foreman序列來說，在封包錯誤率為3%的情形下，SBET較PBET最好可高1.21dB。此外，由於沒有使用強迫性的INTRA更新機制，所以沒有位元率暴增的現象。而如果未來錯誤隱藏機制做得更好，則SBET也會隨之提供更好的表現。 The most recent H.264 video coding utilizes complex predictions in both the temporal and spatial domains to get better performance than other standards. Certainly, such predictions may cause serious error propagation effects when suffering from transmission errors. Therefore, the objective of this paper is to develop a robust error resilient algorithm, named as the Synchronous Backward Error Tracking (SBET) algorithm, to completely terminate the error propagation. If the state of the encoder can synchronize to that of the decoder, the error propagation effects can be entirely terminated. Therefore, we assume that a feedback channel is available and the encoder can be aware of the decoder’s error concealment by external means. The pixel-based Precise Backward Error Tracking (PBET) is utilized to track the error locations and propagate the concealment error of erroneous frame to the corresponding areas to reconstruct the state of the decoder in the encoder. Comparing with the full re-encoding method, the proposed method only involves memory access, simple addition and multiplication operations for the error-contaminated pixels. By observing the simulation results, the rate-distortion performance of the proposed algorithm is always better than that of the conventional algorithms. SBET outperforms PBET up to 1.21 dB under 3% slice error rate for the QCIF Foreman sequence. In addition, without using forced INTRA refreshing, the phenomenon of burst bit rate can be avoided. In the future, if a better error concealment technique is utilized, a better performance of SBET is also expected.
