Abstract
影像壓縮編碼技術是數位化視訊通訊與高畫質電視(HDTV)發展的核心，其中分頻編碼(Subband Coding)即是一種相當有效率的編碼方式。傳統上分頻編碼大都採用以水平與垂直濾波為基礎的可分離矩形頻帶。此種頻帶分割方式雖較簡單，但是卻無法忠實地反應人眼的視覺效應。在本研究中我們採用鑽石形的頻帶分割方式，此鑽石形頻帶能夠更有效地分割二維頻帶的高低頻，並且更接近人類的視覺特性。我們以一維的正交鏡像濾波器組(Quadrature Mirror Filter Bank) 為基礎得到一個二維不可分離的鑽石形濾波器組，再利用樹狀結構以得到四個包含不同頻帶的子頻道。對於最低頻帶由於其時域與空域的資訊重複性仍高，故採用具有運動補償之混合式離散餘弦轉換之編碼方式。運動補償亦使用於其他高頻帶之編碼以降低位元率。由於各個頻帶具有不同的特性，故各頻帶之最佳化編碼須考慮其個別特性。此外位元率分配法則亦應用人類視覺系統(HVS) 模型以符合視覺效應。一種可容易實現，應用時域與空域視覺遮蔽效應的適應性方塊量化亦被提出以提高視覺上之主觀品質。由於分頻編碼本身所具有的階層性與多重解析度特性，使其非常適合應用於寬頻網路上之影像與視訊服務。在ATM 網路中分層編碼可以在資料漏失的情況下提供良好的重建品質。我們提出了一種以鑽石形分頻系統為基礎之適應性分層編碼器，此系統可在資料漏失的情況下可將折疊失真 ( Aliasing Error) 減至最低以維持重建品質。此外我們亦討論了在傳輸錯誤(Transmission Error)發生的情況下一些插補 (Interpolation)以及影像恢復(Image Recovery)的方法。Subband coding (SBC) has been a major technique in datacompression of speeches and images. Instead of conventionalrec- tangular bands in two dimensional subband coding ofimages, a non-separable diamond-shaped frequencydecomposition which splits the original signal into fouruniform subbands along the diagonal directions is discussedin this work. This diamond- shaped frequency decompositionmatches the human visual system more closely than theconventional rectangular bands. The sub- band video codingsystem proposed is built up based on this diamond-shapedsubband filtering system. One motivation of adopting subbandstructures is that each subband can be coded by the optimalcoding algorithm according to its own characteristics. Thecoding algorithms which are adaptive to the differentproperties of the four subbands from diamond-shaped analysisfilter bank are discussed in this work. Subband coding systemis very suitable for the transmission of video or image inthe broadband ISDN for its hierarchical structure. In theATM network the priority scheme is used to provide thebasic service quality. An adaptive layered coding system whichis based on the diamond-shaped subband structure is proposed.This adaptive scheme can provide a superior ability of cellloss resilience. Some other processings which can reduce thereconstruction errors introduced by cell loss or transmissionerrors are also presented.