以適應性位元估計之數位錄放影機影像壓縮技術

Abstract

在本論文中，我們提出一套用於家庭用的數位錄放影機影像壓縮演算法， 此演算法以國際動畫標準為基礎作改良。為滿足數位錄放影機快速放影的 需求，我們將壓縮後的資料單元區分成可獨立解碼及非獨立解碼兩部分， 其中可獨立解碼的部分包含了I-畫面的直流及低頻係數，用以組成快速放 影時的低解析度畫面；可獨立解碼及非獨立解碼兩部分共同組成正常速率 放影時的全解析度畫面。且為了能在各種快速放影下持續更新畫面內容， 我們提出資料放置於磁帶上輪轉的方式。因為磁軌所能儲存之資料量是固 定，故在位元的分配上需要有精確的控制。在有高表現控制之緩衝器 /量 化器架構下，一個重要的要素是以位元數模型來估計壓縮後產生的資料量 。因此，我們發展了一套可適應的位元估計法，使得我們能精確地控制壓 縮後的資料量。此位元估計器是以一樹狀結構的分段式濾波器的型式實現 ，在此數狀結構中的每一節點結合了一線性的關係式，用以描述位元數與 複雜度值/量化度的關係,藉由修改過的 LMS 演算法來調整此關係式中的 參數值。經由電腦模擬的結果顯示，證實結合此影像壓縮演算法與資料排 放在磁帶的方式，產生了相當好的整體表現，在精確的控制輸出位元數的 分布下，此影像壓縮系統不僅提供了正常速率放影時，一致的良好畫面品 質，亦有可接受的快速放影品質。 A source coding algorithm based on MPEG-1 standard is proposed for home-use digital VCR. To meet the requirement of high speed playback of DVCR, the encoded IBP unit data is divided into IDC( independent decodable code) and DDC(dependent decodable code) parts. The IDC part contains the DC and low frequency coefficients of I-frame that can restore low resolution pictures for trick play. For normal play, both IDC part and DDC part are necessary to reconstruct the full resolution pictures. In order to update each segment of a picture frame contiguously, a new data distribution scheme is invented for variable speed playback. Because each tape track can only store a fixed amount of data bytes, it becomes very critical to have a precise control on bits allocation. An essential element in a high performance buffer/quantizer control scheme is the bits model that estimates the compress bits when a certain quantization stepsize is in use. Hence, we develop an adaptive piecewise linear bits estimator whose structure is similar to a tree-structured piecewise linear filter. Each node in the tree is associated with a linear relationship between macroblock bits and activity/stepsize. The parameters in the relationship is adjusted by a modified LMS algorithm. Computer simulations show that this buffer/quantizer control algorithm and the tape data distribution scheme produce a quite good overall performance. Under the precise control of bits distribution, this system not only provides the uniform picture quality at normal speed playback, but also it offers acceptable quality at high speed playback.