數位廣播中多解析訊號的編碼與調變

Abstract

傳統的廣播系統採用了單解析編碼方式，但是由於傳輸耗損，不同距離的 接收端會有隨距離增加而減少的頻道容量，因此畫面品質驟降，此即廣播 系統中的邊際效應。多解析編碼方式可用來設計性能較佳的視訊廣播系統 ，進一步合併考慮訊號編碼中的訊號重要程度和調變訊號點間的尤拉距離 ，將使得所接收到的畫面品質具階梯式減損的優點。在這樣的多解析系統 中，原來影像的分解方式， 以及所使用調變的訊號的級數，將影響系統 的性能。在多解析64級正交調幅調變的傳輸方式中，所分解的兩個傳送頻 道之間有著1:2 的位元率比例。本論文乃根據此傳輸器下的訊號編碼器， 研究子頻帶編碼器、金字塔型編碼器、和分解式離散餘弦轉換編碼器。在 各種不同的位元率下，採用了拉普拉斯分佈的非均勻量化器。為了增加全 解析度的覆蓋範圍，符合此調變器的籬笆碼被建立在所傳送的AWGN頻道上 面。整個系統的模擬結果顯示，分解式的離散餘弦轉換具有編碼器簡化、 不受位元率比例的限制、較高效率的位元分配、並且在整個系統架構的設 計上有較大的彈性等等的優點。 Traditionally,digital broadcast system uses single resolution strategy.Due to the propagation loss, the channel capacity decreases monotonically with increased distance from the transmitter. The picture quality will degrade abruptly. This is so called threshold effect. A better video broadcast system can be designed by the multiresolution strategy. Joint consideration for signal bit stream priority in source coding and the Euclidean distance between signal points in modulation achieves stepwise degradation in the picture quality. In this multiresolution strategy, system performance is affected by the way how the original images are decomposed, and how many signal levels are used in modulation. With the 1:2 ratio of bit-rates between two components in multiresolution 64 QAM transmission, we investigate that subband coder, pyramid coder, and decomposed DCT coder in multiresolution source coding. Optimum quantization is employed with nonuniform quantizer matched to Laplacian distribution in several different bit-rates. To increase full resolution coverage,trellis code that matched to multiresolution 64 QAM modulator is established in AWGN channel. Simulation reveals that decomposed DCT coder is a better candidate for multiresolution transmission, which has the advantages that source coder is simple and not sensitive to bit ratio variation,bit allocation is efficient, and more flexible for system design.