參數立體聲 (Parametric Stereo, PS) 音訊編碼為2004年在ISO/MPEG 制定的HE-AAC v2標準下定義的一音訊編解碼物件。傳統音訊編碼標準MP3 (MPEG-1 Layer 3)、AAC (Advanced Audio Coding)的壓縮技術主要利用聲響心理模型 (Psycoaustic Model)、遮蔽效應(Masking Effect)來提高資料壓縮率，編碼過程仍以個別聲道獨立處理。PS則利用聲道間音訊的特徵關係，可節省將近一半的資料量，使傳統音訊利於以更低位元率在網路傳輸或儲存。 由於傳統PS解碼端複雜度比編碼端複雜度高出幾近兩倍，對使用者的可攜式裝置電力是一很大負擔。因此本論文提出一低複雜解碼的PS編解碼架構以降低解碼端複雜度為目標，先在編碼端利用對冗餘訊號抽取空間參數的方法傳送冗餘訊號參數，然後解碼端利用單聲道訊號重建冗餘訊號後，會進行偵測與評估其暫態訊號對重建冗餘訊號的影響，並使用能量補償演算法，對重建的冗餘訊號補償暫態訊號 所造成的失真。 最後使得解碼後的音訊品質客觀評分工具(Objective Difference Grade,ODG )分數上升約0.6分，並討論與MPEG-4 AAC結合的效能，其低複雜解碼的PS架構也利於對AAC在低位元率時傳輸。The Parametric Stereo (PS) audio coding is an audio coding object of High Efficiency Advanced Audio Coding version 2 (HE-AAC v2) which was standardized by ISO/MPEG in 2004. Traditional audio codec, e.g. MP3 or AAC, utilize “Psycoaustic Model” and “Masking Effect” to achieve high compression efficiency. However, they mainly process the signal with single channel. Different from traditional audio codec, the PS audio coding incorporates the characteristics of two channels, to extract spatial parameters and to down-mixes stereo signals into a mono signal. The PS can save almost half data size which provides great help in storage and transmission. Nevertheless, the complexity of PS decoder is nearly twice larger than that of PS encoder, which causes a serious problem in implementing PS on portable devices. Therefore, this thesis proposes a modified PS coding scheme to reduce the complexity of decoder. The encoder extracts and transmits the additional residual parameters from the residual signal and the mono signal. On the contrary, the decoder reconstructs the residual signal by the mono signal and the transmitted residual parameters. In addition, we detect the existence of transient signal and measure the artifact of reconstructed residual signal. Finally, “Energy compesated algorithm” is proposed to reduce the artifact produced by the transient signal. The proposed scheme can improve the Objective Difference Grade (ODG) of audio quality measurement “EAQUAL” with 0.6 score. Combining with audio coder AAC, the modified PS coding scheme still maintains a good performance at low coding bitrates.
