低功率MPEG-4 HE-AAC編碼器之設計

Abstract

在低功率壓縮的需求之下，本篇論文提出低功率之MPEG-4 HE-AAC編碼器。MPEG-4 HE-AAC編碼器包含AAC編碼器、SBR編碼器和正交鏡像濾波器組(QMF Banks)。藉由analysis QMF bank將輸入訊號分頻成64個子頻帶。低頻訊號在經過synthesis QMF bank還原後，交由AAC編碼器壓縮。而64個子頻帶送往SBR編碼器取出SBR參數，以供解碼器端還原高頻訊號。由於現有的MPEG-4 HE-AAC採用複數結構的analysis/synthesis QMF banks，因此QMF banks和SBR編碼器都是使用複數運算。

本篇論文提出實數結構的QMF banks，無論在QMF banks或在SBR編碼器取出SBR參數的過程都是使用實數運算，因此計算複雜度為傳統方式的二分之一。由於複數和實數結構的QMF banks具有不同的物理特性，因此分析出的64個子頻帶也有不同的特性，造成經由實數結構MPEG-4 HE-AAC編碼器壓縮出的訊號，於複數結構MPEG-4 HE-AAC解碼器還原回的訊號會有失真。因此，本篇論文亦提出音調性校正(Tonality Adjustment)、實化(Realification)和三種技術，以確保降低HE-AAC編碼器的複雜度亦維持壓縮品質。本論文亦會將低功率技術延伸到MPEG-4 HE-AAC version 2之上並證明其可行性。

本篇論文提出的低功率MPEG-4 HE-AAC編碼器架設在NCTU HE-AAC Encoder之上。亦會藉由主觀及客觀測試證明低功率HE-AAC編碼器可維持高品質的壓縮訊號。

Spectral Band Replication (SBR) has been combined with MPEG-4 AAC as bandwidth extension tool. The resulting scheme is referred to as the MPEG-4 High Efficient (HE) AAC or aacPlus. With the SBR module taking care of the high frequency contents, the conventional AAC encoder can compress the low frequency part using most of the available bits. The SBR parameters are all calculated by SBR encoder in complex domain in the architecture of complex QMF. If the components in SBR encoder can be implemented in real domain, the computational complexity of HE-AAC will be reduced by half. This paper proposes the Low Power MPEG-4 HE-AAC encoder to reduce the computational complexity. The objective experiments are conducted to demonstrate the quality of Low Power HE-AAC encoder on critical music tracks. Finally, the paper will extend the Low Power technique to Parametric Stereo (PS) Encoder with HE-AAC.