MPEG-4 HE-AAC 中之高頻調整模組設計

Abstract

高頻調整模組是MPEG-4 HE-AAC中的核心模組，其目標在於回復高頻重建訊號的音調度，使其如同原始訊號一般。兩個決定性的問題為，如何準確的測量訊號的音調度，與如何決定共用的控制參數。控制參數之計算與音調度相關，其值將決定在解碼端的增益大小與額外補償之能量大小，換句話說，重建訊號的品質直接受到高頻調整模組的影響。

本論文提出一有效率的方法來測量子頻帶訊號的音調度，使用線性預測並基於Levinson-Durbin演算法以自動調整預測等級，使其符合不同的訊號內容。進而探討因控制參數之共用所造成的音訊失真，並提出如何有效地決定控制參數之方法。

High frequency adjustment module is the kernel module of spectral band replication (SBR) in MPEG-4 HE-AAC. The objective of high frequency adjustment is to recover the tonality of reconstructed high frequency signal as the original tonality. There are two crucial issues, the accurate measurement of tonality and the decision of shared control parameters. Control parameters, which are extracted according to signal tonality, will be used to determine gain control and energy level of additional components in decoder part. In other words, the quality of the reconstructed signal will be directly related to the high frequency adjustment module.

In this thesis, an efficient method based on Levinson-Durbin algorithm is proposed to measure the tonality by linear prediction approach with adaptive orders to fit the different subband contents. Furthermore, the artifact due to the sharing of control parameter is investigated and the efficient decision criterion of control parameter is proposed.