MPEG-4 HE-AAC 中SBR之開始與截止頻率之探討

HE-AAC是一種繼承AAC優點且增加SBR技術的編解碼演算法，其提高編碼效率之方法在於解碼器僅需對低頻音訊部分編解碼，高頻部分則是利用低頻訊號，配合一組資料量極少的參數，在解碼端重建即可。在判別頻率多少以上為高頻部份方面，3GPP編解碼器是依據取樣頻率與編碼位元率，因此，不會因為原始訊號的不同而有所改變。由於起始與截止頻率會影響SBR之編碼品質，因此本論文在起始頻率的選取上，根據AAC之distortion提出了一套方法，藉由計算出與distortion相關之數據，取得適合之起始頻率，並進一步在編碼中能夠持續變動起始頻率。而在截止頻率選取上，則是依據頻譜高頻變化來決定其數值，與起始頻率相較之下，略微簡單。我們修改後之編碼器，與3GPP之原始編碼器相比，針對不同音樂之測試，在客觀品質(ODG)分數上，大部份都能有所提昇，但是在主觀之聆聽實驗上，則差異較不明顯。

關鍵字

HE-AAC ； SBR ； QMF ； distortion

並列摘要

HE-AAC is a successor AAC with the SBR technology. HE-AAC encodes only low –frequency part of the audio signal, and using parameters analyzed by the SBR tool to represent the high-frequency part of the audio. Among the SBR parameters, the start and the stop frequencies control which parts of low-frequency spectrum should be replicated to high-frequency spectrum. Though these parameters greatly affect the encoded quality, there is no algorithm to describe how to select these parameters based on the content of the music to be encoded. For example, the 3GPP reference software uses sampling rate and bit rate to determine these parameters, regardless the type (genre) of the music. In this thesis, we propose a method to dynamically adjust the start frequency. The method is based on the differences of distortion between successive blocks of frames. To test the performance of our method, we encode several pieces of music in different types. We use the EAQUAL program as the quality assessment tool. The experimental results show that our method has higher ODG scores in most of the music when compared with the 3GPP’s method. However, the results of listening test (using a simplified CMOS) reveal that untrained listeners cannot clearly distinguish the quality differences between ours and 3GPP’s.

並列關鍵字

HE-AAC ； SBR ； QMF ； distortion

參考文獻

[7] 3GPP TS 26.403: “Advanced Audio Coding (AAC) part”, December 2008.

[1] Per Ekstrand, “Bandwidth Extension of Audio Signal by Spectral Band Replication”, Proc.1st IEEE Benelux Workshop on Model based Processing and Coding of Audio (MPCA-2002), Leuven, Belgium, Nov. 15, 2002, pp.53-55.