SBR起始頻率對HE-AAC編碼品質影響之研究

Influences of SBR starting frequency on the encoding quality of HE-AAC

黃威勳(Wei-Hsun Huang)

指導教授 ： 尤信程

國立臺北科技大學/電資學院/電資碩士班/碩士(2012年)

摘要

HE-AAC是使用頻帶複製(Spectral Band Replication)技術來改進音訊壓縮效率的編碼技術。在音頻訊號接收端，SBR模組藉由複製低頻訊號，重建高頻音頻訊號成份。因此，在編碼器端增加了高頻附加控制參數提取的模組，使得在低位元傳輸率時，解碼端依照高頻附加控制參數來做高頻音訊的重建，依然可以保有較高的音訊品質。HE-AAC在建立頻帶複製的頻帶選擇時，SBR模組依據取樣頻率和編碼位元率來決定複製頻帶的起始頻率。經由EAQUAL測試發現到，不同類型的音樂, 使用相同的起始頻率作為頻帶複製的選擇時，所得到的編碼結果並不是最佳的編碼品質，所以不同的起始頻率對於編碼品質是有所影響的。
本文提出一種方法，藉由計算HE-AAC中AAC低頻部分的失真及能量比例來決定起始頻帶，並使起始頻帶可以動態調整。藉由不同音樂類型的比較，經由EAQUAL測試，與原始編碼器相比較，可以得到更好的音樂品質，且非常接近最佳品質。

關鍵字

先進音訊編碼 ； 頻帶複製

並列摘要

HE-AAC use Spectral Band Replication (SBR) module to improve the compression efficiency of perceptual audio codecs. SBR module transposes low frequency components to high frequency components, and then adjustment the high frequency components spectrum to reconstruct the audio signal. Thus, it allows for a much higher audio quality at low datarates. HE-AAC decides the spectral band replication range that depends on SBR module according to sample rate and datarate. After testing via EAQUAL, several kinds of music type use the same start frequency to encode audio file according to sample rate and datarate, the results of audio quality were not the best encoding quality. Thus, encoding quality was influenced by SBR starting frequency.
We propose a method to decide SBR starting frequency. The method calculates the distortions and energies ratio of AAC low band part in HE-AAC, and depends on that, we adjust starting frequency dynamically during eocoding process. After many experiments via EAQUAL, versus to reference encoder, we got better audio quality in new encoder, and encoding quality was very close to the best encoding quality.

並列關鍵字

HE-AAC ； SBR

參考文獻

[11] 蔡欽名，“MPEG-4 HE-AAC中SBR之起始頻率與截止頻率之探討”，碩士論文，國立臺北科技大學資訊工程系碩士班，臺北，2010。

[1] K. Sayood, Introduction to Data Compression 3rd Edition, Morgan Kaufmann, New York, pp515-536, Dec. 2005.

[2] M. Dietz, L. Liljeryd, K. Kjörling, and O. Kunz, “Spectral band replication, a novel approach in audio coding,”in AES 112th Convention, Munich Germany, May 2002.

[3] M. Wolters, K. Kjörling, D. Homm and H. Purnhagen,“A close look into MPEG-4 High Efficiency AAC,”in AES 115th Convention, New York USA, Oct. 2002.

[4] 3GPP TS 26.404 V9.0:2009, Enhanced aacPlus general audio codec; Encoder specification; Enhanced aacPlus encoder SBR part.