MPEG Layer III 與 MPEG-4 AAC 與 MPEG-4 HE-AAC上之有效率的位元儲存分配器設計

Abstract

位元儲存分配器擔負著回收量化後殘餘位元與管控訊框之間位元分配的責任，在目前的音訊壓縮器，如MP3、AAC中，扮演了平衡有限位元與壓縮品質之間的核心角色。位元儲存分配器的設計可以從需求導向與儲量導向兩種方式來探討：需求導向的方法根據音訊內容決定所需分配的位元量；儲量導向的方法則是根據位元儲存器中所累積儲存的位元多寡決定所需分配的位元量。現存的位元儲存分配器設計主要是依循儲量導向的方式來實作。本論文中提出一個綜合需求導向與儲量導向的有效率位元儲存分配器設計：經由需求預測器，我們可以適切的估測出每一個訊框的位元需求；同時透過儲量管理器，我們可以根據壓縮器協定與偏好的模組設定來控制分配的位元量。

更進一步來說，為了在低於96kbps的位元率之下達到良好的壓縮品質，因而提出了結合AAC壓縮器與SBR模組的HE-AAC壓縮器。SBR模組藉由複製訊號的低頻部分來重建高頻部分，使得AAC壓縮器可以專注在處理訊號的低頻部分。因此，妥善的分配位元於AAC壓縮器與SBR模組之間決定了壓縮的品質與效率。在本論文中，我們將位元儲存分配器的概念延伸至HE-AAC上並有效的分配位元於AAC壓縮器與SBR模組之間。同時為了驗證品質的改進與效率的增進，我們採用了主觀的聆聽評量與客觀的軟體量測，並且獲得良好的結果。

Bit reservoir controlling the bits budget among music frames has been the kernel module to have good bits-quality tradeoff in current audio encoders like MPEG Layer III (MP3) and Advanced Audio Coding (AAC). The approaches of bit reservoirs can be investigated from demand-driven approach and budget-driven one. Demand-driven approach determines the required bits according to the audio contents while budget-driven one allocates bits according to the bit budgets accumulated in the bit reservoir. Existing bit reservoirs follow basically the budget-driven approach. This thesis presents an efficient bit reservoir design with concerns from both demand and budget. The bit reservoir includes a demand estimator to adaptively predict the bits required for each frame. Also, there is a budget regulator to control the bits used according to the codec protocol and the preferred scenario.

Furthermore, High efficiency AAC (HE-AAC) has included the Spectral Band Replication (SBR) in combination with AAC to achieve high audio quality at bit rates lower than 96 kbps. SBR reconstructs high frequency signal through replicating the low frequency parts. The bits allocated to AAC encoder module and SBR module decides the quality and compression efficiency. This thesis also extends the concept of bit reservoir to HE-AAC for efficient bits distribution between the AAC encoder and the SBR module. Both subjective and objective tests are conducted to verify the improved quality and efficiency of the new bit reservoir design.