AAC中訊窗轉換方法之設計

Abstract

訊窗轉換方法是為了得到更好的時間或頻率解析度而改變訊窗的大小。當我們對瞬變的訊號進行編碼時，量化誤差會散佈在整個訊窗當中並且無法被遮蔽。訊窗轉換方法會利用較短的訊窗來限制量化誤差所散佈的範圍。本篇論文對於MPEG 2/4 AAC提出一個完整的訊窗轉換設計。 如何成功的設計訊窗轉換方法?主要分為四個設計，分別為訊窗切換的時機，短訊窗的心理聲學模式，短訊窗的編組方法以及與其他AAC模組配合等議題。在論文中，我們將對這四個議題作深入的研究與探討，並且與最佳的位元分配方法結合以達到更好的編碼效率。在品質的量測上，除了主觀的聆聽評量之外，我們還採用了ITU所發展的PEAQ測試系統來評估音訊壓縮後的誤差程度。

Window switch method is to change the window size of the filterbank and achieve the better time/frequency resolution. When a transient signal is being coded the quantization noise is spread out over the entire window length of the filterbank and in this way is not masked by the signal. Window switch method is to use short window length to restrain the spreading of quantization noise. This thesis proposes an integrated design of window switch in ISO/IEC MPEG-2/4 Advanced Audio Coding (AAC). However, the success of AAC window switch is approached by four design aspects: window decision, psychoacoustic model of short window, short windows grouping and combination with other AAC modules. This thesis presents the design method with well concerns on the four issues and jointly integrated with the optimal bit allocation method to have good computing efficiency. Both subjective and objective tests have been conducted to demonstrate the improved quality over the existing method. The objective system adopted is the PEAQ which is the recommendation system by ITU-R Task Group 10/4.