改善MPEG-4音訊編碼之PNS工具

Abstract

MPEG-4是ISO/IEC MPEG (Moving Picture Expert Group)所訂定最新的多媒體壓縮標準。一般音訊 (General Audio)是它的音訊部分。一般音訊可以選擇從12kbit/s ~ 64kbit/s的位元率(bitrate)來編碼雙聲道，甚至在壓縮5聲道音樂時只需320kb/s即可達到‘無法分辨原始訊號及壓縮後訊號’的品質。一般音訊的基本架構與MPEG-2 AAC (Advance Audio Coding)(前一代MPEG音訊壓縮標準)相同，另外它加入許多工具(tool)來增加編碼效率。本篇論文將選擇PNS (Perceptual Noise Substitution)工具作為研究的題目，並藉由適當的使用這個工具以增加壓縮效率。 PNS是一種有效的壓縮聲音中類雜訊部分(noise-like components)的方法。由於標準規定了資料的格式及解壓的方式，因此我們所能改善的只有選擇PNS參數及是否使用PNS。另一方面，我們發現PNS所需的額外資訊(overhead)如果不適當的減少的話，當位元率(bitrate)低時，它將會佔掉太多的位元導致編碼效率的下降。因此，我們提出一個有效的偵測聲音雜訊部分的方法，並在位元分配(bit-allocation)中加入決定是否使用PNS的判斷以減少PNS所需的額外資訊。 實驗結果指出新的的方法在壓縮類雜訊部分可以省下不少位元，藉此，音訊壓縮的效能將可以增加。

MPEG-4 is the latest multimedia compression standard defined by ISO/IEC MPEG (Moving Picture Expert Group). Its audio part, GA (General Audio), aims at data rates from 12kbit/s to 64kbit/s for encoding a pair of stereo channels to 320kb/s for encoding five-channel audio with ‘indistinguishable quality’. The basic structure of MPEG-4 GA is the same as that of the MPEG-2 AAC (Advance Audio Coding), the previous MPEG audio standard. Yet, it adds in several tools to improve its coding efficiency. This thesis selects one of the new tools, PNS (Perceptual Noise Substitution), as the research topic. We try to improve the audio coding efficiency by the proper use of this tool. PNS is an efficient method of coding the noise-like components in an audio signal. Although the data format and the decoder operation of PNS have been standardized, the method of choosing PNS parameters and the decision of using PNS or not are not standardized. In addition, we find that if we do not reduce the overhead of PNS, it would take up too many bits and thus decrease the coding efficiency. Therefore, we propose an effective and efficient procedure that detects the noise part of sound, and add a decision rule inside the bit-allocation loop to decide if PNS should be used. Simulations indicate that the new procedure saves bits in coding the noise-like components and thus may increase the overall audio coding performance.