MPEG-4 AAC 中 高效能TNS的設計

Abstract

TNS是用來消除由激發訊號所造成pre-echo現象的一個模組，它可以控制量化誤差並且將誤差塑形在遮避能力較大的激發訊號裡，進而改進音樂品質。這篇論文會闡明TNS造成的三種人為雜訊。第一種人為雜訊會在激發訊號的邊界產生一個很大的噪音，像是Gibbs phenomenon。第二種則是在時域訊號上的特定位置，會產生一個不尋常的噪音。最後一種則是隨著order數愈高，不尋常的噪音則會愈明顯。因此，這篇論文會提出一個有效率且能降低這三種人為雜訊的方法。

Temporal noise shaping has been defined in MPEG-4 AAC to control the pre-echo noise in attack signals. The module, which is especially important for the MPEG-4 Low Delay AAC due to the absence of window switching mechanism, can shape and control quantization noise spread to improve the quality under bit rate constraint. However, this paper illustrates that the TNS will introduce three artifacts. The first artifact is similar to the Gibbs phenomenon which has high noise level occurring at the edge of the attack signal. The second effect is the time-domain aliasing noise which has unusual noise at a distance from the attack time frame. The third is the noise spreading with the TNS filter orders. This thesis will propose the efficient TNS method which shapes noise with good concerns on the above three artifacts. Also, we provide an efficient computing method to activate the TNS. Both subjective and objective tests are conducted to illustrate the improvement over existing TNS methods.