A Plain Implementation in Frequency Domain for Wideband Active Noise Control Systems

用於寬頻主動式噪音控制系統之易實現的頻率域處理演算法

將訊號轉換到頻率域上做處理需以區塊處理的方式為之，所以會有處理延遲的出現。而主動式噪音消除控制(Active noise control)系統的應用對訊號的相位極為敏感，若是直接應用頻率域的區塊處理方式，則需要選擇高階處理器才能即時處理。而為了處理寬頻雜訊，Morgan便提出了無延遲架構的子頻帶適應性濾波器架構(Delayless filter-x subband adaptive filter)，開啟了可並行處理的無延遲子頻帶適應性濾波器架構之研究。但是，若使用單一運算處理器來實現無延遲子頻帶適應性濾波器架構，則複雜程式的實現效率與程式技巧對工程師而言是一大考驗。因此，本論文利用現今各運算處理器均有提供已優化的離散傅立葉轉換的優勢，提出一個無延遲架構的方便實現方法。所提出的利用快速區塊型最小平方法的無延遲架構(Delayless filter-x fast block LMS, DFX-FBLMS)可以達到與先前技術相同的性能表現且有較低的計算複雜度，因此工程師實現此架構時，可以使用較低成本的運算核心元件，在維護與調整測試時也較為簡單。

Signal processing in frequency domain needs block processing which will cause a longer processing delay. Such processing delay is not permitted for the application of active noise control (ANC) systems because the phase of the anti-noise signal is important for acoustic sound cancellation. However, to perform the adaptation in frequency domain for the wideband ANC systems is necessary; and thus, it needs a high performance processing kernel. The delayless structure of the filter-x subband adaptive filter is proposed by Morgan, who also opens the study in the field of the delayless subband adaptive filtering. To implement such parallel structure over a single processing kernel is hard for engineers. This paper suggests using the discrete Fourier transform (DFT) subroutine which has been optimalized by the chip provider to perform the wideband ANC systems. In this paper, the proposed delayless filter-x fast block LMS (DFX-FBLMS) algorithm can achieve the same performance as previous approaches but with less computational complexity. Once a low level (low cost) controller is applied, the ANC system just updates the weights as the DFX-FBLMS process is finished. The proposed structure also makes the software easily programmed and maintained by engineers.