適用於數位助聽器之超低功耗ANSI S1.11濾波器組

Abstract

由於ANSI S1.11標準中所定義的濾波器組其分頻方式接近人類的聽覺系統，因此適合用在助聽器的應用上，然而因為其運算複雜度高，現今助聽器中的濾波器組大多不採用ANSI S1.11的規格。另一方面，目前的文獻中所提到的ANSI S1.11濾波器組都不是針對低功耗的應用來設計，故不適用於助聽器上。有鑑於此，本篇論文提出一ANSI S1.11濾波器組的設計，並在其演算法及硬體架構上都進行了最佳化來達到低功耗的要求，以適合助聽器的需求。此濾波器組使用了有限脈衝長度的濾波器(FIR filter)，並且採用multirate的架構以大幅降低運算量。我們更提出一個系統化的流程來設計此濾波器組的系數。此multirate濾波器組比起傳統的FIR濾波器組設計只需要4%運算複雜度。另外，本論文同時完成了此multirate濾波器組的硬體架構設計以及降低功率的最佳化，並在TSMC 0.13μm製程下進行實作。根據模擬的結果，此濾波器組在24KHz的取樣頻率下只消耗77.33μW的功率，比起其他針對助聽器應用所設計的濾波器組，本論文所提出的濾波器組功率只有 46% ~ 80%。

Most advanced hearing aids contain a filter bank to decompose the input signal into different frequency bands so that the properly prescribed gains can be applied on those bands to compensate the hearing loss. The ANSI S1.11 1/3-octave filter bank is preferred for hearing aid applications because it well matches the human hearing characteristics. However, most existing filter banks for hearing aids implement proprietary filter banks that are not compliant with the ANSI S1.11 1/3-octave specifications because of the high computational complexity. On the other hand, those existing ANSI S1.1 designs do not target at the power-critical applications, such as hearing aids, so they are not suitable for hearings. Besides, they are all based on IIR filters, which may suffer from the round-off noise and stability issue after quantization. Hence, this thesis presents a low-power FIR-based design of the ANSI S1.11 1/3-octave filter bank, of which the power consumption is minimized through algorithmic, numerical and architectural optimization. The proposed filter bank adopts a multirate architecture to reduce data rates on the bandwidth-limited bands. A systematic coefficient design flow is also proposed to minimize the filter order thereof. In an 18-band digit hearing aid with 24 KHz sampling rate, the proposed design need only 4% multiplications and additions of a straightforward FIR filter bank implementation. The proposed filter bank has been implemented using the cell-based design flow with the Artisan Metro cell library in the TSMC 0.13μm CMOS technology. The simulation result shows this design consumes 77.33μW for 24 KHz and 18-band audio signal processing. Compared with other filter bank designs, the proposed one saves 46% ~ 80% power dissipation.