10-ms 18-Band Quasi-ANSI S1.11 1/3-Octave Filter Bank for Digital Hearing Aids

The ANSI S1.11 1/3-octave filter bank is suitable for digital hearing aids, but its large group delay and high computational complexity complicate matters considerably. This study presents a 10-ms 18-band quasi-ANSI S1.11 1/3-octave filter bank for processing 24 kHz audio signals. We first discuss a filter order optimization algorithm to define the quasi-ANSI filters. The group delay constraint of filters is limited to 10 ms. The proposed design adopts an efficient prescription-fitting algorithm to reduce inter-band interference, enabling the proposed quasi-ANSI filter bank to compensate any type of hearing loss (HL) using the NAL-NL1 or HSE prescription formulas. Simulation results reveal that the maximum matching error in the prescriptions of the mild HL, moderate HL, and severe-to-profound HL is less than 1.5 dB. This study also investigates the complexity-effective multirate IFIR quasi-ANSI filter bank. For an 18-band digital hearing aid with a 24 kHz sampling rate, the proposed architecture eliminates approximately 93% of the multiplications and up to 74% of the storage elements, compared with a parallel FIR filters architecture. The proposed analysis filter bank (AFB) was designed in UMC 90 nm CMOS high-VT technology, and on the basis of post-layout simulations, it consumes 73 μW (@V_DD=1 V). By voltage scaling (to 0.6 V), the simulation results show that the power consumption decreases to 27 μW, which is approximately 30% of that consumed by the most energy-efficient AFB available in the literature for use in hearing aids.

I. Introduction

Hearing loss [1]–[3] can be characterized as conductive, sensorineural, and mixed hearing loss. Conductive hearing loss means the sound is not conducted well through a disordered outer or middle ear. Sensorineural hearing loss (SNHL) means the sensory cells in the cochlea are absent or not functioning appropriately. If both conductive and sensorineural losses are present, the result is mixed hearing loss. Conductive hearing loss can be recovered after some adequate treatments, but most people with SNHL are fitted with hearing aids. SNHL can degrade the functions of human ear in several different ways and introduce phenomena such as a raised hearing threshold, decreased and squeezed hearing range, reduced temporal and spectral resolution, and the loss of noise tolerance [1]. These factors make hearing aids more complex than simply amplifying sound [1]–[3].