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Signal Processing in High-End Hearing Aids: State of the Art, Challenges, and Future Trends V. Hamacher, J. Chalupper, J. Eggers, E. Fischer, U. Kornagel, H. Puder, and U. Rass Siemens Audiological Engineering Group, Gebbertstrasse 125, 91058 Erlangen, Germany Emails: [email protected], [email protected], [email protected], [email protected], [email protected], [email protected], [email protected] Received 30 April 2004; Revised 18 September 2004 The development of hearing aids incorporates two aspects, namely, the audiological and the technical point of view. The former focuses on items like the recruitment phenomenon, the speech intelligibility of hearing-impaired persons, or just on the question of hearing comfort. Concerning these subjects, different algorithms intending to improve the hearing ability are presented in this paper. These are automatic gain controls, directional microphones, and noise reduction algorithms. Besides the audiological point of view, there are several purely technical problems which have to be solved. An important one is the acoustic feedback. Another instance is the proper automatic control of all hearing aid components by means of a classification unit. In addition to an overview of state-of-the-art algorithms, this paper focuses on future trends. Keywords and phrases: digital hearing aid, directional microphone, noise reduction, acoustic feedback, classification, compression.

1.

INTRODUCTION

Driven by the continuous progress in the semiconductor technology, today’s high-end digital hearing aids offer powerful digital signal processing on which this paper focuses. Figure 1 schematically shows the main signal processing blocks of a high-end hearing aid [1]. In this paper, we will follow the depicted signal flow and discuss the state of the art, the challenges, and future trends for the different components. A coarse overview is given below. First, the acoustic signal is captured by up to three microphones. The microphone signals are processed into a single signal within the directional microphone unit which will be discussed in Section 2. The obtained monosignal is further processed separately for different frequency ranges. In general, this requires an analysis filterbank and a corresponding signal synthesis. The main frequency-band-dependent processing steps are noise reduction as detailed in Section 3 and signal amplification combined with dynamic compression as discussed in Section 4. A technically challenging problem of hearing aids is the risk of acoustic feedback that is provoked by strong signal amplification in combination with microphones and receiver This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

being close to each other. Details regarding this problem and possible solutions are discussed in Section 5. Note that feedback suppression can be applied at different s