Feedback suppression in digital hearing instruments

Abstract

Forskningsarbejdet vedrører uønsket feedback i digitale høreapparater. Dette problem skyldes at mikrofonen i høreapparatet opfanger lyden fra høreapparatets højttaler, hvilket kan give anledning til en hyletone som er meget ubehagelig for brugeren af høreapparatet. Man kan kompensere for den uønskede transmission mellem højttaler og mikrofon med et adaptivt (signalafhængigt) digitalt filter som modellerer den uønskede transmissionsvej, men denne ændrer sig drastisk når brugeren af høreapparatet f.eks. taler i telefon. I dette arbejde er problemet angrebet ved at kombinere tre modeller, én for den permanente del af transmissionsvejen, én for den langsomt varierende del, og én for hurtige ændringer. Forskellige løsninger til et andet alvorligt problem for feedbackkompenserende adaptive filtre (”bias-problemet”) er udviklet og afprøvet. Dette problem optræder når signalet har en lang autokorrelationsfunktion, hvilket er tilfældet når det indeholder rene toner (eksempelvis stemte talelyde og musik). Løsningen er en dekorrelationsteknik som reducerer korrelationen mellem apparatets input og outputsignaler.In this dissertation, the feedback whistling problem with digital hearing instruments is investigated. The work focuses on the properties of the feedback path, the modelling of the feedback path and the feedback suppression techniques. The properties and modelling of the feedback path are first discussed. Along the propagation path, different components compose the feedback path. The effects of these components are analyzed and categorized. Accordingly an ideal feedback path model, which consists of a fixed model, a slowing varying model and a fast varying model, is suggested in the dissertation. Methods to extract the fixed model are proposed and proved to be effective in representing the invariant part of the feedback path. Based on the investigation of the dynamic changes of the feedback path in adverse situations, for example when the user picks up the telephone handset, a reflection model is developed as one type of the fast varying models. The techniques to suppress the feedback are then reviewed. To improve the existing feedback suppression systems, two approaches are proposed to address the so-called “bias problem”. The first approach improves the performance of the adaptive feedback canceller with filtered-X adaptation by injecting nearly inaudible noise. The second approach uses a linear predicative coding based vocoder to synthesize the hearing-aid output in order to decorrelate the hearing-aid output signal and the desired input signal. In the end, a discussion about the use of the proposed feedback path models in the feedback cancellation systems is presented