Design and implementation of an interpolation filter for hearing-aid application

Abstract

Abstract – In this master thesis the design and implementation of an interpolation filter for hearing-aid applications will be discussed. The aim of the design will be to minimize the current consumption, hardware demand and area needed for the implementation of the design. Keywords – Hearing aids, interpolation filter, sigma-delta modulator, D/A converter Hearing aids are devices with very strict specifications. They are attached in its owner’s ear, therefore to make them comfortable they must be small and light. The biggest and heaviest part of a hearing aid is the battery. This means that if a significant reduction of the size and weight of the device wants to be achieved, the battery’s dimensions and in consequence, its capacity, must be reduced. This solution conflicts with the fact that since the user will probably be wearing it all day, the working time per charge should be maximized. The design of the hearing aid will aim at the reduction of hardware demands that will lead to a reduction of current consumption that will allow the battery to be smaller. The D/A converter, which is the back-end stage of the audio processing chain of a hearing aid can be seen in Figure 1. It consists of an interpolation filter, a sigma-delta modulator, a digital pulse width modulation, a class-D output stage, a feedback chain and an output filter. The sigma-delta modulator is an oversampled data converter, so a previous oversampling is needed for its correct operation. This oversampling will be performed by the interpolation filter. This oversampling process will increase the sampling frequency by the oversampling ratio needed for the sigma delta modulator, which will lead to an increase of the band of interest. Since the input of the interpolation filter is discrete, its frequency spectrum will be repeated at every multiple of fs. Those frequency spectrum repetitions or images, will appear in the output band of interest of the filter, therefore they will need to be suppressed by the interpolation filter. The interpolation filter of this design will be separated into four stages as it can be seen in Figure 2. The first stages of the filter will be the sharpest ones, and in consequence the most hardware demanding ones since the will have to attenuate the closest images. The last stages of the filter will attenuate the furthest images, so filters with less hardware requirements will be suitable. This multistage approach will also allows the first stages of the filter to work at lower frequency, since the sampling frequency will be increased step by step. This paper will deal with the design of the first stage of the interpolation filter since it is the most hardware demanding one. An optimized design will be critical regarding the overall hardware savings of the interpolation filter.