Wireless Link for Smartphone Based Hearing Aids
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Since their introduction, digital hearing aids have seen continuous improvements in processing speed, as well as the addition of new and more advanced signal processing tools. In order to further develop the capabilities of hearing aids, new methods for improving the devices have to be considered. The goal of this thesis has been to investigate the possibility of adding a radio link between a hearing aid and a smartphone, and to determine the possible uses of such a wireless connection. The first step in establishing a radio link is to select the protocol for the connection. As hearing aids tend to be small devices with limited form factor, the protocol should emphasize low power consumption to prevent the need for additional batteries. In order to utilize the availability of smartphones, the protocol must also be supported by most modern smartphone manufacturers.Based on these requirements, the Bluetooth SMART protocol was determined to be the best fit. From the available manufacturers of Bluetooth SMART radios, the nRF51822 chip by Nordic Semiconductor was chosen for testing. This chip features the lowest peak power consumption, and has a user friendly development kit. To analyse the potential uses of the radio link between a smartphone and a hearing aid, three primary scenarios were defined, high quality audio transfer, low quality audio transfer, and control data. The scenarios are ranked according to the required data rate and involve different primary uses of the radio link. The high quality audio transfer scenario requires a data rate of 1411 kbps with a delay below 10 ms, and transmits live audio from the hearing aid to the smartphone for analysis. In the low quality audio transfer scenario the smartphone is used to periodically analyse small audio samples to improve upon the analysis performed in the hearing aid. To prevent the audio samples being too small, or being transmitted to seldom, the low quality audio transfer scenario requires a data rate of above 200 kbps. In addition to sample analysis, the second scenario would allow the hearing aid to operate as a wireless headset for the smartphone. The control data scenario assumes that the data rate does not support the transfer of recorded audio from the hearing aid. In this scenario the radio link is used to transmit control data such as battery status, audio profile, and volume level. This information can then be displayed on the smartphone, and modified by the user. To determine the viability of the different scenarios, different tests were performed to measure the available data rate and delay of the nRF51822. From these tests it was found that the data rate could reach 128 kbps under optimal conditions. When testing the chip during expected conditions, an average data rate of 120 kbps was achieved. The tests also determined that the average delay of a message is in the area of 5.1 ms between transmission and reception. From the available data rate, the best possible use of a radio link between a hearing aid and a smartphone appears to be a mix of scenario two and three. The hearing aid can then be used as a wireless headset if the proper decoders are added. In addition, control data can be transferred from the smartphone. The radio link could also prove useful in the fitting process, reducing the amount of equipment needed by audiologists.