Research On The Digital System In Hearing Aids

This thesis describes the development of a behind-the-ear digital hearing aid based on the Ezairo 5910 ultra low power audiology processor. The key technologies of the digital hearing aid, as well as the design and implementation of the system, are presented in detail.The emphasis of the thesis is on hardware and software architechture designs.The hardware of the digital hearing aid builds upon the Ezairo 5910 processor which implements all the main functionalities of the system. The software realizes a polling loop architecture and the operations of the two processing cores, the CFX DSP core and the HEAR accelerator, are coordinated through commands and interrupts. An in-depth analysis of load balancing, algorithm partitioning, tonal noise and data flow is also given.The digital hearing aid provides algorithms like multichannel compression, noise reduction, directional microphone and feedback cancellation, as well as functionalities like volume control, beep tones, low battery alarm, operating mode switching, delayed start and programming tool. The implementations of the algorithms and functionalities are presented in detail. Several benchmarking methods used to measure the performance the processing cores are illustrated. The CFX DSP core is measured by counting instructions, observing pulses on the oscilloscope and timing the code execution time with the on-chip timer, while the HEAR accelerator is measured with the timer as well as an infinite loop running on the CFX DSP core. Benchmarking results are given, together with a discussion of the conditions influencing the benchmarking methods. A series of electroacoustic tests of the digital hearing aid is carried out on a testing system and results show the hearing aid complies with industrial standards.