Analogue CMOS Cochlea Systems: A Historic Retrospective

Abstract

For more than twenty-five years, the bionics community (and particularly the VLSI engineering community) has been performing extensive research to understand, model and design in silicon the biological auditory system and specifically the inner ear or cochlea. The aim of this on-going effort is not only towards building the ultimate artificial speech processor or implant, but also to develop systems that can contribute towards a deeper understanding of the underlying engineering strategies that nature chose to espouse. For these reasons, certain parts of the VLSI engineering community believe that trying to mimic certain biological operations will, in principle, yield systems that are somewhat closer to nature’s power-efficient computational ability. However, engineers must identify what they should and what they should not directly replicate in an artificial system inspired from biology. As for example, it does not make sense to create commercial aircraft wings to flap like those of birds (even though unmanned flapping-wing aircrafts are currently being developed – check NASA’s flapping solar aircraft) it is equally meaningful to argue that not all operations of the cochlea can or should be replicated in silicon in an exact manner. Abstractive operational or architectural simplifications dictated by logic and the available technology have been crucial for the successful implementation of useful hearing-type machines. Moreover, the biological cochlea is a threedimensional electro-hydro-mechanical system, whereas most electronic systems are onedimensional systems. Based on these thoughts, it would be wise to try and clarify three terms that are commonly used in the (bio-) engineering literature. Namely, these are: • Neuromorphic: the system level architecture is designed in such a way in order to replicate ‘exact’ basic anatomical identified operations which embody several key features encountered in the biological system. The term ‘neuromorphic’ was introduced by Carver Mead in (Mead, 1989;Mead, 1990) . • Bio-inspired: the design and/or operation are based on (inspired from) the engineering principles underlying its biological counterpart. • Biomimetic: the behaviour/operation/response resembles the one directly observed from the biological system, applying completely, partially or not at all the engineering principles encountered in the biological system. It should be therefore evident that a neuromorphic design is also bio-inspired and biomimetic, whereas a biomimetic (or bio-inspired) design is not necessarily neuromorphic. We shall provide tangible examples on this fact as the chapter unfolds.