Analog Implementations of Auditory Models

ANALOG IMPLEMENTATIONS OF AUDITORY MODELS Richard F. Lyon Apple Computer, Inc. Cupertino, CA 95014 and California Institute of Technology Pasadena, CA 91125 ABSTRACT The challenge of making cost-effective implementations of auditory models has led us to pursue an analog VLSI micro-power approach. Experiments with the first few generations of analog cochlea chips showed some of both the potential and the problems of this approach. The inherent exponential behavior of MOS transistors in the subthreshold or weak-inversion region leads to nonlinear filter circuits, in which the small-signal and large-signal behaviors can be quite different. Early problems with instability, poor dynamic range, and excessive noise are now understood in terms of the transition behavior between these regions, and this understanding has led us to design filter stages with appropriately compressive behavior, resulting in more robust cochea performance. Several types of correlator circuits to follow the cochlea have also been developed into working demonstrations. Videotapes of circuit outputs and simulations illustrate the recent ideas and progress. INTRODUCTION The "Analog Electronic Cochlea" of Lyon and Mead [1] has been presented as a very efficient way to implement a cochlear model in silicon. It was shown to provide some of the basic filtering properties and adaptability potential needed for a comprehensive auditory model. However, we were not able to tune the filters to give a pseudoresonant gain peak greater than about 20 dB, due to poorly understood circuit misbehaviors. In the hydrodynamic model on which our cascade filterbank model of the cochlea is based [2], the signals in the filter cascade correspond to either pressure (across the basilar membrane) or velocity potential at the membrane (the spatial derivative of velocity potential is the fluid velocity vector). For the case of a passive cochlea, the respone of the filters should be monotonically decreasing with frequency; i.e., lowpass. But for a healthy active cochlea in a quiet environment, the active outer hair cells should provide enough gain to change the overall lowpass response to a pseudoresonant bandpass-like response with a broad gain peak of perhaps as much as 60 dB. Over a wide range of sound loudnesses, the filterbank gain should gradually transition between these extremes, acting as an automatic gain control to compress the dynamic range of signals at the output. We have previously discussed the evidence for this kind of function in fact occuring at the mechanical level [3]. We are now making progress on getting this behavior into our circuits. COCHLEA IMPROVEMENTS The original cochlea chip was nothing more than a cascade of two-pole filter stages (omitting the zeros of previous models that had come from a long-wave analysis). Further analysis led us to expect three-pole stages to provide an even better fit to the hydrodynamic short-wave analysis. Most of the circuits and behaviors discussed here apply with minor changes to either type of stage. Tests on early second-order filter stages, including a low- noise version based on the MOSIS low-noise analog BiCMOS process, revealed a problematic nonlinear effect related to the saturation of …

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