Introducing the Differentiated All-Pole and One-Zero Gammatone Filter Responses and their Analog VLSI Log-domain Implementation

The scope of this paper is to introduce two particular filter responses which closely resemble tuning curves at specific set of places on the basilar membrane (BM) of the biological cochlea. The responses are termed Differentiated All-Pole Gammatone Filter (DAPGF) and One-Zero Gammatone Filter (OZGF) and their form suggest their implementation by means of cascades of N identical two-pole systems, which makes them excellent candidates for efficient analog VLSI implementations. The resulting filters can be used in a filterbank architecture to realize cochlea implants or auditory processors of increased biorealism. In addition, their simple parameterization allows the use of conventional automatic gain control (AGC) schemes to model certain important features of the biological cochlea (e.g. level-dependent gain) that are observed physiologically. To illustrate the idea, we present preliminary simulation results from a 4th-order OZGF using novel high dynamic range log-domain biquads in CMOS weak inversion (CMOS-WI). All circuits were designed in Cadence® Design Framework, using the commercially available AMS 0.35μm CMOS process. The reported OZGF structure has a simulated input dynamic range of 122.8dB, while dissipating 3.7μW of static power.