Analog complex gammatone filter for cochlear implant channels

According to recent physiological experiments, the envelope and phase of speech signals are required to enhance the perceptive capability of a cochlear implant processor. In this paper, the design of an analog complex gammatone filter is introduced in order to extract both envelope and phase information of the incoming speech signals as well as to emulate the basilar membrane spectral selectivity. The gammatone impulse response is first transformed into the frequency domain and the resulting 8^th-order transfer function is subsequently mapped onto a state-space description of an orthonormal ladder filter. Using this approach, the real and imaginary transfer functions that share the same denominator can be extracted using two different C matrices. This results in a compact filter structure. The proposed filter is designed using G_m-C integrators and sub-threshold CMOS devices in AMIS 0.35 μm technology. Simulation results using Cadence RF Spectre confirm the design principle and ultra low power operation.