NONLINEAR ALL-POLE AND ONE-ZERO GAMMATONE FILTERS

A nonlinear model of auditory filtering in the digital domain is propo sed. It is given in a general form, because of a missing quantitative description of how output level dependent auditory filter shapes chang e with level and frequency. The nonlinear filter is derived from an 8t h order linear gammatone filter consisting of 4 biquad sections with i dentical pole positions [1]. The order of the nonlinear filter is cons tant. The low frequency branch of the filter rises with increasing con trol levels. The filter center frequency is approximately level indepe ndent. The impulse response becomes shorter with increasing levels. Th e control level of the general model is split into six level dependent control parameters which control three pole pair positions. These con trol parameters are adapted to the auditory filter shapes reported in Rosen and Baker [2] and the resulting magnitude responses are found to provide an excellent fit. For the given example, the control structur e of the general model can be simplified by reducing the number of lev el dependent control parameters to two. In view of a nonlinear auditor y filterbank, a frequency transposition of a single prototype filter i s proposed. Therefore, it is possible to adapt the simplified control to any filter center frequency.