The gammachirp auditory filter was introduced provide an asymmetric, level-dependent version of the gammatone auditory filter (Irino and Patterson, 1997). In this 'analytic' gammachirp filter, the level-dependency was in the chirp parameter. Recently, Carney et al. (1999) reported that, although there is a chirp in the impulse response of the cat's cochlear filter, the form of the chirp does not vary with level. This led Irino and Patterson (1999, 2000) to develop a more physiological version of the gammachirp filter with the following magnitude response.

The first term is a fixed gammachirp filter which represents the passive basilar membrane response; the second term is a highpass, asymmetric function (HP-AF) which represents the active component in the cochlea and produces compression. In the computational version, the HP-AF is simulated by an IIR asymmetric compensation (AC) filter (Irino and Unoki, 1999), which was initially developed to reduce the computational load of the gammachirp filterbank. Irino and Patterson (1999, 2000) have demonstrated the physiological gammachirp fit both the physiological revcor data of Carney et al. (1999) and the physiological masking data of Rosen and Baker (1994).

A filterbank structure for the physiological gammachirp is shown in Fig. 1 (Irino and Unoki, 1999). It is a cascade of three filterbanks: a gammatone filterbank followed by a lowpass AC filterbank, and then a highpass AC filterbank. The gammatone and lowpass-AC filterbanks together produce the fixed gammachirp filterbank which corresponds to the passive basilar membrane whose motion is observed post-mortem or at high sound pressure levels (Recio et al., 1998). The parameter controller estimates signal level to control the highpass AC filterbank which corresponds the active component in the cochlea.

References

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