ACTIVE NOISE CANCELLING IN HEADSETS: LIMITATIONS CAUSED BY NON-MINIMUM PHASE TRANSFER FUNCTION OF THE CANCELLING SOUND SOURCE

摘要

We have investigated active noise canceling in headsets using a Normalized Filtered X LMS algorithm. We believe to have revealed the problems in relation to non-minimum phase transfer functions in the secondary path (the canceling sound source). A systematic investigation is carried out by means of simple but fairly accurate models for the transfer function, S_sec, of the canceling sound source. The model was selected as a second order high-pass filter (minimum phase) cascaded with a second order allpass filter (non-minimum phase). The model parameters is varied in a systematic way, the "generating pole" for the second order allpass filter is varied in the range from 0.4 to 0.8 for the pole radius and 0.1π to 0.8π for the pole angle. The "plant" (sound transmission through the ear cups) is modelled by a second order lowpass filter. In the FXLMS algorithm we use an estimate of the transfer function S_sec for prefiltering the reference signal x[n]. Various estimates, S_est, are generated by simple modification of the parameters for S_sec. The simulations show that even minor amounts of non-minimum phase degrades the adaptive filter performance substantially.