Pre-inverse type active noise control with bias free structure

Filtered-x algorithm is widely used for an active noise control (ANC). However, it has a possibility of making ANC unstable due to the modeling error of secondary path. This problem is caused by a filtered input signal, which is used as tap inputs of adaptive algorithm for a control filter. In order to solve the problem, a pre-inverse ANC (PIANC) has proposed, which uses the flat delayed signal instead of a filtered input signal. Thus, the ANC system becomes stable. However, since the conventional PIANC uses a series adaptive filter to estimate the inverse transfer function of the secondary path, the noise reduction ability is degraded due to bias error. Therefore, this paper introduces a bias free structure into the estimation of the inverse transfer function of the secondary path. The proposed structure takes advantage of the delay of the primary path.