An optimal harmonics extraction filter with pitch fluctuation from the noise-corrupted observation is proposed. It is derived as a solution to the Kalman-Bucy filter in which the pitch fluctuation is treated as a state-dependent noise. The analytical solution provides the frequency characteristic which consists of constant-Q pass-bands just located on each harmonic frequency. This remarkable structure is well suited to the increasing harmonic fluctuation in direct proportion to the harmonic frequency. Simulation results show that this filter has a robustness to the pitch estimation error and fluctuation. Examples of the filter design are presented, and the performance of sound extraction is examined by comparison with conventional filters.
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