a method of setting a transfer function of an adaptive filter . the adaptive filter is used for processing an audio signal . the method includes setting a first , a second , and a third pole , setting a first zero without resting upon the audio signal . a real part of the first zero is a negative value and an imaginary part of the first zero is a positive value . a second zero is set having a real part corresponding a positive value and an imaginary part corresponding to a negative value and a third zero is set according to a key shifting associated with the audio signal .