Allpassphase ~repack~ Online

[ H(z) = \fraca_2 + a_1 z^-1 + z^-21 + a_1 z^-1 + a_2 z^-2, \quad |a_2| < 1 ]

The pole-zero placement (complex conjugate pair) allows tuning of both the center frequency and bandwidth of the phase transition. While phase shift matters, the group delay ( \tau_g(\omega) = -\fracd\phi(\omega)d\omega ) often matters more in practical systems. allpassphase

For a second-order all-pass filter: