Allpass / Phase Corrector Designer
Sections that bend phase without touching magnitude — the tool IIR filters need afterwards.
What an allpass is for
An allpass section has poles and zeros in mirror pairs, so its magnitude is exactly 1 at every frequency — only the phase moves. The second-order form
swings the phase through −180° at f₀, with Q setting how fast. Cascade a few of these after an IIR filter, placing each f₀ where the filter's group delay dips, and you flatten the total delay — the classic delay-equalizer recipe (audio crossovers, data pulse shaping). The same structure is also the building block of fractional delay lines and Hilbert transformers. Tuning tip: start with one section at the filter's cutoff with Q ≈ 0.5–1, watch the summed group delay, then add sections outward; each second-order allpass adds exactly 360° of total phase lag spread around its f₀.