EQ and Phase
A boost moves more than level, since minimum-phase EQ rotates phase around the bell and changes how attacks line up.
Related
Push 3 dB at 80 Hz where the kick and bass overlap. The meter barely moves, yet the attack seems half a step late. EQ changes more than magnitude; it also turns phase around the band.
Why minimum phase must move phase
Most EQs are minimum-phase filters. Boost a band and phase rotates around its centre; the higher the Q, the faster and narrower that rotation. Magnitude and phase come from the same coefficients. An analogue circuit cannot move one without the other, and a digital filter needs a different design to avoid it.
Three ways to make the same move
The same correction can be done several ways, each with its own price.
| Approach | Phase behaviour | Latency | Good for | Cost |
|---|---|---|---|---|
| Minimum phase | Rotates near the centre | Close to zero | Track and bus fixes | Attack edges shift |
| Linear phase | Nearly flat across the band | Grows with block size, often tens of ms | Narrow fixes after summing | Pre-ringing before attacks |
| Dynamic EQ | Rotates only when triggered | Depends on design | Taming occasional resonances | Tone jumps at the trigger |
Where the ear can tell
Low frequency exposes it first. Add 2 dB at 60–100 Hz and listen for whether kick and bass still start together. On a piano recorded with two microphones, a 1–2 dB narrow boost shows whether the edge of the part sharpens or blurs. High-pass filters rotate phase too; the higher the corner, the more group delay piles up below it.
Where it breaks
Linear phase is not automatically cleaner. Its pre-ringing lays a small burst before attacks, which drums and solo piano expose, and the effect grows at low frequencies. Stacking minimum-phase boosts on tracks that were already aligned undoes that alignment. Pile narrow boosts around 2–4 kHz and the rotation concentrates, hardening sibilance and transients together. Confirm the problem is frequency before reaching for EQ; level and performance produce the same symptoms.
Listen in the library
All tracks from midicn-lib — inline badge is the license tier (C1 commercial / C2 non-commercial / C3 study only).
Along the same line
Sources
- FabFilter, Pro-Q 4 user manual, minimum-phase and linear-phase modes.
- Robert Bristow-Johnson, Cookbook Formulae for Audio EQ Biquad Filter Coefficients.
- Zölzer, DAFX.