Phase coherence vs. time alignment

Time alignment and phase coherence get used as if they mean the same thing. They don’t, and the difference decides which fix actually helps.

Time alignment slides one track earlier or later by a single delay, so its transient lands at the same moment as a reference mic. Phase coherence is about how two mics add up across the frequency range once you sum them. A single time slide fixes the first problem. It often can’t fix the second.

What time alignment does

Put two mics on one source and they capture the sound at slightly different times. Time alignment measures that offset and shifts one track to close it, so the attacks line up.

This is the right tool when a fixed delay is the whole story. A top and bottom snare mic sit a few centimetres apart, so the bottom mic’s hit arrives a fraction of a millisecond late. Flip its polarity, nudge it forward, and the two lock together. A DI and a mic on the amp a few feet back have the same kind of clean, constant offset.

It has a cost, though. You have moved the timing you recorded. On a close mic chasing a tight modern sound, that’s exactly what you want. On a room mic, that delay was the room. Slide it up to the close mics and you can flatten the depth you set the mic up to capture.

What phase coherence is

Sliding a whole track assumes the offset is the same at every frequency. Usually it isn’t. Path-length differences, off-axis coloration, and the phase response of the mics and preamps all make the relationship shift with frequency. So you can line up the transients and still lose low end, because the two mics disagree in phase somewhere else in the band. That’s the part a nudge can’t reach.

Phase coherence correction works frequency by frequency. An all-pass process shifts phase by different amounts at different frequencies to bring the two signals into a better relationship across the whole range, without moving the track in time. The magnitude of each track — its EQ — stays put. What changes is how the tracks sum. A fuller combined sound, with the same tone on each mic, is the point.

When you want each

• Time alignment when the arrival offset is the problem and you’re happy to move the timing: top and bottom snare, DI and amp, a close mic you want tucked in tight.
• Phase coherence when the transients already line up but the blend still sounds hollow, or when you want to keep the natural timing of overheads, rooms, and distant mics and only fix how they combine.
• Neither when the recording already sits well. Over-correcting drums can make them small and sterile. Some phase interaction is part of the sound, and the goal is a recording that works, not a flat correlation meter.

The reference choice matters as much as the method. Aligning everything to the overheads keeps the stereo picture they paint and pulls the close mics into it. Aligning to a close mic instead changes the character of the whole kit. Decide what you’re aligning to before you decide how.

How to tell it worked

Trust your ears, then confirm. Sum the mics to mono and listen for the low end coming back. Watch a correlation meter as you make the change. Pull the fader on the mic you suspect and see whether the kit gets fuller without it — if it does, that mic was subtracting, not adding.

Where PhaseLock fits

PhaseLock keeps these as two separate controls, because they’re two decisions. Phase Coherence corrects the frequency-dependent relationship and leaves the timing where you recorded it. Time Alignment moves the arrival when you actually want it moved. You pick which job the recording needs instead of getting both whether they help or not.

If you’re weighing whether to move a mic’s timing at all, start with the reference-mic decision and the room’s natural depth before you reach for a delay.