Key sync in FM synthesis resets each operator's phase on every new note, ensuring consistent sound across retriggers

In FM synthesizers, each operator (oscillator) runs continuously — when a new note triggers, the operator may be at any arbitrary point in its waveform cycle, producing slight variations in sound from note to note. Enabling key sync (phase reset) forces each operator to start from the same phase position on every new note trigger. This ensures that every instance of the note sounds identical — important for sounds like a Reese bass where the wobble movement should start from the same position each time, making the performance predictable and repeatable. Disabling key sync is useful when you deliberately want random, evolving variation in the sound’s start phase (pad sounds, atmosphere). For percussive or rhythmically-groove-sensitive sounds, key sync is typically on.

Examples

In FM8: enable key sync on each operator (F, E, D). Without key sync, each note may sound slightly different depending on where the oscillator cycle was. With key sync, every note starts with the same phase and produces the same sound shape — critical for tight, groove-locked DnB bass lines.

Assessment

Explain what key sync does in FM synthesis and why it matters for a rhythmically tight Reese bass line; then describe a case where disabling key sync would produce a more desirable result.