§2 · One anchor, integer ticks, immutable tempo¶
A session defines its time with three numbers and one instant. Tempo must not change during a session. Every time value in the protocol is a tick; bars are a presentation form and belong in the user interface, not on the wire.
| Field | Meaning |
|---|---|
anchorEpochMs |
Physical instant of tick 0. Immutable. |
bpm |
Quarter notes per minute. Immutable. |
beatsPerBar |
Default 4. Immutable. |
ppq |
Ticks per quarter note. The value is 960. |
ticksPerBar |
Derived: ppq × beatsPerBar — 3840 in 4/4. |
This is what a fractional bar position does not give: loop length is always an integer. Sixteen bars is 61 440 ticks whether the tempo is 118 or 120, so comparison, hashing and equality are exact in every implementation.
N14 · the first thing an implementer gets wrong
A tick must be derived using floored division — toward negative infinity — not truncation toward zero and not rounding. The remainder used for a material position must be non-negative. Both differ from the native operators of several languages:
// JavaScript BigInt: both wrong for this purpose
(-7n) / 2n // -3n — truncates toward zero, want -4n
(-7n) % 2n // -1n — negative remainder, want 1nDone correctly:
// Floored division
let t = q / 60000n;
if (q < 0n && q % 60000n !== 0n) {
t -= 1n;
}A tick may be negative, because a session may be announced before its anchor. That is not an edge case; it is how a set is scheduled.