Electronic music - sync
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Sync in general
Devices can keep pretty damn regular rhythm with an internal clock.
However, multiple devices with same-speed crystals need vary very little which they will for those to go out of sync with each other.
So instead, we often communicate what we want: se wend a pulse to signal when to move on to the next step.
This means there is only one clock, and everything else will keep synchronized to it - no matter how (ir)regular that one clock is.
Sync is electrically very simple.
Typically a square impulse, because that is best defined electrically, and easy to consume.
One practical limitation is that such a pulse alone can't communicate when a measure should start over, so it's not very hard to get two devices playing at the same speed, but start/restart their measures at different times.
One workaround is to, if possible, prime each listening device at the start, and only then start the sync pulses.
PPQN (a.k.a. PPQ and TPQN) is fairly literal: the amount of electronic-level pulses (/ticks) before the listening side moves on to the next quarter note.
Technically, it's only a quarter note in relation to others that intepret it the same way - but that's often true.
PPQN is often seen in the form 24PPQN, e.g. in MIDI beat clock (and the earlier DIN sync).
Sync in the real world
MIDI beat clock, a.k.a. MIDI timing clock, MIDI clock, is using regular MIDI connection to transmit a (single-byte) message at 24PPQN.
- see also MIDI#Beat_clock
MIDI Timecode, a.k.a. MTC, is a variant of SMPTE
- Which isn't really a beat, so somewhere between finicky and useless to use for rhythm. It's useful to point out that this is not the same as beat clock.
- see also MIDI#MTC
 is a PPQN style pulse on a dedicated pin of a DIN socket. (A second pin set whether the drum machine should be running or paused. There are a few pinout variations.)
DIN sync is typically 24PPQN, and also known as Sync24.
Some old drum machines (e.g. some from Linn, Korg) used 48PPQN, many others 24PPQN (e.g. Roland). If DIN-synced between those, they'd go at twice/half speed. Which some people used intentionally, others has a converter box for.
Modular synth sync - most modules trigger as soon as they see an event (trigger or gate), and don't run their own timer at all. Even deriving varied clock signals using modules tends to not do so.
As such, it's usually only the clock generator you use that needs to know about rhythm, everything else just needs to react.