What is MIDI clock?
MIDI clock is how one device keeps others in tempo. The device in charge sends a clock message 24 times for every quarter note, and every device that follows counts those messages to stay in step. Speed the tempo up and the messages arrive closer together. Three more messages, Start, Continue and Stop, tell the followers when to run.
The messages are part of the MIDI standard. I make InputConfig, a free Mac app that sends and receives the transport messages, so one section covers what it does with them.
Read the clock messages
| Message | Byte (hex) | What it does |
|---|---|---|
| Timing Clock | F8 | Sent 24 times for every quarter note |
| Start | FA | Play from the beginning |
| Continue | FB | Play from where it stopped |
| Stop | FC | Stop |
| Song Position Pointer | F2, then two data bytes | Where to be in the song, counted in sixteenth notes |
Each of the first four is a single byte with no channel, so every device on the connection hears it. The clock carries no tempo number: a follower works the tempo out from how fast the messages arrive.
InputConfig is free on the Mac App Store. macOS 14 or later, no account, no tracking, open source.
Set one sender and many followers
- One device sends. Turn on clock output, often named Sync Out or Send MIDI Clock, on the device or app that sets the tempo.
- The others follow. Set each of them to external sync, so it takes its tempo from the clock that arrives.
- Press play on the sender. It sends Start, and the followers begin together.
Only one device should send the clock, or the followers get two tempos at once.
Tell clock from MIDI Time Code
| MIDI clock | MIDI Time Code | |
|---|---|---|
| Measures | Musical time: beats | Clock time: hours, minutes, seconds and frames |
| Changes with tempo | Yes | No |
| Used for | Drum machines, sequencers, arpeggiators, tempo effects | Locking to video, tape and other recorders |
| Message | F8, 24 for every quarter note | F1 quarter frame messages |
Clock says how fast the music is going, and time code says what time it is.
See a clock arrive
Send a device's or a DAW's clock to a browser and it can be measured. The tap tempo tool reads an incoming MIDI clock and shows its tempo, and the MIDI tester shows the tempo beside Start, Continue and Stop as they arrive. On a Mac, a DAW reaches the browser through an IAC Driver bus. If the tempo on screen matches the sender, the clock is getting through.
Use Start and Stop outside music apps
InputConfig treats Start, Continue and Stop as triggers: each one can fire a shortcut, a macro or an app action once. Pressing play in a DAW can start a screen recording in another app. It can also send those three from a game controller button, for a DAW that follows external transport. It does not send or follow the clock itself, because it is a mapper and not a sequencer. The transport messages are the part of MIDI sync that works as a button. MIDI Start and Stop as Mac shortcuts has the steps.
The short version
- Clock. F8, sent 24 times for every quarter note.
- Transport. Start is FA, Continue is FB, Stop is FC.
- Rule. One sender, and every other device set to follow.
MIDI clock is a shared metronome sent down the cable.
Questions
How many MIDI clock pulses are in a beat?
Twenty-four for every quarter note. A sixteenth note is six clock messages.
Does MIDI clock send the tempo as a number?
No. It sends evenly spaced messages, and the receiving device measures their spacing to work out the tempo.
What is the difference between MIDI clock and MIDI Time Code?
MIDI clock counts beats and follows the tempo. MIDI Time Code counts hours, minutes, seconds and frames, and does not change with tempo. Clock syncs instruments, and time code locks to video and recorders.
What is MIDI beat clock?
Another name for MIDI clock: the F8 timing message sent 24 times for every quarter note.
Why do my devices drift or double up on MIDI clock?
Usually two devices are sending a clock, or a follower is still on its internal tempo. Leave clock output on for one device only and set the others to external sync.
Does MIDI clock use a channel?
No. Clock, Start, Continue and Stop are system messages with no channel, so every device on the connection receives them.
Related
Free. macOS 14 or later. No account, nothing leaves your Mac. Open source on GitHub.
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