Category: System Realtime
Purpose
Some master device that controls sequence playback sends this timing message to keep a slave device in sync with the master. A MIDI Clock message is sent at regular intervals (based upon the master's Tempo) in order to accomplish this.
Status
0xF8
Data
None
Errata
There are 24 MIDI Clocks in every quarter note. (12 MIDI Clocks in an eighth note, 6 MIDI Clocks in a 16th, etc). Therefore, when a slave device counts down the receipt of 24 MIDI Clock messages, it knows that one quarter note has passed. When the slave counts off another 24 MIDI Clock messages, it knows that another quarter note has passed. Etc. Of course, the rate that the master sends these messages is based upon the master's tempo. For example, for a tempo of 120 BPM (ie, there are 120 quarter notes in every minute), the master sends a MIDI clock every 20833 microseconds. (ie, There are 1,000,000 microseconds in a second. Therefore, there are 60,000,000 microseconds in a minute. At a tempo of 120 BPM, there are 120 quarter notes per minute. There are 24 MIDI clocks in each quarter note. Therefore, there should be 24 * 120 MIDI Clocks per minute. So, each MIDI Clock is sent at a rate of 60,000,000/(24 * 120) microseconds).
A slave device might receive (from a master device) a Song Select message to cue a specific song to play (out of several songs), a Song Position Pointer message to cue that song to start on a particular beat, a MIDI Continue in order to start playback from that beat, periodic MIDI Clocks in order to keep the playback in sync with the master, and eventually a MIDI Stop to halt playback. See Syncing Sequence Playback.