Outside gear follows Groove's tempo and transport #24

Part of #1

What to build #

Gear plugged into this Mac follows Groove's tempo and transport: a drum machine started from Groove's Play starts on the beat, and stops when Groove stops.

• MIDI outputs are listed in the settings: every MIDI port this Mac has, plus one of Groove's own, ATMOSYNTH Groove, which other apps see as a source without the IAC Driver being turned on.
• Which MIDI outputs are sent the clock is a machine setting, the same for every document.
• Each MIDI output has an offset in milliseconds, a machine setting, positive when the gear is slow. Everything sent to an output leaves early enough that the gear sounds when Groove's own audio for that moment leaves the speakers: ahead by the output device's latency, plus that offset.
• The clock, as atmosynth.at sends it (src/audio/midi-clock.js in atmosynth-audio-engine), from an arrangement and from a patch document:
  • pulses, 24 to a beat, whenever the audio device is open, following the tempo map;
  • Play from the start sends Start; Play from anywhere else sends Song Position, then Continue;
  • Stop sends Stop;
  • the loop range going back, or the playhead moved while playing, sends Stop, Song Position, Continue;
  • in a count-in, the pulses run and Start or Continue lands where the recording begins;
  • in a room, the clock follows the room's count, which is what is heard.
• The out MIDI light flashes for Start, Stop, Continue and Song Position, never for the pulses alone.

Acceptance criteria #

• A MIDI output chosen for the clock receives 24 pulses a beat at the arrangement's tempo, following a tempo change, whether or not the transport moves.
• Play from the start, Play from bar 5, Stop and a loop going back each send what is listed above, in that order.
• A message leaves ahead of the audio by the output device's latency plus the output's offset.
• ATMOSYNTH Groove appears as a MIDI source to another app, with nothing set up in Audio MIDI Setup.
• The out light flashes on Start and Stop and stays dark through the pulses alone.
• A patch document sends the clock as an arrangement does; in a room, from the room's count.