7.10.14. STRUDEL-14 — MIDI Files

daStrudel parses and plays Standard MIDI Files (.mid) — the universal sheet-music format. You get a structured view of tracks, events, and tempo, plus a high-level midi_play entry point that runs the file through either built-in samples or an SF2 SoundFont.

7.10.14.1. Part A: Parsing

load_midi reads a .mid file and returns a MidiFile struct:

require strudel/strudel_midi

let midi = load_midi("fur_elise.mid")
print("Format: {midi.format}\n")         // 0 = single track, 1 = multi-track
print("Ticks/QN: {midi.ticks_per_qn}\n") // timing resolution
print("Tracks: {length(midi.tracks)}\n")

Each track is a MidiTrack containing an array of MidiEvent. Events carry a tick timestamp, a MidiEventKind (note-on, note-off, control change, tempo, etc.), a channel number, and two data bytes.

Format 0 packs everything into one track; format 1 uses parallel tracks — daStrudel handles both. ticks_per_qn is the time resolution: higher values mean finer timing subdivisions.

7.10.14.2. Part B: Merging and Tempo

merge_tracks collapses all tracks into one stable-sorted event stream, which is easier to walk for analysis:

var merged <- merge_tracks(midi)
for (evt in merged) {
    if (evt.kind == MidiEventKind.midi_tempo && evt.tempo > 0) {
        let bpm = 60000000.0 / float(evt.tempo)
        print("Tempo change at tick {evt.tick}: {bpm:.1f} BPM\n")
    }
}

Tempo events store microseconds-per-quarter-note in the tempo field; convert to BPM with 60_000_000 / tempo. Files with no tempo events default to 120 BPM.

7.10.14.3. Part C: Playback with Sample Banks

The midi_play path uses the audio system’s streaming infrastructure and a built-in piano + drum sample bank. The lifecycle:

require strudel/strudel_midi_player

midi_load_samples(media_path)   // load built-in banks from media folder
midi_init()                     // start the playback thread
midi_play("music", "fur_elise.mid", [gain = 0.6, looping = false])
sleep(6000u)
midi_stop("music")
midi_shutdown()

The first argument to midi_play is an arbitrary track name — you can play multiple MIDI files at once and control them independently (cross-fade, stop, set volume) via that name.

7.10.14.4. Part D: Playback with SF2

For higher-quality rendering, swap the sample backend for a General MIDI SoundFont. midi_load_sf2 activates it; subsequent midi_play calls dispatch notes through the SoundFont presets (channel 10 uses the GM drum kit automatically):

midi_load_sf2("FluidR3_GM.sf2")
midi_init()
midi_play("music", "Bach_Air_on_G_string.mid", [gain = 1.0])
sleep(8000u)
midi_shutdown()

The SF2 backend respects program change events — if the MIDI file switches instruments mid-song, the SoundFont follows. Pair with STRUDEL-13 — SF2 SoundFont Playback for per-instrument control from live patterns.