feat: implement src/midi_export.py — .chord → two-track MIDI
chord_file_to_midi() parses the period in the user's original key (no transposition), accumulates held-chord segments, then writes two pretty_midi tracks: chords with root anchored at octave 4 (MIDI 60–71 + intervals) and bass at octave 2 (MIDI 36–47). Extension notes are added as a fifth voice at their standard interval above the root. Tempo is parameterised; the CLI wrapper (python -m src.midi_export) supports --tempo BPM. 10 tests cover: file creation, parseability, instrument count and names, chord/bass note counts for a 4-chord C-major fixture (14 chord + 4 bass), octave placement assertions, and tempo affecting total duration. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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"""MIDI export for .chord files.
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Converts a .chord period to a two-track MIDI file without transposing to
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canonical key — the user's original key is preserved.
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Track 0 "Chords" — chord voicings, root anchored at octave 4 (MIDI 60–71).
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Track 1 "Bass" — bass note (or root when no slash), octave 2 (MIDI 36–47).
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Public API:
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chord_file_to_midi(chord_path, midi_path, tempo=90)
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CLI:
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python -m src.midi_export input.chord output.mid [--tempo BPM]
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Example:
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python -m src.midi_export data/raw_user/2024_001_sea-glass_chorus.chord \\
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out/sea-glass-chorus.mid --tempo 96
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"""
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from __future__ import annotations
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import argparse
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import logging
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from pathlib import Path
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import pretty_midi
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from src.chord_parser import parse_chord_symbol
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from src.tokenizer import parse_chord_file
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log = logging.getLogger(__name__)
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# ---------------------------------------------------------------------------
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# Music-theory tables
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# ---------------------------------------------------------------------------
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_NOTE_SEMITONES: dict[str, int] = {
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"C": 0, "C#": 1, "D": 2, "D#": 3, "E": 4, "F": 5,
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"F#": 6, "G": 7, "G#": 8, "A": 9, "A#": 10, "B": 11,
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}
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# Semitone intervals above the root for each of the 18 canonical qualities.
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_QUALITY_INTERVALS: dict[str, list[int]] = {
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"maj": [0, 4, 7],
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"m": [0, 3, 7],
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"dim": [0, 3, 6],
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"aug": [0, 4, 8],
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"sus2": [0, 2, 7],
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"sus4": [0, 5, 7],
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"maj7": [0, 4, 7, 11],
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"m7": [0, 3, 7, 10],
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"7": [0, 4, 7, 10],
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"m7b5": [0, 3, 6, 10],
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"dim7": [0, 3, 6, 9],
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"mM7": [0, 3, 7, 11],
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"7sus4": [0, 5, 7, 10],
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"aug7": [0, 4, 8, 10],
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"6": [0, 4, 7, 9],
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"m6": [0, 3, 7, 9],
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"add9": [0, 4, 7, 14],
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"m(add9)": [0, 3, 7, 14],
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}
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# Semitones above root for each extension type (added as a fifth voice).
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_EXTENSION_SEMITONES: dict[str, int] = {
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"9": 14,
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"b9": 13,
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"#9": 15,
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"11": 17,
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"#11": 18,
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"13": 21,
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"b13": 20,
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}
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# ---------------------------------------------------------------------------
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# Internal helpers
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# ---------------------------------------------------------------------------
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def _position_duration(time: str, subdivision: int, tempo: int) -> float:
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"""Return the duration in seconds of one subdivision position."""
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num, denom = (int(x) for x in time.split("/"))
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beats_per_bar = num * 4.0 / denom # quarter-note beats per bar
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bar_duration = beats_per_bar * 60.0 / tempo
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positions_per_bar = (num * subdivision) // denom
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return bar_duration / positions_per_bar
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def _emit_chord_notes(
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symbol: str | None,
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start: float,
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hold: int,
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pos_dur: float,
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chord_inst: pretty_midi.Instrument,
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bass_inst: pretty_midi.Instrument,
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) -> None:
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"""Append MIDI notes for one held-chord segment to both instruments."""
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if symbol is None or hold == 0:
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return
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t = parse_chord_symbol(symbol)
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end = start + hold * pos_dur
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# Chord track: root in octave 4, intervals laid above.
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root_base = 60 + _NOTE_SEMITONES[t.root]
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for interval in _QUALITY_INTERVALS[t.quality]:
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chord_inst.notes.append(
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pretty_midi.Note(velocity=80, pitch=root_base + interval, start=start, end=end)
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)
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if t.extension != "none":
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chord_inst.notes.append(
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pretty_midi.Note(
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velocity=70,
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pitch=root_base + _EXTENSION_SEMITONES[t.extension],
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start=start,
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end=end,
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)
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)
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# Bass track: bass note (or root when no slash) in octave 2.
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bass_name = t.root if t.bass == "root" else t.bass
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bass_inst.notes.append(
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pretty_midi.Note(
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velocity=90,
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pitch=36 + _NOTE_SEMITONES[bass_name],
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start=start,
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end=end,
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)
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)
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# ---------------------------------------------------------------------------
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# Public API
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# ---------------------------------------------------------------------------
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def chord_file_to_midi(chord_path: Path, midi_path: Path, tempo: int = 90) -> None:
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"""Convert a .chord file to a two-track MIDI file.
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The period is used in its original (user) key without transposing to
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canonical. Track 0 contains chord voicings (octave 4–5); track 1
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contains the bass line (octave 2).
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Args:
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chord_path: Path to the .chord source file.
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midi_path: Destination .mid file (created or overwritten).
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tempo: Playback speed in BPM (default 90).
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"""
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period = parse_chord_file(chord_path)
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pos_dur = _position_duration(period.time, period.subdivision, tempo)
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pm = pretty_midi.PrettyMIDI(initial_tempo=float(tempo))
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chord_inst = pretty_midi.Instrument(program=0, name="Chords")
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bass_inst = pretty_midi.Instrument(program=32, name="Bass")
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current_chord: str | None = None
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current_start = 0.0
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current_hold = 0
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current_time = 0.0
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for bar in period.bars:
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for pos in bar:
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if pos == ".":
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if current_chord is not None:
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current_hold += 1
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elif pos in ("NC", "?"):
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_emit_chord_notes(
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current_chord, current_start, current_hold,
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pos_dur, chord_inst, bass_inst,
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)
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current_chord = None
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current_hold = 0
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else:
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_emit_chord_notes(
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current_chord, current_start, current_hold,
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pos_dur, chord_inst, bass_inst,
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)
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current_chord = pos
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current_start = current_time
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current_hold = 1
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current_time += pos_dur
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_emit_chord_notes(
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current_chord, current_start, current_hold,
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pos_dur, chord_inst, bass_inst,
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)
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pm.instruments.extend([chord_inst, bass_inst])
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pm.write(str(midi_path))
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log.info("wrote %s (%d bars, %d BPM)", midi_path.name, len(period.bars), tempo)
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# ---------------------------------------------------------------------------
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# CLI entry point
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# ---------------------------------------------------------------------------
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if __name__ == "__main__":
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parser = argparse.ArgumentParser(
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description="Convert a .chord file to MIDI.",
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epilog="Example: python -m src.midi_export input.chord output.mid --tempo 120",
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)
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parser.add_argument("chord_file", type=Path, metavar="input.chord")
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parser.add_argument("midi_file", type=Path, metavar="output.mid")
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parser.add_argument(
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"--tempo", type=int, default=90, metavar="BPM",
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help="playback tempo in BPM (default: 90)",
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)
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args = parser.parse_args()
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logging.basicConfig(level=logging.INFO, format="%(message)s")
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chord_file_to_midi(args.chord_file, args.midi_file, tempo=args.tempo)
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print(f"Written: {args.midi_file}")
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