MIDI

functions.py

from fractions import Fraction
import music21
import glob

# remove leading / trailing whitespace
music21.defaults.ticksAtStart = 0

notes = set([12*j + i for i in [0,2,4,5,7,9,11] for j in [0,1,2,3,4,5,6,7,8,9,10] ]) 

def constrain_pitch(val):
    # keep the pitch in C major, no incidentals
    while val not in notes:
        val = int(val) + 1
        if val > max(notes):
            val - 2
    return val

def get_score(path, **kwargs):
    # return a score object
    if True:
        s = music21.converter.parse(path, 
            forceSource=False, 
            quantizePost=False,
            quarterLengthDivisors=(4,3), # smaller numbers mean fewer different note durations
        ).stripTies(inPlace=True)
    
    else:
        m = music21.midi.MidiFile()
        m.open(path)
        m.read()
        m.close() 
        if kwargs.get('remove_percussion', True):
            tracks = [t for t in m.tracks if not any([e.channel == 10 for e in t.events])]        
        else:
            tracks = m.tracks        
        s = music21.stream.Score()
        music21.midi.translate.midiTracksToStreams(tracks,                                            
            inputM21=s,
            forceSource=False, 
            quantizePost=False,
            ticksPerQuarter=m.ticksPerQuarterNote,
            quarterLengthDivisors=(4,3),
        )       
    return s

def midi_to_string(path, **kwargs):
    d = {
        '32nd': 1/32,
        '16th': 1/16,
        'eighth': 1/8,
        'quarter': 1/4,
        'half': 1/2,
        'whole': 1,
    }
    score = get_score(path, **kwargs)  
    # transpose to c major / a minor
    key = score.analyze('key')
    assert key.mode in ['major', 'minor']  
    interval = 60 - key.tonic.midi
    if key.mode == 'minor': interval -= 3
    score = score.transpose(interval)
    # convert midi to string
    s = ''
    last_offset = 0
    for n in score.flat.notes:
        # get the note/chord duration
        duration = n.duration.components[0].type # n.duration.type returns complex for tied notes
        if not duration or duration == 'zero': continue
        duration = float(Fraction(d.get(duration, duration)))
        # get the offset since the previous note/chord
        delta = float(Fraction(n.offset - last_offset))
        if delta: s += 'w{} '.format(round(delta, 4))
        # add the note/chord to the string
        for i in [n] if isinstance(n, music21.note.Note) else n.notes:
            try:
                pitch = i.pitch.midi
                if kwargs.get('constrain'): pitch = constrain_pitch(pitch)
                s += 'n{}_{} '.format(pitch, round(duration, 4))
            except:
                print(' * could not parse note', i)
        last_offset = n.offset
    return s

def string_to_midi(s):
    # convert string back to midi
    time = 0
    stream = music21.stream.Stream()
    for i in s.split():
        if i.startswith('n'):            
            note, duration = i.lstrip('n').split('_')
            n = music21.note.Note(int(note))
            n.duration.quarterLength = float(duration) * 4
            stream.insertIntoNoteOrChord(time, n)
        elif i.startswith('w'):
            duration = float(Fraction(i.lstrip('w')))
            time += duration
        else:
            print('did not expect', i)
    return stream

midi.ipynb