Bass Transcribe

Bass Transcribe — played take → MIDI / Live clip / chart

Default is the offline CREPE pipeline (validated: CREPE full+viterbi beats pyin on bass and vocals). For in-Live plugin options (NeuralNote, Jam Origin) read references/realtime-options.md.

1. Get the audio

• User points at a file — wav/mp3/aiff of the take. Mono preferred; DI cleaner than mic.
• Record in Live — audio track, arm, record the take. MCP cannot export audio: the user does File → Export, or point at the raw recording under the Set's Samples/Recorded/ folder.
• Ask for (or detect) the take's BPM if the output will go into a Live clip.

2. Default pipeline — CREPE → MIDI

import crepe, librosa, numpy as np, pretty_midi

y, sr = librosa.load(AUDIO, sr=16000, mono=True)
time, freq, conf, _ = crepe.predict(y, sr, model_capacity='full', viterbi=True, step_size=10)

CONF, MIN_LEN, GAP = 0.55, 0.08, 0.05   # confidence gate, min note s, merge gap s
midi_f = 69 + 12*np.log2(freq/440.0)
notes, cur = [], None
for t, m, c in zip(time, midi_f, conf):
    p = int(round(m)) if c >= CONF and np.isfinite(m) else None
    if cur and p == cur['p'] and t - cur['end'] <= GAP:
        cur['end'] = t
    else:
        if cur and cur['end'] - cur['start'] >= MIN_LEN:
            notes.append(cur)
        cur = {'p': p, 'start': t, 'end': t} if p is not None else None
if cur and cur['end'] - cur['start'] >= MIN_LEN:
    notes.append(cur)

pm = pretty_midi.PrettyMIDI()
inst = pretty_midi.Instrument(program=33)     # Electric Bass (finger)
for n in notes:
    if 26 <= n['p'] <= 67:                    # octave sanity: drop-D D1 to 24th-fret G4
        inst.notes.append(pretty_midi.Note(velocity=96, pitch=n['p'],
                                           start=n['start'], end=n['end']))
pm.instruments.append(inst)
pm.write(OUT_MID)

Tuning knobs: CONF up to 0.6–0.7 for noisy takes; MIN_LEN down to 0.05 for fast lines; step_size=10 ms is fine for bass.

3. Cleanup rules

• Drop blips < 80 ms (tracker flicker, fret noise).
• Octave sanity: 4-string bass ≈ MIDI 26–67 (drop-D D1 … 24th-fret G4). CREPE octave errors land a 5th/octave out — flag and correct outliers to the nearest in-range octave.
• Legato-join same-pitch notes split by < 50 ms gaps.
• Ghost notes / slides show as short low-confidence fragments — either drop (chart use) or keep (feel/groove analysis). Ask which the user wants.
• Double-stops or chords played? CREPE is monophonic and will pick one note — rerun with basic-pitch (basic-pitch <out_dir> <audio> --save-midi) for those passages.

4. Outputs

1. .mid file — the pipeline's direct product; drag into any DAW.
2. Live clip via the ableton-mcp skill: convert seconds → beats (beats = sec * BPM / 60), size the clip in BARS (round up to 1/2/4/8…), push with add_notes_to_clip in ≤140-note chunks (never two pushes to the same clip in parallel).
3. Tab / chart — bass tab or bar chart generation: chart craft lives in the song-analysis skill (references/chart-and-lyrics.md); pitch-class per half-bar aggregation as in its Phase 3.

5. Verify before declaring done

Report note count, pitch range, and duration vs. take length. Sanity signals: a bass take yields roughly 1–4 notes/second; a pitch histogram dominated by 2–5 pitch classes (the song's roots). Wildly more notes → lower CONF gate is letting flicker through; wildly fewer → gate too high or the take is quieter than expected.

Local environment (this machine)

• Python with crepe 0.0.16 + basic-pitch 0.4.0 + pretty_midi (verified 2026-07-16): /Users/peripan/dev/abletonAI/audio-analysis/.venv-bp/bin/python
• Prior art: scripts/transcribe_bass_pyin.py in the tooling root (pyin — superseded by CREPE).