LoRA adapter (r=16) that conditions the F1 base (PearlLeeStudio/TheArtist-MusicTransformer-ft-pop80) toward bossa nova chord progressions — Brazilian bossa nova jazz harmony. One of eleven per-genre adapters from the paper How Far Can Chord-Symbol Time-Series Adaptation Carry Genre Identity? (Lee, 2026). The released snapshot is the best rank of a 5-point sweep (r ∈ {4, 8, 16, 32, 64}); the vocabulary is extended 351 → 359 with [GENRE:X] tokens shipped in embedding_extension.pt.

Paper · Code · Demo · All models

Base-weights note. The released F1 base is weight-identical to the Phase-0 pop baseline (a checkpoint-selection artifact — see the note on the base card). Every "F1 base" column below was measured against those exact weights, so the Δ shown is the adapter's gain over a pure-pop harmonic prior.

Usage

Requires torch, huggingface_hub, peft, safetensors. Both repos bundle model.py and tokenizer.py, so nothing needs to be cloned from GitHub.

python

import sys
import torch
import torch.nn as nn
from huggingface_hub import snapshot_download
from peft import PeftModel

# 1. Download the base + LoRA repos. Both bundle model.py and tokenizer.py.
base_dir = snapshot_download(repo_id="PearlLeeStudio/TheArtist-MusicTransformer-ft-pop80")
lora_dir = snapshot_download(repo_id="PearlLeeStudio/TheArtist-MusicTransformer-lora-bossa")
sys.path.insert(0, base_dir)  # so the next two imports resolve

from model import MusicTransformer
from tokenizer import ChordTokenizer

# 2. Extended tokenizer (351 base + 8 new genre tokens = 359). The PAD id
#    is unchanged across base and extended tokenizers.
tokenizer = ChordTokenizer(include_extra_genres=True)

# 3. Build the model at the BASE vocab size (351) so F1's state_dict loads
#    cleanly; we grow the embedding rows immediately after. Passing the
#    extended tokenizer's pad_id is safe because PAD is shared (see step 2).
BASE_VOCAB = 351
model = MusicTransformer(
    vocab_size=BASE_VOCAB,
    d_model=512, n_heads=8, d_ff=2048, n_layers=8,
    max_seq_len=256, dropout=0.0, pad_id=tokenizer.pad_id,
)
ckpt = torch.load(f"{base_dir}/best.pt", map_location="cpu", weights_only=False)
model.load_state_dict(ckpt["model_state_dict"])

# 4. Grow token_emb + out_proj from 351 -> 359 (new rows init from
#    [GENRE:none]), then overlay the LoRA's trained extension rows.
def _grow_to_extended_vocab(m, new_vocab, none_id):
    d = m.token_emb.embedding_dim
    new_emb = nn.Embedding(new_vocab, d, padding_idx=m.token_emb.padding_idx)
    with torch.no_grad():
        new_emb.weight[:m.token_emb.num_embeddings] = m.token_emb.weight
        for i in range(m.token_emb.num_embeddings, new_vocab):
            new_emb.weight[i] = m.token_emb.weight[none_id]
    m.token_emb = new_emb
    new_out = nn.Linear(d, new_vocab, bias=False)
    with torch.no_grad():
        new_out.weight[:m.out_proj.out_features] = m.out_proj.weight
        for i in range(m.out_proj.out_features, new_vocab):
            new_out.weight[i] = m.out_proj.weight[none_id]
    m.out_proj = new_out

_grow_to_extended_vocab(model, tokenizer.vocab_size, tokenizer.encode_genre("none"))

ext = torch.load(f"{lora_dir}/embedding_extension.pt",
                 map_location="cpu", weights_only=False)
model.token_emb.load_state_dict(ext["token_emb_state"])
model.out_proj.load_state_dict(ext["out_proj_state"])

# 5. Apply the LoRA adapter (the adapter files live at lora_dir/adapter/).
model = PeftModel.from_pretrained(model, f"{lora_dir}/adapter")
model.eval()

# 6. Generate a bossa nova continuation. With LoRA injected,
#    PeftModel.forward routes through the adapted attention layers.
song = {
    "key": "Cmaj", "time_signature": "4/4", "genre": "bossa",
    "bars": [["Cmaj7"], ["Fmaj7"]],
}
prompt_ids = tokenizer.encode_sequence(song)[:-1]
ids = torch.tensor([prompt_ids])
with torch.no_grad():
    for _ in range(32):
        logits = model(ids)  # routed through LoRA via PeftModel
        next_id = torch.multinomial(
            torch.softmax(logits[:, -1, :] / 0.8, dim=-1), 1,
        )
        ids = torch.cat([ids, next_id], dim=-1)
        if next_id.item() == tokenizer.eos_id:
            break
print(tokenizer.decode(ids[0].tolist()))

Evaluation

Teacher-forced token-level metrics on the bossa nova val split (1,431 sequences, no key augmentation). Both columns use the same dataloader and the same [GENRE:none]-initialised embedding extension — only the adapter weights and trained embedding rows differ.

Metric	F1 base	F1 + this LoRA (r=16)	Δ
Top-1 accuracy (%)	78.33	82.30	+3.97
Top-5 accuracy (%)	93.64	95.99	+2.35
Cross-entropy loss	0.9635	0.6589	-0.3046

Rank sweep the released adapter was selected from (minimum val loss, top-1 tiebreak):

Rank	val_loss	Top-1 (%)	Δ Top-1 vs F1
r=4	0.6593	81.13	+2.80
r=8	0.6601	81.15	+2.82
r=16	0.6589	82.30	+3.97 ← released
r=32	0.6601	81.13	+2.80
r=64	0.6604	81.12	+2.79

Real-song check — mean over 10 held-out bossa nova songs (24–78 bars each; titles are Spotify track IDs by upstream Chordonomicon policy — the progressions are real songs):

Model	Top-1 (%)	Top-5 (%)	Loss
F1 base	81.43	95.47	0.7825
F1 + this LoRA	84.02	97.53	0.5604
Δ	+2.59	+2.07	-0.2221

The 10 songs are this genre's slice of a 130-song eval set (10 per genre × 13 genres, seed 42) drawn from the held-out val/test partitions only — pop from McGill Billboard (CC0), jazz from public standards corpora, classical from Bach chorales, the other ten genres from the matching Chordonomicon subsets (CC BY-NC 4.0).

Training data

14,315 sequences from 12,011 songs — the bossa nova subset of the Chordonomicon dataset, song-level 80/10/10 split (seed 42), 12-key augmentation on train. Chordonomicon is licensed CC BY-NC 4.0; see the dataset card for full terms.

Adapter: LoRA on the Q/K/V projections (w_q, w_k, w_v), r=16, α=32, dropout 0.05. The adapter file holds 393,216 LoRA parameters (1.5 MB). Training also updates the token-embedding and output matrices (367,616 parameters, shipped in embedding_extension.pt), so the trained set totals 760,832 parameters, 3.0% of the 25,665,536 that full fine-tuning updates. Best checkpoint by minimum val loss.

License

CC BY-NC 4.0 (matching Chordonomicon, the upstream training corpus). Research, paper replication, portfolio, and demo use are permitted; commercial use is not.

Citation

bibtex

@misc{lee2026chordmix,
  title         = {Empirical Study of Pop and Jazz Mix Ratios for Genre-Adaptive Chord Generation},
  author        = {Lee, Jinju},
  year          = {2026},
  eprint        = {2605.04998},
  archivePrefix = {arXiv}
}

@misc{lee2026chordtimeseries,
  title         = {How Far Can Chord-Symbol Time-Series Adaptation Carry Genre Identity?},
  author        = {Lee, Jinju},
  year          = {2026},
  eprint        = {2606.07334},
  archivePrefix = {arXiv}
}