Instruct-MusicGen-MLX-4bit

• speech-swift — Apple SDK
• soniqo.audio — website
• blog — blog

MLX port of Instruct-MusicGen — text-instructed music editing. Built on MusicGen-large (3.3B params, 48-layer autoregressive transformer over EnCodec 32 kHz tokens) with cross-attention base weights from the upstream checkpoint, LoRA-merged on Q/V (α/r = 2.0), plus a 48-layer CPTransformer adapter that injects the input audio's per-layer Q/K/V via prefix-attention into every self-attention block.

Inputs / Outputs

• Input: text instruction (e.g. "Music piece. Instruct: Only Drums.") + input audio (mono float32 @ 32 kHz, ≤ 10 s window)
• Output: edited audio (mono float32 @ 32 kHz, matches input length)

Performance (Apple Silicon, INT4)

Metric	Value
Bundle size	~2.2 GB on disk
RTF (wall / audio)	~1.21 (for 5 s output @ 250 AR steps)
Peak RSS	~3 GB

Quality (CLAP score vs instruction)

Mean CLAP score (laion/clap-htsat-unfused) across 4 edit instructions on a MusicGen-generated input clip — output vs the instruction text:

Variant	mean CLAP	"Only Drums"	"Only Piano"	"Remove Drums"	"Only Bass"
FP16	+0.352	+0.40	+0.36	+0.42	+0.22
INT4 (this bundle)	+0.311	+0.45	+0.17	+0.40	+0.21
INT8	+0.311	+0.44	+0.20	+0.39	+0.21

INT4 ≈ INT8 in CLAP, both within ~12 % of FP16. The "Only X" instructions generally produce a positive Δ vs the input clip's CLAP score — i.e. the edit moves the audio toward the instruction. "Only Bass" remains the hardest case.

Usage (sketch)

from huggingface_hub import snapshot_download
bundle = snapshot_download("aufklarer/Instruct-MusicGen-MLX-4bit")

# Production loader: https://github.com/soniqo/speech-swift
# Minimal MLX sketch:
#   1. Read bundle/config.json (HF MusicGen config + cp_transformer metadata)
#   2. Construct InstructMusicGen MLX class
#   3. Replay quantization on linear projections (mlx.nn.quantize, bits=4)
#   4. Load weights from bundle/model.safetensors
#   5. audio = model.generate(text, input_audio, max_steps=250)

Architecture details

text instruction ── T5-base ── [LoRA-merged] cross-attn ──┐
                                                          │
input audio ─ EnCodec encode ─ CPTransformer ─ prefix Q/K/V ──┐
                                                              │
                                       ┌──────────────────────┘
                                       ▼
              MusicGen-large LM (48 AR layers, delay pattern)
                                       │
                                       ▼
                              EnCodec decoder → 32 kHz wav

• CPTransformer: shares the base LM's transformer blocks (norm/self-attn/FFN) but adds learned pos_emb (49, 501, 2048), merge_linear[i] per layer (2048 → 2048), and a zero-init gate[i] scalar.
• Prefix injection (per self-attn): second SDPA over the input audio's K/V, with dt_q = prefix_q[step] + main_q, gated add before out_proj: attn = main_attn + dt_attn × gate[i].

Files

• model.safetensors — quantized LM (INT4 affine, group size 64) + adapter weights
• config.json — architecture + quantization + instruct metadata
• compression_state_dict.bin — passthrough of upstream EnCodec for offline init

Source

• Upstream: ldzhangyx/instruct-MusicGen (CC-BY-NC, re-trained on public datasets)
• Paper: Instruct-MusicGen (arxiv 2405.18386)
• Base: facebook/musicgen-large

License

CC-BY-NC 4.0 — inherited from MusicGen + the upstream checkpoint. Non-commercial use only.