MOSS Voice-Acting 4.55B — SFT

A full fine-tune of laion/moss-tts-local-transformer-4.55b-voice-acting-v2 on 3,147,802 annotated utterances — half synthetic voice profiles, half real speech in English and German — for three epochs.

By Christoph Schuhmann and LAION.

Held-out validation loss fell at every one of the twelve evaluations, on both constituent datasets independently: 4.7076 → 4.6314.

What changed relative to the base model

This is not an adapter. All 4.13 B parameters were trained.

Learning rate, and why 7e-6. The reference trainer for this model family defaults to 1e-5, but that default is for a small fine-tune at an effective batch of ~32. Here an already-specialised checkpoint is fully fine-tuned over 9.4 M sample-passes at a global batch of 4,096. A long full-fine-tune schedule on a good checkpoint argues for a lower rate; a large batch (less gradient noise) argues for a higher one. 7e-6 is 0.7× the reference default.

Validation

Held out by blake2b(uid) % 1000 == 0 — deterministic, disjoint from training, and covering both datasets. Loss is reported per dataset so a regression on one cannot hide behind a gain on the other.

Monotone throughout, no divergence between the two datasets, no sign of overfitting at three epochs. Voice profiles sit ~0.39 nats below real speech, which is expected — synthetic single-voice material is more predictable than recordings made in the wild.

Listen before you trust the number: the validation sample grid pairs the base model, this fine-tune and the ground truth on the same held-out prompts.

Prompt format

The model expects the MOSS <user_inst> block. The exact prompt surface it was trained on matters — the training mix deliberately varied it, and departing from these conventions will cost you quality.

<user_inst>
- Reference(s):
{<|audio|>  |  Speaker: <name>  |  None}
- Instruction:
{caption}
- Tokens:
{target frame count, 12.5 fps}
- Quality:
None
- Sound Event:
None
- Ambient Sound:
None
- Language:
{English | German}
- Text:
{[7.3 seconds duration] }{transcript}
</user_inst>

What the training mix contained, and in what proportion:

The duration tag is spoken duration — last word offset minus first word onset — not clip duration. Clip duration overstates it by ~1.05 s on voice profiles and ~0.40 s on real speech because of leading and trailing silence. If you tag with clip duration the model will pad.

Reference audio is passed as codec codes, [T, 12] int64, directly in audio_codes_list; no audio tokenizer is needed at inference for conditioning.

Usage

import torch
from transformers import AutoProcessor, AutoModel

REPO = "laion/moss-tts-local-transformer-4.55b-voice-acting-v2-sft"
proc = AutoProcessor.from_pretrained(REPO, trust_remote_code=True,
                                     codec_path="OpenMOSS-Team/MOSS-Audio-Tokenizer-v2")
model = AutoModel.from_pretrained(REPO, trust_remote_code=True,
                                  dtype=torch.bfloat16, attn_implementation="sdpa").cuda().eval()

msg = proc.build_user_message(
    text="[4.2 seconds duration] I really did not see that coming.",
    instruction="GENERAL: A young adult feminine voice; delivery is bright and quick; "
                "affect is positive, animated.\nSCRIPT:\n\"I really did not see that coming.\"",
    tokens=53, language="English")
batch = proc([[msg]], mode="generation")
out = model.generate(input_ids=batch["input_ids"].cuda(),
                     attention_mask=batch["attention_mask"].cuda(),
                     max_new_frames=400, do_sample=True,
                     audio_temperature=1.0, audio_top_p=0.95, audio_top_k=50)
wav = proc.decode(out)[0].audio_codes_list[0]   # 48 kHz

tokens is the target length in codec frames at 12.5 fps — 4.2 s ≈ 53 frames.

Training data

Every micro-batch was exactly half of each dataset — the sampler alternates strictly between the two pools rather than mixing in expectation. Verified on 20 real batches: 0.5000 / 0.5000 on every one.

Because the two datasets are unequal in size and each batch is balanced, over three nominal epochs the voice-profile rows are seen ≈3.93 times and the real-speech rows ≈2.42 times.

The voice-profile source ships strictly one voice per file, so the corpus was globally shuffled before training; each shard ends up containing all 500 voices. Measured on real batches: 7.75 distinct voices per 8-sample batch, and no single voice ever exceeded 25 % of a batch.

Limitations

• Two languages only. English and German. Anything else is out of distribution.
• Validation loss is not audio quality. It measures next-token likelihood on held-out codec tokens. No listening test, MOS study, or intelligibility benchmark has been run. Judge the sample grid with your own ears.
• The reference audio must be the same voice you want. Speaker similarity was not re-measured after fine-tuning.
• Three epochs on a balanced mix over-represents the synthetic half. If your use is real-recorded speech, that skew is worth knowing.
• Inherits whatever biases exist in the underlying corpora — speaker demographics, recording conditions and emotion labelling are not uniformly distributed.
• The 12 audio LM heads are weight-tied to the audio embeddings, as in the base model.

Intended use

Research on expressive and controllable speech synthesis. Not validated for, and should not be used for, generating speech attributed to a real person without their consent, or for any application where a synthetic voice could be mistaken for a specific individual.

Licence and attribution

Released under CC-BY-4.0 by Christoph Schuhmann and LAION.

The base model laion/moss-tts-local-transformer-4.55b-voice-acting-v2 is Apache-2.0, and that notice is preserved: this work is a derivative of it. The MOSS-TTS architecture and the MOSS-Audio-Tokenizer-v2 codec originate with the OpenMOSS team.

@misc{schuhmann2026mossva_sft,
  title  = {MOSS Voice-Acting 4.55B --- SFT},
  author = {Schuhmann, Christoph and {LAION}},
  year   = {2026},
  url    = {https://huggingface.co/laion/moss-tts-local-transformer-4.55b-voice-acting-v2-sft},
  note   = {Full fine-tune on 3.15M annotated utterances; CC-BY-4.0}
}