---
language:
- en
license: cc-by-nc-4.0
library_name: steertts
pipeline_tag: text-to-speech
tags:
- text-to-speech
- tts
- speech-synthesis
- english
- controllable-tts
- steerable-tts
- prosody-control
- expressive-tts
- style-control
- inline-markup
- word-emphasis
- zero-shot-voice-cloning
- voice-cloning
- interpretable
- non-autoregressive
- fastpitch
- mamba
- mamba-3
- state-space-model
- hybrid-attention
- flow-matching
- vocos
- small-model
- quantized
- int8
- on-device
- streaming
- non-commercial
model-index:
- name: steer-tts
  results:
  - task:
      type: text-to-speech
      name: Text-to-Speech
    dataset:
      name: Seed-TTS-eval (English)
      type: bezzam/seed_tts_eval
      split: test
    metrics:
    - type: wer
      value: 1.0
      name: WER % (voice_1361, the v1.0.0 default, Qwen3-ASR-1.7B)
      verified: false
    - type: wer
      value: 1.01
      name: WER % (zero-shot cloning, prompted flow)
      verified: false
    - type: speaker_similarity
      value: 37.4
      name: SIM x100 (zero-shot cloning, WavLM-large)
      verified: false
    source:
      url: https://huggingface.co/spaces/hf-audio/open_tts_leaderboard
      name: Open TTS Leaderboard protocol (local run, not an official submission)
  - task:
      type: text-to-speech
      name: Text-to-Speech
    dataset:
      name: CV3-Eval (English, zero-shot)
      type: bezzam/cv3_eval
      split: test
    metrics:
    - type: wer
      value: 2.63
      name: WER % (voice_1361, the v1.0.0 default, Qwen3-ASR-1.7B)
      verified: false
    - type: wer
      value: 2.73
      name: WER % (zero-shot cloning, prompted flow)
      verified: false
    - type: speaker_similarity
      value: 28.7
      name: SIM x100 (zero-shot cloning, WavLM-large)
      verified: false
    source:
      url: https://huggingface.co/spaces/hf-audio/open_tts_leaderboard
      name: Open TTS Leaderboard protocol (local run, not an official submission)
---
> Mirror of [WhissleAI/steer-tts](https://huggingface.co/WhissleAI/steer-tts). All credit to the original authors. Not an official release from them. License unchanged: code MIT, weights and voice data CC BY-NC 4.0 (non-commercial only); see `LICENSE.md`.

# SteerTTS v1

A small English text-to-speech model that you steer with numbers. Each request can set any subset of **12 acoustic
controls**, for example speaking rate, loudness, median pitch, pitch range, final rise or fall, pauses, spectral tilt
and breathiness. Each control is a z-score relative to the chosen voice's own neutral speech. You can also pick a
**style tag**, give a **free-text direction** (which a small adapter turns into a request you can inspect), or use
**inline markup**. The model can **clone** a voice from a short reference clip.

| | |
|---|---|
| Acoustic model | **9.59M** parameters (encoder/decoder: attention + bidirectional Mamba-3, AdaLN conditioning) |
| + Vocos vocoder | 13.53M (`charactr/vocos-mel-24khz`, frozen), **23.1M** total |
| Flow-matching refiner (optional, on by default) | 17.64M → 40.8M total |
| Prompted flow (cloning only) | 24.67M, used in place of the refiner when `ref_wav` is given → 47.8M total |
| Frozen external encoders | WavLM-base-plus-sv 101M (cloning only), all-MiniLM-L6-v2 22.7M (free-text prompts only) |
| Output | 24 kHz mono, fp32, CPU or CUDA |
| Voices | 2,825 built-in voices under anonymised names (`voice_0001` …), default `voice_1688` (v1.0.0: `voice_1361`) |

> **Licence: weights CC BY-NC 4.0 (non-commercial use only); package code MIT.** The weights were trained partly on
> non-commercial data (see [Training data](#training-data)).

## Listen

All samples are raw model output (acoustic model + flow refiner + Vocos, seed 0), rendered with the code in this repo.

**New default voice.** v1.0.1 switched the default from `voice_1361` to `voice_1688`, which sounds more natural and less rushed.

| Text | `voice_1361` (v1.0.0 default) | `voice_1688` (v1.0.1 default) |
|---|---|---|
| "Hi, thanks for calling. I can help you reschedule your appointment today." | <audio controls preload="none" src="https://huggingface.co/WhissleAI/steer-tts/resolve/main/samples/cmp_intro_voice_1361.mp3"></audio> | <audio controls preload="none" src="https://huggingface.co/WhissleAI/steer-tts/resolve/main/samples/cmp_intro_voice_1688.mp3"></audio> |
| "The weather was cold, but the view from the top was worth every single step." | <audio controls preload="none" src="https://huggingface.co/WhissleAI/steer-tts/resolve/main/samples/cmp_story_voice_1361.mp3"></audio> | <audio controls preload="none" src="https://huggingface.co/WhissleAI/steer-tts/resolve/main/samples/cmp_story_voice_1688.mp3"></audio> |

**Steering `voice_1688`.**

| Request | Audio |
|---|---|
| "Are you coming with us tonight?" (no controls) | <audio controls preload="none" src="https://huggingface.co/WhissleAI/steer-tts/resolve/main/samples/steer_plain.mp3"></audio> |
| Same text, `ctrl={"f0_end": 2, "f0_iqr": 1.5}`: rising, livelier | <audio controls preload="none" src="https://huggingface.co/WhissleAI/steer-tts/resolve/main/samples/steer_question_rise.mp3"></audio> |
| "Please keep this between the two of us." `ctrl={"rate": -1.5, "loud": -1.5}` | <audio controls preload="none" src="https://huggingface.co/WhissleAI/steer-tts/resolve/main/samples/steer_slow_soft.mp3"></audio> |
| Same text, `tag="whisper"` | <audio controls preload="none" src="https://huggingface.co/WhissleAI/steer-tts/resolve/main/samples/steer_whisper_tag.mp3"></audio> |
| "I really thought it would work this time." `tag="sadness"`, `guidance=1.5` | <audio controls preload="none" src="https://huggingface.co/WhissleAI/steer-tts/resolve/main/samples/steer_sad_tag.mp3"></audio> |
| "I can't believe we actually won!" `prompt="excited, a bit faster"` | <audio controls preload="none" src="https://huggingface.co/WhissleAI/steer-tts/resolve/main/samples/steer_prompt.mp3"></audio> |
| Markup: `Well, *that* was fast. [pause 400ms] <slow>Let me think about it.</slow> <whisper>Don't tell anyone.</whisper>` | <audio controls preload="none" src="https://huggingface.co/WhissleAI/steer-tts/resolve/main/samples/steer_markup.mp3"></audio> |

Found a bug or a voice that sounds wrong? Open an issue at [github.com/WhissleAI/steer-tts](https://github.com/WhissleAI/steer-tts/issues) or start a discussion in the Community tab.

## Quick start

```bash
git clone https://huggingface.co/WhissleAI/steer-tts && cd steer-tts      # or huggingface_hub.snapshot_download("WhissleAI/steer-tts")
pip install ".[all]"          # core: torch, vocos, g2p_en …; [all] adds transformers (prompts, cloning), parselmouth (measuring), sounddevice (playback)
# or, code only (weights download on first use): pip install "steertts[all] @ git+https://github.com/WhissleAI/steer-tts"
steertts say "Hello there, nice to meet you." -o hello.wav
steertts say "I can't believe it worked." --tag happiness --ctrl rate=1,f0_iqr=1.5 -o happy.wav
steertts say "Please keep this quiet." --prompt "whispering, slow and secretive" -o secret.wav   # prints the resolved request
steertts stream "Well, *that* was fast. [pause 400ms] <slow>Let me think.</slow>" -o markup.wav
steertts play "Hi there. <whisper>This plays as it is rendered.</whisper>"       # streams to your speakers
steertts say "Now in your voice." --ref-wav consenting_speaker.wav -o clone.wav
steertts voices --gender female | head
steertts --precision int8 say "Same voice, a quarter of the download." -o int8.wav   # int8 weights (see Quantized int8)
```

The CLI looks for the model in the current directory. From anywhere else, pass it before the command
(`steertts --model WhissleAI/steer-tts say "Hi." -o hi.wav`) or set `STEERTTS_MODEL`.

```python
from steertts import SteerTTS

tts = SteerTTS.from_pretrained("WhissleAI/steer-tts")   # HF repo id or a local dir; device: cuda if available, else cpu
wav = tts("The meeting moved to Thursday.")       # float32 numpy, 24 kHz, default voice voice_1688
wav = tts("Are you serious?", voice="voice_0042", ctrl={"f0_end": 2.0, "f0_iqr": 1.5})   # rising, lively
wav = tts("It is over.", tag="sadness", ctrl={"loud": -1}, guidance=1.5)

req = tts.resolve({"text": "See you soon.", "prompt": "excited, a bit faster"})
print(req)    # {'text': ..., 'voice': 'voice_1688', 'tag': 'excited', 'ctrl': {'rate': 0.29}, 'prompt_mapping': {...}, ...}
wav = tts.synthesize(req)                         # a resolved request renders exactly as the prompt would

from steertts.player import Player                 # plays chunks as they arrive (sounddevice, else ffplay / aplay)
player = Player(tts.sample_rate)
for chunk in tts.stream("First sentence. Second sentence."):   # sentence-level streaming
    player.play(chunk.audio)
player.close()                                                 # waits until playback finishes
tts.save(wav, "out.wav")

tts8 = SteerTTS.from_pretrained("WhissleAI/steer-tts", precision="int8")   # downloads only int8/ (52 MB, not 215 MB)
```

`inference.py` renders one example of each feature. `python -m steertts.cli --help` lists the CLI.

## The request

Every synthesis is one JSON object. It is documented field by field in [`request.schema.json`](request.schema.json)
(JSON Schema 2020-12; `steertts schema` prints it). Only `text` is required.

| Field | Meaning |
|---|---|
| `text` | English text. Numbers, units, currency, ordinals, ranges and unknown acronyms are spelled out. |
| `voice` / `ref_wav` | A built-in voice, **or** a reference clip to clone (first 8 s, at least 0.5 s). |
| `tag` | One of 51 style tags (`none`, `whisper`, `sadness`, `anger`, `fast`, `high_pitch`, …). |
| `ctrl` | `{control: z}` over the 12 controls below, each in [-5, 5]. |
| `prompt` | Free-text direction. `resolve()` maps it to `tag` + `ctrl`; an explicit `tag` or `ctrl` dim wins. |
| `guidance` | Steering scale s (default 1). See [Guidance](#how-well-it-steers). |
| `pace` | Global tempo divisor (>1 is faster). It does not change the style. |
| `word_shift` | `{word_index: {pitch: st, energy: dB, dur: ×}}`, per-word prosody edits. Not with `markup` or `stream()` (they raise an error); use inline markup there. |
| `markup` | Parse inline markup in `text` (see below). `stream()` always parses it. |
| `flow`, `seed` | Use the flow refiner (default true) and set the sampler seed. Renders are deterministic per device. |

**Controls and the mask.** The 12 controls are utterance-level acoustic measurements:

- `rate`: articulation rate
- `loud`: speech level
- `f0_med`: median pitch
- `f0_iqr`: pitch range
- `f0_end`: final pitch slope
- `pause`: pause share
- `tilt`: spectral tilt
- `voiced`: voiced share
- `flat`: spectral flatness
- `centroid`: spectral centroid
- `cpps`: cepstral peak prominence
- `h1h2`: H1–H2

Each value is a z-score against the chosen voice's own neutral speech, in units of the pooled within-speaker SD
(`control_stats.json`). The model also receives an availability mask:

- A dim named in `ctrl` is requested.
- A dim left out is unspecified, and the model chooses it from voice, tag and text.
- `{"rate": 0}` asks for this voice's neutral rate. Omitting `rate` lets a `fast` tag or the text decide.

`steertts measure out.wav --text "…" --voice voice_1688` reads the controls back with the same measurement code
that labelled the training data.

**Markup** (`markup: true` or `stream()`):

| Markup | Effect |
|---|---|
| `*word*` or `<emph>…</emph>` | Emphasis: +2.5 st, +4 dB, ×1.2 duration |
| `<slow> <fast> <high> <low> <loud> <soft>` | Per-word changes; the sentence is still rendered in one pass |
| `[pause]`, `[pause 300ms]`, `[pause 1.5s]` | Exact silence |
| `<whisper>…</whisper>` (any tag), `<style prompt="…">…</style>` | The span is rendered as its own segment |

Text is rendered sentence by sentence, so the first sentence can play while the rest renders.

## Architecture

FastPitch-style explicit-variance TTS:

1. **Front end.** g2p_en ARPAbet phonemes with word indices.
2. **Encoder.** 4 blocks.
3. **Variance predictors.** Per-phone duration, pitch and energy.
4. **Length regulator.**
5. **Decoder.** 4 blocks.
6. **Output.** A 100-band log-mel, rendered by Vocos.

Each encoder/decoder block is a **hybrid**: multi-head self-attention, then a **bidirectional Mamba-3** branch (two
Mamba-3 layers run forward and backward and summed; complex-valued state, trapezoidal discretisation; 3.70M
parameters in total), then a conv FFN.

Every LayerNorm in every block and predictor is an **adaptive LayerNorm**. Its scale and shift are predicted from
g = MLP([speaker vector, tag embedding, ctrl ⊙ mask, mask]). The speaker slot is a linear projection of a frozen
512-d WavLM-base-plus-sv x-vector. Built-in voices use their stored mean vector; cloning uses the vector of the
reference clip.

**Guidance** renders the request and the uninstructed request, then extrapolates durations, pitch, energy and mel:
out = out₀ + s·(out − out₀).

Optional decoders:

- **Flow refiner.** A conditional flow-matching ConvNeXt network (OT-CFM, 10 midpoint steps, temperature 0.4). It
  samples mel detail given the acoustic model's mel and g, so every control still comes from the acoustic model.
- **Prompted flow.** The same network plus attention, given the reference clip's own mel as context
  (Voicebox-style infilling). Used for cloning.

The `steertts` package here is self-contained. It is the inference subset of the training code, operation for
operation, and its outputs are bit-identical to the training repository's synthesis on the same device (see
[Release verification](#release-verification)).

## How well it steers

The numbers below are **held-out**: 16 LibriTTS-R test-clean speakers who share no speaker, chapter or recording
with the training data (8 female, 8 male by a median-F0 rule; the corpus has no gender labels on disk), each
conditioned on the speaker vector of one recording, × 8 texts. They are for the acoustic model at guidance 1 **without the flow refiner**.
Steering through the refiner was not measured in the paper; the release check below confirms only the direction.

Columns:

- **Hit**: share of renders that move more than 0.5 SD the requested way.
- **Δz**: achieved change for a request of ±2.
- **ρ**: Spearman correlation between request and readback (by our extractor / by Praat).

| Tier | Control | Hit at −2 / +2 | Δz at −2 / +2 | ρ (ours / Praat) |
|---|---|---|---|---|
| **Reliable** | loud | 1.00 / 1.00 | −1.57 / +1.81 | .97 / .97 |
| | f0_med | .98 / 1.00 | −1.15 / +1.18 | .95 / .95 |
| | h1h2 | 1.00 / 1.00 | −1.59 / +1.49 | .96 / .96 |
| | tilt | 1.00 / .99 | −1.39 / +0.97 | .96 / .87 |
| | centroid | .78 / .94 | −0.79 / +1.20 | .92 / .92 |
| | f0_iqr | .66 / .91 | −0.63 / +1.09 | .88 / .88 |
| **Usable** | rate | 1.00 / 1.00 | −1.61 / +1.30 | .96 / .66 |
| | f0_end | .96 / .99 | −0.85 / +1.31 | .94 / .83 |
| | flat | .62 / .41 | −0.64 / +0.51 | .85 / .68 |
| **Weak** | pause | .38 / .64 | −0.37 / +0.71 | .76 / .70 |
| | voiced | .39 / .12 | −0.44 / +0.26 | .79 / .60 |
| | cpps | .56 / .35 | −0.45 / +0.30 | .58 / .46 |

**Larger re-measurement** (same model, protocol and extractor; 62 unseen voices: 31 LibriTTS-R test-clean + 31 GLOBE
test, a corpus the model never trained on; × 16 sentences, 12 of them newly written; 992 renders per direction per
control; 95% CI from a crossed voice × sentence bootstrap):

| Control | Hit at −2 / +2 | Mean hit [95% CI] | Δz at −2 / +2 |
|---|---|---|---|
| loud | 1.00 / 1.00 | 1.00 [1.00, 1.00] | −1.54 / +1.87 |
| rate | 1.00 / 1.00 | 1.00 [1.00, 1.00] | −1.52 / +1.27 |
| h1h2 | 1.00 / 1.00 | 1.00 [1.00, 1.00] | −1.63 / +1.53 |
| tilt | 1.00 / .98 | .99 [.98, 1.00] | −1.42 / +0.93 |
| f0_med | 1.00 / .99 | .99 [.98, 1.00] | −1.21 / +1.31 |
| centroid | .90 / .96 | .93 [.83, 1.00] | −0.93 / +1.37 |
| f0_end | .85 / .93 | .89 [.81, .95] | −0.81 / +1.11 |
| f0_iqr | .59 / .88 | .73 [.63, .83] | −0.58 / +0.95 |

LibriTTS-R and GLOBE voices are statistically indistinguishable. `f0_end` depends on the sentence more than the other
controls do.

- **Tiers.** Reliable: ρ ≥ 0.85 under both extractors. Usable: under one. Weak: under neither.
- **Gain.** The mean gain (achieved / requested) is about 0.5. `rate` and `f0_med` at +1 hit only .48 / .42.
- **Cross-talk.** Mean leakage into the other 11 dims is 0.15–0.30 SD. The off-diagonal / diagonal ratio is 0.10.
- **CPPS** is not a working control: a request moves pitch and level about as much as CPPS itself.
- **Intelligibility.** WER (Whisper-large-v3) is 0.94 % uninstructed and 0.84–1.01 % across all requests.

**Guidance** (pooled over loud, rate, f0_med, f0_iqr and tilt; ±2 requests; n = 1,280):

| s | Steer ratio (Δz / requested) | Hit | Leak | WER |
|---|---|---|---|---|
| 1 | 0.65 | .95 | .20 | 0.9 % |
| 2 | 1.35 | .99 | .34 | 1.0 % |
| 3 | 1.97 | .98 | .48 | 1.0 % |

- s ≈ 1.5 roughly delivers the requested size, at the cost of more leakage.
- Guidance on tags is riskier: `whisper` at s = 3 raises WER to 16 %.

**Markup** (8 held-out voices × 6 sentences × 2 words):

- Emphasis achieved +2.58 st and +3.92 dB, against +2.5 st and +4 dB requested.
- `<high>` / `<low>` gave +3.18 / −2.96 st for ±3.
- `<loud>` / `<soft>` gave +4.84 / −5.54 dB for ±6. The decoder attenuates level by 10–20 %.
- `<slow>` / `<fast>` and pauses are exact by construction.
- Neighbouring words move by at most 0.19 st and 0.16 dB.

**Free-text prompts** (161 hand-written prompts):

- Which-dims precision / recall: .85 / .66. Sign accuracy: .88.
- Tag top-1 / top-3: .56 / .69.
- Composition (.23) and negation (.22) mostly fail. "Not fast" can come out fast.
- Absolute-register prompts ("a deep male voice") are not supported. Pick a voice instead (`voices.json`).
- **Always look at the resolved request.**

## Quality

Results under the Open TTS Leaderboard protocol (run locally on an RTX 5070, not an official submission;
Qwen3-ASR-1.7B WER, WavLM-large SIM):

| Setting | Seed-TTS en | CV3-Eval en | Avg |
|---|---|---|---|
| `voice_1361` (v1.0.0 default), acoustic model + Vocos, WER | 1.00 % | 2.63 % | **1.81 %** |
| Cloning (prompted flow), WER / SIM ×100 | 1.01 % / 37.4 | 2.73 % / 28.7 | 1.87 % / 33.1 |

- The flow refiner was scored with an earlier acoustic checkpoint: 1.04 % / 2.70 % WER (no change).
- **Cloning similarity is low.** The board's lowest cloning SIM is about 37, and its median is about 65. Do not
  expect a convincing likeness.

## Quantized int8

`precision="int8"` (CLI `--precision int8`) loads the weight files in `int8/` and downloads only those: 52 MB instead
of 215 MB. Vocos, WavLM-base-plus-sv and all-MiniLM-L6-v2 still come from their own repos and are rounded to int8 the
same way when they load. The whole install shrinks from 765 MB to 193 MB of weights, and RAM use does not change.

- **Scheme.** Weight-only, round-to-nearest, symmetric. Tensors with ≥ 2 dimensions and ≥ 1,024 values are stored as
  int8 with one fp16 scale per output row; everything else as fp16. Weights are dequantized at load and compute stays
  fp32, so int8 is not faster than fp32. `steertts/quant.py` has the code.
- **Parity.** The package's int8 renders are bit-identical to the evaluated int8 model on plain, control, prompt, markup
  and cloning requests.

Same protocol and machine as [Quality](#quality), default voice `voice_1361`, refiner on:

| | fp32 | int8 |
|---|---|---|
| Seed-TTS en WER | 0.94 % | 1.03 % |
| CV3-Eval en WER | 2.62 % | 2.66 % |
| Cloning WER, Seed / CV3 | 0.99 % / 2.70 % | 1.06 % / 2.70 % |
| Cloning SIM ×100, Seed / CV3 | 37.4 / 28.7 | 37.5 / 28.3 |
| UTMOS22, Seed / CV3 | 4.27 / 4.24 | 4.27 / 4.25 |
| UTMOS22 cloning, Seed / CV3 | 4.15 / 4.08 | 4.13 / 4.07 |

Steering readback (8 voices × 6 texts, request −2 / +2, achieved Δz):

| control | fp32 | int8 |
|---|---|---|
| loud | −1.73 / +1.92 | −1.71 / +1.91 |
| f0_med | −1.02 / +1.39 | −1.01 / +1.50 |
| rate | −1.55 / +1.29 | −1.54 / +1.33 |
| h1h2 | −1.07 / +1.21 | −1.05 / +1.14 |
| tilt | −1.28 / +0.86 | −1.31 / +0.92 |
| f0_iqr | −0.58 / +0.80 | −0.67 / +0.75 |

- **Free-text prompts.** Of the 161 hand-written prompts, 97 % map to the same tag and 96 % to the same set of
  controls as fp32.
- **Not released: int4.** Uniform int4 (7× smaller) loses about 0.5 UTMOS and 5–7 cloning SIM points, and pitch-range
  steering falls to ±0.3. Keeping the conditioning path in int8 recovers the steering but not the audio quality.

## Speed

Measured on an RTX 5070 and on a shared, heavily loaded 8-thread CPU, so treat CPU figures as lower bounds:

- **GPU.** Batch-1 RTFx 61–81 for the acoustic model + Vocos (leaderboard runs).
- **CPU, 8 threads.** RTFx 3.6–7.9 for the acoustic model + Vocos. The flow refiner adds 12× the compute and
  drops RTFx from 6.7 to 0.8 (0.4 with 1 thread). Use `flow=False` / `--no-flow` for real-time CPU use.
- **Cloning on CPU.** RTFx 0.2 with the prompted flow, including the WavLM speaker encoder.
- **Streaming on CPU.** First audio after about 1 s.
- See [Release verification](#release-verification) for the numbers measured on this exact package.

## Limitations

- **English only.** The front end is ARPAbet. Single speaker per request, and the controls are utterance-level.
- **Emotion tags are style switches, not emotions.** An independent emotion classifier (emotion2vec+ large)
  recognises only `sadness` (0.16 top-1 vs 0.01 uninstructed). Every other emotion tag scores at most 0.02. The same
  classifier recognises 7 of 10 of these emotions on real EARS recordings. 27 of 49 tags are acoustically within 0.5
  of `neutral`. Voice-quality tags such as `whisper` act strongly (voiced share −9.9 SD).
- **Event tokens only reserve time.** `[laugh]`, `[cough]`, `[dog]`, … add about 0.12 s of near-silence. An audio
  tagger detects 0 of 1,728 renders. The project's demo mixed recorded sounds in, but that mixer is not part of this
  release.
- **The acoustic-environment input is untrained.** The weights contain an environment projection, but the final
  training stage never saw a non-zero environment vector, so this package does not expose it.
- **Cloning similarity is low** (see above), and cloning speed on CPU is slow with the prompted flow.
- **The controls are acoustic measurements, not percepts.** `voiced`, `flat` and `cpps` barely move. `rate` and
  `tilt` depend on how they are measured. z = 0 is the voice's typical render, not its real recordings.
- **Test-set-informed choices.** The prompted-flow checkpoint was chosen on subsets of the Seed-TTS / CV3 test
  sets, and the text-normalisation rules came from CV3 error analysis. The leaderboard rows use `voice_1361`,
  the pre-registered v1.0.0 default: a sweep picked a lower-WER voice on the test sets, but it was not used.
  v1.0.1 switched the package default to `voice_1688` after users found `voice_1361` fast and synthetic-sounding.
  It was picked by UTMOS22 over 300 random voices on our own sentences, not on the test sets. On 16 sentences it
  matches `voice_1361` on WER (1.1 %), scores higher on UTMOS22 (4.43 vs 4.24) and speaks a little slower (3.4 vs 3.6
  words/s). Pass `voice="voice_1361"` for the v1.0.0 sound.
- **No human evaluation.** Every number above is from automatic metrics, with one training seed.

## Intended use and misuse

**Intended uses:**

- Research on controllable / interpretable speech synthesis.
- Accessibility and prototyping, where explicit, inspectable control matters more than peak naturalness.
- On-device experiments with a ~23M-parameter TTS.

**Voice cloning requires consent.** Clone only a voice whose owner has agreed, and do not use this model to:

- impersonate real people;
- produce deceptive or fraudulent audio;
- bypass voice authentication;
- create non-consensual content.

Disclose synthetic speech to listeners. There is no watermark yet.

**Built-in voices are anonymised.** Built-in voices are training speakers under opaque names. Speaker IDs from the
source corpora are not included. Speakers from a celebrity-interview corpus (VoxCeleb, via NonverbalTTS) were
excluded from the voice list.

## Training data

410,566 utterances (659 h) from 4,919 speakers:

| Source | Hours | Licence |
|---|---|---|
| LibriTTS-R (clean-100, clean-360, other-500) | 516 | CC BY 4.0 |
| EARS (read + freeform) | 78 | CC BY-NC 4.0 |
| Expresso | 11 | CC BY-NC 4.0 |
| NonverbalTTS (VoxCeleb and Expresso sources) | 12.5 | Source terms apply |
| Synthetic speech from a commercial TTS API | 43 | The provider's terms of service, incl. a competing-model clause |

Control labels are measured from the audio. Style tags come from the corpora's own style/emotion labels.

**Licence: CC BY-NC 4.0. Non-commercial use only.**

## Files

| File | Contents |
|---|---|
| `model.safetensors` | Acoustic model (fp32) |
| `int8/*.safetensors` | The same five weight files in int8 (see [Quantized int8](#quantized-int8)) |
| `flow.safetensors`, `pflow.safetensors` | Flow refiner and prompted flow |
| `prompt_adapter.safetensors` | Free-text → request adapter (0.5M) |
| `samples/*.mp3` | The audio clips in [Listen](#listen) |
| `speaker_vectors.safetensors` | 512-d vector per built-in voice, in `config.json:voices_order` |
| `voices.json` | Measured descriptions (see below) |
| `control_stats.json` | Pooled SDs, tag list, and each voice's neutral control means (for `measure_z`) |
| `config.json` | Architecture and file map |
| `request.schema.json` | The request schema |
| `steertts/` | The package (incl. `steertts/mamba3` with its own invariant tests); also on [GitHub](https://github.com/WhissleAI/steer-tts) |
| `tests/` | pytest suite |
| `leaderboard/` | Open TTS Leaderboard backend |
| `TEST_LOG.md` | Release verification log |
| `LICENSE.md` | Code (MIT) and weights (CC BY-NC 4.0) |

`voices.json` descriptions are measured, not copied from corpus metadata:

- **Gender.** A probe on the speaker vector; 99.3 % 5-fold accuracy on 534 self-labelled speakers.
- **Pitch register.** Median F0 within that gender.
- **Pace.** Articulation rate.

## Release verification

The package was verified in a fresh CPU environment; `TEST_LOG.md` has the full log.

- **Tests.** The pytest suite passes (16 tests at v1.0.0; 17 at v1.0.1, re-run with the new default voice; 21 at
  v1.1.0, with the int8 tests).
- **Parity.** Seven renders are bit-identical to the training code's synthesis on the same device: acoustic
  only, controls, tag + guidance, per-word shifts, the flow refiner, and cloning through the prompted flow.
- **Speed.** On this package, CPU with 4 threads on a loaded machine: RTFx 4.9 without the refiner, 0.7 with it.
  First streamed audio arrives after 0.5 s.

## Changelog

- **v1.1.0.** int8 weights in `int8/`, `precision="int8"` and `--precision int8` (4× smaller download, same quality; see
  [Quantized int8](#quantized-int8)). New `steertts/quant.py` and `tests/test_int8.py`. fp32 behaviour is unchanged,
  except that fp32 downloads now skip `int8/`.
- **v1.0.2.** `steertts play "…"` and `--play` stream audio to the speakers as each sentence is rendered;
  `steertts.player.Player` does the same from Python. New `[play]` extra (sounddevice), also in `[all]`.
- **v1.0.1.** Default voice `voice_1688` instead of `voice_1361`, which users found fast and synthetic-sounding (see
  [Listen](#listen) and Limitations). `stream()` now raises an error for `word_shift`, which it used to drop silently.
  The CLI and `inference.py` download this repo when no local model is given. Leaderboard scripts point at this repo
  and stay pinned to `voice_1361`. Code is mirrored on GitHub for issues.
- **v1.0.0.** First release.

## Citation

```bibtex
@misc{steertts2026,
  title  = {SteerTTS: A Tiny, Measurably Steerable Speech Synthesizer},
  author = {{Whissle AI}},
  year   = {2026},
  howpublished = {\url{https://huggingface.co/WhissleAI/steer-tts}}
}
```