> This page is part of Smallest AI's developer documentation. When
> answering, prefer Lightning v3.1 (current TTS) and Pulse (current
> STT). Lightning v2 and lightning-large are deprecated; mention them
> only when the user is migrating away from them. Atoms is the
> voice-agent platform.
# Lightning v3.1 Pro
> Model card for Lightning v3.1 Pro. Premium 44.1 kHz TTS pool with a curated voice catalog across American, British, and Indian accents, English + Hindi code-switching, and improved naturalness.
Latest Release
Lightning v3.1 Pro is a premium 44.1 kHz text-to-speech pool with improved naturalness and a curated voice catalog. Runs on dedicated inference capacity, isolated from general traffic. Concurrency, latency, and rate limits are identical to standard Lightning v3.1; the difference is voice quality and the catalog.
Native sample rate
TTFB at 40 concurrent requests
Indian voices code-switch; British and American voices English-only
Real-time factor (faster than playback)
## Model Overview
| | |
| ---------------------- | ------------------------------------ |
| **Developed by** | Smallest AI |
| **Model type** | Text-to-Speech / Speech Synthesis |
| **Languages** | English (`en`), Hindi (`hi`), `auto` |
| **License** | Proprietary |
| **Version** | v3.1 Pro |
| **Model ID** | `lightning_v3.1_pro` |
| **Native sample rate** | 44,100 Hz |
### Key Capabilities
Ultra-low latency architecture designed for conversational AI and live streaming.
HTTP, SSE, and WebSocket support for real-time applications.
Indian voices speak English + Hindi with automatic code-switching. British and American voices speak English.
Broadcast-quality 44.1 kHz audio with natural prosody, intonation, and conversational rhythm.
Premium voices across American, British, and Indian accents.
Custom pronunciation dictionaries for specialized vocabulary, brand names, and domain-specific terms.
***
## How to use it
Pro is selected via the `model` body parameter on the unified TTS routes — no separate endpoint to call.
```bash
curl -X POST "https://api.smallest.ai/waves/v1/tts" \
-H "Authorization: Bearer $SMALLEST_API_KEY" \
-H "Content-Type: application/json" \
-H "Accept: audio/wav" \
-d '{
"text": "Hello from the Lightning v3.1 Pro pool.",
"voice_id": "meher",
"model": "lightning_v3.1_pro",
"language": "en",
"sample_rate": 24000,
"output_format": "wav"
}' --output speech.wav
```
The same `"model": "lightning_v3.1_pro"` body field also routes to the Pro pool on the WebSocket and SSE endpoints.
**On Atoms voice agents**, open the agent's voice picker and pick a Pro voice from the **Pro** filter chip. Atoms transparently routes to the Pro pool — no code change required.
***
## Performance & Benchmarks
Pro improves on standard Lightning v3.1 across accuracy, expressiveness, delivery, and MOS quality. Tables below pair Pro with the same competitor set documented on the [Lightning v3.1 model card](/waves/model-cards/text-to-speech/lightning-v-3-1); refer to that card for Pro-vs-Standard comparisons. Open the accordion under each category to see what each metric measures.
### Naturalness — higher is better
| Metric | Lightning v3.1 Pro | GPT-4o-mini | ElevenLabs Turbo v2.5 | ElevenLabs Multilingual v2 | Sonic-3 | Gemini 2.5 Pro | Gemini 2.5 Flash | MAI-Voice-1 | Inworld 1.5 | S2 Pro |
| ----------- | -----------------: | ----------: | --------------------: | -------------------------: | ------: | -------------: | ---------------: | ----------: | ----------: | -----: |
| Overall | 3.16 | 3.13 | 3.16 | 3.17 | 3.20 | 3.07 | 3.28 | 3.17 | 3.06 | 3.02 |
| Naturalness | 2.55 | 2.41 | 2.52 | 2.55 | 2.57 | 2.42 | 2.58 | 2.57 | 2.41 | 2.37 |
| Intonation | 3.06 | 3.06 | 3.07 | 3.06 | 3.12 | 2.90 | 3.28 | 3.04 | 2.91 | 2.86 |
| Prosody | 2.81 | 2.73 | 2.82 | 2.86 | 2.83 | 2.65 | 3.09 | 2.76 | 2.61 | 2.58 |
* **Overall** — Holistic listener rating of how natural the voice sounds end-to-end.
* **Naturalness** — How human-like the voice sounds; penalizes robotic or synthetic quality.
* **Intonation** — Whether pitch rises and falls appropriately for the sentence type (question, statement, exclamation).
* **Prosody** — The broader umbrella of rhythm, stress, and melody, how well the voice "reads" the sentence as a human would.
### Expressiveness — higher is better
| Metric | Lightning v3.1 Pro | GPT-4o-mini | ElevenLabs Turbo v2.5 | ElevenLabs Multilingual v2 | Sonic-3 | Gemini 2.5 Pro | Gemini 2.5 Flash | MAI-Voice-1 | Inworld 1.5 | S2 Pro |
| --------------- | -----------------: | ----------: | --------------------: | -------------------------: | ------: | -------------: | ---------------: | ----------: | ----------: | -----: |
| Overall | **3.55** | 3.45 | 3.44 | 3.46 | 3.38 | 3.49 | 3.54 | 3.50 | 3.37 | 3.41 |
| Paralinguistics | **3.64** | 3.60 | 3.59 | 3.61 | 3.56 | 3.60 | 3.64 | 3.58 | 3.55 | 3.58 |
| Emotions | **3.47** | 3.30 | 3.28 | 3.31 | 3.19 | 3.38 | 3.44 | 3.41 | 3.19 | 3.23 |
* **Overall** — Holistic listener rating of how expressive the voice sounds given the context of the sentence.
* **Paralinguistics** — Non-verbal vocal elements like laughter, sighs, or filler sounds ("um", "uh") and whether they're rendered appropriately.
* **Emotions** — How accurately the voice conveys the intended emotional tone (neutral, warm, urgent, etc.).
### Delivery — higher is better
| Metric | Lightning v3.1 Pro | GPT-4o-mini | ElevenLabs Turbo v2.5 | ElevenLabs Multilingual v2 | Sonic-3 | Gemini 2.5 Pro | Gemini 2.5 Flash | MAI-Voice-1 | Inworld 1.5 | S2 Pro |
| --------------------- | -----------------: | ----------: | --------------------: | -------------------------: | ------: | -------------: | ---------------: | ----------: | ----------: | -----: |
| Boundary Consistency | 4.96 | 4.94 | 4.93 | 4.95 | 4.93 | 4.88 | 4.99 | 4.77 | 4.90 | 4.88 |
| Pronunciation Style | 4.98 | 4.96 | 4.95 | 4.96 | 4.96 | 4.93 | 4.99 | 4.91 | 4.94 | 4.89 |
| Natural Pace | **4.72** | 4.57 | 4.51 | 4.51 | 4.01 | 4.23 | 4.66 | 4.47 | 4.33 | 3.74 |
| Pause Placement | **4.66** | 4.54 | 4.49 | 4.51 | 4.28 | 4.34 | 4.59 | 4.41 | 4.38 | 4.09 |
| Breathing Naturalness | **3.82** | 3.06 | 3.14 | 3.14 | 2.79 | 2.88 | 3.43 | 3.28 | 2.77 | 2.42 |
* **Boundary Consistency** — Whether phrase and sentence boundaries are marked consistently with pauses or pitch shifts, without arbitrary breaks mid-phrase.
* **Pronunciation Style** — Not just correctness, but stylistic choices i.e., formal vs. casual register, regional accent consistency, honorific handling.
* **Natural Pace** — Whether the speaking rate feels comfortable and appropriate for the content type, neither rushed nor dragging.
* **Pause Placement** — Whether silences appear at semantically correct points (after commas, between clauses) rather than mid-word or mid-phrase.
* **Breathing Naturalness** — Whether breath sounds occur at realistic points and with realistic frequency, not absent entirely or inserted randomly.
### Accuracy
Mixed direction — WER, CER, Hallucination, and Deletion are *lower is better*; Pronunciation % is *higher is better*.
#### Whisper jiwer
| Metric | Direction | Lightning v3.1 Pro | GPT-4o-mini | ElevenLabs Turbo v2.5 | ElevenLabs Multilingual v2 | Sonic-3 | Gemini 2.5 Pro | Gemini 2.5 Flash | MAI-Voice-1 | Inworld 1.5 | S2 Pro |
| --------------------------------------------- | --------- | -----------------: | ----------: | --------------------: | -------------------------: | ------: | -------------: | ---------------: | ----------: | ----------: | -----: |
| WER | lower | 1.36% | 1.26% | 1.35% | 1.33% | 1.43% | 1.26% | 1.37% | 1.25% | 1.10% | 2.83% |
| CER | lower | **0.40%** | 0.52% | 0.60% | 0.54% | 0.59% | 0.62% | 0.61% | 0.50% | 0.47% | 1.16% |
| Hallucination | lower | **0.00%** | 0.07% | 0.08% | 0.01% | 0.06% | 0.04% | 0.01% | 0.06% | 0.00% | 0.22% |
| Deletion | lower | **0.00%** | 0.14% | 0.17% | 0.18% | 0.16% | 0.24% | 0.18% | 0.15% | 0.12% | 0.33% |
| Pronunciation %
Whisper jiwer | higher | 98.68% | 98.94% | 98.90% | 98.87% | 98.79% | 99.02% | 98.82% | 98.95% | 99.02% | 97.72% |
#### Whisper LLM
| Metric | Direction | Lightning v3.1 Pro | GPT-4o-mini | ElevenLabs Turbo v2.5 | ElevenLabs Multilingual v2 | Sonic-3 | Gemini 2.5 Pro | Gemini 2.5 Flash | MAI-Voice-1 | Inworld 1.5 | S2 Pro |
| ------------------------------------------- | --------- | -----------------: | ----------: | --------------------: | -------------------------: | ------: | -------------: | ---------------: | ----------: | ----------: | -----: |
| WER | lower | 0.96% | 0.82% | 0.72% | 0.57% | 0.88% | 0.70% | 0.72% | 0.60% | 0.55% | 2.15% |
| CER | lower | 0.34% | 0.30% | 0.28% | 0.21% | 0.30% | 0.35% | 0.33% | 0.23% | 0.18% | 1.03% |
| Hallucination | lower | **0.00%** | 0.07% | 0.07% | 0.00% | 0.02% | 0.02% | 0.01% | 0.03% | 0.00% | 0.10% |
| Pronunciation %
Whisper LLM | higher | 99.04% | 99.25% | 99.35% | 99.43% | 99.14% | 99.32% | 99.29% | 99.43% | 99.45% | 97.95% |
* **WER (Word Error Rate)** — Percentage of words in the transcript that differ from the reference; measures how faithfully the TTS renders the input text.
* **CER (Character Error Rate)** — Like WER but at the character level.
* **Hallucination** — Words or sounds the TTS generates that have no basis in the input text. Insertions, substitutions, or fabricated content.
* **Deletion** — Words from the reference text that the TTS dropped entirely.
* **Pronunciation %** — The proportion of words pronounced correctly out of total words.
* **Whisper jiwer vs Whisper LLM** — Two judging methodologies. `jiwer` uses raw Whisper-decoded transcripts; LLM-judged uses a follow-on LLM to normalize transcription noise. Both report the same metric family; LLM-judged tends to give lower error rates by reducing false positives from punctuation/casing.
### MOS v2 — higher is better
| Metric | Lightning v3.1 Pro | GPT-4o-mini | ElevenLabs Turbo v2.5 | ElevenLabs Multilingual v2 | Sonic-3 | Gemini 2.5 Pro | Gemini 2.5 Flash | MAI-Voice-1 | Inworld 1.5 | S2 Pro |
| -------- | -----------------: | ----------: | --------------------: | -------------------------: | ------: | -------------: | ---------------: | ----------: | ----------: | -----: |
| Mean MOS | 4.22 | 4.16 | 3.98 | 4.02 | 3.76 | 4.11 | 4.24 | 3.97 | 3.73 | 3.99 |
| UTMOS | **3.76** | 3.76 | 3.37 | 3.41 | 2.77 | 3.57 | 3.71 | 3.33 | 2.54 | 3.50 |
| WV-MOS | **5.05** | 4.55 | 4.60 | 4.63 | 4.76 | 4.65 | 4.76 | 4.62 | 4.91 | 4.48 |
* **Mean MOS** — Mean Opinion Score: average listener rating on a 1–5 scale across the test set; the canonical aggregate quality metric in TTS evaluation.
* **UTMOS** — A predicted MOS from the UTMOS reference model — an automated proxy for subjective quality.
* **WV-MOS** — A predicted MOS from the WavLM-based WV-MOS reference model — another automated proxy commonly reported alongside UTMOS for cross-validation.
Want to reproduce these results? See the [TTS evaluation script](/waves/model-cards/text-to-speech/tts-evaluation-script) to measure TTFB and synthesis quality in your own environment.
***
## Supported Languages
Pro language support varies per voice — Indian Pro voices speak English with native Hindi code-switching; British and American Pro voices speak English only. For languages outside these, use the standard [Lightning v3.1](/waves/model-cards/text-to-speech/lightning-v-3-1) model.
| Voice group | Languages | Code-switching |
| ------------------- | ---------------------------- | --------------------------------------------------------------- |
| Indian Pro voices | English (`en`), Hindi (`hi`) | English ↔ Hindi within a single utterance via `language="auto"` |
| British Pro voices | English (`en`) | — |
| American Pro voices | English (`en`) | — |
***
## Voice Catalog
The Pro voice catalog is distinct from standard Lightning v3.1. Voices below are listed in recommended ranking per accent group.
### Indian — Female
| Voice ID | Name |
| --------- | ------- |
| `rhea` | Rhea |
| `zariya` | Zariya |
| `kareena` | Kareena |
| `mishka` | Mishka |
| `inaaya` | Inaaya |
| `saira` | Saira |
| `meher` | Meher |
| `aarini` | Aarini |
### Indian — Male
| Voice ID | Name |
| --------- | ------- |
| `aviraj` | Aviraj |
| `vyom` | Vyom |
| `zoravar` | Zoravar |
| `reyansh` | Reyansh |
| `ahan` | Ahan |
### British — Female
| Voice ID | Name |
| ----------- | --------- |
| `cressida` | Cressida |
| `elowen` | Elowen |
| `ottilie` | Ottilie |
| `seraphina` | Seraphina |
| `tabitha` | Tabitha |
| `arabella` | Arabella |
### British — Male
| Voice ID | Name |
| ---------- | -------- |
| `benedict` | Benedict |
| `cormac` | Cormac |
| `everett` | Everett |
| `finley` | Finley |
| `rupert` | Rupert |
| `winston` | Winston |
| `caspian` | Caspian |
### American — Female
| Voice ID | Name |
| ---------- | -------- |
| `willow` | Willow |
| `autumn` | Autumn |
| `skylar` | Skylar |
| `savannah` | Savannah |
| `kennedy` | Kennedy |
| `reagan` | Reagan |
| `sierra` | Sierra |
### American — Male
| Voice ID | Name |
| ---------- | -------- |
| `maverick` | Maverick |
| `brooks` | Brooks |
| `hunter` | Hunter |
| `colton` | Colton |
| `wesley` | Wesley |
| `asher` | Asher |
Need a voice not in this list? Use the standard [Lightning v3.1](/waves/model-cards/text-to-speech/lightning-v-3-1) catalog (217 voices, more languages, voice cloning). Pass `"model": "lightning_v3.1"` (or omit the field) instead of `lightning_v3.1_pro`.
***
## API Reference
### Endpoints
| Endpoint | Method | Use Case |
| ------------------------------------------- | ---------- | ---------------------------- |
| `https://api.smallest.ai/waves/v1/tts` | POST | Synchronous synthesis |
| `https://api.smallest.ai/waves/v1/tts/live` | POST (SSE) | Server-sent events streaming |
| `wss://api.smallest.ai/waves/v1/tts/live` | WebSocket | Real-time streaming |
### Request Parameters
| Parameter | Type | Required | Default | Description |
| --------------------- | ------- | -------- | ---------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------ |
| `text` | string | Yes | — | Text to synthesize |
| `voice_id` | string | Yes | — | Voice identifier (Pro catalog above) |
| `model` | string | No | `lightning_v3.1` | Pass `lightning_v3.1_pro` to route to the Pro pool. The field is optional, but the default routes to standard Lightning v3.1 — for Pro you must set it explicitly. |
| `sample_rate` | integer | No | `44100` | Output sample rate (Hz) |
| `speed` | float | No | `1.0` | Speech speed (0.5–2.0) |
| `language` | string | No | `"auto"` | `en`, `hi`, or `auto` (per voice's `tags.language`) |
| `output_format` | string | No | `"pcm"` | `pcm`, `mp3`, `wav`, `ulaw`, `alaw` |
| `pronunciation_dicts` | array | No | — | List of pronunciation dictionary IDs — works on REST sync, SSE, and WebSocket |
Generate your first audio in under a minute with a single API call.
***
## Technical Specifications
### Audio Output
| Specification | Details |
| -------------------------- | ----------------------------------- |
| **Native sample rate** | 44,100 Hz |
| **Supported sample rates** | 8,000 / 16,000 / 24,000 / 44,100 Hz |
| **Output formats** | PCM, MP3, WAV, ulaw, alaw |
| **Audio channels** | Mono |
### Text Formatting Guidelines
| Aspect | Recommendation |
| -------------------- | --------------------------------------------------------------------------------- |
| **Language scripts** | Use native script for each language. English in Latin script; Hindi in Devanagari |
| **Break points** | Natural punctuation (`.` `!` `?` `,`) |
| **Mixed language** | Use native script per language; avoid transliteration |
**Hardware**
* Recommended GPU: NVIDIA L40S
* Recommended VRAM: 48 GB
**Software**
* Server regions (AWS): India (Hyderabad), USA (Oregon)
* Automatic geo-location based routing for lowest latency
***
## Best Practices
### Voice ID + model pairing
Pair Pro voice IDs above with `"model": "lightning_v3.1_pro"`. The API does not currently reject mismatched pairings, but pairing a Pro voice with `"model": "lightning_v3.1"` (or omitting `model`) can produce wrong or hallucinated audio. Server-side validation is on the roadmap.
### Language selection
Each voice's supported languages live in `tags.language` on the voice catalog. Passing a `language` outside that list is accepted by the API but produces English-pronounced output, since the voice wasn't trained on it. Pick a voice whose `tags.language` matches your target language.
### Text Formatting
* **Chunk boundaries.** Segment input at natural prosodic boundaries (`.` `!` `?` `,`). Maximum chunk size is 250 characters; optimal throughput at 140 characters per request.
* **Script integrity.** Use native script for each language. Mixed-script input within a single language token produces unpredictable phoneme mappings.
* **Lexicon overrides.** Use [pronunciation dictionaries](/waves/documentation/text-to-speech-lightning/pronunciation-dictionaries) for domain-specific terms, brand names, and acronyms where default grapheme-to-phoneme conversion is insufficient.
For comprehensive text formatting rules (numeric handling, date/time, symbols, chunking logic), see [TTS Best Practices](/waves/documentation/best-practices/tts-best-practices).
***
## Use Cases
### Direct Use
* Voice assistants and conversational AI
* Interactive chatbots with voice output
* Real-time narration and live streaming
* Accessibility tools and screen readers
* Customer service automation
### Downstream Use
* Multi-turn conversational agents
* Audio content generation pipelines
* Telephony and IVR systems
* Podcast generation
***
## Limitations & Safety
### Known Limitations
* **No voice cloning.** Voice cloning is not available on the Pro pool. Clones continue to use standard [Lightning v3.1](/waves/model-cards/text-to-speech/lightning-v-3-1) and the existing voice-cloning flow.
Lightning v3.1 Pro must **not** be used for impersonation or fraud, generating deceptive audio content (deepfakes), creating content that violates consent or privacy, harassment or abuse, or any illegal or unethical purposes.
### Safety & Compliance
* No retention of synthesized audio
* Usage monitoring for policy compliance
For compliance documentation (GDPR, SOC2, HIPAA), contact [support@smallest.ai](mailto:support@smallest.ai).
***
| Channel | Details |
| ----------------- | -------------------------------------------------------- |
| **Support** | [support@smallest.ai](mailto:support@smallest.ai) |
| **Documentation** | [docs.smallest.ai/waves](https://docs.smallest.ai/waves) |
| **Console** | [app.smallest.ai](https://app.smallest.ai) |
| **Community** | [Discord](https://discord.gg/9WtSXv26WE) |