> 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. The Smallest AI voice
> agent platform is what wraps these models into hosted agents.

# Math notation

> Opt-in Lightning TTS flag that reads digit-flanked math operators (x, ×, ÷, spaced -, ^, =) as spoken words instead of leaving them for the number reader. Off by default because dimensions, 24x7, and vehicle-reg codes look like math on paper.

Pass `math_notation: true` on a Lightning TTS request to have the normalizer read digit-flanked math operators as spoken words. Off by default because in real traffic `NxN` is more often a product dimension (`18x24 Inch`), the `24x7` idiom, or a vehicle-registration code than an actual multiplication.

## Endpoint support

Available on Lightning v3.1 and v3.1\_pro across every transport:

| Surface                                               | `math_notation` |
| ----------------------------------------------------- | --------------- |
| `POST /waves/v1/tts` (sync HTTP)                      | Supported       |
| `POST /waves/v1/tts/live` (HTTP SSE)                  | Supported       |
| `wss://api.smallest.ai/waves/v1/tts/live` (WebSocket) | Supported       |

## Operator lexicon

The operator word follows `number_pronunciation_language`, not the synthesis `language`. So `language=en` with `number_pronunciation_language=hi` reads `6 x 7` as "छः गुणा सात". Localized only for Hindi and Marathi. Every other language falls back to the English words.

| Glyphs             | en (default / fallback) | hi     | mr     |
| ------------------ | ----------------------- | ------ | ------ |
| `×` `x` `X` `*`    | times                   | गुणा   | गुणिले |
| `÷` and spaced `/` | divided by              | बटा    | भागिले |
| `+`                | plus                    | प्लस   | अधिक   |
| spaced `-` `–` `−` | minus                   | माइनस  | वजा    |
| `=`                | equals                  | बराबर  | बरोबर  |
| `^` `**`           | to the power of         | की घात | ची घात |

## Matching rules

**Unambiguous glyphs** fire whether glued or spaced.

* `×` `x` `X` `*` `^` `**` `=` `+`
* Wrong-glyph `x`/`X` counts as unambiguous when a digit sits on both sides.
* `5x3` and `5 x 3` both become "5 times 3".
* `2^10` becomes "2 to the power of 10".

**Ambiguous glyphs** fire only when space-padded.

* `-` `–` `−` and `/`.
* `5 - 3` becomes "5 minus 3".
* `5-3` stays untouched so range and date readers still work (`2-3`, `2026-07-24`, `020-4825-9631`).
* `6 / 2` becomes "6 divided by 2".
* `1/2` stays a fraction.

**Operand handling.** Unicode digits (including Indic digits) count as operands. The right operand may carry a currency prefix (`2 x ₹99` reads correctly). An operator never bridges a line break. The pass runs after the date, phone, and version handlers, so those readings still take precedence.

## Example

```bash
curl -X POST "https://api.smallest.ai/waves/v1/tts?model=lightning_v3.1" \
  -H "Authorization: Bearer $SMALLEST_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "text": "The answer is 6 x 7 and the ratio is 3 - 1.",
    "voice_id": "avery",
    "language": "en",
    "math_notation": true,
    "output_format": "mp3"
  }' \
  --output out.mp3
```

With `math_notation: false` (default), the client hears "six ecks seven" and "three one". With `math_notation: true`, the client hears "six times seven" and "three minus one".

## Protected patterns (ON == OFF)

The following patterns produce identical audio whether the flag is on or off:

* ISO dates: `2026-07-24`
* Slash dates: `12/04/2026`
* Ranges and scores: `2-3`, `2-1`
* Version strings: `1.2.3`
* Fractions: `1/2`
* Phone numbers: `020-4825-9631`
* Hindi year ranges and dates

## Known limitations

Read the flag as opt-in for a reason. Enable it only if your text is genuinely math-heavy and does not carry the patterns below.

* **`24x7` idiom.** Free-text `24x7` reads as "twenty four times seven", not the "24 by 7" idiom. Channel names carrying `24x7` are handled separately via the phrase dictionary and are unaffected.
* **Product dimensions.** `18x24 Inch` reads as "eighteen times twenty four Inch". There is no unit-context guard.
* **Vehicle registration and codes.** `GJ 27 X 2850` reads as multiplication.
* **Glued `-` and `/` never convert (by design).** `5-3` stays a range and `1/2` stays a fraction.

## European languages

The `normalize_eu` block localizes `×`, `÷`, and `=` per language: `it` reads `×` as "per", `es` as "por", `fr` as "fois". The remaining wrong-glyph and extra cases (`x`, `*`, `^`, `**`, `+`, `-`) read in English via the fallback.

## Related

* [`number_pronunciation_language`](/api-reference/models/text-to-speech/tts) sets the language for spoken numeric content, including math operator words.
* [Pronunciation Dictionaries](/models/text-to-speech/pronunciation-dictionaries) let you override specific tokens directly.

> Opt-in flag that reads math operators aloud.