voxnorm

Text normalization for TTS: written forms into spoken forms, multilingual, pure Python. Currently speaks Traditional Chinese (zh-TW), Simplified Chinese (zh-CN), English, Japanese, and Korean.

Language models and databases write for the eye — 12:00, NT$1,200, 0912-345-678 — and a small TTS frontend handed that text verbatim has to guess, and guesses badly. voxnorm rewrites the written forms into what a speaker would actually say, and leaves everything else character-for-character alone:

>>> import voxnorm
>>> voxnorm.normalize("會議在12:00開始")
'會議在十二點開始'
>>> voxnorm.normalize("The meeting starts at 12:00")
"The meeting starts at twelve o'clock"
>>> voxnorm.normalize("会議は12:00からです")      # kana → Japanese
'会議は十二時からです'
>>> voxnorm.normalize("회의는 12:00에 시작합니다")  # hangul → Korean (native-numeral hours)
'회의는 열두 시에 시작합니다'
>>> voxnorm.normalize("電話是0912-345-678。")
'電話是零九一二三四五六七八。'
>>> voxnorm.normalize("NT$1,200", lang="zh")
'新台幣一千兩百元'
>>> voxnorm.normalize("会议在12:00开始")        # simplified glyphs → zh-CN
'会议在十二点开始'

Design

Two layers, split where languages stop sharing:

• Detection is language-neutral (voxnorm/tokens.py): one scanner finds the spans — times, time ranges and am/pm times, ISO and four-digit-year slash dates, phone numbers (with area codes in parentheses, + country codes, #/轉 extensions and North American 3-3-4 numbers), currency, percentages, temperatures, metric units, room numbers, letter-glued identifiers and masked tails, emails and URLs, decimals, integers — and names their kind. Angle-bracket markup (<break time="500ms"/>) passes through untouched.
• Verbalisation is per-language (voxnorm/zh.py, cn.py, en.py, ja.py, ko.py): a new language is a new module and one registry entry. English, Japanese and Korean ride on num2words for their number words, each adding the readings the library cannot know — a Japanese year without the era default, Korean native-numeral hours (한 시, never 일 시), the irregular Korean months 유월/시월. Chinese is spelled out in-repo, Traditional first, because the 兩/二 reading choice (一千兩百, never 一千二百 in zh-TW speech) is exactly what general-purpose converters get wrong; the Simplified variant re-spells that one output vocabulary (兆 → 万亿, and ¥ reads as renminbi rather than yen).

Language is decided per call: an explicit hint wins (normalize(text, lang="zh-TW")), otherwise script evidence decides — kana means Japanese, hangul Korean, a Han character Chinese (Simplified when distinctive simplified glyphs appear, Traditional otherwise), else English. (A Japanese sentence written entirely in kanji is script-identical to Chinese and lands on the Chinese reading — pass lang="ja" when you know better.)

The charter is structural only, never guess: a form whose reading is ambiguous does not convert (1/2 the fraction and 8/17 the date are the same slash string, so a slash form converts only with a four-digit year in front, 2026/10/16; 7-11 the shop and 3-5天 the range are the same hyphen string, so no hyphen form does either, unless its shape is an ISO date, a clock-time range or a phone number). What is not converted is simply spoken as written — which is where it started.

Codes the text does not mark

Structure settles most readings: a leading zero, a letter glued on the left (A123456789), a 號 after three or more digits all say "code, digit by digit". What structure cannot settle is vocabulary — that the 4820 after 車牌 or plate is a plate tail, not a quantity. That word list is open-ended and belongs to the caller, so the package takes no guess and instead accepts the words that settle it:

>>> voxnorm.normalize("車牌4820的車款已逾期12天。", code_words=["車牌"])
'車牌四八二零的車款已逾期十二天。'
>>> voxnorm.normalize("請問您4820的車款繳了嗎?", code_words=["的車款"])
'請問您四八二零的車款繳了嗎?'
>>> voxnorm.normalize("plate 4820 is overdue", code_words=["plate"])
'plate four eight two zero is overdue'
>>> voxnorm.normalize("請撥打 1234-5678，交易序號是12345678901。", code_words=["撥打", "序號是"])
'請撥打 一二三四五六七八，交易序號是一二三四五六七八九零一。'

Each word is matched literally, directly before the digit run (whitespace allowed) or directly after it; include any connective in the word itself (末四碼是, not 末四碼). The words are per-call and per-language — the same API carries 車牌, plate, 会員番号 and 계좌. A word also licenses hyphen-joined groups (1234-5678), which without it stay written: the same string is the year range 1990-2000.

Why not an FST toolkit

nemo_text_processing and WeTextProcessing were measured before this package was written. Both depend on pynini, which publishes no aarch64 wheel — on an ARM box they need OpenFst compiled from source and a permanently vendored libfst.so — and both mis-read the very classes this package exists for: phone numbers as quantities, 302號房 as a date, and (WeTextProcessing) force-simplifying every Traditional character in the sentence it touches. The classes a spoken product needs are a bounded set; implementing them directly costs ~1.5 MB of pure Python (num2words included) and stays debuggable.

Install

uv add voxnorm      # or: pip install voxnorm

Development

uv sync
uv run pytest
uv run ruff check .

Apache-2.0.