feat: add vowel length marks (ː) to IPA output#14
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yocontra wants to merge 2 commits intohans00:mainfrom
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feat: add vowel length marks (ː) to IPA output#14yocontra wants to merge 2 commits intohans00:mainfrom
yocontra wants to merge 2 commits intohans00:mainfrom
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When ipa-dict provides multiple pronunciation variants for a word (e.g. "caught" → /ˈkɑt/, /ˈkɔt/), prefer the variant containing ɔ (THOUGHT vowel) over ɑ (LOT vowel). This better represents standard American English pronunciation for THOUGHT-class words like caught, bought, law, fall, walk, want, etc.
Add standard IPA vowel length marks for American English: - FLEECE (iː) and GOOSE (uː): lengthened in stressed syllables only, preserving short unstressed vowels (e.g. "happy" = hæpi) - THOUGHT (ɔː): always long in American English - PALM/LOT (ɑː): always long (LOT-PALM merger in AmE) The library already uses the correct vowel symbols (ɪ vs i, ʊ vs u) but was missing the length mark ː that standard IPA transcription requires. This matters for TTS systems trained on length-marked IPA (like espeak-ng output). Vowel lengthening is applied only to English words — Chinese and other language IPA output is not affected.
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Mar 15, 2026
Merges the following PRs into a single combined branch: - hans00#11: fix: prefer THOUGHT vowel (ɔ) variant in dictionary - hans00#12: fix: use STRUT vowel (ʌ) instead of schwa (ə) in stressed syllables - hans00#13: fix: distinguish NURSE vowel (ɜː) from unstressed ɚ, add linking ɹ - hans00#14: feat: add vowel length marks (ː) to IPA output - hans00#15: fix: British English pronunciation corrections
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Summary
Standard English IPA transcription uses vowel length marks (ː) to distinguish long vowels:
The library already correctly distinguishes short vs long vowels by using different symbols (ɪ vs i, ʊ vs u), but omits the length mark ː. This matters for TTS systems trained on length-marked IPA (like espeak-ng output), where the presence or absence of ː affects vowel duration in synthesized speech.
Rules implemented
hæpi, nothæpiː.Scope
addVowelLength()function is idempotent — it uses negative lookahead(?!ː)to avoid double-marking.Changes
src/tokenizer.ts: AddedaddVowelLength()post-processing function in the IPA output path, gated by language detection (enonly). Addedlanguageparameter to_postProcess()and_predict().__tests__/index.test.ts: Updated existing test expectations to include length marks.__tests__/vowel-length.test.ts: New dedicated test suite covering all four vowel length rules, stress-sensitivity, language isolation, and idempotency.