What is phonetic conversion in TTS?

Phonetic conversion in text-to-speech (TTS) systems is the process of translating written text into a sequence of phonetic symbols that represent how words should be pronounced. This step is critical because written language often doesn’t map directly to spoken sounds. For example, the letter combination “ough” in English can be pronounced differently in words like “through,” “cough,” or “bough.” Phonetic conversion resolves these ambiguities by using rules or pre-defined dictionaries to generate accurate pronunciations. Without this step, a TTS system might mispronounce words, leading to unnatural or confusing speech output.

The process typically involves two main stages: text normalization and grapheme-to-phoneme (G2P) conversion. Text normalization handles formatting issues, such as expanding abbreviations (“Dr.” to “Doctor”) or converting numbers to words (“123” to “one hundred twenty-three”). After normalization, G2P algorithms map each character or group of characters (graphemes) to their corresponding sounds (phonemes). For example, the word “example” might be split into phonemes like /ɪɡˈzæmpəl/. Some systems use rule-based approaches with linguistic guidelines, while others rely on machine learning models trained on pronunciation datasets. Hybrid approaches, which combine rules and statistical data, are common in modern TTS systems to balance accuracy and flexibility.

Developers working with TTS systems often interact with phonetic conversion through tools like the International Phonetic Alphabet (IPA) or system-specific phonetic notations. For instance, Amazon Polly uses SSML (Speech Synthesis Markup Language) tags to let developers manually adjust pronunciations, such as specifying that “read” should be pronounced as /rid/ (present tense) instead of /rɛd/ (past tense). Understanding phonetic conversion helps developers debug issues—like mispronunciations in domain-specific terms—by inspecting intermediate phonetic outputs or customizing pronunciation dictionaries. While many TTS APIs abstract this layer, knowing how it works is essential for fine-tuning synthetic speech quality, especially for specialized vocabularies or languages with irregular spelling rules.