TTS definition
Text-to-speech (TTS) is technology that converts written text into spoken audio. Modern neural TTS systems produce natural, expressive voices in many languages, with control over speed, pitch and style, and some can clone a voice from short samples. TTS powers voice assistants, AI phone agents, accessibility features, audiobooks and in-app voice guidance.
How neural text-to-speech works
TTS starts with text processing: expanding numbers, dates and abbreviations into words, so 3/4 becomes three quarters or March fourth depending on context, choosing pronunciations for ambiguous words, and predicting prosody, the rhythm, stress and intonation of the sentence. Older systems then stitched together recorded fragments of speech, which often sounded choppy and robotic.
Neural systems generate audio instead. An acoustic model predicts a spectrogram or audio tokens from the processed text, and a vocoder converts that into a waveform. Current end-to-end and language-model-based TTS systems produce speech with natural pauses, breaths and emotion, and can switch speaking styles on request, which is why synthetic voices now often sound close to human recordings.
Speech Synthesis Markup Language (SSML) gives developers finer control, marking pauses, emphasis, the pronunciation of names and how numbers should be read, which matters for brand names, addresses and phone numbers in customer-facing voice products.
Quality, latency and languages
For conversational uses such as voice AI agents, latency is as important as quality: users notice delays of even a second before a reply starts. Streaming TTS begins playing audio while the rest is still being generated, and pairing it with streaming LLM output keeps conversations feeling natural.
Language coverage varies. Major languages have many high-quality voices, while regional languages, code-mixed speech such as Hinglish and local accents may have fewer options, so test with real scripts in each language. Mispronounced names, places and product terms are the most common quality complaint, usually fixed with custom lexicons.
Tools and use cases
TTS is available from cloud platforms, specialist voice companies and open-source projects, with different trade-offs in quality, price per character, latency and data handling, so it is worth testing several. Common options and uses include:
- Cloud APIs: Amazon Polly, Google Cloud Text-to-Speech and Azure AI Speech, with many languages and SSML support
- Specialist providers: ElevenLabs, Cartesia and OpenAI's TTS models for highly natural, expressive voices
- Open source: models such as Piper and other community projects for self-hosting and offline devices
- Phone agents and IVR: natural voice replies in support and booking calls
- Accessibility: read-aloud features for people with visual impairments or reading difficulties, supporting web accessibility
- Content: audiobooks, news narration, e-learning and in-app guidance
Voice cloning and responsible use
Voice cloning creates a synthetic voice from recordings of a real person, sometimes from short samples. It enables branded voices and personal accessibility uses, such as preserving the voice of someone losing the ability to speak, but it also enables fraud: scammers have used cloned voices to impersonate executives and family members.
Use cloned voices only with explicit, documented consent, disclose AI voices where rules or expectations require it, and prefer providers with consent checks and watermarking. Nexzem builds voice features and phone agents with licensed voices and clear disclosure, as in our AI voice agent solution.