glossary
verified July 2026
Streaming text-to-speech
Streaming TTS returns audio incrementally — the first chunk while the rest is still being synthesized — instead of one complete file at the end. It is the difference between an agent that starts speaking and an agent that buffers.
01
Why agents cannot batch
A four-second sentence synthesized as one file arrives after the whole four seconds are rendered. Streamed, the first chunk can arrive in about a hundred milliseconds and playback overlaps synthesis. Same model, same audio — the caller’s wait drops by an order of magnitude.
Fig. — The same four-second sentence, batched vs streamed
Batched — wait for the file
~4,000 ms of silence
Streamed — wait for the first chunk
~107 ms of silence
Playback then overlaps the remaining synthesis — the caller never hears the render finish.
02
What to check in a streaming API
- Time to first chunk under load, not just on an idle box — ask for the percentile pair at your concurrency.
- Raw PCM output at your telephony stack’s sample rate, so no transcode sits in the hot path.
- A clean end-of-stream signal with timing metadata — ours reports ttfa_ms and audio_ms per utterance.
- Which transports carry the stream, and what each assumes about your infrastructure — worked through on the SSE vs WebSocket page.
See also
Related sheets.
glossary
1 shape
SSE vs WebSocket for TTS
When to stream synthesis over server-sent events and when to hold a WebSocket: connection lifetime, turn structure, and infrastructure fit.
glossary
107 / 108
Time to first audio (TTFA)
TTFA is the delay from sending text to the first playable audio byte. How it differs from model latency and TTFB, and how to measure it honestly.
capability
107 ms
Streaming synthesis that keeps up with a call
Chunked PCM from the first hundred milliseconds: SSE and WebSocket streaming TTS measured at 107 ms to first audio on the production API.
Every term on this page is measurable on a live call — watch the readout while your own script synthesizes.
See the measured figures live