# Streaming TTS produces choppy, syllable-by-syllable audio

**Status:** open, not started. `AI_VOICE_V2_STREAMING_TTS` is `0` (disabled) in
`deployment/aimaq.env.production` until this is fixed — see commit `26d718b`
(enabled) and `b8c922c` (reverted after live testing on the Creator plan).

## Symptom

With `AI_VOICE_V2_STREAMING_TTS=1`, live calls sound robotic / read
syllable-by-syllable ("роботизированно, читает по слогам"), reported by
Didar 2026-08-29 after testing on a paid ElevenLabs Creator plan (so it is
not a quota/concurrency artifact — that was ruled out separately the same
week).

## Root cause

`services/ai_voice_runtime_service/media_runtime.py`, `_speak_reply`
(~line 1969) only buffers once, at the very start of a reply:

```python
if prebuffered:
    interrupted = await _write_pcm_frames(pcm_8k)   # fed straight through
    ...
```

`_tts_stream_prebuffer_ms` (200ms) absorbs jitter only until the first
`_tts_stream_prebuffer_bytes` have arrived. After that, every network chunk
from ElevenLabs is written to the AudioSocket the moment it arrives, with no
ongoing cushion. `eleven_turbo_v2_5` (and flash models generally) deliver
audio over the wire in uneven bursts, not a smooth constant stream — any
gap between bursts mid-utterance becomes literal dead air in the outbound
audio, which is what reads as "robotic"/"syllable by syllable".

## Fix direction

Replace the one-shot prebuffer with a rolling buffer maintained for the
whole utterance: keep ~150-200ms of decoded PCM queued ahead of what's
being paced out via `_FramePacer`, refilling from the producer queue
continuously, instead of switching to pass-through after the first fill.

## Before re-enabling

1. Implement the rolling buffer above.
2. Re-test live with `AI_VOICE_V2_STREAMING_TTS=1` on the current Creator
   plan and confirm no gaps/choppiness across a few real calls.
3. Only then flip `AI_VOICE_V2_STREAMING_TTS` back to `1` in
   `deployment/aimaq.env.production`.
