The universal sync fallback in stream_tts_to_speaker ran strictly serially
per sentence — synthesize, play, and only then start synthesizing the next
sentence — so every sentence boundary added a full synthesis-time of dead
air. Chunked streamers (elevenlabs/openai/gemini/xai) already avoid this;
every other provider (edge, piper, plugin providers) paid it on each reply
in voice mode and the wake-word loop.
_SyncSentencePipeline overlaps the two: one single-threaded synthesis
worker (sentences stay FIFO; providers never see concurrent calls from
this loop — same effective concurrency as before) feeds one playback
worker through a small bounded queue, so sentence n+1 synthesizes while
sentence n plays. Lookahead is bounded (backpressure + at most a couple of
temp files), stop_event short-circuits both stages, synthesis failures are
isolated per sentence, temp files are always unlinked, and the finally
block flushes the pipeline BEFORE tts_done_event fires so continuous voice
mode never reopens the mic over its own voice. synthesize/play are
resolved late so existing monkeypatch-based tests work unchanged.
Measured with a real local model provider (OmniVoice plugin, Apple
Silicon), same 3-sentence reply, playback simulated at the produced clips'
true durations, best-of-2 interleaved runs under identical load:
serial pipelined
time to first word 10.8s 4.4s
mid-reply dead air 11.2s 1.8s (second gap: 0.03s)
full reply wall 33.2s 17.0s
Tests: 4 new (timestamp-proven overlap, order + per-sentence failure
isolation, stop skips queued playback, temp-file hygiene); the existing
sync-fallback and display-callback tests pass unchanged.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Mahdi Hedhli
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