Turns, Silero VAD & Interruptions

1. Plain-English UI Settings GuideCopy Anchor Link

Open any workflow's Settings -> General panel and Canvas Node Properties:

Voice Sensitivity Presets (Quick Tuning)Copy Anchor Link

(Clicking a card updates the 4 fields below simultaneously while leaving VAD Silence Hold (vad_stop_secs) untouched. Brand-new workflows default to 0.20 / 0.50 / 0.8s / 3.5s, which the UI highlights under High Sensitivity.)

Turn Detection & Interruption Controls ExplainedCopy Anchor Link

• Interruption Strategy (turn_start_strategy — When the caller can interrupt the bot):
  • Default (default): Stops the bot whenever Silero VAD (~0.2s voice onset) or any interim/final transcript fires. Lowest latency, most sensitive to backchannels.
  • Minimum words (min_words, 1–10 words, default 3): When the bot is silent, opens the turn immediately on VAD (0 ms delay) with a 1-word threshold; when the bot is speaking, ignores coughs and short backchannels ("hmm", "okay") until interim/final text reaches N words (and resets sub-threshold filler so it doesn't bleed into the next turn).
  • Provisional VAD (provisional_vad, 0.1–5.0s, default 1.5s): Pauses bot output playback cleanly on speech onset while waiting up to N seconds for a real transcript; if none arrives, resumes playback seamlessly and locks out late false alarms for the rest of that bot utterance.
• Detection Strategy (turn_stop_strategy — When the bot decides the caller finished speaking):
  • Transcription-based (transcription): Waits for Silence Before Responding (0.8s default) + finalized transcript (or STT deadline).
  • Smart Turn Analyzer (turn_analyzer): Evaluates a local ML prosody model over 500ms pre-speech + up to 8s of speech. Even if it predicts COMPLETE, it waits Silence Before Responding; if INCOMPLETE ("Wait, my number is..."), it waits up to Incomplete Turn Timeout (smart_turn_stop_secs, 0.5–10s, default 2.0s), bypassing extra debounce once that timer fires (capped by a secondary safety fallback).
• Idle & Call Duration Guardrails:
  • Max User Idle Timeout (max_user_idle_timeout, 10.0s default; <= 0 disables): Two-strike check-in after bot finishes speaking—Strike 1 asks "Are you still there?", Strike 2 politely hangs up. Resets whenever caller speaks.
  • Max Call Duration (max_call_duration, runtime default 300s): Hard call limit capped by tier/org duration limit (note: UI helper copy notes 600s, while API/runtime default is 300s).
• Per-Node Allow Interruption (allow_interrupt):
  • Start Call: Defaults false so line noise or "Hello?" doesn't break the opening greeting.
  • Agent: Defaults true (turn false only on mandatory disclosures/readbacks).
  • End Call: Always forced false and hidden in the node editor so closing goodbyes finish cleanly.

2. Quick Recipes by SymptomCopy Anchor Link

(Detailed step-by-step fixes live in Audio & Turn Problems.)

• Bot cuts caller off mid-sentence: Switch Detection Strategy to Smart Turn Analyzer or raise Silence Before Responding (1.0–1.4s).
• Traffic/fan noise interrupts bot: Choose Noisy Room preset, raise Minimum Words to 3–4, or turn Allow Interruption off on key disclosures.
• Bot misses quiet "yes" / "haan": Switch to High Sensitivity preset (0.20 volume / 0.50 confidence) and keep Boost Short Affirmations ON.
• 4-second awkward silence after caller finishes: Lower Silence Before Responding (0.7–0.8s) and lower Safety Watchdog Timeout toward 2.5–3.5s.

3. How Turns Work Under the HoodCopy Anchor Link

A Turn is one complete speaker floor interval (User Turn or Agent Turn). Local Silero VAD checks both vad_min_volume AND vad_confidence every 32 ms (plus a 1.0s audio-idle failsafe if a mic mutes mid-sentence), and VAD Silence Hold (vad_stop_secs) should stay short (0.20–0.35s, default 0.25s) while pacing is tuned via Silence Before Responding.

All Turn & Timer Behaviors at a GlanceCopy Anchor Link

• Voice User Turn: Triggered by Silero VAD (or External STT Turn) -> transcribed by STT -> passed to LLM.
• DTMF Keypad Turn (Plivo): When Enable DTMF is on, keypad presses buffer and flush to the LLM on # or after 3.0s of keypad inactivity.
• Delayed Start (Start Call): When delayed_start = true, waits delayed_start_duration (0.1–10.0s, default 2.0s) after call answer before playing the opening greeting.
• Safety Watchdog Timeout (user_turn_stop_timeout, default 3.5s / 4.0s external): Fires at 1x timeout if VAD is already quiet on a stalled transcript, or 2x timeout if line static kept VAD stuck open.
• Bot Output VAD Tail (bot_vad_stop_secs): Defaults to 3.0s in standard pipeline mode and 0.5s in Realtime S2S mode for snappy bot-turn closure.

4. Local Silero VAD vs. Provider Turn OwnershipCopy Anchor Link

5. Three Active User Muting Layers Preventing False InterruptionsCopy Anchor Link

False interruptions aren't stopped at TTS—incoming caller audio/transcription frames are filtered at the front door by three OR'd muting layers:

1. Startup Greeting Mute: Always mutes caller barge-in from second zero until the bot's very first utterance completes, preventing pickup clicks or "Hello?" from cancelling your greeting regardless of allow_interrupt.
2. Function / Tool Execution Mute: Mutes user speech turns while mid-call HTTP, transfer, or DB tools are running—with a dedicated speech listener exception on wait_for_user so callers can resume early by speaking.
3. Node, Transition & Teardown Mute: Mutes caller frames whenever allow_interrupt = false on the active node, whenever queued transition or tool-filler speech is playing, or while tearing down at hangup.