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TTS and Voice Output

Tryll supports server-side text-to-speech synthesis using the Sherpa-ONNX engine. Two graph nodes produce speech:

  • GenerateAndSpeak fuses LLM inference with TTS synthesis, streaming PCM audio frames alongside the text reply — lowest latency for a generated spoken answer.
  • Speak is TTS-only: it voices the latest assistant text produced upstream (e.g. a CannedResponse) with no LLM pass.

Both stream the same PCM audio frames to the client and use the same voice parameters and playback components; they differ only in whether generation is involved.

The streaming pipeline

When a turn starts on a GenerateAndSpeak node, three concurrent things happen:

  1. Token generation — the language model produces tokens, forwarded to the client as AnswerText deltas (identical to Generate).
  2. Sentence segmentation — tokens accumulate until a sentence boundary is detected or min_sentence_chars is reached. Each completed sentence is dispatched to the TTS strand immediately — no waiting for the full response.
  3. TTS synthesis — the TTS model synthesizes the sentence to mono int16 PCM at the model's native sample rate, then streams the audio back as a sequence of TtsAudioFrame wire messages, preceded by a single TtsAudioFormatFrame message that declares the sample rate.

This pipeline keeps time-to-first-audio low: the NPC starts speaking as soon as the first sentence is complete, while the language model is still generating the rest of the answer.

Client                      Server
  │                            │
  ├─ SendMessage ─────────────►│
  │                            ├─ LLM tokens → AnswerText chunks ──────────►│
  │                            ├─ Sentence ready → TTS synthesis
  │◄── TtsAudioFormatFrame ────┤  (first sentence)
  │◄── TtsAudioFrame … ────────┤
  │◄── AnswerText … ───────────┤  (LLM still running)
  │◄── TtsAudioFrame … ────────┤  (second sentence)
  │◄── AnswerText … ───────────┤
  │◄── TurnComplete ───────────┤

Audio format

All TTS audio is mono signed-16-bit PCM at the sample rate reported in the TtsAudioFormatFrame frame. The sample rate is model-specific (commonly 22 050 Hz or 24 000 Hz). The client's speaker component converts int16 samples to float32 before passing them to the audio engine.

End of stream

TtsAudioFrame carries no is_final flag. The last frame arrives before TurnComplete. The audio queue on the client drains naturally — the playback component fills the audio buffer with silence on underrun until the queue is empty.

Session configuration

You must declare a TTS engine when you create the session. Without one, a GenerateAndSpeak node produces text only and audio frames are not emitted.

// Unity
client.CreateSession(
    TryllInferenceEngine.LlamaCpp,
    ttsEngine: TryllInferenceEngine.SherpaOnnx);

// Unreal (C++)
Subsystem->CreateSession(
    ETryllInferenceEngine::LlamaCpp,
    /*GameName=*/ TEXT(""),
    /*SttEngine=*/ ETryllInferenceEngine::Mock,
    /*TtsEngine=*/ ETryllInferenceEngine::SherpaOnnx);

Voice configuration

TTS voice settings live on the GenerateAndSpeak and Speak nodes (identical fields on both):

Parameter Structural Runtime-mutable Description
tts_model_name Yes No Catalog voice name. Optional — empty uses the first TTS voice in models.json.
speaker_id No Yes Voice index, 0 to the model's speaker count − 1 (the shipped Supertonic 3 bundle has 10 voice styles, 0–9). Pocket TTS is single-speaker: must stay 0 there — pick a Pocket voice with tts_voice instead. Out-of-range values fail agent creation.
speed No Yes Speech-rate multiplier. 1.0 = native speed, range 0.1 – 3.0.
tts_lang No Yes Synthesis language on multilingual models (Supertonic 3: 31 languages). Empty = catalog default.
tts_voice No Yes Reference-voice WAV (storage-root-relative) for voice-cloning models (Pocket TTS). Empty = model default voice. See Clone a Voice.
min_sentence_chars No Yes Minimum characters before a sentence fires to TTS. Lower = shorter first-audio latency.

Structural parameters are locked at agent creation time. Mutable parameters can be changed between turns with ChangeAgentParam (see Change Agent Parameters at Runtime).

Client-side playback components

The Tryll Unity and Unreal packages ship a ready-made playback component that wires the TTS audio stream to the platform audio engine:

Platform Component Audio backend
Unity TryllSpeaker (MonoBehaviour) Streaming AudioClip with pcmReaderCallback
Unreal UTryllSpeakerComponent (UActorComponent) USoundWaveProcedural + hidden UAudioComponent

Both are non-spatialized (2D) by default, suitable for NPC narration. For spatial audio, subscribe to TryllAgent.OnTtsAudioFormat and OnTtsAudio directly and feed the raw PCM chunks to a custom spatializer.

See also