Voice Activity Detection

Before running transcription on audio, it's important to know when to stop listening and start transcribing (and answering). This can be done in a simple way (a silence timeout), but that can often be quite clunky and feel slow. Voice activity detection uses a small model that understands the shape of speech and can reliably tell speech and silence apart.

Streaming

The main use case for VAD is streaming audio into the model chunk by chunk. This is useful, for example, to detect when to stop listening to the microphone and start running transcription/answer generation.

To do that, we provide a push interface:

import 'package:nobodywho/nobodywho.dart' as nobodywho;

// ... after NobodyWho.init().
final vad = await nobodywho.VoiceActivityDetection.load(
  sampleRate: 16000,
  source: 'hf://onnx-community/silero-vad',
);
final stt = await nobodywho.SpeechToText.load(source: 'hf://onnx-community/whisper-base');

while (true) {
  final chunk = readMic(); // however you're reading from the microphone
  if (vad.push(chunk) == nobodywho.VoiceActivityDetectionEvent.speechEnded) break;
}

final speech = vad.finish();
final transcription = await stt.transcribePcm(speech, 16000).completed();
print(transcription);

VoiceActivityDetection acts as a buffer: every push() call tells you the current state (speechStarted, speechEnded, speech, or silence), so you can decide when to stop listening. Once you do, finish() gives you back the buffered audio for just the speech segment, and resets internal state so it's ready for the next turn.

Segmentation

Another use case is segmenting speech out of audio you already have. A good example is a long, mostly-silent recording with a few short occurrences of speech you want to transcribe. That's what the segment() method is for:

import 'package:nobodywho/nobodywho.dart' as nobodywho;

// ... after NobodyWho.init().
final vad = await nobodywho.VoiceActivityDetection.load(
  sampleRate: 16000,
  source: 'hf://onnx-community/silero-vad',
);
final stt = await nobodywho.SpeechToText.load(source: 'hf://onnx-community/whisper-base');

final audio = readWavPcm('recording.wav'); // a full recording as i16 PCM samples

for (final speech in vad.segment(audio)) {
  final transcription = await stt.transcribePcm(speech, 16000).completed();
  print(transcription);
}

Configuring sensitivity

We try to provide reasonable defaults to capture most situations. However, especially in the case of voice activity detection, manual tuning is often needed to reach better performance. For that, we provide numerous params that you can tweak:

// ... after NobodyWho.init().
final vad = await nobodywho.VoiceActivityDetection.load(
  sampleRate: 16000,
  // VAD is currently fixed to Silero ONNX, but you can change the source.
  source: 'hf://onnx-community/silero-vad',
  // Determines the sensitivity to what counts as speech. Moving it up will make the VAD stricter.
  threshold: 0.5,
  // Determines the minimum duration classified as speech.
  minSpeechDurationMs: 250,
  // Determines the minimum duration classified as silence.
  minSilenceDurationMs: 250,
  // Determines how much audio to keep before the official speechStarted, to avoid cutting off the start.
  prerollDurationMs: 500,
);