Technology & privacy
Voice activity detection, and why your first word goes missing
A small model runs before the speech model and decides which parts of the audio are speech. It saves time and battery, and it is responsible for the most common dictation complaint.
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What it does, and why it exists
When you hold a dictation shortcut, the app captures everything the microphone hears — including the half-second before you start, the pauses while you think, and the moment after you finish but before you release the key.
Handing all of that to a speech model is wasteful in three ways. It costs time you notice as latency. It costs battery. And it costs accuracy, because a model asked to transcribe a fan and a distant conversation will sometimes produce words from them.
VAD solves this by being small and cheap. Silero VAD, which several dictation apps use including ours, is a few megabytes and runs in a fraction of the time the speech model takes. It marks the speech regions; the recogniser only sees those.
| Voice activity detection | Speech recognition | |
|---|---|---|
| Question it answers | Is this speech? | What words are these? |
| Model size | A few megabytes | Hundreds of megabytes to gigabytes |
| Speed | Effectively instant | The slow part |
| Output | Timestamps | Text |
| Runs | First, on everything | Second, on the trimmed audio only |
Why the first word goes missing
This is the most useful practical consequence and it explains a complaint that otherwise looks like a bug.
The detector needs a moment of audio to decide that speech has begun. If you press the shortcut and start talking in the same instant, the beginning of your first word arrives while it is still deciding, and part of it can be trimmed with the preceding silence.
The same happens at the end, in a slightly different way: people drop their volume on the last few words of a sentence, the detector reads the quiet tail as silence, and "...before the meeting" becomes "...before the".
The trade-off VAD is tuned against
Every detector sits on a dial between two failure modes, and moving away from one moves towards the other.
| Setting | Failure mode |
|---|---|
| Too eager (marks more as speech) | Room noise, keyboard clicks and a colleague's conversation get transcribed. Stray words appear in your text |
| Too strict (marks less as speech) | Quiet starts, soft speakers and the ends of sentences get trimmed away |
There is no setting that is right for every room and every voice, which is why an app's default is a judgement about its typical user rather than a correct answer. Softly spoken people in quiet rooms and loud people in busy offices genuinely want different tuning.
Where else VAD shows up
- Latched dictation. When the microphone is held open rather than push-to-talk, VAD decides where one utterance ends and the next begins.
- Video calls. The same class of model drives the speaking indicator and noise suppression.
- Voice assistants. Deciding when you have finished talking is a VAD problem, and it is why an assistant sometimes cuts you off mid-sentence.
- Transcription tools. Chunking a long recording into segments before transcribing it.
Troubleshooting VAD-shaped problems
| Symptom | Likely cause | Fix |
|---|---|---|
| First word missing or clipped | Speaking as you press | Half a second of pause first |
| Last few words missing | Trailing off in volume | Keep the level up to the end; release a beat after you finish |
| Words appear that you did not say | Background speech being marked as yours | Closer microphone, or a quieter room |
| Long pauses split a sentence in two | The detector read the thinking pause as an ending | Speak in complete phrases; think before you press |
| Nothing is transcribed at all | Wrong input device, so the detector hears nothing | System Settings → Sound → Input |
Questions
What is VAD short for?
Voice activity detection. It is a small model that identifies which parts of an audio stream contain speech, running before the speech recogniser so the expensive model only sees the parts that matter.
Why does my dictation cut off the first word?
Almost always because you started speaking as you pressed the shortcut. The detector needs a moment of audio to decide speech has begun, and part of the first word can be trimmed with the leading silence. Pause half a second before speaking.
Does VAD mean my microphone is always listening?
Not in a push-to-talk app. The microphone is live only while you hold the shortcut, and VAD runs on that captured audio. That is a different design from an always-listening assistant waiting for a wake word.
Can I turn VAD off?
Most dictation apps do not expose it, and turning it off would make things worse — the speech model would be asked to transcribe silence and room noise, which is slower and produces stray words.