SpeechRecognition­Result.audio­StartTime

syntax

readonly attribute DOMHighResTimeStamp? audioStartTime;

Proposed spec text (PR #192, quoted verbatim): “A nullable DOMHighResTimeStamp representing the start of the audio segment corresponding to this recognition result, in milliseconds relative to the time origin. Returns null if the underlying recognition engine does not support audio segment start timestamps.”

The shipping Chromium IDL gates the same member on the runtime feature (speech_recognition_result.idl):

[RuntimeEnabled=WebSpeechTimestamps] readonly attribute DOMHighResTimeStamp? audioStartTime;

value

inputs

None. This is a read-only attribute: no setter, no parameters, no options. What the engine supplies is what the getter returns (after quantization); script cannot influence the value other than by choosing a recognition engine/backend that supports segment timestamps.

errors

No error surface. The getter never throws: unsupported engines yield null, not an exception. Reading the attribute on a browser without the runtime flag fails earlier — the member does not exist on the prototype at all — so feature-detection ('audioStartTime' in SpeechRecognitionResult.prototype) is the correct guard, not try/catch.

context and exposure

• Exposed on every SpeechRecognitionResult — the objects in SpeechRecognitionEvent.results delivered to result event handlers — when the WebSpeechTimestamps runtime feature is enabled.
• The host Web Speech API requires a secure context and microphone permission (or a MediaStreamTrack source); these attributes add no new requirements of their own.
• Both interim (isFinal === false) and final results can carry the value: Chromium's aggregation path constructs provisional and final results from the same engine struct (speech_recognition.cc).

lifecycle

1. The recognition engine emits a result with timing information (TimingInformation.audio_start_time in the mojom struct).
2. Blink stores it on the result object at construction (SpeechRecognitionResult::Create).
3. Each getter call returns the stored value floored to 2 ms, or null if unset. The value for a given result object does not change afterwards — results are immutable snapshots delivered through the event.
4. Interim results for the same utterance may be replaced by later interim/final results with updated bounds as the engine refines its hypothesis; read the value from the result object you are actually rendering.

examples

if (!('audioStartTime' in SpeechRecognitionResult.prototype)) {
  // Flag off or engine too old — fall back to untimed captions.
}

recognition.onresult = (event) => {
  const result = event.results[event.resultIndex];
  if (result.audioStartTime !== null) {
    // Same clock as performance.now(): position the caption on the timeline.
    showCaptionAt(result.audioStartTime / 1000, result[0].transcript);
  }
};

browser compatibility

Interim table. No BCD key exists for this member (verified 2026-07-29); rows compiled from primary sources.

security and privacy

• High-precision timing is a hardware-fingerprinting vector; the spec PR requires fuzzing/precision reduction before script exposure (“e.g. by rounding to 2ms precision”) and Chromium floors values to 2 ms multiples in the getter.
• The attribute reveals nothing beyond timing metadata about audio the page already submitted for recognition under the existing permission model — no new audio, device, or engine access.
• A null result distinguishes “engine cannot time segments” from “segment starts at 0”, preventing fabricated-zero misreads.

see also

• SpeechRecognitionResult Timestamps — overview (feature page)
• audioEndTime — the segment end, and the latency-computation input
• Explainer — SpeechRecognitionResult timestamps (motivation + usage patterns)
• Chrome Platform Status — feature 5811907077472256
• MDN — SpeechRecognitionResult (no coverage of this member, 2026-07-29)
• Showcase — caption timeline demo (HEAD-checked 200, 2026-07-29)