# TASK 22 — Audio sync offset

> Catalog: [`TASKS.md`](../TASKS.md) — status and requirements live here.


**Goal:** per-source audio delay compensation to prevent lip-sync drift from USB mics and capture cards.

### Status: ✅ Done (shipped TASK 22, 2026-08-31)

1. ☑ `AudioSyncOffsetMs` on `MainViewModel` (global, default 0, range 0..500 ms) — positive-only: OBS's documented fix delays the audio so it lands on video when it runs ahead; advancing audio would need a video-side delay (out of the audio layer's scope, v1.1+).
2. ☑ Applied in `AudioMixer` — post-mix interleaved-stereo delay via the pure `AudioSyncDelay` line (`Services/Audio/AudioSyncDelay.cs`), fed through a `Func<int> syncOffsetMs` seam each mix tick.
3. ☑ UI: compact "SYNC" slider (0..500) on the mic bar with a status dot (green = no-op, amber = offset set) via `IntToSyncBrushConverter`. **Provenance (recorded 2026-09-01 after a creator "I never ordered this" scare): the creator explicitly asked for OBS's delay-filter lip-sync fix BUILT NATIVELY — the feature is his, not AI drift. Its placement (preview-rail vs mic-settings/gear) remains an open design question; do not remove the capability.**
4. ☑ Persisted in `LayoutStore.Settings` (`Audio.SyncOffsetMs`) via `LoadAudioSyncOffsetMs`/`SaveAudioSyncOffsetMs`; saved from `SaveLayoutNow`.
5. ☑ Tests: `AudioSyncDelayTests` — zero-delay identity, negative→0 clamp, >500 ms clamp to 500 ms, and 10 ms → 960 interleaved-sample shift.
6. ⚠ **Post-ship regression (found 2026-09-01, first real launch):** this task's sync-delay line `_delayedMix` was declared nullable and never initialized — the first `delayed.Length` deref NRE'd EVERY live-mix tick, silently killing all live/record audio, hanging `AudioPipelineTests`, and being mislabeled a "known failure". Fixed in the recording-verification pass (init + null-check + throttled loop errors logged with stack); `AudioPipelineTests` 25/25 green afterward. Lesson recorded in MyMistakes.

### Design decisions

- **Global offset first** — one setting for all audio sources. Per-source is v1.1+.
- **Signed offset (2026-09-14)** — WIDENED from positive-only 0..500 to **−500..+500**.

  > The "positive-only / advance needs video-side delay / out of scope" line below is
  > RETIRED. Negative offsets now ADVANCE the audio by eating the head of the live stream
  > (OBS's negative-sync behavior): `AudioMixer.StartLive` arms `_advanceSamplesRemaining`
  > = |N| ms → samples, and `LiveLoopAsync` drops that many samples off the write head.
  > Positive keeps using the `AudioSyncDelay` line live-reactive. Slider relabelled
  > "AUDIO SYNC", `Min="-500"`, and **locked (`IsEnabled = IsEditMode`) while live or
  > recording** — a negative advance can only be armed at go-live, so it must not move
  > mid-session. Clamps: `Pre-viewModel` + `LayoutStore` ±500; `AudioSyncDelay` still
  > clamps negative→0 internally (pure delay line, unchanged).
  > **Provenance (2026-09-14): creator-directive** — asked for both directions after the
  > ring-backlog fix surfaced the residual (audio can run late too: capture cards, BT,
  > webcams). Regression test: `StartLive_NegativeOffset_AdvancesAudio_ByDroppingTheStreamHead`.
- **Positive-only (delay audio)** — the physically-correct direction (audio runs ahead of the video). True "advance" needs a video-side delay and is PERMANENTLY OUT with per-source sync (TASKS.md → "Out of product" — v1.x phrasing retired 2026-09-01).
- **Simple slider** — 0 to +500 ms, default 0. No numeric input needed.
- **Visual feedback** — "sync OK" status dot shows when an offset is dialled in.

### ❗ REQUIRED before 1.0 (creator directive 2026-09-14)

A **detailed user-doc tutorial** on the audio-sync feature (what -500..+500 means, the
clap-calibration recipe both directions, and that it locks while live). `docs/` currently
holds only the README image — the tutorial is unstarted. Add it to the 1.0/gold-pass checklist.