# LANE E — Audio, FX & UI

**Read first:** `docs/DESIGN.md` (§5–§7), `docs/CONTRACTS.md`, everything in
`src/core/`.
**You own:** `src/audio/**`, `src/fx/**`, `src/ui/**`, `src/demo/audioDemo.ts`,
`demo-audio.html`.
**You may not touch anything else.**

## Mission

Make the booth alive: a fully synthesized house groove in stems that unmute as
the quest progresses, positional audio from the decks, beat events driving
LED pulses/VU meters/particles, all diegetic SFX, and the HUD. "Sound is the
win state" — you own the payoff of the whole game.

## Provides

- `AudioEngine` — WebAudio graph, `init()` on first user gesture,
  `setStemCount(n)` (0–5 stems audible), `setChannelGain(ch, v)` (live fader
  ducking), `setPlaybackState(deck, playing, rpm)` (pitch-bend spin-up/brake),
  `playSfx(name, at?: Vec3)`, positional listener sync `updateListener(view:
  IPlayerView)`.
- `FxSystem` — `update(dt)`, needs `{ scene, setEmissiveBoost }` injected
  (Lane A's hook), owns particles + LED pulse logic + VU animation hooks.
- `Hud` — DOM overlay: crosshair, hotbar (renders Lane D's hotbar model via
  events/injected getter), quest tracker (5 nodes), subtitles line ("SIGNAL
  RESTORED: CROSSFADER — 3/5"), win screen, start/pause overlay (pointer-lock
  gate + "click to drop the needle" splash).

## Consumes

`bus` events (`signal:repair`, `game:win`, `platter:state`, `fader:move`,
`audio:*` are yours to emit, `block:break/place`, `player:step/landed`,
`machine:interact`), `IPlayerView` interface, `blockDef().sound` categories.

## Tasks

### E1. The groove (all synthesized, no samples)
- 118 BPM, 8-bar loop, five stems built from oscillators/noise through a
  shared WebAudio clock (lookahead scheduler, ~25 ms tick — do not use
  `setInterval` naively for note timing; schedule on `AudioContext.currentTime`):
  1. **drums** — kick (sine drop + click), hats (filtered noise, offbeat),
     clap on 2/4
  2. **bass** — detuned saw through lowpass, one-bar riff, sidechain-ducked
     from the kick (a simple gain envelope is fine)
  3. **chords** — filtered saw stabs, minor 7th vamp, ping-pong-ish delay
  4. **lead** — square/sine hook, sparse (every 4th bar), long release
  5. **sweeps** — noise riser + downlifter each 8 bars, vinyl crackle bed
- Stems map to quest progress (`setStemCount`): silence → drums → +bass →
  +chords → +lead → +sweeps/full. Keep musical when partial.
- Vinyl-truth: master through a gentle highshelf + crackle when playing off
  the blue record; `setPlaybackState` bends a master playbackRate-equivalent
  (detune all stems' pitch AND tempo via a global rate param — simplest:
  scale scheduler tempo and per-voice detune together) for spin-up/brake, and
  pitch-fader moves detune ±.
- Positional: stems output into two `PannerNode`s at the deck spindle world
  positions (constants LAYOUT), plus a dry low bed so it never fully
  disappears; `updateListener` follows the player eye/orientation.

### E2. Analysis & beat events
- Drive `audio:beat { energy }` from the scheduler itself (you know when the
  kick fires — no FFT needed; energy = current kick gain), `audio:bar`.
  Additionally an `AnalyserNode` on master for VU levels (expose
  `getLevels(): { low, mid, high }` for FX).

### E3. SFX (synthesized, short)
- Footsteps by `blockDef.sound` category (metal/wood/plastic/soft/glass —
  filtered noise bursts with different bodies), land thump scaled by
  impactSpeed, block break/place clicks per category, fader zip, button
  clunk, RCA "clunk-clunk-CLICK" (rising), fuse zap, tonearm cue lever creak,
  win: needle-drop crackle → the full mix slams in (brief lowpass sweep open).

### E4. FX systems
- **LED pulse**: on `audio:beat`, pulse Lane A's `setEmissiveBoost` (1.0 →
  ~1.8 decay ~150 ms) — but only after ≥1 repair (dead booth = dim: start
  boost at 0.35, step toward 1.0 per repair).
- **VU towers**: Lane C built LED columns at known LAYOUT-derived positions;
  animate them by swapping emissive boost per-level is Lane-A-internal, so
  instead: overlay thin emissive sprite quads you own, positioned over the
  tower faces, height-keyed to `getLevels()`. Same trick for the patch-bay
  blink and signal-path trace on repairs (a moving bright sprite running the
  cartridge→tonearm→cable→mixer path — hardcode the polyline from LAYOUT).
- **Particles**: drifting dust motes in light shafts (a few hundred point
  sprites, gentle brownian), block-break puff at break position, record-rim
  sparkle when a platter spins at 45.
- **Win sequence**: on `game:win` — 2 s: kill boost, silence… then needle
  drop, full mix, boost pulse ×2 for 8 bars, every LED sprite strobing to
  levels, confetti of tiny vinyl-black quads over the decks. Then settle to
  living-booth steady state.

### E5. HUD/UI
- DOM (not canvas): crosshair; hotbar strip (9 slots, block tint swatches +
  counts, active ring); quest tracker top-right (5 icons, lit as repaired);
  event subtitle line; start overlay (title "TURNCRAFT", "click to drop the
  needle") gating AudioContext + pointer lock; pause on lock-loss; win
  banner. Styling: dark, minimal, DJM-font-ish (system mono is fine),
  amber/green LED accent colors from `blocks.ts` tints.

## Demo (`demo-audio.html`)

No other lanes: a mock stage (dark plane, two glowing discs at deck
positions, a camera you orbit) + a control panel: stem count slider 0–5,
play/stop per deck with spin-up bend, pitch slider, channel-fader sliders
(live ducking), every SFX as a button, beat-pulse visual (the discs flash via
the same code path as `setEmissiveBoost` — inject a mock), quest simulator
buttons firing `signal:repair` 1→5 then `game:win` to preview the entire
audio-visual arc, HUD rendered live with a mock hotbar.

## Acceptance

- [ ] Groove runs indefinitely with zero timing drift (scheduler, not
      setInterval); stems musically stack 0→5
- [ ] Spin-up/brake audibly bends tempo+pitch together; pitch fader detunes
- [ ] Walking between decks audibly pans/attenuates (test with camera move)
- [ ] Every SFX distinct and non-clipping; footsteps vary by surface category
- [ ] Beat pulse, VU sprites, dust motes, break puffs all run at 60 fps
- [ ] Full win sequence previews correctly from the quest simulator
- [ ] No audio starts before user gesture; `npm run typecheck` clean; `HANDOFF.md`

## Out of scope

Gameplay logic (D), world/meshes (A/C), player (B). You react to events and
render overlays/sprites you own — you never `setBlock` and never mutate
another lane's materials beyond the provided `setEmissiveBoost` hook.