# WAI Extension: Object-Based Spatial Audio

> Status: Draft. Phase 1 of the cargo roadmap. The audio analog of
> interactive-worlds: ship sound *objects* + a scene, render to the
> listener's own layout. Conformance is **mixdown-equivalence** (bit-exact
> PCM for a declared layout), the audio counterpart of replay-equivalence.
> Reference impl: the `audio_scene` feature of `wai-rs` (mixer + receipt);
> conformance corpus: `audio-conformance/`; live sink: `/audio`.
Keywords **MUST**, **MUST NOT**, **SHOULD**, **MAY** are RFC 2119/8174.

## 1. Scope and model

A `wai.audio.scene` object describes a set of **audio objects** — each a
content-addressed source plus a position trajectory and gain — that a
sink renders to *its own* speaker layout. Today's audio ships a fixed
channel mix (stereo, 5.1); this ships the scene and lets the sink make
the mix it can actually play. It is lever 3 (instructions-at-the-sink)
for hearing.

In scope: the scene container, the deterministic mixdown render
contract, the **mixdown-equivalence** conformance criterion, the
fallback floor, and the receipt profile.

Out of scope: a transport; the source codecs (sources are ordinary
`wai.audio.*` WAI objects, decoded by the existing path); HRTF design
(an HRTF is named as a requirement-of-existence, never shipped).

Relationship to the core spec: this mirrors interactive-worlds exactly.
A scene is a content-addressed artifact; sources are content-addressed
WAI assets (§4.1 discipline, domain `wai:asset\x01`); the render is
deterministic under a declared numeric profile; receipts are a JWP
group profile. `media` takes the informational value `"audio-scene"`;
the capability `wai.audio.scene` is authoritative.

## 2. Container (`WSCN`)

```
"WSCN" | u16 n_sections | section table | sections
        | n × (u8 kind, u32 off, u32 len), 8-byte-aligned bodies
```

| kind | section | required |
|---|---|---|
| `0x01` | scene contract (canonical JSON) | REQUIRED |
| `0x02` | object table | REQUIRED |
| `0x03` | asset table (§4.1, the source `wai.audio.*` objects) | REQUIRED |
| `0x04` | seal: duration + final mixdown hash + Ed25519 sig | OPTIONAL (a sealed scene pins its reference mix) |

Unknown section kinds MUST be ignored (forward compat within MINOR).

### 2.1 Scene contract (canonical JSON)

```json
{
  "sample_rate": 48000,
  "numeric":     "wai.det.fixed64",
  "pan_law":     "wai.audio.pan.sqrt_constpower",
  "distance":    "wai.audio.dist.inverse_clamped",
  "duration":    240000,
  "floor_layout":"stereo"
}
```

`numeric` is the same Q32.32 fixed-point floor as worlds — the mixdown
is bit-exact on every machine. `floor_layout` (`stereo`) is the
mandatory render target; richer layouts (`5.1`, `binaural@<hrtf-id>`)
are OPTIONAL and, for binaural, perceptual (the HRTF is a
requirement-of-existence, not bit-exact across HRTFs).

**Reader limit.** A reader MUST NOT refuse a scene for its length while `duration` is
at most 67 108 864 (2²⁶): its stereo floor mixdown is then at most 2²⁷ samples, the audio
limit of SPEC §2. It MAY refuse one past it (`duration_too_long`), when it reads the
contract, before it renders anything. A richer layout is a render the sink chooses, at
its own cost. A conforming encoder writes no scene past the limit.

### 2.2 Object table

`n_objects u32`, then per object:

```
source_hash (32 B)        // content hash of a wai.audio.* WAI envelope (asset table)
gain        (i64 Fx)
n_keys      (u16)
keys: n × ( pos_sample u64 | x i64 | y i64 | z i64 )   // listener-relative position, Fx metres
```

Position between keyframes is linear-interpolated in `Fx` (so it is
reproducible). One keyframe ⇒ a static source.

## 3. Render contract (mixdown)

For output channel `c` and sample `t`, in `Fx`:

```
out[c][t] = Σ_objects  src_o[t] · gain_o · dist_atten(|p_o(t)|) · pan_gain(p_o(t), c)
```

- `src_o[t]`: object `o`'s source as mono PCM at `sample_rate`. v0
  requires sources already at `sample_rate` (a deterministic resampler
  is a registered sub-detail, not yet in the reference impl).
- `dist_atten(d) = 1 / (1 + d)` clamped to `[0,1]` (`inverse_clamped`),
  where `d = sqrt(x² + y² + z²)`.
- `pan_gain` for `sqrt_constpower` stereo — chosen to be exactly
  computable in the `Fx` floor (**sqrt only, no transcendental
  tables**): `pan = clamp(x / (|x| + |z|), -1, 1)` (0 when `|x|+|z|=0`),
  then `gL = sqrt((1 - pan)/2)`, `gR = sqrt((1 + pan)/2)` — constant
  power (`gL² + gR² = 1`).
- Accumulate in `Fx`, then quantize to the canonical output (interleaved
  **i16 LE**) by round-half-away-from-zero in integer space.

## 4. Conformance — mixdown-equivalence

> Given the same scene and the same **declared output layout**, a
> conforming sink MUST produce the identical canonical PCM, hence the
> identical `BLAKE3` over it — on every machine, with no tolerance
> parameter.

```
mixdown_hash = BLAKE3("wai:audio-mixdown\x01" || layout_id || pcm_i16le)
```

The mixdown is a function of the sources' decoded PCM, so the hash is portable
only as far as that PCM is. A source whose capability promises the same bytes on
every sink (`wai.audio.flac`, an integer codec) keeps it portable; a source decoded
through a codec whose standard conforms decoders to a tolerance (`wai.audio.opus`,
SPEC §7, `StandardTolerance`) may decode differently on two conforming sinks, and
then so does the mixdown. The reference registry reports this:
`codecs::is_cross_decoder_exact("wai.audio.scene")` is false, because a scene's
payload names its sources by hash and not by capability, and
`codecs::is_cross_decoder_exact_with_sources` is true once the capability of
every source envelope is known and each promises the same bytes on every sink. It
answers false when it cannot tell: when no source is given (a scene with no
objects included), or when a source is not a `wai.audio.*` capability (§2.2).

The `stereo` floor is mandatory and bit-exact. `binaural@<hrtf>` is
perceptual (conformance is similarity, not bytes — a sink advertising
it states the metric, like the `wai.semantic.*` path). The conformance
corpus ships scenes + the expected `stereo` mixdown hash; pass = hash
equality.

## 5. Fallback floor

`model_requirement.fallback` SHOULD name `wai.audio.opus` (or `flac`),
and the encoder SHOULD ship the `WAI2` multi-rendition form: the scene
plus a pre-rendered stereo recording. A sink without the scene runtime
plays the recording. Scene-capable sinks render to their real layout;
below-floor sinks hear a fixed mix — both opened one envelope.

## 6. Session receipts (JWP profile)

A render session receipts as a JWP group (jwp-receipts.md) — the worlds
`SessionReceipt` transposed to audio (`SceneReceipt` in the reference
impl):
- identity `scene_hash` = `BLAKE3("wai:audio-scene\x01" || contract_bytes
  || object_table_bytes)` (the `world_hash` analog).
- the group's objects = the rendered mix's **PCM blocks** (fixed span of
  frames), each `BLAKE3("wai:audio-block\x01" || block_index || i16le)`,
  **Merkle-bound** with JWP's exact leaf/node domains.
- one Ed25519 signature (verified as SPEC §7 *Signatures* requires) over a
  profile payload carrying `scene_hash`,
  the Merkle root, the block list, `work_mac` (= output samples ×
  objects mixed — exact, signed) and `joules_micro` (carried; the native
  `wai_world_meter` pattern supplies a measured figure, `0` if
  unmetered), with the figure's acquisition class as an optional signed
  label ([energy-measurement](energy-measurement.md) §2.8) — absent, the
  receipt keeps its legacy bytes and the figure is unlabelled.
- `parent_receipt_hash` chains successive renders into an append-only
  signed timeline, exactly as worlds' receipts do.

## Appendix — why this is the first cargo class

Mixdown-equivalence is replay-equivalence with the tick replaced by the
audio sample; the `Fx` floor, the content-addressed assets, the seal,
the receipt profile, and the fallback floor are all reused verbatim. A
spatial-audio scene is, structurally, a world whose "sim" is a mixer,
so the interactive-worlds rules carry over to a second sense.