hyperframes/docs/reference/audio-effects.mdx

---
title: "Audio effects implementation"
sidebarTitle: "Audio effects"
description: "The four audio attributes, every effect and parameter, automation targets, the voiceover carve, audio groups, and how preview and render stay identical."
---

Use this reference when Studio controls are not enough: you need to write a mix
into HTML, know a parameter's exact range, automate a knob, or understand why
something behaves differently in the render than in preview. For everyday use
start with [Mix audio and apply effects](/studio/audio-effects), and for what to
ask an agent for, [Audio effects and mixing](/prompting/audio-effects).

A composition carries its whole mix in the HTML. There is no session file and
nothing to load beside the markup.

## The four attributes

| Attribute | Holds | Shape | Goes on | Read at playback |
| --- | --- | --- | --- | --- |
| `data-fx-chain` | The effects, in signal order | JSON | `<audio>`, `<video>`, `<hf-audio-group>` | Yes |
| `data-automation` | Envelopes on volume or an effect parameter | JSON | `<audio>`, `<video>`, `<hf-audio-group>` | Yes |
| `data-fx-carve` | A voiceover carve's own settings | JSON | `<audio>`, `<video>` | No — see [below](#data-fx-carve) |
| `data-audio-group` | The id of the group this clip belongs to | A plain id, not JSON | `<audio>`, audible `<video>` | Yes |

`data-audio-group` is the odd one: a bare string, and **ignored on a muted
`<video>`** (`muted`, or `data-has-audio="false"`). The other three are JSON. Of those, only `data-fx-chain` and `data-automation`
can also sit on an `<hf-audio-group>`, where they apply to the group's whole bus;
`data-fx-carve` belongs to a clip.

**Write the JSON attributes double-quoted, escaping the JSON's own quotes as
`&quot;` and `&` as `&amp;`** — as every example on this page does. The browser
reads them through `getAttribute` and does not mind either way, but
`carve.mjs` finds them with a `name="..."` regex, so a single-quoted
attribute is invisible to it: a carve reports no existing chain and overwrites
work it could not see.

Nothing static validates a chain. Preview plays an unreadable chain **dry** so
the composition stays workable; the render **fails the whole mix** rather than
shipping a dry track that sounds plausible and is wrong.

### `data-fx-chain`

```html
<audio
  id="narration"
  src="assets/vo.wav"
  data-start="0"
  data-duration="12"
  data-fx-chain="{&quot;version&quot;:1,&quot;nodes&quot;:[
    {&quot;type&quot;:&quot;highpass&quot;,&quot;id&quot;:&quot;n1&quot;,&quot;label&quot;:&quot;Remove Rumble&quot;,
     &quot;params&quot;:{&quot;frequency&quot;:120,&quot;q&quot;:0.707,&quot;poles&quot;:&quot;2&quot;}},
    {&quot;type&quot;:&quot;peaking&quot;,&quot;id&quot;:&quot;n2&quot;,&quot;label&quot;:&quot;Reduce Mud&quot;,
     &quot;params&quot;:{&quot;frequency&quot;:250,&quot;gain&quot;:-3,&quot;q&quot;:1.2}},
    {&quot;type&quot;:&quot;limiter&quot;,&quot;id&quot;:&quot;n3&quot;,&quot;enabled&quot;:false,
     &quot;params&quot;:{&quot;limit&quot;:-1,&quot;attack&quot;:5,&quot;release&quot;:50,&quot;level_out&quot;:0}}
  ]}"
></audio>
```

Unescaped, that chain reads: a high-pass labelled Remove Rumble, a peaking cut
labelled Reduce Mud, and a bypassed limiter.

- **Order is signal order.** Each node processes what the one before produced.
- `type` is an effect id from [the registry](#effect-registry). `params` are in
  the units a person thinks in — dB, ms, Hz. Out-of-range values are clamped on
  read, so a chain that parses is always safe to realise.
- `id` is a stable handle. Automation addresses nodes by id, never by position,
  so reordering a chain cannot re-point a lane at a different effect. A node
  with no id loads fine but cannot be automated. Studio hands out the first free
  `n1`, `n2`, …
- `label` replaces the effect's own name in the rack. Write one whenever a node
  is doing a named job — two `peaking` nodes otherwise show the same row twice
  and you cannot tell the mud cut from the clarity lift.
- `enabled: false` is bypass: the node stays in the chain, out of the signal
  path. Absent means enabled.
- `fromCarve: true` marks a node the carve analysis generated. `fromPreset`
  carries the id of the preset that wrote the node, which is how re-applying a
  preset replaces its own nodes in place. **Do not set `fromCarve` by hand** —
  the next carve deletes exactly those nodes.

### `data-automation`

```html
data-automation="{&quot;version&quot;:1,&quot;lanes&quot;:[
  {&quot;target&quot;:&quot;volume&quot;,&quot;points&quot;:[{&quot;t&quot;:0,&quot;v&quot;:1},{&quot;t&quot;:2.5,&quot;v&quot;:0.4}]},
  {&quot;target&quot;:&quot;fx.n2.gain&quot;,&quot;points&quot;:[{&quot;t&quot;:0,&quot;v&quot;:0},{&quot;t&quot;:1,&quot;v&quot;:-6,&quot;curve&quot;:0.4}]}
]}"
```

That is two lanes: the track's volume dropping to 0.4 by 2.5 s, and node `n2`'s
gain moving to −6 dB by 1 s along a bent segment.

| Field | Meaning |
| --- | --- |
| `target` | `volume` for the track's own level, or `fx.<nodeId>.<param>` |
| `t` | Seconds — on **which clock depends on where the lane lives**, see below |
| `v` | The parameter's own unit — dB for a gain, Hz for a frequency, a linear multiplier for volume |
| `curve` | −1 to 1, bends the segment *leaving* a point; positive holds low then rises late |
| `viaX` / `viaY` | An interior point the segment passes through (progress 0–1, value travelled 0–1); supersedes `curve` when both are present |

### Two clocks

| Lane on | `t: 0` means |
| --- | --- |
| A clip (`<audio>` / `<video>`) | The start of **that clip**. A bed with `data-start="8"` has `t: 0` at composition time 8 |
| A group (`<hf-audio-group>`) | **Composition time.** A group has no `data-start`, so its clock starts at zero |

Getting this backwards is the most expensive mistake on the page: a group lane
written in clip-relative time lands wherever the first member happens to start.
Preview and render agree on both clocks.

### Volume is not capped at 1

`v` on a `volume` lane is a linear gain multiplier with a ceiling of **+12 dB**,
which is about `3.981` — the same ceiling `data-volume` uses. A lane that boosts
above unity is valid and will play; `1` is unity, not the maximum.

A lane holds its first value backwards to the start of its clip and its last
value forward to the end. **A bed that begins before the voice therefore needs an
explicit "no cut" point at `t: 0`, or it starts out already ducked.** Maximum 512
points per lane. A lane whose node is gone is pruned on read rather than
erroring — so a mistyped `nodeId` costs you the envelope silently. Read ids back
out of the chain rather than assuming what was minted.

A `volume` lane and a GSAP tween on `volume` conflict: the lane wins and the
tween is ignored. The linter reports that as `audio_volume_double_automation`.

### `data-fx-carve`

```html
data-fx-carve="{&quot;enabled&quot;:true,&quot;sources&quot;:[&quot;voiceover&quot;],&quot;strength&quot;:0.25}"
```

Unescaped: `{"enabled":true,"sources":["voiceover"],"strength":0.25}`.

- `sources` names **what this bed makes room for** — element ids, or a group id,
  which expands to its current members on every analysis.
- `strength` is 0–1 and derives the whole mechanism.
- `enabled: false` keeps the settings and stops the carve. It exists because a
  bed with exactly one candidate voice is carved by default; with "off" as an
  absent attribute, switching it off would read as never-configured and the
  default would put it back.

This attribute is **not read at playback** — the chain and lanes it produced are
what play. It exists so strength can be changed on an existing carve instead of
being guessed back out of the filters.

Older projects may carry six mechanism numbers (`maxCutDb`, `bands`, `q`,
`intelligibilityBias`, `duckDb`, `headroomDb`) instead of `strength`. They still
load: the depth maps back onto a strength and the rest is re-derived. A stored
carve with no `enabled` reads as on, a single `source` reads as a one-voice
`sources` list, and a stored `dynamic` is ignored — every carve follows the
speech now.

### `data-audio-group`

```html
<style>hf-audio-group { display: none !important; }</style>

<hf-audio-group id="voiceover" data-label="Voiceover" data-volume="1"></hf-audio-group>

<audio id="vo-intro"  data-audio-group="voiceover" src="assets/vo1.wav" …></audio>
<audio id="vo-middle" data-audio-group="voiceover" src="assets/vo2.wav" …></audio>
```

Membership is held by the **member**, not by the group nesting its members, so a
deleted clip simply disappears from the group on the next resolve and nothing
dangles.

- `<hf-audio-group>` is optional metadata: `data-label`, `data-volume`,
  `data-hidden`, and its own `data-fx-chain` / `data-automation`. A group with
  members but no element still resolves, using its id as the label.
- **Audio and audible video.** A `<video>` with sound (`data-has-audio="true"`,
  no `muted`) joins its group in preview and render; on a muted video
  `data-audio-group` is ignored. Groups do not nest.
- `data-audio-group=""` is no group at all.
- The element must be inert. The runtime injects
  `hf-audio-group{display:none!important}` so an unknown custom element cannot
  take a flex gap or shift `:nth-child` — but author the rule yourself so a bare
  preview never lays it out.
- Group ids and element ids share one namespace.

### The master bus: `data-fx-chain` on the composition root

```html
<div id="root" data-composition-id="main" data-width="1920" data-height="1080"
  data-fx-chain="{&quot;version&quot;:1,&quot;nodes&quot;:[
    {&quot;type&quot;:&quot;truepeak&quot;,&quot;id&quot;:&quot;m1&quot;,&quot;label&quot;:&quot;Delivery ceiling&quot;,
     &quot;params&quot;:{&quot;ceiling&quot;:-1.5,&quot;lookahead&quot;:3,&quot;release&quot;:80}}
  ]}"
>
```

The master bus is the `data-fx-chain` on the top-level composition root. It runs
once over the sum of every clip and every group, after each group's own chain and
fader, so a limiter here sees the peak of the whole mix. A clip can belong to
only one group, so this is the way to put one effect across everything.

- Only the **top-level** composition root is read as the master bus. On a clip or
  a group `data-fx-chain` keeps its own meaning, and on a sub-composition's root
  it is not applied while a parent composition contains it. A sub-composition file
  previewed or rendered on its own is the top-level root, so its chain is the
  master bus there.
- There is no master automation: `data-automation` on the root is not read as
  the master's.
- The render sums the tracks to a float WAV, runs the chain, and only then
  encodes AAC, so the chain reads unclipped samples. Preview inserts the chain
  between the clips' sum and the monitor.
- With a master chain on the root, preview plays every audible `<video>` through
  that bus too, as the render does. Without one, a video at volume 1 with no
  chain, group or automation of its own stays on native output.
- An unreadable master chain **fails the whole mix**: the render stops with an
  `audio_processing_failed` error under every strictness setting, and exports
  nothing. Preview plays dry with no message, and no lint rule checks a chain's
  contents, so one typo on the root blocks every export of that composition.
  Check a master chain in preview before rendering.

## Effect registry

Sixteen effects. Values outside a range are clamped on read. **AUTO** marks a
parameter an automation lane can drive; anything unmarked cannot move over time.
`HF_AUDIO_FX` in `@hyperframes/core/audio-fx` is the source of truth.

### Filter — which frequencies a track may occupy

| Effect | Parameters |
| --- | --- |
| `highpass` | `frequency` 20–20000 Hz (300, log) **AUTO** · `q` 0.1–20 (0.707, log) **AUTO** · `poles` `1`\|`2` (2) |
| `lowpass` | `frequency` 100–20000 Hz (8000, log) **AUTO** · `q` 0.1–20 (0.707, log) **AUTO** · `poles` `1`\|`2` (2) |
| `peaking` | `frequency` 20–20000 Hz (1000, log) **AUTO** · `gain` −40–40 dB (0) **AUTO** · `q` 0.1–20 (1, log) **AUTO** |
| `lowshelf` | `frequency` 20–2000 Hz (200, log) **AUTO** · `gain` −40–40 dB (0) **AUTO** |
| `highshelf` | `frequency` 500–20000 Hz (4000, log) **AUTO** · `gain` −40–40 dB (0) **AUTO** |

`q` is bandwidth — higher is narrower. `poles` is the slope: `2` is the usual
biquad (12 dB/oct), `1` is gentler (6 dB/oct). Shelving filters have no `q`; the
Web Audio spec leaves it unused for them.

### Dynamics — how level behaves over time

| Effect | Parameters |
| --- | --- |
| `gain` | `gain` −60–12 dB (0) **AUTO** |
| `compressor` | `threshold` −60–0 dB (−24) · `ratio` 1–20 (4) · `attack` 0.01–2000 ms (20, log) · `release` 0.01–9000 ms (250, log) · `knee` 1–8 (2.83) · `makeup` 0–36 dB (0) · `mix` 0–1 (1) |
| `limiter` | `limit` −24–0 dB (−1) · `attack` 0.1–80 ms (5) · `release` 1–8000 ms (50, log) · `level_out` −24–24 dB (0) |
| `truepeak` | `ceiling` −24–0 dBTP (−1) · `lookahead` 0.5–10 ms (3) · `release` 10–2000 ms (80, log) |
| `gate` | `threshold` −80–0 dB (−35) · `range` −80–0 dB (−24) · `ratio` 1–20 (10) · `attack` 0.01–9000 ms (1, log) · `release` 0.01–9000 ms (100, log) · `knee` 1–8 (2.83) |

Cuts on `gain` reach −60 dB, boosts stop at +12: it is a level stage for making
room, and a chain that could add 40 dB would clip long before that was useful.
`knee` of 1 is a hard corner. `mix` below 1 blends the dry signal back in
(parallel compression). `range` is how far down the gate pulls when closed.

`limiter` is an envelope follower: it reacts to the level of the signal, has no
lookahead, and can let a sample peak or an inter-sample peak through. `truepeak`
is the one to use for a delivery ceiling. It estimates the true peak on a 4x
oversampled signal, reads `lookahead` ms ahead so the gain is already down when
the peak arrives, and holds that estimate at `ceiling`. Channels share one
gain. It delays its output by `lookahead` plus about 0.3 ms (3.3 to 10.3 ms across
the range): the offline render trims that delay, live preview plays it, so the
sound is that much behind the picture. A seek clears the limiter's delay line and
held gain on a group or master bus. Changing `lookahead` rebuilds the node.

The ceiling is held against that 4x estimate, not against the waveform itself. A
4x interpolator cannot see the peaks of content close to Nyquist, so the true peak
can end above `ceiling`. Measured against an ideal 16x interpolation, tones, pink
noise and a dense mix ended at most 0.6 dB over. Full-band white noise driven 8 dB
or more into the limiter ended 1.2 to 1.7 dB over (median 1.4 dB, 28 runs of 10
to 30 s). A meter that also reads at 4x, such as ffmpeg's `ebur128=peak=true`,
reports that same noise about 0.1 dB over, so it will not show the miss. Under a
hard delivery limit set `ceiling` at least 2 dB below it: −3 dBTP for a −1 dBTP
limit.

### Nonlinear — changes the waveform's shape

| Effect | Parameters |
| --- | --- |
| `saturate` | `type` `tanh`\|`atan`\|`cubic`\|`exp`\|`alg`\|`quintic`\|`sin`\|`erf`\|`hard` (tanh) · `threshold` −40–0 dB (−6) · `output` −24–24 dB (0) **AUTO** · `oversample` 1–8× (4) |
| `bitcrush` | `bits` 1–32 (8) · `samples` 1–250× (1) · `mix` 0–1 (1) |

`tanh` is the gentlest curve, `hard` is outright clipping. Higher `oversample`
costs more CPU and keeps aliasing down. `samples` repeats each sample N times — a
crude downsample, which is where the lo-fi character comes from.

### Time — where a track sits, and how it moves

| Effect | Parameters |
| --- | --- |
| `delay` | `time` 1–5000 ms (250, log) **AUTO** · `feedback` 0.01–0.95 (0.35) **AUTO** · `mix` 0–1 (0.4) **AUTO** |
| `reverb` | `size` 0.05–1 (0.7) · `damping` 0–1 (0.5) · `wet` 0–1 (0.35) **AUTO** · `dry` 0–1 (0.7) **AUTO** |
| `chorus` | `delay` 1–100 ms (7) **AUTO** · `depth` 0–10 ms (2) **AUTO** · `speed` 0.01–10 Hz (1) **AUTO** · `mix` 0–1 (0.5) **AUTO** |
| `phaser` | `in_gain` 0–1 (0.4) **AUTO** · `out_gain` 0–2 (0.74) **AUTO** · `delay` 0.1–5 ms (3) · `decay` 0–0.99 (0.4) · `speed` 0.1–2 Hz (0.5) **AUTO** · `type` `0`\|`1` (0) |
| `pitchshift` | `semitones` −12–12 st (0, whole steps) · `mix` 0–1 (1) |

`pitchshift` moves pitch without changing playback speed, which is what
separates it from a playback-rate change. Neither of its parameters can be
automated — it is a worklet.

Reverb convolves a *generated* impulse and both preview and render generate the
same one, so a room is reproducible without shipping an impulse file. Higher
`damping` rolls the top off the tail faster. `feedback` is bounded below 1
because at 1 it never decays.

## Why some parameters cannot be automated

Automation is handed to the audio thread once, as native `AudioParam` ramps and
curves — that is what keeps it sample-accurate and identical between preview and
render. A parameter can therefore only be automated if an `AudioParam` backs it.
Three kinds do not:

| Kind | Effects | Consequence |
| --- | --- | --- |
| Worklet processor options | `compressor`, `limiter`, `truepeak`, `gate`, `bitcrush`, `pitchshift` | **No parameter is automatable at all** |
| A WaveShaper curve | `saturate`'s `type`, `threshold`, `oversample` | Only its `output` stage is a real param |
| A convolution impulse | `reverb`'s `size`, `damping` | `wet` / `dry` are gain stages and automate fine |

**A lane on a non-automatable parameter is silently inert.** To make one of those
behave differently over time, automate a `gain` stage around it instead: a lane
on a `gain` before a compressor changes how hard the compressor is driven, which
is most of what automating its threshold would have done.

## Presets, jobs, and one-knob profiles

Every one of these is a shortcut to a chain you could have built by hand. Open
any of them and you find ordinary effects with their parameters showing.

### Presets

Twenty-two, in four families. Applying one **appends**; re-applying one already
present replaces its own nodes in place, because position is signal order.

| Family | Ids |
| --- | --- |
| Voice | `voice-clean`, `voice-broadcast`, `voice-warm` |
| Repair | `rumble-cut`, `room-gate`, `boom-tame`, `harsh-tame` |
| Character | `telephone`, `radio-am`, `megaphone`, `lofi-tape`, `pa-system`, `intercom`, `doofus-worble`, `chipmunk`, `giant`, `monster` |
| Space | `room-tight`, `room-natural`, `hall`, `slap-echo`, `dub-throw` |

`chipmunk`, `giant`, and `monster` are built on `pitchshift`, so they change the
speaker rather than the channel.

`voice-clean` is Remove Rumble → Reduce Mud → Even Out Loudness → Add Clarity →
Peak Ceiling. `voice-broadcast` is denser and more forward; `voice-warm` adds
body rather than cutting it.

A preset's nodes are wrapped in a wet/dry blend, so `presetAmount` (0–1) fades
the whole thing, and `fx.preset.<id>` is an automation target that ramps it over
time. That is the only way to automate a preset as a unit — its nodes share no
common parameter.

### Jobs

Five named `peaking` filters with the frequency already chosen. Picking the job
is picking the range, which is what makes a single "how much" knob honest.

| Job | Symptom | Sets |
| --- | --- | --- |
| Tame Boominess | Too much chest — it booms | 200 Hz, −4 dB, Q 1.4 |
| Reduce Mud | Muffled, like it is behind cardboard | 250 Hz, −3 dB, Q 1.2 |
| Reduce Boxiness | Sounds like a small room, or a box | 400 Hz, −3 dB, Q 1.4 |
| Add Clarity | Words are hard to make out | 3 kHz, +2.5 dB, Q 1 |
| Soften Harshness | Harsh and tiring to listen to | 3.2 kHz, −3 dB, Q 1.6 |

Writing one by hand, carry the name in `label` — the parameters alone are not
the job.

**Every job also ships inside a preset at identical settings**, which is where
the five came from. So check what a preset already contains before adding a job
on top: `voice-clean` plus a Reduce Mud job is −6 dB at 250 Hz where −3 was
meant.

### One-knob profiles

Five effects have no single parameter that can honestly be their face — a
compressor's threshold means nothing without its ratio. They get a derived
control instead, 0–1, setting several parameters together.

| Effect | Knob | 0 → 1 | Sets |
| --- | --- | --- | --- |
| `compressor` | Evenness | Barely touched → very even, quite squashed | threshold, ratio, attack, release, makeup |
| `gate` | Tightness | Only true silence → cuts quiet words too | threshold, range, release |
| `saturate` | Warmth | Just a sheen → openly distorted | threshold, output |
| `reverb` | Space | A small tight room → a big open hall | size, wet, dry |
| `bitcrush` | Crush | Slightly gritty → destroyed | bits, samples, mix |

Evenness, Warmth, and Space are **level-matched**: the make-up gain, output trim,
and dry leg move with the drive, so turning the knob up does not also turn the
track up. Tightness and Crush are not, because neither has a trim to move.

The chain stores the mechanism values, not the knob position — the knob is read
back by inverting the curve, so hand-editing a parameter under a profile is
allowed and simply moves the knob.

## Voiceover carve

A carve is a **relationship, not an effect**. The settings live on the *bed* —
the track that gets processed — and name the voices to listen to, exactly as a
sidechain compressor does. Never put a carve on a voice track.

`strength` derives six numbers that move together in any real mix: how deep to
cut, how many bands, how wide, how far to favour intelligibility over raw voice
energy, how far the level may drop, and how far under the voice to aim.

| Strength | Result |
| --- | --- |
| `0` | Spectral only — one band, no level match at all |
| `0.25` (default) | A 6 dB dip in three bands with 6 dB of level room |
| `0.5` | The dip reaches 10 dB — where a carve starts being heard as an effect |
| Above `0.5` | Deliberate territory for a loud bed under a quiet voice |

Every value becomes an envelope of the speech's own level: silence leaves the bed
alone, a loud passage pushes the carve to full depth. There is no static mode.

Inside a carved bed the signal runs through the dips first, then the level match,
then anything you built yourself — which is why a limiter you add still acts as
the last ceiling.

Voices are summed onto the bed's clock before anything is measured, so one
analysis covers all of them. Voices that never play while the bed does are left
out; they cannot mask it.

**Name a group, not a run of clip ids.** A list of ids has to be exhaustively
right and stays right only until the next edit — a fourth narration clip added
later plays outside the carve's awareness and the bed silently fails to duck
under it. A `sources` list naming two or more plain clip ids is reported as
`audio_carve_ungrouped_sources`.

There are two arrangements where naming the group is **worse** than naming ids,
and both come from the group form resolving later and wider than the analysis
that wrote it:

| Arrangement | What would happen |
| --- | --- |
| The bed is itself a member of the voice group | The bed is handed to itself as a voice and carved against its own content |
| The group also holds a member classified music or SFX | That member enters the sidechain on a later analysis, ducking the bed under a whoosh |

Neither shows up on the run that writes it — the analysis sums the voices it
detected, so the first pass is correct however wrong the stored attribute is. It
surfaces on the next re-analysis, against a file the previous run declared good.
Keep the bed in its own group.

What a carve writes is an ordinary chain of `peaking` filters plus a `gain`
stage, tagged `fromCarve`, and one automation lane per carved parameter. That
tagging is the whole trick: a re-run replaces the previous carve and leaves every
effect and lane you built by hand exactly where it was.

### From the command line

The script ships with the `media-use` skill; `<SKILL_DIR>` is that skill's folder.

```bash
node <SKILL_DIR>/audio/scripts/carve.mjs --comp index.html
```

That is the whole command — it finds the voice and the bed itself and prints what
it decided:

```text
bed    music-bed (name looks like music)
voice  narration (only track left)
carve  strength 0.25
bands  400Hz -6dB q1.4, 1000Hz -3dB q1.4, 1600Hz -3.17dB q1.4
level  216-point envelope, floor -6 dB
```

Name tracks with `--bed` / `--voice` (repeatable) when the automatic choice is
wrong, `--strength` to push it, `--dry-run` to see the report and write nothing.
It refuses when it cannot tell which track is the bed rather than carving the
wrong one.

**What it records in `sources`.** When every voice it analysed shares exactly one
group, and that group is safe to name, it writes the group id. It falls back to
the individual clip ids — and says so — when the group contains the bed or a
member classified music or SFX, for the reasons in the table above. A group
member classified voice or unknown that this run did not analyse is *not* a
reason to fall back: picking up a clip that starts playing later is the whole
point of naming the group.

Track choice is by name first, using the same classifier Studio's picker uses, so
the two cannot disagree: an id or filename that looks like music (`music`, `bgm`,
`bed`, `score`…) is the bed, and everything else playing over it that is not
SFX-shaped is a voice. Needs `ffmpeg` on `PATH` and `@hyperframes/core`
installed in the project.

## Groups, mute, and solo

A group sums its members through one bus, and the bus carries the group's own
volume, FX chain, and automation. In the render each group is sub-mixed into a
single track at full composition length before entering the main mix, so a
group-level effect hears the sum rather than each member separately.

**Mute and solo are not symmetric, on purpose.**

| Control | Where it lives | Reaches the export |
| --- | --- | --- |
| Mute | `data-hidden` on the group element or the clip | **Yes** — muted members are dropped from the rendered mix |
| Solo ("hear only this") | Studio session state | **No** — it never reaches the engine |

Solo is a listening tool. An element is audible while any solo is active only if
it is itself soloed or its own group is soloed; a group bus is never itself
attenuated by solo, so a soloed member's path out stays open. A group that is not
soloed while one of its members is shows as half-lit — the display-only signal
that some of what is under it still plays.

## Preview and render

Both read the same two attributes through the same builders, which is why
preview predicts the render:

| | Preview | Render |
| --- | --- | --- |
| Context | Live `AudioContext` | `OfflineAudioContext` in the headless browser |
| Unreadable chain | Plays **dry**, so the composition stays workable | **Fails the whole mix** |
| Volume lane | Scheduled on the graph | Baked into the PCM by the mixer |
| Editing an attribute mid-playback | Picked up by a `MutationObserver` | n/a |

Effects with a tail — `reverb`, `delay` — make the rendered track **longer** than
its source, and the mix is told how much. A bed with reverb therefore no longer
ends exactly at its `data-duration`. That is expected.

## What the linter checks

Almost no static gate covers a mix. Three rules exist:

| Rule | Reports |
| --- | --- |
| `audio_carve_ungrouped_sources` | A carve naming two or more plain clip ids instead of a group |
| `audio_volume_double_automation` | A `volume` lane on a track that also has a GSAP tween on `volume` — the lane wins, the tween is ignored |
| `audio_volume_tween_overrides_gain` | An authored `data-volume` on a track whose `volume` is tweened — the tween's values are absolute and replace that gain rather than scaling it |

Nothing validates the chain or the effect lanes at all. What enforces those is
the render. Beyond that, a mix is verified by rendering and listening.