feat(audio): graphic equalizer with presets and custom bands
Adds a 10-band graphic equalizer to AudioSettings (enabled flag + per-band dB gains, normalised to 10 entries and clamped to range). Presets return gain curves; the settings page gains EQ UI. libmpv applies the filter on Linux (Android parity pending). Old persisted settings without EQ fields load as disabled + flat. Also includes the requirements/traceability/ux-flows doc updates for this feature and the home long-press routing (UR-058/DR-087). TRACES: UR-027 | IR-020, DR-030 | UT-079, UT-080, UT-081, UT-082
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# Spec: Audio equalizer
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**Status:** Accepted
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**Requirements:** UR-027 → DR-030 (EQ UI), IR-020 (MPV EQ integration).
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**UX spec:** n/a (extends the Settings › Audio section, ux-flows §8.1 instant-apply).
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**Supersedes / revises:** —
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## Summary
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Add a graphic audio equalizer to playback. Users pick a preset (Flat, Rock,
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Pop, Jazz, Classical, Bass Boost, Treble Boost, Vocal) or set custom per-band
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gains, from a new block in Settings › Audio. On Linux the gains apply live via
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MPV's audio-filter chain; the settings persist and re-apply on the next track
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and at startup, exactly like crossfade/gapless/normalize do today. Android is a
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no-op for now (documented parity gap, same as those three features).
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## Motivation
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UR-027 is one of the few still-unbuilt audio features. The audio-settings
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pipeline it needs already exists — `AudioSettings` + `set_audio_settings` on the
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`PlayerBackend` trait, the `player_set_audio_settings` command, and the Settings
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› Audio UI with instant-apply. Crossfade, gapless, and volume normalization all
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ride that pipeline. The equalizer is the same shape: N more fields on
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`AudioSettings`, an `af` filter on the MPV backend, one more block in the
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settings panel. No new command, no new state machine.
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## Layer assignment
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| Logic / responsibility | Layer | Why it belongs there |
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|------------------------|-------|----------------------|
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| EQ band count, centre frequencies, gain range/clamping | Rust | Domain of the audio engine; the bands must match what the MPV filter expects. Changing the DSP must not require a frontend change. |
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| Preset name → per-band gain curve | Rust | A preset *is* a domain gain curve, not a label. It changes with the audio engine's band layout, never with the UI. Placing it in the frontend would be the scoped-search taxonomy mistake again (values that look like config but are domain data). |
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| Translating gains → MPV `af` filter string | Rust | Platform playback detail; lives with the other `set_audio_settings` filter code in `mpv_backend.rs`. |
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| Persisting the chosen settings, re-pushing on load | Rust/existing | Same path crossfade/etc. already use; the controller re-applies `AudioSettings` per track. |
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| Rendering band sliders, the preset chips, live readouts | Frontend | Pure presentation; changes only if the settings UI is redesigned. |
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| Which preset chip is highlighted; instant-apply on change | Frontend | Presentation/input handling (UR-057), the same as the normalize preset picker. |
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Tie-breaker note: the preset→curve map is the one tempting boundary leak. It goes
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in Rust because a preset is a set of band gains defined *by the band layout*,
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which is an engine property. The frontend only ever names a preset and renders
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the resulting gains; it never defines them.
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## Design
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### `AudioSettings` (Rust, `settings.rs`)
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Add two fields (both `#[serde(rename_all = "camelCase")]` via the existing
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struct attribute):
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```rust
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/// Equalizer enabled. When false, no `af` EQ filter is applied.
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pub equalizer_enabled: bool,
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/// Per-band gains in dB, one per FIXED band (see EQ_BANDS). Length is
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/// validated/normalised to EQ_BANDS.len(); clamped to [-12, +12] dB.
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pub equalizer_bands: Vec<f32>,
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```
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Fixed 10-band ISO layout (domain constant in `settings.rs`):
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```rust
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pub const EQ_BANDS: [f32; 10] =
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[31.0, 62.0, 125.0, 250.0, 500.0, 1000.0, 2000.0, 4000.0, 8000.0, 16000.0];
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pub const EQ_GAIN_MIN: f32 = -12.0;
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pub const EQ_GAIN_MAX: f32 = 12.0;
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```
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- `Default`: `equalizer_enabled: false`, `equalizer_bands: vec![0.0; 10]` (flat).
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- New `with_equalizer_normalised(self)` clamps each gain to `[EQ_GAIN_MIN,
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EQ_GAIN_MAX]` and pads/truncates the vec to 10 bands. Applied in the command
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alongside `with_crossfade_clamped` (add that call too — it's currently missing).
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- Backward compat: both fields `#[serde(default)]` so old persisted JSON loads.
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### Presets (Rust, `settings.rs`)
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```rust
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#[derive(specta::Type, Serialize, Deserialize, Clone, Copy, PartialEq)]
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#[serde(rename_all = "camelCase")]
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pub enum EqPreset { Flat, Rock, Pop, Jazz, Classical, BassBoost, TrebleBoost, Vocal }
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impl EqPreset {
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/// The 10-band gain curve (dB) for this preset.
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pub fn gains(&self) -> [f32; 10] { /* table */ }
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}
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```
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Preset selection is a *frontend* convenience: tapping a chip sets
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`equalizer_bands = preset.gains()` and pushes settings. The curve tables live in
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Rust; the frontend reads them via a tiny `player_get_eq_presets` command
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returning `Vec<(EqPreset, Vec<f32>)>` (or a map), so the frontend never encodes
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the numbers. (If exposing the whole table is awkward through specta, expose
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`player_eq_preset_gains(preset) -> Vec<f32>` instead — pick at implement time.)
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### MPV application (Rust, `mpv_backend.rs::set_audio_settings`)
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Build an `equalizer` / `anequalizer` filter from the bands and set the `af`
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property. When `equalizer_enabled` is false or all gains are 0, clear the EQ
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filter (leave any other `af` entries intact). Use `af add`/`af remove` or a
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rebuilt `af` string; keep it isolated so it doesn't stomp a future crossfade
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filter. Errors map to `PlayerError` like the gapless code.
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### No new persistence table
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`AudioSettings` is already round-tripped by the frontend settings store and
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re-pushed via `player_set_audio_settings` on change and on load. The two new
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fields ride along. `NullBackend`/Android inherit the trait default (no-op).
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### Wire summary
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- Command names unchanged: `player_set_audio_settings`,
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`player_get_audio_settings` (now carry the EQ fields).
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- New (optional) read-only command for preset curves — kebab n/a (it's a
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command): `player_get_eq_presets` (or `player_eq_preset_gains`).
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- Regenerate `bindings.ts` from the Rust types; never hand-edit.
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## Out of scope
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- Android/ExoPlayer EQ (parity gap tracked with crossfade/gapless/normalize).
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- Per-track or per-library EQ profiles — one global profile only.
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- Automatic loudness/room correction; only manual bands + presets.
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- Changing the crossfade/normalize TODOs in `set_audio_settings` beyond wiring
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the missing `with_crossfade_clamped` call.
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## Acceptance criteria
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- [ ] Settings › Audio has an Equalizer block: enable toggle, preset chips, 10
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band sliders with live dB readouts, instant-apply (no Save button).
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- [ ] Choosing a preset sets the bands from the Rust-defined curve; editing a
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band switches the highlighted preset to "Custom" (frontend-only label).
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- [ ] Gains clamp to [-12, +12] dB; the band vector always normalises to 10.
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- [ ] On Linux, enabling EQ audibly changes output and persists across tracks
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and app restart; disabling clears the filter without affecting other audio.
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- [ ] Old persisted settings (no EQ fields) load without error, defaulting flat.
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- [ ] `bun run check` and `bun run test` pass.
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- [ ] `cargo fmt` clean, `cargo clippy` clean, `bun run test:rust` passes.
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- [ ] `bun run check:boundary` passes (no preset curve numbers in the frontend).
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- [ ] New requirement-implementing code carries `// TRACES:` comments.
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- [ ] `bindings.ts` regenerated.
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## Testing
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- Rust (`settings.rs`): default is flat + disabled; `with_equalizer_normalised`
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clamps out-of-range gains and pads/truncates band length; serialization
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round-trips the camelCase fields; backward-compat load of pre-EQ JSON; each
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preset returns a 10-length curve; Flat is all zeros.
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- Rust IPC param naming for any new command (camelCase rule per CLAUDE.md).
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- Frontend (`settings` page or an extracted helper): selecting a preset sets the
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expected band array; editing a band flips the label to Custom; enable toggle
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gates the sliders. Keep DSP untested on the frontend (it's Rust's).
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## TRACES
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- `AudioSettings` EQ fields + normalise + presets: `UR-027 | DR-030` (+ unit tests)
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- MPV EQ filter application: `UR-027 | IR-020`
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- Settings EQ UI block: `UR-027 | DR-030`
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- Preset-curve command: `UR-027 | DR-030`
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## Notes for the implementer
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- A parallel Claude session is active in this repo (it has touched
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`tauri.conf.json`, `Dockerfile`, `package.json`, home components, and added
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build scripts, and the Rust build is currently broken by its
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`tauri.conf.json` bundle-target change). `git diff` before "repairing"
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anything you didn't write; keep EQ changes isolated to `settings.rs`,
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`mpv_backend.rs`, `backend.rs` (trait default already covers it),
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`commands/player/settings.rs`, and the settings page.
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- Mirror the volume-normalization block in the settings page for the toggle +
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preset-picker pattern; mirror the gapless code in `set_audio_settings` for the
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MPV property handling.
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- Confirm the exact MPV filter name available in the linked libmpv
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(`equalizer` vs `anequalizer`/`superequalizer`) before committing the filter
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string; gate cleanly if unavailable.
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