244367ccb9
§11 — annotated Phase 7 and Phase 8 (a–e) inline with what landed and where, so the section now reads as a commit-log index rather than a forward-looking todo. The "Tracked follow-ups" subsection keeps the two trigger-gated dormant items (ephemeral overlay, sub-ms dispatch primitive) and strikes through the filter-playback BUSY spike — 8e's ~3 min release-build capture didn't reproduce the ~8×-baseline outlier pattern from the original 6c smoke finding, so the work to be done collapsed to "instrumentation kept, no code change." §11 preamble now notes "all planned phases (0–8) are done as of 2026-05-21"; §12 picks up the same theme by pointing to team-memory `headroom-project` for current per-risk status. Memory bumps go to ~/.claude-amaan: `headroom-project` description + Phase 7 entry + revised "How to apply" (no more "next planned work"), `headroom-routing-link-bug` moves both Phase 4k "still-open follow-ups" to a "Closed (4l)" section, and MEMORY.md's project hook is updated to reflect "all phases shipped, audio threads validated alloc-free, packaging modules in place".
1127 lines
53 KiB
Markdown
1127 lines
53 KiB
Markdown
# Headroom
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A Rust AGC + compressor + true-peak limiter for PipeWire. Per-application
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exclusion, profile-based presets, single-binary daemon, scriptable over a
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Unix-domain socket.
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This document is the canonical plan. It supersedes the earlier
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conversational sketch.
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---
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## 1. Goals & non-goals
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### Goals
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- **Hard safety net on the processed route.** Audio routed through
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`headroom-processed` is guaranteed to leave the filter below a
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configurable ceiling (default **−0.1 dBTP**) with proper inter-sample
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peak handling. The guarantee is enforced inline in the filter,
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downstream of every control-plane code path, and survives daemon
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misbehaviour, profile reloads, and bad routing decisions. Streams
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routed `bypass` ride the real sink directly and are **not** subject
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to this contract (see §2 path ①); the contract also does not extend
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to whatever resampling or post-processing the downstream device
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path applies after the filter's output.
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- **Per-application exclusion.** Music players, games, and DAWs route
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around the processor; browsers, voice chat, and "everything else" go
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through it. Rules are app-level and live in profiles.
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- **Drop-in defaults.** First-run experience: install, enable user
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service, done. No mandatory config. Power users edit TOML or use the
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CLI.
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- **Profiles** for distinct listening scenarios (default / night /
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speech / transparent / bypass-all).
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- **Single binary.** Daemon, filter, routing, and control loop all live
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in one process. The DSP kernels are a separate crate so they can be
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reused (LV2/standalone) later.
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- **Scriptable.** Unix-domain-socket IPC with a documented JSON schema
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so anyone can write an alternative client (Qt/QuickShell panel, Eww
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widget, scripts). A first-party Rust crate (`headroom-ipc`) wraps it.
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- **Rust, lean dep tree.** No NIH where mature crates exist, no bloat
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where they don't.
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### Non-goals (v0)
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- Surround / >2-channel content. v0 is stereo only; >2ch is routed
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directly to the real sink, untouched by Headroom's filter chain.
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- LV2/CLAP plugin distribution. The DSP crate is plugin-shaped so this
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is cheap to add later, but it's not a v0 deliverable.
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- GUI. Third parties can build one against the IPC.
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- Capture-side processing (microphone). v0 is playback only.
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---
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## 2. Architecture
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Each app's audio takes one of four end-to-end paths, chosen by two
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**orthogonal** profile flags: a routing decision (processed vs.
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bypass) and a per-app level-control flag (on vs. off).
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```
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┌─── optional, opt-in per app (Layer A) ────────────────┐
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│ │
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│ ┌─► passive tap ─► peak + RMS ─► AppLevelController │
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│ │ (sibling link in same quantum) │ │
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│ │ │ │
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│ │ Props.channelVolumes write ◄──────┘ │
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│ │ │
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└───┼───────────────────────────────────────────────────┘
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│
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│ APP STREAM NODE
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│ ┌──────────────────────────┐
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│ │ raw output │
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app's audio ───►├──►│ × channelVolumes │──► output port
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│ └──────────────────────────┘
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│ │
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└────────────────────────────────────────────│
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│
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routing decision (Layer B) │
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target.object set by daemon │
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│
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┌─────────────────────────────────────────┴┐
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▼ ▼
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route = "bypass" route = "processed"
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target.object = target.object =
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preferred_real_sink headroom-processed
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│ │
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│ ▼
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│ ┌─────────────────────┐
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│ │ headroom-processed │
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│ │ (virtual sink, the │
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│ │ system default) │
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│ └─────────┬───────────┘
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│ ▼
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│ ┌─────────────────────┐
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│ │ headroom-filter │
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│ │ (pw_stream pair) │
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│ Layer C (bus DSP) │ AGC → compressor │
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│ │ → soft → hard │
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│ └─────────┬───────────┘
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│ │
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▼ ▼
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preferred_real_sink ◄──────────────────────► (DAC)
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```
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### The four end-to-end paths
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| | Routing = bypass | Routing = processed |
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|---|---|---|
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| **per-app off** | ① **true bypass** — Headroom touches nothing on the signal path. Same latency as if Headroom weren't installed. | ③ **bus DSP only** — stream flows through `headroom-processed` and the inline chain. `channelVolumes` left at whatever the user/app set. |
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| **per-app on** | ② **per-app only** — level-reactive `channelVolumes` writes, no graph hop. Zero added signal-path latency. | ④ **full stack** — per-app level control *and* bus DSP. Maximum protection. |
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Path-by-path properties:
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| Path | Signal-path latency added | Limiter contract? | Per-app gain ride? |
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|---|---|---|---|
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| ① bypass / per-app off | 0 | no | no |
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| ② bypass / per-app on | 0 | no | yes (Layer A) |
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| ③ processed / per-app off | filter hop + ~2 ms lookahead | yes (Layer C hard tier) | no |
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| ④ processed / per-app on | filter hop + ~2 ms lookahead | yes (Layer C hard tier) | yes (Layer A) |
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The two flags are independent. A competitive game's typical config
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is ①: zero Headroom involvement in its audio. A user concerned about
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notification dings on top of that game would put Discord on ② or ④
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(so notifications get tamed via Discord's own `channelVolumes`)
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while leaving the game on ①.
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```
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headroom-core (daemon, one process)
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• per-app level controllers (Layer A)
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• routing engine + preferred_real_sink (Layer B)
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• slow AGC loop, profile manager (Layer C)
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• IPC server
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│
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▼
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$XDG_RUNTIME_DIR/headroom/control.sock
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│
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┌───────────┴───────────┐
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▼ ▼
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headroom CLI third-party clients
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(Qt panel, widgets, …)
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```
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See §4 for Layer A's mechanics and §5 for the PipeWire-level details
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of Layers B and C.
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### One virtual sink, one daemon process
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- `headroom-processed` — virtual sink. Set as the system default so
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new streams land in it by default. Its monitor is captured by
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`headroom-filter`, pushed through the DSP graph, and emitted to the
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current `preferred_real_sink`.
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- **No bypass sink.** Streams marked `route = "bypass"` are pointed
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directly at `preferred_real_sink` via a `target.object` metadata
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write. They pay zero added latency vs. running without Headroom
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installed at all — there's no extra graph hop, no extra DSP. The
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word "bypass" in the profile DSL means "route directly to the real
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sink, untouched."
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- The **daemon** owns:
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- the one virtual sink (created on startup, torn down on exit);
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- the filter (a pair of `pw_stream`s — capture + playback — running
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on PipeWire's realtime audio thread, with the playback half
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targeting `preferred_real_sink`);
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- one **`AppLevelController`** per managed app stream (§4), each
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with its own passive `pw_stream` tap, peak/RMS envelopes, and
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`Props.channelVolumes` writer. Created/destroyed on stream
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lifecycle events.
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- **`preferred_real_sink` tracking.** The daemon watches the
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`default.audio.sink` metadata key. When the user changes the
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system default (via pavucontrol, `wpctl set-default`, etc.) to a
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hardware sink, the daemon (a) treats that sink as the new
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`preferred_real_sink`, (b) re-links `headroom-filter`'s playback
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stream to it, and (c) rewrites `target.object` for every
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currently-bypassed stream so they follow. Hotplug / Bluetooth
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handoffs use the same machinery.
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- the slow AGC loop (reads loudness, writes gain target into the
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filter via an `rtrb` channel);
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- the routing engine (subscribes to the PipeWire registry, evaluates
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rules on new streams, writes `target.object` to the `default`
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metadata: either `headroom-processed` for processed streams or
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`preferred_real_sink` for bypassed streams);
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- the IPC server.
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### Why no `headroom-bypass` sink
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An earlier iteration of the design had a second virtual sink
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(`headroom-bypass`) that loopback'd to the real sink, so "bypassed"
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streams routed to it. This added one PipeWire quantum of latency to
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every bypassed stream for no functional benefit — `module-loopback`
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buffers across the quantum boundary even when the DSP is a no-op.
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Direct routing via `target.object` skips the hop entirely. The win is
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real for competitive games, DAW monitoring, and music players: they
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now ride exactly the same path they'd take if Headroom weren't
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installed.
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### Why this is *not* the "analytical sink + adjust master volume"
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### shape originally proposed
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Volume control via SPA `Props` updates is not sample-accurate. A true-peak
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limiter needs a small internal delay line so gain reduction is applied
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to the same samples that were analyzed. Therefore the **brickwall must
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be inline**. The analytical-monitor approach is still used — for the
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*slow* AGC loop, where multi-second time constants make control-plane
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latency irrelevant — but it cannot own the ceiling.
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### Why a `pw_stream` pair, not an LV2 plugin in `module-filter-chain`
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LV2 is not native to PipeWire; it's one of several plugin formats
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`module-filter-chain` happens to host (via lilv). Using LV2 would split
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Headroom into a plugin + a daemon + a filter-chain JSON, pull in a lilv
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runtime, and force gain-target updates through a 32-bit-float control-port
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abstraction. A `pw_stream` capture+playback pair is the same pattern
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`module-filter-chain` itself uses internally, but written directly in
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Rust against `pipewire-rs`, in the same process as the rest of the
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daemon. One binary, no IPC for parameter updates, idiomatic Rust audio
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thread. An LV2 wrapper of `headroom-dsp` remains a viable optional
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deliverable for use in DAWs.
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---
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## 3. DSP
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### 3.1 Two-tier true-peak limiter (`headroom-dsp::limiter`)
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The limiter has **two parallel tiers** sharing the same upsampler,
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downsampler, delay line, and sliding peak buffer. Both run at the
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oversampled rate.
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**Hard tier — the safety contract.** Output ceiling default
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**−0.1 dBTP**, configurable. Instant attack on the gain envelope plus a
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brief hold and a slow release. Two defensive `clamp` stages downstream
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(once in the oversampled domain, once at the input rate after
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downsampling) guarantee the contract numerically — the envelope can
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misbehave and the contract still holds. Never bypassed, never
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disabled.
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**Soft tier — the comfort cap.** Targets a *dynamic* ceiling computed
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as `program_lufs + max_psr_db`. Smooth attack/release envelope so the
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gain reduction sounds like volume riding, not a slap. Pulls transients
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to a comfortable peak-to-loudness ratio (default 14 dB) *before* they
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ever threaten the hard ceiling. When the AGC hasn't yet provided a
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program loudness (startup, after reset), the soft tier falls back to a
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static ceiling. Disabled by omitting `[limiter.soft]` in a profile —
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useful for the `transparent` profile where users want pure brickwall
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behavior.
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Algorithm (per oversampled sample, after upsampling):
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1. Push raw `|s|` into the sliding-window peak buffer; read the
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max-of-window.
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2. **Soft tier** computes target = `soft_ceiling / window_peak` (clamped
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to ≤ 1), runs through the smooth attack/release envelope, yields
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`soft_gain`.
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3. **Hard tier** predicts the worst-case effective peak after the soft
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tier acts (max of `window_peak * soft_gain` and the asymptote
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`min(window_peak, soft_ceiling)`), then sizes `hard_target` to keep
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that under the hard ceiling. Instant attack, hold, exponential
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release. Yields `hard_gain`.
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4. `total_gain = min(soft_gain, hard_gain)`.
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5. Multiply the delayed sample by `total_gain`.
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6. Clamp at hard ceiling (defense-in-depth).
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7. Downsample, clamp again at hard ceiling at the input rate.
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When the soft tier is doing its job, the hard tier's "predicted-post-soft"
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target stays above 1.0 and the hard tier never engages. When the soft
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tier is mid-attack (peak just arrived), the hard tier snaps in as a
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safety, then releases as the soft tier catches up.
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The compressor and AGC stages run *before* the limiter.
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### 3.2 Feed-forward compressor (`headroom-dsp::compressor`)
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Standard shape: log-domain detector (peak or RMS, switchable) →
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ratio + soft knee → attack/release envelope smoother → makeup gain →
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linear gain → apply to (small) delayed input. ~150 lines of clean code.
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Defaults aimed at "gentle, transparent": threshold −24 dBFS,
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ratio 2.5:1, knee 6 dB, attack 10 ms, release 100 ms, makeup auto.
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### 3.3 Slow AGC (`headroom-core::agc`)
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Algorithmic descendant of EasyEffects' `autogain.cpp`. Runs *outside*
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the audio thread, on a ~50 ms control tick.
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- Feeds the audio thread's monitor tap into `ebur128` with
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`Mode::M | S | I | TRUE_PEAK`.
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- Computes `target_gain_dB = target_lufs − measured_lufs`.
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- Smooths with separate attack/release coefficients (leaky integrator).
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- Gates when momentary loudness < silence threshold.
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- Soft-clamps so the AGC can never push more than ±N dB (profile knob).
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- Writes the new gain target into the audio thread via an `rtrb` queue.
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The AGC's gain is applied *before* the compressor. The compressor and
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limiter still own their own behaviour and ceilings.
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### 3.4 Measurement: `ebur128`
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`Mode::M | S | I | TRUE_PEAK`. EBU TECH 3341/3342 conformant via the
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`ebur128` crate. Constructed on the daemon thread; fed from a ring-buffer
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consumer that pulls from the audio thread. The audio thread allocates
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nothing.
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This is **bus-level** measurement only — used to drive the slow AGC
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loop and meter the processed sink output. Per-app measurement (§4)
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uses a different, much cheaper metric.
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---
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## 4. Per-application level control (Layer A)
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An opt-in, near-zero-latency feedback loop that watches each managed
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application's output stream and adjusts its `Props.channelVolumes`
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multiplier in response to **two parallel level metrics**:
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- a **fast peak envelope** that catches short bursts and sustained
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loud passages (think: a notification ding, a video that just got
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louder), and
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- a **slow RMS envelope** that catches *sustained loudness*
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mismatches (think: "Discord is permanently louder than everything
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else even when nobody's shouting").
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A stream's applied gain reduction is `max(peak_reduction,
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rms_reduction)` — whichever path is asking for more cut wins, and
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recovery only happens when *both* paths agree the stream has settled.
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This is the layer's whole point: the peak path handles transients
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within one quantum; the RMS path keeps long-term inter-app loudness
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balanced. Neither alone is enough.
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Orthogonal to bus routing — a stream can be processed *or* bypassed
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*and* level-controlled independently. Its goal is "tame noisy apps
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without startling the listener and without making the chronic
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loudmouth permanently dominate," while the signal path itself stays
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untouched.
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### 4.1 Why this is zero-latency
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The per-app multiplier is the `channelVolumes` value PipeWire already
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applies inside the app's stream node — it's the same number
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`pavucontrol`'s per-app slider writes to. Adjusting it doesn't insert
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a graph node; nothing new sits between the app and its destination
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sink. The only cost is that **the analysis happens via a sibling
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fanout link**, not in the playback path: PipeWire schedules fanout
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consumers in parallel within the same quantum, so the playback path's
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timing is identical to the no-tap case.
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```
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┌──► passive tap (analysis only)
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│ │
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│ ▼
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│ peak + RMS envelopes
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│ (audio thread, sub-ms)
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app stream ──────┤ │
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(output port) │ ▼
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│ rtrb push
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│ │
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│ ▼
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│ AppLevelController (daemon thread)
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│ │
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│ │ Props.channelVolumes write
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│ ▼ (back into the app stream node)
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│ ┌─────────────────────┐
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└──►│ app stream multiplies
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│ by channelVolumes, │──► (its sink — Layer B)
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│ then publishes. │
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└─────────────────────┘
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```
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### 4.2 The metrics: peak + RMS, no LUFS
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LUFS is the wrong measurement here. Its shortest window (momentary,
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400 ms) blurs out exactly the transients we want to catch, and the
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K-weighting filter adds CPU for no benefit when we're trying to react
|
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fast. We also explicitly want a *second* path that targets sustained
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loudness — for that, plain mean-square RMS is the right cheap stand-in,
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not LUFS.
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||
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| Metric | Window | Job |
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|---|---|---|
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| **Peak envelope** — `max(\|samples\|)` per block, smoothed | ~100 ms attack window, ~500 ms release | Fast: catches a notification ding, a clip getting louder, a partner standing up and shouting. Triggers cut on `peak_threshold_db` (default −6 dBFS). |
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| **RMS envelope** — block mean-square, smoothed | ~1–2 s | Slow: catches "this app is just chronically louder than everything else." Triggers cut on `rms_target_db` (default ≈ −20 dBFS RMS). |
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Both are computed from the *same* raw buffer in the audio thread, so
|
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the audio-thread cost is one additional MAC accumulator and a max-
|
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scan per sample. Cost analysis in §4.7.
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|
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### 4.3 Architecture
|
||
|
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For each managed playback stream (matched by routing rule — see §6):
|
||
|
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1. **Audio thread (tap stream's process callback):**
|
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- Pull the buffer from the fanout link.
|
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- `peak = max(|samples|)` over the block.
|
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- `mean_sq = Σ(x*x) / n` over the block.
|
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- Push `{node_id, peak, mean_sq}` to a per-stream `rtrb`.
|
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2. **Daemon thread (`AppLevelController` per stream):**
|
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- Drain the rtrb.
|
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- Update peak envelope (one-pole, fast α — attack within a block,
|
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release ~500 ms).
|
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- Update RMS envelope (one-pole, slow α — window ~1–2 s).
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- Compute `peak_reduction_db` and `rms_reduction_db` independently,
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||
then `proposed = max(peak_reduction_db, rms_reduction_db)`.
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- Smooth toward `proposed`.
|
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- If the smoothed value is significantly different from
|
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last-written AND we're not rate-limited (~10 Hz max writes per
|
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stream), submit `Props.channelVolumes` update.
|
||
|
||
The recovery condition is intentionally *both*-paths-agree: a
|
||
release on the peak path only counts toward unwinding gain
|
||
reduction if the RMS path also reads quiet. This avoids the pumping
|
||
artefact where a transient-heavy stream would rapidly release
|
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between transients only to be slapped back down on the next one.
|
||
|
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### 4.4 Honouring user-set volumes
|
||
|
||
The daemon subscribes to `Props` param-change events on each managed
|
||
stream. When a `channelVolumes` change arrives that's meaningfully
|
||
different from `last_written_volume`, it wasn't us — the user
|
||
adjusted via pavucontrol, a hotkey, an app's own UI, etc. The
|
||
controller then either:
|
||
|
||
- **defers entirely** (stops adjusting the stream until the user opts
|
||
back in via `headroom per-app reset <app>`), or
|
||
- **treats the user value as a ceiling** (continues to cut on spikes
|
||
but never raises above what the user wanted).
|
||
|
||
Default is the ceiling behaviour — it's the principle-of-least-surprise
|
||
choice. Users who want strict deference set a profile flag.
|
||
|
||
#### A historical concern: apps that fight back
|
||
|
||
Some PulseAudio-era apps (Discord most famously) used to read and
|
||
re-assert their own `channelVolumes` periodically, fighting any
|
||
external volume manager. The pattern produced a visible ping-pong
|
||
loop and effectively disabled per-app management.
|
||
|
||
The pattern is largely absent from modern PipeWire-native and
|
||
Electron-based apps in 2024+: in-app sliders write `channelVolumes`
|
||
only on user interaction, not on a timer. From Headroom's
|
||
perspective, those user-interaction writes are indistinguishable from
|
||
a pavucontrol slider move — both are legitimate external changes the
|
||
deference policy correctly yields to.
|
||
|
||
If a fight-back app does appear, the **ceiling** deference mode
|
||
degrades gracefully:
|
||
|
||
- App produces hot output → Headroom cuts to 0.5.
|
||
- App writes `channelVolumes = 1.0` back over our cut.
|
||
- Headroom detects the external change, marks the new value
|
||
(1.0) as the ceiling, and stops actively writing.
|
||
- Layer A becomes effectively inert for that stream — there is no
|
||
ping-pong, the user just doesn't get the per-app cut they were
|
||
hoping for. The bus-level Layer C limiter (if engaged) still
|
||
enforces the absolute output ceiling regardless.
|
||
|
||
Explicit pattern detection and rate-limiting of ceiling updates
|
||
(e.g., "ignore ceiling-restoring writes that arrive within N seconds
|
||
of our own writes") is deferred to v1, pending evidence from
|
||
real-world testing that any modern app warrants it. The graceful
|
||
degradation property is the v0 contract.
|
||
|
||
### 4.5 Reaction-time honesty
|
||
|
||
The signal-path latency is **zero**. The reaction latency to a spike
|
||
is bounded by:
|
||
|
||
```
|
||
spike in block N ─► analysis (same quantum)
|
||
─► rtrb push (ns)
|
||
─► controller computes (μs)
|
||
─► Props write to pw main loop
|
||
─► applied to block N+1 of the app stream
|
||
```
|
||
|
||
So sustained loud passages are attenuated within ~one quantum
|
||
(5–20 ms depending on the system's quantum). **Isolated one-block
|
||
transients still leak through** — the first block carrying the spike
|
||
plays with the old gain; subsequent blocks see the reduction. This
|
||
is the irreducible cost of "no lookahead allowed." For absolute
|
||
spike prevention you need lookahead, which means latency, which
|
||
contradicts the constraint of this layer.
|
||
|
||
On the processed route the bus-level Layer C limiter (§3.1) catches
|
||
anything that would exceed the ceiling regardless of whether Layer A
|
||
has caught up; on bypass routes Layer A is the only thing watching, so
|
||
isolated one-block transients reach the real sink. Layer A reduces
|
||
*workload* on Layer C where Layer C is in the path, and is a
|
||
best-effort comfort filter where it isn't; it doesn't replace the
|
||
limiter.
|
||
|
||
### 4.6 Layered budget summary
|
||
|
||
| Layer | Metric | Time scale | Signal-path latency added |
|
||
|---|---|---|---|
|
||
| A: per-app peak | sample peak per block | tens of ms | **0** |
|
||
| A: per-app RMS | block mean-square | seconds | **0** |
|
||
| C: inline soft tier | true-peak, lookahead | sub-ms | shared with hard tier |
|
||
| C: inline hard tier | true-peak, lookahead | sub-ms | ~2 ms lookahead |
|
||
| C: bus AGC | LUFS (ebur128) | many seconds | — (control plane only) |
|
||
|
||
Five distinct jobs, five distinct time scales, no two layers
|
||
duplicate each other. Layer A is the cheapest line of defense and
|
||
the only one that costs zero latency on the audio path.
|
||
|
||
### 4.7 Resource budget per stream
|
||
|
||
| | No TRUE_PEAK (recommended for Layer A) |
|
||
|---|---|
|
||
| Audio thread per quantum | ~10 μs (peak + RMS pass) |
|
||
| Daemon thread per measurement | ~few μs (HashMap lookup + envelope math) |
|
||
| Memory per controller | ~100 bytes |
|
||
| Memory per ebur128 (if enabled) | — N/A; Layer A doesn't use ebur128 |
|
||
|
||
At realistic stream counts (2–5 managed apps): **<0.5% CPU total,
|
||
<1 KB RAM total**. Doesn't move the needle.
|
||
|
||
### 4.8 Lifecycle
|
||
|
||
- **Stream appears** with `media.class = Stream/Output/Audio`
|
||
matching a `[[per_app.rules]]` pattern: create tap link
|
||
(`pw_link_create`), spawn controller, register rtrb.
|
||
- **Stream disappears** (`pw_registry::global_removed`): tear down
|
||
tap, drop controller, clean up rtrb.
|
||
- **App restarts**: new `node_id` → fresh controller. User-volume
|
||
deference state is per-stream-instance, which is the right default.
|
||
|
||
---
|
||
|
||
## 5. PipeWire integration
|
||
|
||
### 5.1 Sinks
|
||
|
||
Created on daemon startup by emitting a `pipewire.conf.d` fragment into
|
||
`$XDG_CONFIG_HOME/pipewire/pipewire.conf.d/headroom.conf` (if not already
|
||
present) and reloading. Alternative: create them at runtime via
|
||
`pw-loopback` equivalents using `pipewire-rs`. v0 ships with the
|
||
runtime-creation path so the install footprint is "one binary, one
|
||
unit file."
|
||
|
||
Sink properties:
|
||
|
||
- `headroom-processed`: `node.name=headroom-processed`,
|
||
`media.class=Audio/Sink`, `audio.position=[FL,FR]`,
|
||
`node.description="Headroom (processed)"`. Promoted to system
|
||
default on startup so new streams land in it by default.
|
||
|
||
There is no second sink. Bypassed streams are routed directly at the
|
||
current `preferred_real_sink` via `target.object` metadata writes
|
||
(see §4.3).
|
||
|
||
### 5.2 The filter
|
||
|
||
Two `pw_stream`s:
|
||
|
||
- **Capture stream** linked to `headroom-processed`'s monitor. Format:
|
||
`F32 LE`, channels 2, rate matched to real sink, latency-quantum
|
||
matched (default 1024 frames; configurable).
|
||
- **Playback stream** linked to the current `preferred_real_sink`.
|
||
Same format.
|
||
|
||
`process` callback: pull a buffer from capture, run AGC gain →
|
||
compressor → limiter → push to playback. Allocation-free. Parameter
|
||
updates arrive over an `rtrb` SPSC queue from the control thread.
|
||
|
||
### 5.3 Routing
|
||
|
||
- On startup, write `default.audio.sink` in the `default` metadata to
|
||
point at `headroom-processed` so new streams default to the
|
||
processor. The previous value (the user's hardware sink) is
|
||
captured as the initial `preferred_real_sink`.
|
||
- Subscribe to `pw_registry` global-added events.
|
||
- On any new node with `media.class == "Stream/Output/Audio"` and
|
||
`node.dont-move != true`:
|
||
- Read `application.process.binary`, `application.name`,
|
||
`pipewire.access.portal.app_id`, `media.role`.
|
||
- Evaluate routing rules from the active profile to decide
|
||
`processed` vs. `bypass`.
|
||
- Write `target.object` into the `default` metadata for the new
|
||
stream:
|
||
- `processed` → `headroom-processed`'s `object.serial`.
|
||
- `bypass` → `preferred_real_sink`'s `object.serial`.
|
||
WirePlumber honours this for any movable stream.
|
||
- Watch `default.audio.sink` metadata changes. When the user switches
|
||
the system default to a hardware sink, the daemon:
|
||
- records that sink as the new `preferred_real_sink`,
|
||
- re-links `headroom-filter`'s playback stream to it,
|
||
- rewrites `target.object` for every currently-bypassed stream so
|
||
they follow the new hardware,
|
||
- re-asserts `headroom-processed` as the *default for new streams*
|
||
(so subsequent app launches still land in the processor).
|
||
- Hotplug (sink appears/disappears) goes through the same code path.
|
||
|
||
### 5.4 Stream identification
|
||
|
||
| Property | Reliability | Use |
|
||
|---|---|---|
|
||
| `application.process.binary` | high (kernel-sourced) | primary key |
|
||
| `application.name` | medium | secondary / display |
|
||
| `pipewire.access.portal.app_id` | high (Flatpak only) | match sandboxed apps |
|
||
| `media.role` | low (most apps omit) | bonus signal only |
|
||
| `media.class` | structural | gate to playback streams |
|
||
|
||
---
|
||
|
||
## 6. Profiles
|
||
|
||
Profile files live in `$XDG_CONFIG_HOME/headroom/profiles/*.toml`,
|
||
shadowing shipped defaults in `/usr/share/headroom/profiles/` by
|
||
name. Profile files are user-authored configuration — they're the
|
||
thing you open in `$EDITOR`. File-watcher hot-reload via
|
||
`notify-debouncer-mini` is planned; in the meantime `profile.reload`
|
||
re-scans on demand.
|
||
|
||
Daemon-managed user state — active profile name, per-app route
|
||
overrides made via `route.set`, dotted-key tweaks made via
|
||
`setting.set`, the global bypass flag — is *not* mixed in with the
|
||
profile TOMLs. It lives in a single `overlay.toml` at
|
||
`$XDG_STATE_HOME/headroom/overlay.toml`, written atomically by the
|
||
daemon (stage to `overlay.toml.tmp-…`, then rename). The overlay
|
||
rides on top of whichever profile is active, so `route.set obs
|
||
bypass` persists across `profile.use night` — that's a user
|
||
preference, not a tweak of `default`. If the overlay names an active
|
||
profile that's not on disk, the daemon falls back to the built-in
|
||
default and surfaces a warning; it does not refuse to start.
|
||
|
||
Each profile is a complete listening scenario. Schema (`headroom-core::profile`):
|
||
|
||
```toml
|
||
name = "default"
|
||
description = "Gentle transparent processing for everyday use."
|
||
|
||
[agc]
|
||
enabled = true
|
||
target_lufs = -18.0 # ITU-R BS.1770 integrated target
|
||
attack_ms = 2000.0
|
||
release_ms = 800.0
|
||
silence_threshold_lufs = -70.0
|
||
max_boost_db = 12.0
|
||
max_cut_db = 12.0
|
||
|
||
[compressor]
|
||
enabled = true
|
||
detector = "peak" # "peak" | "rms"
|
||
threshold_db = -24.0
|
||
ratio = 2.5
|
||
knee_db = 6.0
|
||
attack_ms = 10.0
|
||
release_ms = 100.0
|
||
makeup_db = "auto" # number or "auto"
|
||
|
||
[limiter]
|
||
ceiling_dbtp = -0.1
|
||
lookahead_ms = 2.0
|
||
release_ms = 80.0
|
||
hold_ms = 5.0
|
||
oversample = 4 # 1 | 2 | 4 | 8 (1 disables ISP detection)
|
||
link = "stereo" # "stereo" | "dual-mono"
|
||
|
||
[meters]
|
||
publish_hz = 20.0
|
||
|
||
[[rules]]
|
||
match = { process_binary = ["spotify", "mpv", "ardour", "reaper", "qpwgraph"] }
|
||
route = "bypass"
|
||
|
||
[[rules]]
|
||
match = { process_binary = ["firefox", "chromium", "google-chrome", "Discord", "discord", "element-desktop", "Slack", "zoom", "WEBRTC VoiceEngine"] }
|
||
route = "processed"
|
||
|
||
[default_route]
|
||
route = "processed" # safe default: anything unmatched is processed
|
||
|
||
# ----------------------------------------------------------------------
|
||
# Per-application level control (Layer A). Orthogonal to routing — you
|
||
# can enable per-app on bypass-routed streams to get zero-latency
|
||
# level control (e.g. tame Discord notifications without touching
|
||
# the game's audio path).
|
||
# ----------------------------------------------------------------------
|
||
[per_app]
|
||
enabled = true # master switch; false disables Layer A entirely
|
||
default_enabled = false # for streams not matched by any rule below
|
||
|
||
# Per-rule knobs. Matches use the same key set as [[rules]] above.
|
||
[[per_app.rules]]
|
||
match = { process_binary = ["Discord", "discord", "element-desktop", "Slack", "zoom"] }
|
||
enabled = true
|
||
peak_threshold_db = -6.0 # short-window peak above this triggers cut
|
||
rms_target_db = -20.0 # long-term RMS target (slow path)
|
||
max_cut_db = 12.0 # never cut more than this
|
||
peak_attack_ms = 5.0
|
||
peak_release_ms = 500.0
|
||
rms_window_ms = 1500.0
|
||
# Controller-side knobs (all optional; defaults shown).
|
||
smoother_ms = 30.0 # anti-bounce smoother on max(peak,rms)
|
||
write_db_threshold = 0.5 # dB diff below which we don't fire a write
|
||
min_write_interval_ms = 100.0 # min ms between writes per stream (10 Hz cap)
|
||
defer_to_user = "ceiling" # "ceiling" | "strict"
|
||
|
||
[[per_app.rules]]
|
||
match = { process_binary = ["firefox", "chromium", "google-chrome"] }
|
||
enabled = true
|
||
peak_threshold_db = -3.0 # browsers run hotter; raise the trigger
|
||
rms_target_db = -18.0
|
||
|
||
# Music, DAWs, games default to per-app off — they're either trusted
|
||
# to set their own level or routed bypass for a reason.
|
||
[[per_app.rules]]
|
||
match = { process_binary = ["spotify", "mpv", "ardour", "reaper", "qpwgraph", "carla"] }
|
||
enabled = false
|
||
```
|
||
|
||
### Shipped profiles
|
||
|
||
| name | one-liner |
|
||
|---|---|
|
||
| `default` | Gentle transparent processing, sensible for daily use. |
|
||
| `night` | Aggressive: −20 LUFS, 4:1, fast release, narrow dynamic range. |
|
||
| `speech` | VoIP-focused; short attack, fast release, slight rumble cut. |
|
||
| `transparent` | Limiter only. Compressor + AGC bypassed. Safety net only. |
|
||
| `bypass-all` | Routes everything directly to the real sink. The kill switch. |
|
||
|
||
The limiter section of `bypass-all` is irrelevant in practice (nothing
|
||
flows through `headroom-processed`), but its ceiling field is still
|
||
respected as a fail-safe in case a stream lands on the processed sink
|
||
anyway.
|
||
|
||
---
|
||
|
||
## 7. IPC
|
||
|
||
Transport: Unix-domain socket, `SOCK_STREAM`, `0600`, at
|
||
`$XDG_RUNTIME_DIR/headroom/control.sock`.
|
||
|
||
Wire protocol: **see `IPC.md`** for the full normative schema.
|
||
Summary: u32 BE length prefix + UTF-8 JSON payload. Three message
|
||
shapes — `Request` (id + op + args), `Response` (id + result|error),
|
||
`Event` (topic + data). Subscribers signal interest by topic; events
|
||
fan out to all subscribers with bounded per-subscriber queues. Slow
|
||
subscribers have events **dropped** (overflow events count is itself
|
||
published on the `daemon` topic so clients know they fell behind).
|
||
|
||
The first-party Rust wrapper is `headroom-client`, mirroring how
|
||
[`niri-ipc`](https://github.com/YaLTeR/niri/tree/main/niri-ipc) wraps
|
||
Niri's socket: a thin, no-magic crate that re-exports the wire types
|
||
from `headroom-ipc` and adds a blocking `Client` (and an optional async
|
||
`AsyncClient` behind a feature flag).
|
||
|
||
---
|
||
|
||
## 8. CLI
|
||
|
||
```
|
||
headroom status # current profile, sinks, levels
|
||
headroom daemon # run the daemon (systemd Type=simple)
|
||
headroom profile list | use <name> | show [name]
|
||
headroom route list
|
||
headroom route set <app> processed|bypass # persists in user profile
|
||
headroom route unset <app>
|
||
headroom route stream <node-id> processed|bypass # ad-hoc
|
||
headroom set <key> <value> # tweak active profile in place
|
||
headroom get <key>
|
||
headroom bypass on|off # global kill switch
|
||
headroom reload # reload profiles from disk
|
||
headroom monitor # live meter TUI (uses subscribe)
|
||
```
|
||
|
||
CLI is sync, blocks on `UnixStream`. Talks the same JSON wire as any
|
||
other client.
|
||
|
||
---
|
||
|
||
## 9. Crates
|
||
|
||
```
|
||
headroom/
|
||
├── flake.nix # devshell + package
|
||
├── Cargo.toml # workspace
|
||
├── PLAN.md # this file
|
||
├── IPC.md # wire-protocol schema (normative)
|
||
├── README.md
|
||
└── crates/
|
||
├── headroom-dsp/ # AGC + compressor + limiter (pure DSP, no PW)
|
||
├── headroom-ipc/ # wire types, framing, serde; no I/O
|
||
├── headroom-client/ # blocking client (+ optional async); thin
|
||
├── headroom-core/ # daemon: PW integration, routing, profiles, IPC server
|
||
└── headroom-cli/ # `headroom` binary; depends on headroom-client
|
||
```
|
||
|
||
### External crates (final v0 dep list)
|
||
|
||
**Audio / DSP**
|
||
- `pipewire`, `libspa` — official PipeWire bindings.
|
||
- `ebur128` — measurement.
|
||
- `rtrb` — SPSC ring buffer (audio ↔ control).
|
||
- `basedrop` — RT-safe shared ownership.
|
||
- `assert_no_alloc` — debug-build tripwire.
|
||
|
||
**Plumbing**
|
||
- `serde`, `serde_json` — IPC + profile (de)serialization.
|
||
- `serde-toml` (`toml`) — profile files.
|
||
- `clap` (derive) — CLI.
|
||
- `tracing`, `tracing-subscriber`, `tracing-journald` — logs.
|
||
- `notify`, `notify-debouncer-mini` — profile hot-reload.
|
||
- `crossbeam-channel` — control-plane channels.
|
||
- `parking_lot` — mutexes.
|
||
- `signal-hook` — clean shutdown.
|
||
- `thiserror` — error types.
|
||
|
||
No `tokio`, no `zbus`, no `dbus-*`.
|
||
|
||
---
|
||
|
||
## 10. Nix
|
||
|
||
`flake.nix` ships:
|
||
|
||
- A **devshell** with rust toolchain (via `rust-overlay` for pinned
|
||
channel; default to a stable release pinned in
|
||
`rust-toolchain.toml`), `pkg-config`, `pipewire`'s dev outputs,
|
||
`clang` (for bindgen if invoked by deps), `socat` (handy for poking
|
||
the IPC), `jq`.
|
||
- A **package** output (`packages.<system>.default`) that builds the
|
||
daemon + CLI with `rustPlatform.buildRustPackage`. v0 uses
|
||
`cargoLock.lockFile`. Crane can come later if incremental builds in
|
||
CI become a bottleneck.
|
||
- A `nixosModules.default` placeholder so packagers can wire the user
|
||
unit later. Not implemented in v0 of the flake itself.
|
||
|
||
Intermediate dev work uses plain `cargo` inside `nix develop`. Final
|
||
builds and any CI go through `nix build`.
|
||
|
||
---
|
||
|
||
## 11. Phased implementation
|
||
|
||
The phases are roughly token-of-work units, not calendar weeks. **All
|
||
planned phases (0–8) are done as of 2026-05-21**; this section is
|
||
preserved as historical context + a reading guide to the commit log.
|
||
See [[headroom-project]] in team memory for the per-commit ledger.
|
||
|
||
**Phase 0 — scaffolding.** Flake, workspace, crate skeletons, README,
|
||
PLAN/IPC docs. *(done as part of this commit)*
|
||
|
||
**Phase 1 — IPC + client.** `headroom-ipc` (types, framing, codec) and
|
||
`headroom-client` (blocking `Client`) implemented against the schema in
|
||
`IPC.md`. Round-trip tests, fuzz the codec. *(this commit)*
|
||
|
||
**Phase 2 — DSP kernels.** `headroom-dsp` with limiter, compressor, AGC,
|
||
oversampler, envelope. Tested in isolation against synthesized
|
||
signals; limiter validated to hold a −0.1 dBTP ceiling on EBU TECH
|
||
3341 generators. *(this commit: limiter first)*
|
||
|
||
**Phase 3 — daemon core.** `headroom-core` brings up the
|
||
`headroom-processed` virtual sink, the filter (pw_stream pair),
|
||
the `preferred_real_sink` tracker, the registry subscriber, and the
|
||
routing engine. Hardcoded profile, no IPC server yet.
|
||
|
||
**Phase 4 — IPC server + profile manager.** Wire `headroom-core` to the
|
||
IPC schema. Profile loading + hot-reload. Slow AGC loop ticking on
|
||
real loudness measurements.
|
||
|
||
Sub-stages used in commits / TODOs:
|
||
|
||
- **4a–4d** — Unix socket server, op dispatch, mutating ops, event
|
||
broadcaster.
|
||
- **4e** — `ProfileStore`: shipped + user profiles, atomic reload,
|
||
user overlay at `$XDG_STATE_HOME/headroom/overlay.toml`. `profile.use`,
|
||
`profile.reload`, `setting.set`, `route.set` all dispatch through it.
|
||
- **4f** — DSP parameter propagation: `setting.set` reaches the running
|
||
filter via the `rtrb` control queue, so live profile/setting edits
|
||
take effect without restart.
|
||
- **4h** — `preferred_real_sink` tracking: subscribe to
|
||
`default.audio.sink`, snapshot the prior default, promote
|
||
`headroom-processed`, retarget every bypassed stream on
|
||
default-sink change, on hotplug, and on Bluetooth handoff. Also
|
||
pins the filter's playback to the tracked real sink so processed
|
||
audio follows when the user switches default, and resolves the
|
||
real sink's node id from the registry for `status` reporting.
|
||
- **4i** — `route.stream <node-id> processed|bypass`: ad-hoc per-stream
|
||
override that doesn't write a profile rule. Crosses the
|
||
IPC-thread → PipeWire-thread boundary via a `crossbeam` channel
|
||
drained by a 50 ms timer source on the main loop. State updates
|
||
synchronously; metadata write follows ≤ ~50 ms later.
|
||
|
||
- **Slow AGC loop** — wraps up Phase 4. Audio-thread `AgcGain` stage
|
||
sits at the head of the DSP chain (anti-zipper smoother around a
|
||
per-sample multiplier). Filter pushes *pre-AGC* input samples into a
|
||
dedicated measurement ring. A `AgcController` on the PipeWire main
|
||
loop ticks at 50 ms: drains the ring into `ebur128` (Mode S | M |
|
||
TRUE_PEAK), reads `[agc]` config from the active profile, computes
|
||
`target_lufs − short_term_lufs` clamped to `[-max_cut_db,
|
||
+max_boost_db]`, gates below `silence_threshold_lufs`, slow-smooths
|
||
via leaky integrator, and pushes the result through `FilterControl`
|
||
on the same `rtrb` channel `setting.set` uses.
|
||
|
||
### Tracked follow-ups (carried past their sub-stage)
|
||
|
||
Items deliberately deferred from earlier sub-stages so they don't get
|
||
lost. Pick up by name when the trigger that gates them fires.
|
||
|
||
- **Ephemeral overlay mutations.** *(4e follow-up.)* All `route.set`
|
||
/ `setting.set` changes are persisted to `overlay.toml`. A
|
||
`--ephemeral` flag (or `--volatile`) on the CLI for one-shot tweaks
|
||
that don't outlive the daemon was considered and dropped from v0
|
||
for simplicity. Revisit if real users ask for it; the store-level
|
||
change is a flag on the setter methods. **Dormant** — no user has
|
||
asked through Phase 8.
|
||
- ~~**Filter playback BUSY spikes (periodic, ~10 s cadence).**~~
|
||
**Closed in 8e (`d52cd6d`).** The instrumentation added by 8e
|
||
did not reproduce the ~8×-baseline outlier pattern in a ~3 min
|
||
release-build capture; steady state was ~2.2 ms / call at this
|
||
hardware's quantum with max growing only to 1.3× baseline.
|
||
`PlaybackTiming` stays so future regressions surface at WARN.
|
||
Original observation may have been a transient WP/PW housekeeping
|
||
artefact under a different config; no actionable code change.
|
||
- **Sub-millisecond dispatch primitive for spike-reactive writes.**
|
||
*(Phase 6 optimisation, downgraded from prerequisite.)* The 4i
|
||
`PwCommand` channel uses a 50 ms polling timer, fine for
|
||
`route.stream` and slow AGC. Layer A's per-app
|
||
`Props.channelVolumes` writes were originally feared to need a
|
||
sub-ms wake primitive. After 6a/6b benches landed (see
|
||
§11.6 below) we re-evaluated: at a 5 ms polling timer and 21 ms
|
||
PipeWire quantum, the worst-case detection-to-write latency stays
|
||
well inside one quantum, which is what PLAN §4.5 actually
|
||
promises. Polling reuses existing infrastructure and is cheap
|
||
(controller tick is ~30 ns; even at 200 Hz it's lost in the
|
||
noise). The tighter primitive — `EventSource::signal` with an
|
||
`unsafe impl Send` shim around `spa_loop_utils.signal_event`, or a
|
||
pipe + `IoSource` — stays on the table as an optimisation if
|
||
manual testing shows audible spike-leak artefacts. `pw::command`
|
||
module docs still carry the constraint warning for future variants
|
||
that might be tempted to share the 50 ms timer.
|
||
|
||
**Phase 5 — CLI + monitor TUI.** `headroom-cli` implements all the
|
||
subcommands above, plus a `monitor` TUI built on the meters
|
||
subscription.
|
||
|
||
**Phase 6 — Per-application level control (Layer A).** Per-managed-stream
|
||
tap creation, `AppLevelController` with peak + RMS envelopes,
|
||
`Props.channelVolumes` writer, user-volume deference logic,
|
||
`[per_app]` profile parsing, `headroom per-app …` CLI verbs, and a
|
||
per-stream meter event on the IPC. Land after the bus path is stable
|
||
so we have a baseline to compare against.
|
||
|
||
Sub-stages:
|
||
|
||
- **6a** — Pure DSP. `headroom_dsp::LevelEnvelopes`: two-tier (peak
|
||
+ RMS) block-rate detector, `max(peak_reduction, rms_reduction)`
|
||
combined, clamped to `max_cut_db`. Allocation-free,
|
||
block-rate-driven (audio thread emits one `(peak, mean_sq)` pair
|
||
per quantum).
|
||
- **6b** — Daemon-side glue.
|
||
`headroom_core::app_level::AppLevelController`: rule snapshot,
|
||
envelopes, 30 ms anti-bounce smoother, 0.5 dB / 100 ms write
|
||
gate, ceiling vs strict deference state.
|
||
`app_level::evaluate` matches `[[per_app.rules]]` against
|
||
`PwNodeInfo` using the same matcher the routing engine uses.
|
||
- **6c** — PipeWire tap + audio-thread analysis. **Mechanism**:
|
||
per managed stream we create our own `pw_stream` (Direction::Input,
|
||
F32LE stereo, rate left unspecified to negotiate with the source,
|
||
`AUTOCONNECT` off, `NODE_DONT_RECONNECT`, `node.dont-move`),
|
||
`connect()` with no target, `set_active(true)`. PipeWire creates
|
||
our input ports from the declared format. We then build **explicit
|
||
passive port-level links** via `link-factory` with
|
||
`link.output.port` / `link.input.port` set to the source's and
|
||
tap's port global IDs respectively, plus `link.passive = true`.
|
||
**Why not `target.object` or `target_id`**: empirically (6c manual
|
||
smoke) WirePlumber's policy refuses to wire `Stream/Output →
|
||
Stream/Input` via any session-manager-mediated path — it logs no
|
||
error, just doesn't act. The stream-level target was getting set
|
||
on the node (`node.target = <source-id>`) but no link ever
|
||
appeared. Going through `link-factory` with explicit port IDs
|
||
bypasses the session manager entirely and uses PipeWire core
|
||
directly. **Per managed stream**: one `pw_stream`, two `Link`
|
||
proxies (one per channel), one `MeasurementSample` `rtrb`
|
||
(capacity 64). Audio-thread `process` runs `peak = max(|x|)` and
|
||
`mean_sq = Σx²/N` over the block, pushes one sample to the ring.
|
||
**Lifecycle**: registry watcher sees a `Stream/Output/Audio`
|
||
matching a `per_app` rule → spawn tap (ports come up
|
||
asynchronously) → the Layer A drain timer (6d) retries link
|
||
creation each tick until both port sets are visible on the
|
||
registry → links built, stream transitions to `Streaming`,
|
||
samples flow. On registry `global_remove` of the source, drop the
|
||
`ManagedStream`; declaration order severs links first, then the
|
||
tap stream + listener.
|
||
- **6d** — `Props.channelVolumes` writes + controller drain timer.
|
||
A polling timer source on the PipeWire main loop ticks every 5 ms
|
||
(200 Hz, CPU cost ≪ 0.1% of one core per the benches), iterates
|
||
active controllers, drains each measurement ring, calls
|
||
`process_block`, and on a `Some` return writes
|
||
`Props.channelVolumes` via the bound `default` metadata
|
||
(subject = source node id). The 5 ms tick guarantees a spike
|
||
detected at quantum boundary `N` is written before quantum `N+1`
|
||
starts on typical 21 ms quanta — see §4.5 reaction-time honesty
|
||
table.
|
||
- **6e** — User-volume deference + per-stream meter events.
|
||
Subscribe to `Props` param-change events on each managed stream.
|
||
Distinguish daemon writes from external by comparing against
|
||
`last_written_lin` (within 1e-4) — external changes apply
|
||
ceiling-mode or strict-mode deference per the matched rule's
|
||
`defer_to_user` field. Per-stream meters publish on the `meters`
|
||
topic with the smoothed reduction, the peak/RMS envelope values,
|
||
and the current applied `channelVolumes`.
|
||
|
||
**Validated cost budget (criterion microbenches, run 2026-05).**
|
||
PLAN §4.7 budgeted "~10 μs/quantum audio thread, few μs/measurement
|
||
daemon thread." Reality on this hardware:
|
||
|
||
| Bench | Time |
|
||
|---|---|
|
||
| Audio-thread peak + mean_sq scan, 1024-frame stereo block | 1.33 μs |
|
||
| `LevelEnvelopes::process_block` (daemon) | 18 ns |
|
||
| `AppLevelController::process_block` hot signal | 29 ns |
|
||
| `AppLevelController::process_block` quiet signal | 22 ns |
|
||
|
||
5 managed streams: audio thread ≈ 6.6 μs/quantum (0.03% of one
|
||
core at 21 ms quanta); daemon ≈ 145 ns/quantum. ~7-10× under the
|
||
PLAN budget, so the design has room for many more managed streams,
|
||
or for adding ebur128 / TRUE_PEAK to Layer A later if useful.
|
||
|
||
**Manual latency validation (post-6c implementation).** PipeWire
|
||
scheduling can't be benched from Rust alone. Use:
|
||
|
||
- **`pw-top`** — note the source-node `QUANT` and any WAIT/BUSY or
|
||
delay column before attaching the tap; attach Layer A; confirm
|
||
the source-node numbers don't change. The tap appears as a new
|
||
row with its own quantum; the test is whether the *app's* numbers
|
||
degrade.
|
||
- **`qpwgraph`** / **`helvum`** — visually confirm the source node
|
||
has two outgoing links (one to its original destination, one to
|
||
our tap), both terminating correctly.
|
||
- **Ear** — connect/disconnect the tap on live audio. Crackles or
|
||
dropouts on attach indicate the §4.1 sibling-fanout claim doesn't
|
||
hold and the design needs revisiting.
|
||
|
||
If those three say "fine," the §4.1 promise is upheld in practice
|
||
and 6c is acceptance-tested. `jack_iodelay` and other true-round-trip
|
||
tools are overkill.
|
||
|
||
**Phase 7 — Packaging.** *Done — `c65c75b`.* `contrib/systemd/headroom.service`
|
||
(user-scope, Type=simple, After=pipewire.service, Restart=on-failure,
|
||
journald, LimitRTPRIO=20). The package's `postInstall` substitutes
|
||
the unit's `@bindir@` placeholder with an absolute store path and
|
||
copies `profiles/*.toml` to `share/headroom/profiles/`. Two Nix
|
||
modules: `nixosModules.default` (`programs.headroom.enable` —
|
||
binary on global PATH + `systemd.packages` for `systemctl --user`
|
||
discovery + hard assertion on `services.pipewire.enable`) and
|
||
`homeModules.default` (`services.headroom.enable` — symlinks
|
||
shipped profiles into `$XDG_CONFIG_HOME/headroom/profiles/`,
|
||
`extraProfiles` attrset for per-user overrides, writes the systemd
|
||
user unit). README rewritten with install + usage sections.
|
||
|
||
**Phase 8 — Hardening.** *Done — `9220143` + `d52cd6d` + verification.*
|
||
- **8a — `assert_no_alloc` on audio-thread callbacks (`9220143`).**
|
||
`#[global_allocator] AllocDisabler` in `headroom-cli/src/main.rs`
|
||
behind `cfg(debug_assertions)` (release strips it via the crate's
|
||
default `disable_release`). The three RT callbacks
|
||
(`capture_process`, `playback_process`, `tap_process`) wrap their
|
||
body in `assert_no_alloc(|| inner(...))`. Verified by a deliberate
|
||
`Vec::with_capacity` injection → SIGABRT on first audio callback;
|
||
reverted before commit. Audio thread proven alloc-free under
|
||
multi-thousand-callback live load.
|
||
- **8b — live profile-reload under signal flow (verification only).**
|
||
Edit `$XDG_CONFIG_HOME/headroom/profiles/<active>.toml` while a
|
||
sine plays: notify-debouncer-mini fires, `ProfileStore::reload`
|
||
runs, `setting.set` propagates via `FilterControl`'s rtrb to the
|
||
audio thread. Compressor GR went 0 → −9.3 dB ≈ 1 s after edit
|
||
and back to 0 after restore; 180 meter ticks over 9 s with max
|
||
inter-tick gap = exact 50.0 ms (the AGC period). No glitches.
|
||
- **8c — sink hotplug / default-sink change (verification only).**
|
||
`wpctl set-default <other-sink>` while daemon runs:
|
||
`on_metadata_property` fires, `adopt_new_real_sink` runs,
|
||
filter.playback re-pinned via 4k explicit-link enforcement,
|
||
`routing/real_sink_changed` emitted on the wire. Bounces back
|
||
cleanly.
|
||
- **8d — multi-rate hardware (partial / deferred).** Filter is
|
||
hardcoded F32 stereo @ 48 kHz; PipeWire's link layer inserts a
|
||
resampler at the filter.playback → real-sink edge when rates
|
||
differ; bus DSP stays at 48 kHz internally. Architecture is
|
||
sound; real-hardware validation (USB DAC at 96k etc.) deferred
|
||
until available.
|
||
- **8e — playback callback timing instrumentation (`d52cd6d`).**
|
||
Lock-free `PlaybackTiming` atomics in `meters.rs`; AGC controller
|
||
drains once per second and logs at WARN above
|
||
`SPIKE_THRESHOLD_US = 5000`. The original ~10 s-cadence ~8×
|
||
spike pattern from §11 follow-ups *did not reproduce* in a ~3 min
|
||
release-build capture; steady state 2.2 ms / call at ~4 Hz,
|
||
max climbed to only 1.3× baseline. Instrumentation kept so
|
||
future regressions surface.
|
||
|
||
---
|
||
|
||
## 12. Risks & open questions
|
||
|
||
These are the original v0 design risks — still useful as a checklist
|
||
for new contributors. Phase 4k/4l/8c have exercised the routing /
|
||
hotplug / default-sink branches; the bullets below are unchanged
|
||
since several of them remain live concerns for non-NixOS distros
|
||
and multi-rate hardware. See [[headroom-project]] in team memory
|
||
for current status per risk.
|
||
|
||
- **WirePlumber re-linking on device hotplug.** When a Bluetooth
|
||
headset connects, WP re-evaluates linking. Headroom must re-pin its
|
||
routed streams. Tractable; the registry events surface this.
|
||
- **Latency budget.** Processed path: one quantum hop (the filter)
|
||
plus lookahead (~2 ms) plus 4× oversampling buffering ≈ 8–15 ms
|
||
added to processed-path latency. Fine for video/voice. Bypass path:
|
||
**zero added latency** — the stream rides the real sink directly.
|
||
- **Default-sink changes.** When the user switches the system default
|
||
to a hardware sink, the daemon adopts it as `preferred_real_sink`,
|
||
re-links the filter's playback, retargets bypassed streams, and
|
||
re-asserts `headroom-processed` as the default for new streams.
|
||
Watching `default.audio.sink` in the metadata is the trigger.
|
||
- **Sample-rate mismatch.** `headroom-processed`, the filter, and the
|
||
real sink must agree, or PipeWire resamples behind our back. The
|
||
filter should source its rate from the real sink and convert on the
|
||
capture side only.
|
||
- **Surround content downmix vs. passthrough.** v0 punts: anything
|
||
>2ch is routed directly to the real sink (bypass behaviour)
|
||
regardless of profile rule. Documented behaviour.
|
||
|
||
---
|
||
|
||
## 13. License
|
||
|
||
GPL-3.0-or-later for the daemon and CLI. `headroom-dsp` and `headroom-ipc`
|
||
are MPL-2.0 so third-party clients and plugin hosts can link them
|
||
without GPL contagion. (Re-evaluate when LSP-derived code is
|
||
introduced; current plan does not pull any.)
|