soundcraft/docs/architecture.md

Architecture

Last reviewed: 2026-10-10 · Last updated: 2026-10-12 · Change: minor (bounces stream to disk) · Target: Avid Pro Tools Ultimate 2026.4.1

SoundCraft is a Cargo workspace of small crates with enforced layering (cargo xtask layers). Nothing below the UI knows about egui, so the interface can be replaced.

The UI package pins craft-ui for common icon-button and track record-enable interactions. SoundCraft supplies each state's colors, border, radius and icon painter; selection and command dispatch remain in the toolbar and track controls. The single-shape frame option preserves the existing fill/border blending when controls are disabled. Specialized transport Play/Stop/Record styling, nonlinear meters, faders, pan gestures, fonts and the live theme resolver remain local. Shared adapters read the current palette. The UI requires Rust 1.95 or later without raising the core workspace's Rust 1.90 declaration.

L0  time        audio-io        midi        video   (standalone: no workspace deps)
L1  dsp         clap-host   vst3-host   au-host     (plugins; the three hosts are the only crates
                                                     allowed `unsafe`, confined to their FFI modules)
L2  model                                       (the session document)
L3  mix                                         (the mix engine)
L4  engine      playback                        (commands, undo, I/O · audio devices)
L5  automation                                  (MCP server, control-channel client)
L6  ui-egui                                     (the user interface)
    apps: soundcraft (desktop) · soundcraft-cli · soundcraft-web (wasm)

The document

soundcraft_model::Session is plain data (serde): tracks with playlists of clips, mixer state (inserts, sends, routing, automation lanes), tempo and meter maps, memory locations, groups, busses, and EditState (selection, tools, modes, view flags). Decoded audio lives in a SourcePool of Arcs that is not serialised; cloning a session is cheap. Sessions save as .scraft JSON plus an Audio Files/ folder.

Commands and undo

Every user action is a CommandSpec (id, label, menu path, shortcut, params doc, enabled, run) registered in crates/engine/src/cmd/. Engine::execute(id, params) runs it, catches any escaped panic, and pushes an undo snapshot (the previous Arc<Session>) when the document changed. Continuous gestures use execute_merged so a fader drag is one undo step. Menus are built from the incumbent's menu catalog (crates/engine/catalog/menus.txt); a menu item lights up when a command has the same menu path and label (or an alias maps it), which also drives the parity report.

Audio

soundcraft_mix::MixEngine::render(session, pos, frames, out) renders one block: clips (gain, fades, clip effects) → trim → inserts → pre-fader sends → fader/mute (automation, VCA, trim automation) → post-fader sends → pan → busses → aux inputs → master faders. Independent strips process in parallel; plugin delay compensation aligns every path. The same engine renders bounces offline (RangeRender, streamed to the file block by block) and realtime playback (soundcraft_playback::Player, which owns it on the audio thread and receives new session snapshots through a channel).

Device I/O today is the system's default output and default input device, opened as two separate cpal streams; the mix renders 512-sample blocks and resamples to the device rate when they differ. There is no device or buffer-size selection, no ASIO, and no MIDI device layer yet (see hardware-parity.md). Sources are decoded whole into the SourcePool and recordings are captured in memory until stop.

soundcraft-video holds our own H.264, ProRes and Motion JPEG decoders and the ISO-BMFF demuxer for the video track. Hosted plugins (clap-host, vst3-host, au-host) are scanned, created and destroyed off the audio thread; only process runs on it.

The cpal callbacks mark their thread (soundcraft_playback::mark_audio_thread). Code that may run there can ask on_audio_thread() before doing anything that blocks; the desktop app's logger does, so a log:: record from the audio thread (a stream error, a full synth event queue, a hosted plugin's failed process, a CLAP plugin's own log call) is kept without waiting and written later by the UI thread (see README › Logs).

Agent control

The desktop app's control channel (JSON lines over TCP) exposes engine commands, inspection, synthetic input and screenshots; soundcraft-cli mcp wraps either a headless engine or a running app as an MCP server. See control-protocol.md and mcp.md.

Revision history

Date	Change	Summary
2026-10-10	minor	Added video, vst3-host and au-host to the layer diagram; documented device I/O limits and in-memory media
2026-10-05	major	First architecture overview