xtc/docs/TRACK_INSERTS.md

Track inserts

The sequencer's Create dropdown groups track creation, WAV import and instrument loading. View → Track inspector opens the optional inspector; it is hidden when the application starts to leave more room for editing.

Project format 17 stores up to eight effect slots per track, their order, bypass, wet amount and chain output gain. Formats 1–16 read with empty chains. Metadata load, clone and history never open executable plugin modules. Track move, duplicate and removal retain the association with stable track IDs. Nonempty bus chains are rejected. The model accepts mono effect specifications for LADSPA, VST2, VST3, CLAP and internal effects; this alone does not establish that a third-party binary is compatible with the host.

The chain API is declared in xtc_plugin.h. Setter errors preserve the old chain; plugin identity and layout are checked when preparing playback.

Playback prepares an independent serial graph for each affected track. Audio clips on the same track sum before its effects; instrument and scheduled MIDI contributions also pass through their track graph. Effects run before track and bus gain/pan, and meters observe their output. Wet/dry uses parallel gain branches. Internal effect history resets at timeline discontinuities.

Each renderer owns its plans and preallocated raw buffers (up to 256 KiB per affected track); processing uses blocks of at most 256 frames, with no host allocation or I/O. Mono effect adapters require zero reported latency. External plugin state on seeks follows the existing host contract. Bus inserts and tracks containing both audio assets and MIDI reject preparation explicitly. Live unassigned sine monitoring remains a separate master input.

Open View → Track inserts (or Ctrl+5 for the selected track) and choose a track. Add effect offers internal Delay/Reverb and the plugin library. The left rack shows signal order; the graph displays the actual generated graph compiled by playback. Wiring in this view is read-only: edit the rack to preserve the serial chain. The global Modular Graph remains independent. A failed plugin probe or playback preparation preserves the current project; successful additions are undoable.

The library searches configured Preferences paths recursively (including VST3 bundle contents), with case-insensitive search and format filters. LADSPA and VST2 both use .so, so those filters share candidates until inspection identifies the ABI. Listing files executes no plugin code. Only inspect trusted binaries.

Select a slot in the rack, then use Up, Down, Bypass or Remove. The selection follows reordered effects. Bypassed slots retain their settings and appear marked in the rack; their nodes are omitted from the compiled graph. Each successful operation creates one project undo step.

Inspect / edit verifies the selected plugin and opens its identity and generic controls. Reset defaults clears parameter automation on Apply. Cancel leaves the project unchanged. Internal effects and LADSPA expose controls; modern adapters currently expose none and run default configurations. Native plugin editor windows and opaque plugin-state chunks are not implemented.

Preset → Save chain / Load chain reuses the atomic versioned project file container with one track. Order, bypass, wet amounts, output gain and exposed controls/automation are retained. Loading metadata executes no plugin code; applying it verifies all enabled plugins before replacing the current chain. Preset files reference absolute installed binary paths, so another machine must provide compatible binaries at those paths. No plugin binaries or opaque state chunks are embedded.

Insert wet blends the selected effect with its input (0 dry, 1 fully wet). Chain gain scales the complete chain before the track fader (0–4 linear). Edits are prepared and undoable, with finite range validation. Internal sync parameters follow project tempo. This is mono amplitude blending; stereo track insert layouts, latency compensation and gapless live insert edits are future work. Editing stops transport before preparing the replacement plan.

The CLI xtc-check --inspect project.xtc lists insert format, slot order, bypass/wet settings and missing binary references without loading plugins. VST3 processing supplies a fixed output-parameter sink (64 IDs, final point per ID/block) for plugins that emit meter/parameter notifications; notifications are not recorded or exposed as controls.

Host qualification on 2026-11-06 used the installed Linux x86_64 ZamEQ2, ZamAutoSat and ZamDelay VST3 binaries under /usr/lib/vst3. All three were prepared together, rendered with nonzero finite output and exported to WAV. This verifies those default mono configurations, not arbitrary plugins or physical hardware. The repository fixtures cover LADSPA, CLAP, VST2 and VST3 insert adoption and VST3 output notification bounds. Internal-plan allocation failure and callback guards, sanitizers, GUI/JACK lifecycle and install checks cover host ownership and cleanup.

Reproduce focused verification with:

make BUILD_DIR=build/audio-editor test-track-fx test-gui-track-fx
XTC_TEST_HOST_VST3=1 ./build/audio-editor/test_track_fx

The host opt-in requires the three named Zam bundles; the normal tests use internal devices and controlled fixtures. The screenshot tool's XTC_CAPTURE_HOST_VST3=1 mode captures those real host plugins; see screenshots/README.md.