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JUCE SDK integration.

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# JUCE integration (Phase 107)
JUCE is used by optional, runnable examples alongside xtc's C99 engine and FLTK
application. It is an external SDK dependency, not vendored, fetched, linked by
normal builds or automatically installed. The integration uses:
| JUCE capability | Implemented use |
| --- | --- |
| AudioProcessor and plugin clients | mono/stereo VST3 instrument and standalone application |
| Audio/MIDI devices | standalone wrapper's device selection and MIDI routing UI |
| AudioProcessorValueTreeState | stable host parameters and validated versioned presets |
| GUI, graphics, events | custom native editor, parameter attachments, keyboard, meters and asynchronous preset chooser |
| DSP | low-pass filter, stereo reverb and smoothed output gain |
| MIDI | host event offsets, GUI event FIFO, Standard MIDI File parsing and tempo conversion |
| Audio formats | stereo float32 WAV export |
| Plugin hosting | explicit VST3 inspection, parameter discovery and state/processing qualification |
Synthesis is the real xtc **Soft Keys** implementation, called through
`xtc_plugin_*`. C++ remains confined to the optional integration. The SDK build
uses a separate PIC build of the C archive with JACK/FLTK disabled; the installed
application and existing build directories are not modified.
## Licensing and dependency impact
The inspected JUCE checkout is 9.0.4, revision
`3fb5cbc0340758596dac39f7c8de47d562b4896e`. Its modules are dual-licensed under
**AGPLv3 and the commercial JUCE licence**. A commercial JUCE licence may be
required for distribution; choose and comply with the applicable SDK licence.
xtc's MIT licence for original integration code does not relicense JUCE or a
combined binary. Read the actual SDK `LICENSE.md`, its third-party inventory
`JUCE.spdx.json`, and [JUCE licensing](https://juce.com/legal/juce-9-licence/)
before distributing those binaries. No SDK source or JUCE-linked binary is added
to the normal xtc install/package targets.
JUCE requires C++17 and its Linux development dependencies, including ALSA,
Fontconfig/FreeType and X11 libraries. CMake >= 3.25 and GNU Make build these
examples. Xvfb runs automated checks without a desktop. The SDK's `juceaide`
tool is compiled at configure time. Builds are substantially heavier than the
C-only core. Browser/network features are disabled; normal builds never fetch
SDKs or install system packages. Only the inspected SDK revision has been
qualified; use the documented revision for reproducibility.
## Processing and ownership
The processor creates/destroys its C synth and prepares JUCE DSP in quiescent
`prepareToPlay`/`releaseResources`. Host calls must obey AudioProcessor's lifecycle
contract. The callback splits arbitrary host buffers into 256-frame C blocks.
MIDI note, controller, pitch-bend and all-notes-off messages keep their sample
offsets; other messages, including SysEx/program change, are ignored. Events
outside the block are not rendered. More than 256 supported events in a chunk
silences the whole host block and clears voices. GUI notes enter a bounded
single-producer/single-consumer FIFO at offset zero in the next callback;
GUI queue overflow requests panic and discards queued notes. No keyboard-state
locks or GUI calls occur in the callback.
Five APVTS parameters control output gain, low-pass cutoff, room size, wet mix
and effect enable. Values are read atomically per host block; output gain ramps
for 10 ms. This is block-level parameter adoption, not sample-accurate automation.
Effects default off, making the wrapper output identical to the C synth at
unity gain. Mono/stereo work at 8–192 kHz; float64 is not exposed. The synth has
32 voices, attack/release, sustain and pitch bend from the C implementation.
The keyboard shows local GUI notes; it does not mirror external MIDI notes.
Presets contain the five parameter values and schema version 1, not voices,
plugin binaries, device choices, project arrangements or reverb history. Restore
validates the complete state before adoption (64 KiB maximum). Saving uses a
same-directory temporary file; the chooser asks about replacing an existing
preset. Host state APIs use the same schema. The wrapper's `.xtcjuce` presets
are separate from xtc project/chain presets.
## Limits and qualification
See [build and example commands](../examples/juce/README.md). These examples do
not migrate the main DAW to JUCE, add a new public backend, extend the C VST3
host's editor/controller support, or finish Phase 106's native ASIO work.
The JUCE VST3 can be loaded by xtc's existing instrument workflow in default
configuration. The C host translates all-off into individual note-offs, allowing
the synth release tail rather than an immediate hard stop. Its custom editor/parameters require a capable JUCE/VST3 host or
the standalone example. Explicit plugin inspection executes trusted plugin code;
it is not a scanner sandbox and must not run inside an audio callback.
Linux build/offline/editor/plugin results are recorded in Phase 107. Physical
audio/MIDI devices, Windows/macOS, ASIO and AU/AAX/LV2/VST2 exports require their
own platform, SDK, license and hardware qualification. Unrelated JUCE modules
such as video, network services and mobile UI are outside this audio phase.