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docs/BUILTIN_DEVICES.md

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+# Internal instruments and effects
+
+No plugin downloads, search paths or external libraries are needed. The original
+built-in sine remains available for existing projects.
+
+Choose **Internal: Soft Keys** or **Internal: Sub Bass** in a MIDI track's
+instrument dropdown. Soft Keys blends a sine with a third harmonic; Sub Bass
+uses a pure sine. Both are fixed presets with 32 voices, velocity sensitivity,
+short attack/release envelopes, sustain pedal, note-off and panic handling.
+High harmonics are omitted near Nyquist. These basic instruments have no preset
+editor, pitch bend, sample library or oscillator/filter controls.
+
+In **Modular Graph**, select a wire and choose **Edit → Insert internal delay on
+wire** or **Insert internal reverb on wire**. This preserves the route and inserts
+the effect into the signal path. **File → Add internal delay/reverb** creates an
+unconnected device for manual wiring. Select its node and use **Edit → Selected
+plugin control** to edit it, or **Selected plugin automation** to automate it.
+Changes use the existing stopped-engine preparation and undo workflow.
+
+- **Delay:** time (1–2000 ms), feedback (0–0.95), dry/wet mix (0–1).
+- **Reverb:** four parallel damped feedback combs, with decay (0–0.95), damping
+  (0–0.95) and dry/wet mix (0–1). Decay is a feedback coefficient, not seconds.
+
+These are basic mono processors; stereo plans use independent instances. Reverb
+is deliberately simple and has no room model or stereo diffusion. Delay time
+changes can click; there is no crossfade or tempo sync. Transport seeks retain
+existing effect history, like LADSPA. Effect tails require continued rendering;
+playback/export does not automatically extend the song for tails.
+
+Project format 13 stores internal devices as format 4 with reserved module path
+`/xtc/internal`, stable numeric identity and index (0 keys, 1 bass, 2 delay,
+3 reverb). This is a virtual identifier, never an external binary. Effect controls
+and automation are saved using the existing plugin records; instruments are fixed
+presets. Readers still accept formats 1–12. Older xtc builds reject format 13.
+All buffers are allocated during preparation; processing performs no allocation,
+file access, library loading, locking or GUI calls.
+
+Validation: `make BUILD_DIR=build/phase38 all gui test test-gui-instruments
+ test-gui-modern-plugins test-gui-plugins analyze` (join onto one line).
+`test-builtins` checks impulse tails, dry mix, feedback bounds, block partition
+consistency, note release/panic and instrument/effect project round trips.