xtc/docs/MIDI.md

# MIDI clips and instrument

Phase 7 supports drawing persisted notes and live JACK MIDI input. Record still
captures audio to WAV: MIDI capture and Standard MIDI File import are deferred.

## Workflow

Add/select a track in Sequencer, choose Add MIDI clip, then Piano roll (Ctrl+4).
Creation adds an instrument-to-output route if no instrument exists. Existing
instruments retain their routes; use Modular Graph to connect them as needed.
Scheduled MIDI clips now apply their track mix/automation before summing to
paired stereo instrument buffers; live input stays centered and unassigned. No hardware ports
are connected automatically. Connect a MIDI source to the backend's `midi_in`
JACK port, and route audio output explicitly to hear it while Play is running.

The piano roll shows all 128 pitches and edits clip-source time. Click blank
space to draw a one-beat note (velocity 100, filtered channel or channel 0); drag its body to move
it, drag the right edge to resize, or right-click/Delete to remove it. Edits
snap to the configured grid (default 1/16). Escape cancels a drag. Trimmed-out source notes remain editable
and are preserved. Arrangement/note edits stop playback and are blocked during
audio recording. Save/Open preserves MIDI flags, notes and stable IDs.

## Timing and ownership

The C model owns up to 4096 notes per project, with source-relative ticks at
960 ticks/quarter and constant tempo. Control-thread preparation intersects
notes with clip trims and converts boundaries to integer sample positions.
Scheduled rendering uses absolute note age: block partitions do not change
samples, and seeking into a note chases its phase. Clip ends cut notes off.
The instrument source has its own buffer, separate from external audio input
and WAV timeline playback, and routes through the ordinary graph.

Prepared ranges are immutable; only their audio owner changes voice/panic state.
Preparation allocates; rendering uses bounded preallocated storage and performs
no allocation, freeing, I/O, locks, logging or GUI calls. Project preparation
builds audio and MIDI candidates before committing the stopped backend state.

JACK events have sample offsets within the current block. Supported messages
are complete note on/off (including velocity-zero note on) and CC120/123 on
channels 0..15, plus CC7 volume, CC10 pan and CC64 sustain (see CONTROL.md).
Equal-offset events retain input order. Bend, program changes, running status
and SysEx are unsupported.
At most 256 events are accepted per block. Malformed/overflowed input discards
that block's events and triggers panic, with a MIDI-drop status counter.

## Recovery and limits

Panic/F9 clears live voices and suppresses scheduled notes already started;
future timeline notes can still start. CC120/123 does this for its channel.
Stop/start and seek clear live voices and reset suppression, allowing timeline
notes to be chased again. MIDI port disconnection, xruns and callback processing
failures request panic. Control requests use lock-free generation counters;
voice state changes on the audio thread, or while stopped.

The sine instrument scales amplitude by 0.1 times velocity/127. It supports
32 simultaneous scheduled notes and a separate 32 live voices; live overflow
steals the oldest voice, and repeated channel/pitch retriggers it. Preparation
rejects more than 32 overlapping scheduled notes with CAPACITY. Such metadata
can be saved by the C model, but GUI Open/Play rejects it until overlap is
reduced. Immediate note-off/stealing can click; there is no envelope or limiter,
and summed voices can exceed unity. Above-Nyquist pitches are silent.
There is no per-track/channel instrument routing, MIDI recording, SMF import
or piano-roll zoom yet. Note edits participate in project snapshot undo/redo.

## Verification

`make test` checks exact onset/off samples, block partition independence, trim,
seek, panic, channel handling, voice/event limits, persistence migration and
malformed files, plus callback fault recovery. `make test-gui` draws, moves and
resizes notes with FLTK events, then saves/reopens and renders them.
`make test-jack-midi` uses only software MIDI/audio ports to verify live on/off,
panic, disconnection and saved/reopened scheduled playback. JACK port discovery
may lag activation under PipeWire; the test waits before connecting.
Physical MIDI devices, bridge/device loss behind an unchanged port, latency and
long-running performance remain unverified. Panic/Stop provides manual recovery.

Phase 8 adds track mixing, automation and project undo/redo; see MIXER.md.

## Phase 21 composition commands

The piano-roll **Notes** menu exposes these commands. Shortcuts apply while the
piano roll has focus; use the main window Edit menu for undo/redo.

| Command | Shortcut | Behavior |
| --- | --- | --- |
| Grid | G | 1, 120, 160, 240, 320, 480 or 960 ticks |
| Filter | F | Channel -1 (all) or 0..15, inclusive pitch range 0..127 |
| Velocity and channel | V | Set either property; -1 preserves it |
| Quantize | Q | Snap starts to nearest configured grid; ties go later |
| Transpose | T / Up / Down | Semitone count / one semitone up / down |
| Repeat | Ctrl+D | Additional copies at a positive source-tick spacing |
| Select filtered | Ctrl+A | Select every note matching the current clip filter |
| Delete | Delete | Remove the single selected note or all filtered notes |

160 ticks is a sixteenth-note triplet; 320 is an eighth-note triplet.
The grid governs drawing, drag movement and minimum resize duration. At 1-tick
snap the visual grid remains 120 ticks to stay legible at the fixed zoom.
The header shows current grid, channel/pitch filter and selection scope.
Velocity also affects unselected note brightness. Preferences are session-only.

Click selects one visible note; Ctrl+A selects the whole filtered set. A click
or filter change clears that set selection. Hidden notes cannot be mouse-edited.
A channel filter also sets the channel for newly drawn notes. No lasso or
arbitrary disjoint multi-selection is provided. Moving/resizing uses the mouse;
V edits velocity/channel without changing timing, pitch or IDs. Quantization
changes starts only, preserving durations and other properties. Repetition
keeps originals and gives every copy a fresh stable ID; spacing need not equal
note length, so both overlapping patterns and rests are possible. The existing
clip source must have room for every result; extend it in Sequencer first if
needed. Source notes outside the visible arrangement trim remain editable.

All batch edits validate before mutation. Invalid pitch/range, copy counts,
capacity overflow or a note crossing the source boundary rejects the entire
operation; notes are never silently clamped, merged or shortened. Unmatched
notes, notes in other clips and unrelated properties are preserved. Each
successful command records one undo step and stops playback before model edits;
Play prepares the new engine plan. Undo/redo reinstalls the corresponding plan.
These edits use the existing persisted note fields; format remains v7.

Scheduled notes occupy [start,end), with ends counted before starts at an equal
sample for the 32-voice limit. Overlapping same-channel/pitch scheduled notes
are independent sine voices; adjacent repetitions restart phase. Live repeated
notes instead retrigger the matching channel/pitch voice, and a note-off releases
that voice (or defers release under sustain). Equal-sample live on/off/control
messages execute in caller/JACK order, with no implicit priority reordering.
The renderer rejects malformed/unsorted/out-of-range events before touching
output or runtime; raw velocity-zero note-ons decode to note-off.

`make test-composition` covers filters, property preservation, triplet rounding,
repeats/fresh IDs, rejected batch rollback, capacity, undo snapshots, equal-offset
live order, adjacent/overlapping notes, exact block partitions and seek chase.
`make test-gui` covers mouse edits and the new commands with undo/redo and engine
reinstallation. Listening and physical MIDI-device validation remain pending.