Instruments

Per-instrument physical modeling was removed

Earlier versions included detailed physical performer models (bow direction, hand spans, fret positions, fatigue, harmonics, and so on) that scored how playable each note was. The composer engine does not use these. Instrument handling is now intentionally simpler: a General MIDI program, a playable range, and an ornament-density profile. This page describes what the engine actually does today.

What an Instrument Selects

Choosing an instrument affects three things:

1. General MIDI program — the sound written into the MIDI file.
2. Playable range — the pitch window the candidate search keeps the generated lines within. Notes are folded back into range rather than modeled for ergonomic difficulty.
3. Ornament density — how heavily the opt-in ornament pass decorates the line (combined with the character).

The voice count, meter, and structure come from the form, not from the instrument. Each form has a default instrument, but any instrument may be substituted.

Instrument vs voice

An instrument is the playback sound and range profile. A voice is the musical line being generated. A single instrument, such as organ, can carry several voices.

General MIDI Programs

Instrument	String	GM Program	Sound
Organ	"organ"	19	Church Organ
Harpsichord	"harpsichord"	6	Harpsichord
Piano	"piano"	0	Acoustic Grand Piano
Violin	"violin"	40	Violin
Cello	"cello"	42	Cello
Guitar	"guitar"	24	Acoustic Guitar (Nylon)

An unknown instrument string is rejected (it throws) rather than silently falling back to a default.

Default Instrument per Form

Form	Default Instrument
fugue, prelude_and_fugue, trio_sonata, chorale_prelude, toccata_and_fugue, passacaglia, fantasia_and_fugue	Organ
cello_prelude	Cello
chaconne	Violin
goldberg_variations	Harpsichord

Ornament Density

The deterministic ornament pass adds trills, mordents, and Nachschlag. How densely it decorates depends on both the character and the instrument, and some lines are never ornamented:

Trill, mordent, Nachschlag

These are short Baroque decorative figures around a structural note. They are added after the main composition is validated, so they are tagged as source: "ornament" rather than "material" or "compose".

• Ground-bass lines (passacaglia, chaconne, Goldberg bass) are never ornamented.
• Cantus-firmus lines (chorale prelude) are never ornamented.

Notes added here carry the source: "ornament" provenance tag.

Expression Output

Beyond ornaments, the engine writes form-appropriate expression:

• Organ registration — a CC#7/#11 curve that follows the form's energy arc.
• Closing ritardando — tempo events that slow the final cadence.
• Time signature — 3/4 for passacaglia and chaconne, 4/4 otherwise.

TIP

See Voice Architecture for how voices map to instruments and tracks, and the Generation Pipeline for where the ornament and expression passes sit.