Voice Architecture

Each voice in a MIDI Sketch Bach composition is an independent melodic line. The form decides how many voices there are and what each one does; there is no voice-count option.

What is a Voice?

In Baroque music, a "voice" is an independent melodic line — whether sung or played. A three-voice fugue has three simultaneous lines, each with its own rhythm, contour, and identity. In MIDI Sketch Bach, each voice becomes a separate MIDI track.

Track is the export container; voice is the musical role

The MIDI file uses tracks because DAWs understand tracks. The composition engine uses voices because counterpoint rules care about independent musical lines. Usually one voice maps to one track.

The Form Decides the Voice Count

Voice count is fixed per form. Picking the form picks the texture:

Form	Voices
fugue, prelude_and_fugue, trio_sonata, toccata_and_fugue, fantasia_and_fugue, goldberg_variations	3
chorale_prelude, passacaglia, chaconne	2
cello_prelude	1

Passing numVoices/num_voices is accepted and ignored for backward compatibility. See Forms for the full table including meter and natural length.

Voice Intents

The form director assigns each voice a voice intent over the bar spans of the piece — the role that voice plays at each point. The engine defines 30 named intents (voice_intent.h); they group into a handful of families:

Family	Representative intents	Role
Fugal entries	SubjectCarrier, AnswerCarrier, CountersubjectCarrier	Fugue theme entries (tonic), their answers (dominant), and the recurring companion line
Development	MiddleEntryCarrier, StrettoCarrier, PedalCarrier, CodaCarrier	Related-key entries, overlapping entries, pedal points, closing material
Episodes	Episode, FortspinnungSpan	Sequential travel material derived from subject motifs
Fixed lines	GroundCarrier, PassacagliaGround, CantusFirmusCarrier	Immutable repeating basses and chorale melodies
Figural	FigurationCarrier, ArpeggioFlow, ToccataCarrier, FantasiaCarrier	Continuous idiomatic figuration and sectional solo writing
Variation	VariationCarrier and role-specific variants	Reinvented upper material over a fixed bass
Texture	RhythmCarrier, NctCarrier, free counterpoint	Anacrusis/hemiola rhythm shapes, declared non-chord-tone figures, and accompaniment selected by the candidate search

Material-bearing intents (subjects, grounds, cantus firmus) are fixed; the remaining lines are filled in by the candidate search.

Subject, answer, ground, cantus firmus

A subject is the main fugue theme. An answer is that theme restated in another voice, usually shifted toward the dominant. A ground bass is a repeating low pattern. A cantus firmus is a fixed long-note melody.

Voice Independence

Maintaining independence between voices is a core principle of Baroque counterpoint. The engine sustains it through:

Rhythmic Differentiation

Voices use complementary rhythmic patterns so they do not move in lockstep — when one voice sustains, another tends to be active. This complementarity is a hallmark of Bach's writing.

Melodic Differentiation

Each voice has its own intervallic profile and directional tendencies, and develops its thematic material independently.

Register Differentiation

Voices keep to distinct registers. Voice crossing is minimized; when voices approach each other, they tend to move apart. The candidate search anchors each beat to chord tones, keeping the harmony clear while voices stay independent.

Register and voice crossing

Register means pitch range: bass is low, soprano-like writing is high. Voice crossing happens when a lower voice moves above a higher voice, or a higher voice moves below a lower one. That makes the streams harder to follow.

Voice-to-Track Mapping

Each voice maps to a separate MIDI track with its own channel and program:

const events = generator.getEvents()

for (const track of events.tracks) {
  console.log(`Voice: ${track.name}`)      // track name
  console.log(`Channel: ${track.channel}`)  // MIDI channel (0-15)
  console.log(`Program: ${track.program}`)  // GM program number
  console.log(`Notes: ${track.note_count}`) // number of notes
}

Every note also carries a source provenance tag ("material", "compose", or "ornament") — see the JavaScript API.

General MIDI Program Mapping

Instrument	GM Program	Sound
Organ	19	Church Organ
Harpsichord	6	Harpsichord
Piano	0	Acoustic Grand Piano
Violin	40	Violin
Cello	42	Cello
Guitar	24	Acoustic Guitar (Nylon)

The output is a Type 1 Standard MIDI File with one track per voice, so you can reassign instruments per voice in your DAW.