Architecture

Overview

What is MIDI Sketch Bach?

MIDI Sketch Bach is an algorithmic composition engine that generates Baroque-style instrumental MIDI using rule-based counterpoint, not machine learning. Every note is determined by music theory constraints and deterministic algorithms, producing editable MIDI data you can import into any DAW.

MIDI Sketch Bach compiles a core composition engine into WebAssembly, providing a high-level JavaScript API on top:

Two Composition Systems

Organ System (Forms 0--6)

The Organ System handles the large-scale organ works -- seven forms spanning the major genres of Bach's organ repertoire:

• Fugue (0) -- standalone fugue with exposition, development, and stretto
• Prelude and Fugue (1) -- flowing prelude leading into an elaborate fugue
• Trio Sonata (2) -- three-voice texture (two upper voices + pedal bass)
• Chorale Prelude (3) -- cantus firmus treatment with independent accompaniment
• Toccata and Fugue (4) -- multi-section piece with free toccata and strict fugue
• Passacaglia (5) -- continuous variation form over an ostinato bass
• Fantasia and Fugue (6) -- free-form fantasia paired with a structured fugue

These forms typically use 3--4 voices and default to organ registration.

Solo Instrument System (Forms 7--8)

The Solo Instrument System handles works for unaccompanied string instruments:

• Cello Prelude (7) -- flowing prelude for solo cello, modeled on Bach's Cello Suites
• Chaconne (8) -- monumental variation form for solo violin, inspired by the Partita No. 2 Chaconne

These forms use 3 voices and default to cello or violin.

Generation Pipeline

When you call generator.generate(config), a six-step pipeline executes. For a detailed breakdown of each step, see the Generation Pipeline page.

1. Configuration Resolution -- Resolve all options, apply form defaults for unspecified fields
2. Subject/Theme Generation -- Generate primary melodic material based on key, mode, and character
3. Formal Structure -- Build the macro-level structure (exposition, episodes, variations, etc.)
4. Voice Generation -- Generate each voice independently within its register
5. Counterpoint Rules -- Validate and adjust interval relationships between voices
6. MIDI Encoding -- Map to Standard MIDI File format

TIP

See Generation Pipeline for detailed explanations of each step, including how different form families handle structure differently.

Voice Architecture

Each voice in a MIDI Sketch Bach composition operates as an independent melodic line, following Baroque conventions for voice independence and voice leading. For a deep dive, see the Voice Architecture page.

Key principles:

• Registral separation -- voices have distinct ranges that minimize overlap
• Rhythmic independence -- different rhythmic patterns maintain voice identity
• Stepwise motion preferred -- voices move by step more often than by leap
• Leap resolution -- large leaps are followed by stepwise motion in the opposite direction
• Cadential patterns -- standard formulas close phrases and sections

TIP

See Counterpoint & Voice Leading for the music theory rules governing how voices interact.

WASM Integration

The composition engine is compiled to WebAssembly for portable, high-performance execution:

• Initialization: init() loads and instantiates the WASM module
• Memory management: BachGenerator allocates WASM memory; destroy() frees it
• Data transfer: MIDI output is copied from WASM memory to a JavaScript Uint8Array
• Event data: Structured event data is serialized from WASM and parsed in JavaScript