Architecture

Reading the musical terms

This page uses music terms as data-model terms: voice means a melodic stream, harmony means the chord plan over time, and form means the composition template. See the Music Primer for Engineers for the short definitions.

Overview

What is MIDI Sketch Bach?

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

A C++ composer engine is compiled to WebAssembly, with a JavaScript API on top:

The Composer Engine

A single composer subsystem handles every form through dedicated per-form builders. Each builder expresses its form as a layout of voice intents over a harmonic plan, then the composer assembles, validates, and renders that layout. The form decides the texture (voice count, meter, natural length); there is no voice-count option.

Voice intent and harmonic plan

A voice intent is the role assigned to one melodic stream over a span of bars: subject, answer, ground bass, figuration, and so on. A harmonic plan is the chord roadmap used when building and validating that material.

The pipeline is:

1. Compose Request — resolve and validate the config; resolve the seed; fix voice count / meter / length from the form.
2. Form Director — assign per-form voice intents (subjects, grounds, cantus firmus, figuration, variations) to bar spans.
3. Candidate Search — dispatch each span; all default shipped spans replay authored carrier material verbatim.
4. Generation Validation & Renderer — accumulate counterpoint and structure failures, then render the assembled voices to tracks.
5. Ornament & Final Validation — apply deterministic ornaments, then validate the complete score.
6. Velocity, CC & Tempo — apply velocity and re-render; add CC 7/CC 11 and tempo events.
7. MIDI & Event Export — transpose pitches into the output key and emit the Standard MIDI File and public event data.

The scored search branch is off by default and contributes no notes to default output. --free-counterpoint reroutes only passacaglia V1 (voice == 1); other forms report that free counterpoint is unavailable.

See the Generation Pipeline for a step-by-step breakdown.

Design-Value Arc

Structure follows a fixed design arc — establish → develop → climax (at ~80% of the span) → resolve — that controls density, register, and velocity tiers. The arc is a property of the form, not something searched per seed, which keeps output musically shaped and reproducible.

Density, register, velocity

Density is how many notes happen in a span. Register is pitch height, such as low bass or high treble. Velocity is MIDI note intensity. The arc raises and lowers these values so the output has phrase shape instead of a flat stream of notes.

Form Families

The form director handles several layout families:

• Fugal (fugue, prelude_and_fugue, toccata_and_fugue, fantasia_and_fugue) — subject/answer entries with episodes.
• Ground-bass variation (passacaglia, chaconne, goldberg_variations) — an immutable bass with successive variation cycles; the natural 20-bar Goldberg layout contains the aria and four variations, while the full 128-bar layout contains the aria, all thirty variations, and the aria da capo. passacaglia/chaconne run their cycles in 3/4.
• Cantus firmus (chorale_prelude) — a fixed chorale line with a figuration voice and independent bass.
• Linear / figural (trio_sonata, cello_prelude) — interacting or continuous figuration.

Determinism

The engine is fully deterministic: the same config and seed produce byte-identical output. Composition and validation run internally in C. Output serialization applies the requested key and any output-octave shift to both the MIDI file and the pitches returned by getEvents(). The lower-level generated.v1 artifact retains the internal C pitches.

Two event representations

Use getEvents() when you need the notes as they sound in the selected output key. Use generated.v1 for internal diagnostics whose pitches must stay comparable across keys.

WASM Integration

• 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 is serialized from WASM and parsed in JavaScript.