MIDI Sketch Bach

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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:

What runs whereThree stacked layers. On top, the JavaScript API that application code calls. In the middle, the Emscripten bridge that moves memory across the boundary. At the bottom, the composer engine compiled from C++ to WebAssembly, drawn as five stages in order: form director, candidate search, generation validation and the ornament pass followed by FinalScore and the form counterpoint budget, expression preparation, and export. Expression preparation applies velocity, preserves a notated-note snapshot, applies articulation, re-renders tracks, assigns instruments, and adds CC and tempo events. Nothing calls back up the stack.JavaScript APIinit()BachGeneratorpresets and enumsthe only surface your code touchesEmscripten bridgeallocates and frees WASM memory; copies MIDI bytes and serialized events out to JavaScriptcomposer engineC++ compiled to WebAssembly — one deterministic pass, with no callback into JavaScriptForm Directorvoice intents assignedto bar spansCandidate Searchcarrier replay by default;scored search is opt-inValidate & Budgetgeneration → ornament →FinalScore + form budgetExpression & MIDIvelocity & articulationtracks & instrumentCC & tempoExportStandard MIDI File andpublic event dataNothing crosses back up the stack: the engine runs once and hands back bytes — no callbacks, no file I/O, no audio rendering.

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 & Initial Renderer — accumulate counterpoint and structure failures, then render the assembled voices to initial tracks.
  5. Ornament — apply deterministic ornaments to eligible notes.
  6. Final Validation & Expression — run FinalScore and the form budget, apply velocity, save the notated snapshot, apply articulation, re-render, then 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.

The design-value arcA curve across the whole span of a piece. It rises slowly through the establish phase, keeps climbing through the develop phase, peaks at roughly eighty percent of the span, then falls through the resolve phase. The height of the curve stands for density, register and velocity tier together. The arc belongs to the form, so the same form always has the same shape.design valueclimax at ≈ 80% of the spandensity — notes per spanregister — how high or lowvelocity — MIDI intensityestablishthe material is stated plainly:thin texture, middle registerdevelopentries in related keys, busier rhythm,a widening register and louder tiersresolvethe tension is discharged;most forms add a ritardandoThe arc is a property of the form, not something searched per seed — which is why output stays shaped and reproducible at the same time.

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 ​

For an explicit non-zero seed, the same config and seed produce byte-identical output. With seed: 0, the runtime chooses a new seed. 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 and the notated durations captured before articulation.

Two event representations

Use getEvents() when you need the notes as they sound in the selected output key, including articulated playback durations. Use generated.v1 for internal diagnostics whose pitches and notated durations 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.

Dual-licensed: AGPL-3.0 · commercial licensing available. Generated MIDI is yours to use freely.

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