ARP8

A web-based MIDI arpeggiator for the Korg phase8 acoustic synthesizer. Runs in Chrome on macOS via the Web MIDI API — no installer, no patching, no DAW.

Live: https://johnusher.github.io/Arp8/ — open in Chrome, allow MIDI, plug in your phase8.

What it is

The phase8 is Korg Berlin's eight-voice acoustic synth: eight steel resonators, electromechanically excited, swappable, tuneable. It's a beautiful instrument with a built-in step sequencer, but for "play around with chord progressions" the on-board UI is overkill. ARP8 is the opposite of that — a single page where you click a chord pad and a chord arpeggiates onto your phase8 over USB-MIDI.

It's built around the assumption that you're a music novice (chord pads labelled with both Roman numerals and names; key/scale aware; nothing outside the phase8's installed resonators), but the engine is general enough to drive any MIDI device.

The SONG button — 28 bundled compositions

Press SONG and ARP8 plays a complete generative composition. The transport has a song picker (◀ / amber title screen / ▶) — eight pieces, each a different musical territory using the same engine vocabulary differently:

#	Title	Style	Key	BPM range	The hook
1	Tines & Time	axis pop + Pachelbel + Cage	C maj	88 → 105 → 30 → 140 → 50	tempo crashes, psychedelic chaos bridge, sudden burst climax
2	Prelude	after Bach BWV 846	C maj	76 (locked)	continuous broken-chord arpeggios, no chaos, mathematical
3	Pulse	after Steve Reich	A min	132	one chord held for 12+ bars, octaves slowly morph, hypnotic
4	Drift	after Brian Eno	A min	48	chord pads with vast AIR, sparse phrases, lots of silence
5	Driver	after John Carpenter	A min	96	descending ostinato, build / drop / drive, cinematic minor
6	Gymnopédie	after Erik Satie	D min/dor	60	tender, simple, lots of pauses, soft velocity
7	Indeterminacy	after John Cage	C maj	35 → 200 (chance)	mostly single tines + silence, wild tempo swings, chaos = 0.8
8	Choral	after Arvo Pärt	C maj	50	tintinnabuli — held chord pads, long gates, sacred minimalism

All eight are written for the default phase8 install (C major C3-C4). The minor-key songs run in A minor and the modal piece in D Dorian — both share the C-major scale notes so every chord lands on installed tines.

Cycle past these eight and you reach the sacred / organ / drone catalogue — 20 hand-arranged pieces leaning into the phase8's strengths (sustained tones, modal harmony, slow-evolving texture). Most are played as actual melodies, not just rolled chord arps — the engine's pattern: "played" mode plays explicit MIDI note sequences in order, so you hear melodic lines instead of broken chords:

• Organ icons. Bach Toccata in D minor (BWV 565), Bach Passacaglia in C minor (BWV 582), Pachelbel Chaconne in F minor, Bach Cello Suite No. 1 Prelude.
• Sacred & choral. Bach Jesu, Joy of Man's Desiring · Schubert Ave Maria · Allegri Miserere mei, Deus · Tavener The Lamb · Hildegard von Bingen O Virtus Sapientiae (~1150 medieval chant).
• Pärt tintinnabuli. Spiegel im Spiegel · Für Alina · Cantus in Memoriam Britten · Fratres — slow, sparse, drone-based.
• Special / atmospheric. Pachelbel Canon (with melodic variations) · Bach Air on the G String · Satie Gymnopédie No. 1 · Albinoni Adagio in G minor · Carl Orff "O Fortuna" (with the iconic chord stabs and chant) · Górecki Symphony of Sorrowful Songs · Terry Riley In C (minimalist drone).

All transposed to C major (or A minor for the modal/minor pieces) so every note lands on an installed C-major C3-C4 resonator. Compositions are in src/song.js — each is hand-authored as a sequence of c() (chord-pad) and m() (played-melody) scenes, with rest() for silences.

What the engine drives during a song

• AIR slider (CC 30) automated per-scene — tight + dry intros, near-max during chorus territory, fully-open during ambient / Cage sections
• Mod Depth / Mod Rate (CC 28/29) swell from settled to psychedelic
• Envelope per resonator (CC 20-27) and Velocity per resonator (CC 12-19) broadcast at scene boundaries with a small per-slot random offset so the eight tines never behave identically
• Per-scene BPM overrides — sudden tempo drops to 30 BPM ("almost stopped"), bursts to 200 BPM
• Pauses — notes: [] empties the arp; the scheduler keeps ticking but nothing fires
• Cage-style single-tine repetition — notes: [48] makes the arp hammer just C3 for two bars; combined with a low BPM = stuck-tone trance
• chaos parameter (0..1) — random-walks global CCs every beat. 0 in the verses, spiked to 0.65-0.8 in the bridges and Indeterminacy

Each composition lives in src/song.js as a list of scenes; the engine is SongPlayer and runs every song end-to-end against the virtual-clock test harness (test/songs.test.js) to validate timing, CC payload, and pattern/rate correctness.

The song picker stops the current song and starts the new one if you cycle while playing. Pressing a chord pad mid-song aborts the song and resumes normal arp playback. The song's key/mode is restored to your previous selection when it ends.

Inspirations

The visual language is industrial / Berlin / Korg-Berlin. Deep concrete bg, brushed-steel panels, brass tine resonators, amber phosphor for the screens. A mix of:

• The phase8 itself. The brass-and-concrete colour palette, the eight tine bars in the top-left of the UI mirroring the physical resonators, the warm wood-inlay accent under the header.
• Make Noise faceplates. Bold, monospace typography. High-contrast labels. Confidence over polish.
• Mutable Instruments. Restraint. Nothing extra. Each control earns its space.
• Eurorack tactility. Knurled SVG knobs with conic arc indicators that you drag vertically (Shift = fine). Scroll wheel works too.
• Ableton Live's Arpeggiator. Its pattern library is the canonical one — Up, Down, UpDown, Converge, Diverge, Played, Random, Random-Other, Random-Once, plus a "Chord" pattern that fires every note simultaneously. ARP8 implements all of them.
• Vintage gear amber displays. The BPM screen, knob value readouts, and now-playing readout are all glowing amber on near-black, with subtle phosphor bloom.
• Berlin-techno colour-coded clip launchers. The seven diatonic chord pads each take a colour from a curated nine-stop palette so a chord progression becomes a visual phrase.
• John Cage's prepared-piano sonatas + "Music of Changes" — single-tine repetition, sudden tempo shifts, near-silence bordered by sudden bursts. The "Cage interlude" in Tines & Time and the entire Indeterminacy song are direct nods.
• Bach's Prelude in C, BWV 846 — the first known great arpeggiator demo from 1722. Prelude in the song catalogue follows its locked-tempo, no-chaos broken-chord aesthetic.
• Steve Reich's "Music for 18 Musicians" / Brian Eno's "Music for Airports" — long single-chord stretches with parameters slowly morphing, no harmonic motion. Pulse and Drift respectively.
• John Carpenter's score for "Halloween" / "Escape from New York" — minor-key descending ostinato, cinematic synth bass, dynamic builds and drops. Driver leans into this.
• Erik Satie's Gymnopédies / Arvo Pärt's tintinnabuli — tenderness through restraint, the spaces between notes carrying as much weight as the notes themselves. Gymnopédie and Choral respectively.

Features

• SONG button + 8-song catalogue — pieces inspired by Bach, Reich, Eno, Carpenter, Satie, Cage, Pärt; cycle with the ◀/▶ chevrons in the transport (see above)
• 11 arpeggiator patterns including all the Ableton standards
• 7 rates from 1/4 down to 1/32, plus 1/4T, 1/8T, 1/16T triplets
• Octave stacking 1×–4×
• Gate, swing, velocity as drag-knobs (Shift for fine)
• 16-step rhythm grid — click to mute / unmute steps; rests punch holes in the arp without losing position
• Latch toggle — chord-on-press vs. chord-stays-armed
• Snap-to-tines — quantises any chord note onto your installed phase8 resonators (preserving note class; F4 with no F4 tine becomes F3, never the chromatically nearest)
• Key + mode picker — generates the seven diatonic chords for any key in major or natural minor
• Phase8 mode selector — handles Korg's three "MIDI Note Assignment" modes (see below)
• Tap tempo + ±BPM, MIDI channel select, all-notes-off panic, and a TEST button that fires a single C3 to verify your MIDI path
• Live MIDI monitor in the footer showing the last byte sent
• Keyboard shortcuts: Space play/stop, 1-7 chord pads, Esc panic

Quickstart

The easiest way: open https://johnusher.github.io/Arp8/ in Chrome. That's the live version of main. You'll be prompted for MIDI access; allow it and plug in your phase8.

To run locally instead (for hacking on the source):

git clone git@github.com:johnusher/Arp8.git
cd Arp8
npm run serve     # http://localhost:8080

If you don't have a phase8: append ?demo to either URL to see the UI populated with mock state.

Single repo, single source of truth

There is exactly one repo (this one) and one branch that matters (main). Every push to main triggers .github/workflows/pages.yml, which runs the full 80-test suite and — if it passes — deploys the working tree as-is to GitHub Pages. There is no separate "live" repo, no build artifact to commit, no manual deploy step. The site is the source.

Browser support: Chrome / Chromium / Edge / Brave. Safari and Firefox don't ship Web MIDI as of 2026.

Retuning your phase8

The phase8 ships with 13 chromatically tuned steel resonators and you install 8 at a time in the eight slots. They're physically swappable — loosen the stabiliser screw with the included 2.5 mm hex key, slide one out, slide another in. Per manual §7.1.

You can also fine-tune each resonator by sliding it in or out of its mount: longer = lower pitch, shorter = higher pitch (manual §7.2). Tune by ear or against a tuner. After any swap or tune-up, calibrate with SELECT + power-on so the synth re-learns its scale.

This makes the phase8 a serious instrument for alternative tunings — equal-tempered scales beyond C major, just intonation, microtonal layouts, custom modes, even percussive setups where you intentionally detune the resonators away from any musical pitch. Korg Berlin has signalled future "resonator drops" with new tonal palettes (extra-bass packs, etc.) and the launch edition includes three precision-crafted experimental shapes. Communities at Superbooth showings have already shown people building their own scales on the unit.

What this means for ARP8: the chord engine currently assumes the default C major C3-C4 layout (phase8Tines() in src/chords.js returns [C3, D3, E3, F3, G3, A3, B3, C4]). If you've retuned to something exotic, you have two options:

1. Edit phase8Tines() in src/chords.js to list the MIDI pitches you've actually installed. Diatonic chord generation, snap-to-tines, and the on-screen tine labels will then reflect your custom scale. The chord pads may go out-of-range for many keys — that's not a bug, it's the constraint of your install. (Future: a UI editor for tine layout.)
2. Switch the phase8 to "Frequency Based" MIDI mode (SELECT + power-on calibration auto-assigns the MIDI map), then set ARP8's p8 mode dropdown to pitch · calibrated. Now MIDI on the wire matches your real pitches. Combined with edit (1) above, you get a fully consistent setup.

For non-pitched / percussive resonator setups, you're better off bypassing the chord engine entirely: use the "played" pattern, click chord pads as triggers (each pad sends a fixed slot set), and treat ARP8 as a polyrhythmic step trigger.

Phase8 MIDI mode — important

Per the phase8 manual §8.4 ("MIDI Note Assignment"), the synth ships in STATIC mode where the eight resonator slots receive on MIDI 36-43 chromatically, regardless of the pitch of the resonators you've installed. So a "C3 Note On" (MIDI 48) will be ignored by a default-configured phase8 even if you have a C3 resonator in slot 1.

ARP8 defaults to STATIC mode — pitches generated by the chord engine get translated to slot-MIDI before they hit the wire. The dropdown labelled p8 mode in the switches panel lets you switch:

Mode	What phase8 expects	When to pick it
slot · 36-43	MIDI 36-43 → slots 1-8	Phase8 factory default. Use this unless you've changed the synth's globals.
pitch · calibrated	MIDI matches resonator pitch	You've put your phase8 in "Frequency Based" mode (auto-calibrated on power-on).
slot +2oct · 60-67	MIDI 60-67 → slots 1-8	You've enabled the "MIDI Transpose" global parameter.

The tine UI always shows pitch (e.g. "C3"); the wire MIDI is whatever the selected mode dictates.

Architecture

Pure ES modules, no build step.

index.html, styles.css        — single-page UI
src/chords.js                 — note theory, diatonic chord generator, snap-to-tines, phase8 slot mapping
src/arp.js                    — pattern engine (Up / Down / UpDown / Converge / Diverge / Random*3 / Played / Chord)
src/scheduler.js              — Chris-Wilson-style lookahead scheduler (25 ms tick, 100 ms ahead),
                                clock and sender are injectable so it tests headlessly
src/song.js                   — "Tines & Time" composition + SongPlayer; per-scene BPM, explicit
                                notes for pauses & Cage repetition, phase8 CC table, chaos jitter
src/midi.js                   — Web MIDI bridge (device list, panic, sendCC, send-with-log)
src/app.js                    — UI controller; knobs, chord pads, tine animations, keyboard
serve.js                      — 30-line static-file server

The engine modules know nothing about the DOM or about Web MIDI. The scheduler takes { now, send, schedule, cancel } as its dependencies, so the test harness drives it with a VirtualClock and a MockMIDIOutput that records every byte stream sent. That's how 57 tests run in 200 ms with deterministic output — no flake, no setTimeout in the tests, no real MIDI required.

Testing

npm test

79 tests across 9 suites:

• syntax — node --check every src/*.js, plus an import-resolution sanity test for src/midi.js (which the unit tests don't otherwise touch)
• chords — note theory, diatonic chord generation in major / minor, snapToTines correctness including the note-class-preservation rule
• arp — every pattern, octave expansion, randomness-with-seed determinism
• scheduler — interval math, gate/swing/channel/velocity, per-step grid, onStep UI hook fires for rests too
• integration — full pipeline from chord pad to wire bytes
• flow — multi-bar progressions, latch behaviour, panic semantics
• phase8 mode — STATIC / FREQUENCY / TRANSPOSED translation tables
• song — composition shape (BPM variation, pauses, Cage scenes), scene timing under variable BPM, CC payload structure, chaos behaviour with seeded RNG
• songs — catalogue-wide validation: every song in SONGS has valid pattern/rate/octaves/gate/velocity/BPM at every resolved scene, sensible total duration, and plays end-to-end against the virtual clock

Browser support

• ✅ Chrome / Chromium / Edge / Brave on macOS
• ❌ Safari (no Web MIDI)
• ❌ Firefox (Web MIDI is behind a flag and unstable)

Web MIDI requires a secure context — localhost qualifies, so no HTTPS needed for local use.

License

MIT.