A rock workstation with a generative songwriter. The electric guitar and the drums are sampled — velocity layers and round robins, shipped with the app, so it runs with no network. The bass and the acoustic are generated by a four-operator FM engine. What the guitars go through is modelled: valve preamps, tone stacks, power-amp sag, speaker cabinets, and a pedalboard wired in pedalboard order.

Quick start

1. Hit Write a Song — it arranges a whole song and plays it.
2. Change Style, Key, Scale, Tempo or Density, then write another.
3. Export it as MP3, WAV or MIDI.

Loop repeats the song; Auto-next writes a new one when it ends. Auto-next waits out the ring-out first — measured from where you left the delay and reverb, not guessed.

Styles

• Grunge — chorused clean verse, fuzz chorus, half-time drums, the quiet-loud drop.
• Metal — down-tuned palm-muted gallops, scooped mids, double kick, a solo that runs.
• Heavy Metal — harmonic minor, gallop into an anthem chorus, British valve amps.
• Death Metal — tremolo verses over blast beats, phrygian, down three semitones. The least dynamic thing here, on purpose.
• Groove Metal — slower and heavier, stop-start riffs locked to a syncopated kick.
• Rap Metal — single-note riffs full of stops, a sixteenth-note funk chorus, and a wah parked open in front of the gain as a giant mid filter.
• Progressive — clean arpeggios, organ underneath, a ride-led drummer, a smooth lead.
• Rock Ballad — cross-stick verse, arena chorus, the biggest quiet-loud gap here.
• British Rock — jangly, major-key, mid-forward guitars and a Hammond.
• Country Rock — boom-chick guitar over a two-step bass and kit, a twangy clean amp, the II7 and the ragtime circle, and a lead that bends like a pedal steel.
• Alternative — mixolydian, stop-start, sixteenth hats, more bite than gain.
• Punk — downstroke eighths, no bridge, no solo, over in two minutes.
• Stoner / Sludge — one riff, very slowly, through a fuzz with the low end left in.

Scales

The rock vocabulary — pentatonic, blues, the modes, harmonic minor — plus Spanish, Hungarian minor, Byzantine, Egyptian and Japanese. The scale writes the melody; the chords underneath still come from the style. So hirajoshi over a grunge progression is a grunge song with a very strange lead over it, which is the interesting version.

How a song is put together

Intro, Verse, an optional Pre-Chorus, Chorus, a second verse and chorus, a Solo or Bridge, a long final chorus, and an ending — riff-out, a guitar left ringing, or a dead stop on one hit. Two to three minutes, longer in the slow styles.

Verse and chorus are not one part at two volumes. Where a style has a quiet mode the verse plays the clean guitar through a clean amp and the chorus the rhythm guitar through two distorted ones — an arrangement decision, not a mix one. The metals, punk and stoner keep the wall up and take their contrast from the drums instead, because a death metal verse on a clean guitar is a different band.

Each song writes its own fader ride, one level per part per section, under your mixer. Dynamics have to live there: a driven amp is a compressor, so past the first few dB of gain, playing softer makes a guitar cleaner rather than quieter. Velocity still changes the tone; the level is set after the amp, where an engineer sets it.

The guitars

The electric is recordings — nearest note wins, so nothing is shifted more than a semitone and a half. The acoustic is generated instead, for a licensing reason rather than a musical one, and takes the same note events. What is synthesized is the playing: the pick attack, the palm mute, the bend, the slide, the vibrato. A sampler that only maps pitch to a file is a guitar-shaped organ, and the ear catches it at once.

The rhythm guitar is built twice — two amps panned hard apart, a few milliseconds and a few cents from each other. The width comes from the takes genuinely differing, which no stereo widener fakes. Everything distorted is voiced as power chords: two notes a fifth apart share most of their harmonic series and barely intermodulate, while a major third through the same clipping turns to mush.

The bass, and why it isn't just an amp

A bass through one amp disappears in a rock mix, and turning it up only makes the low end louder. What makes a line legible sits around 200–1000 Hz, exactly where two distorted guitars and a snare already are. So it runs a split rig: below 150 Hz clean, mono and gently compressed — the weight — and above it a driven amp and cabinet, which adds harmonics the ear reads as pitch and levels every note. That is what a SansAmp does, and why rock bass survives two guitars.

The part is played as well as placed: dead notes in the gaps, a slide into an off-beat note, hammer-ons, the odd octave, vibrato on anything held — all cut hard under the heavy styles, where ornaments turn to mud.

Four guitars, one mix

Two rhythm tracks, a lead and a clean share the same three octaves, so each owns a different place rather than a different volume. The rhythm pair goes hard left and right and dips a few dB at 1 kHz; the lead sits just right of centre with its mid hump intact, playing into the hole they left; the clean sits well left of the lead. All are high-passed above 105–130 Hz, which belongs to the bass and the kick — three instruments in that octave is the commonest reason a rock mix turns to porridge.

Amplifiers

Each channel is a full chain: an input high-pass that climbs with the gain (the low end is tightened before the distortion), a treble bleed, one or two clipping stages with a filter between them, a tone stack, power-amp sag and a cabinet.

The cabinet matters most and is most often left out: a waveshaper alone sounds like a fuzz box, and a 4×12 throwing away everything above about 5 kHz is what turns it into a guitar. The impulses are built by modal synthesis, identical every run, which is what lets an export match what you heard.

Gain drives the stage harder rather than bending the curve, which is what turning an amp up does — so a clean channel gets quieter when you play softly and a cranked one gets cleaner. Mid is the other one: scooped sounds enormous alone and vanishes under a band, which is the whole argument between a metal record and a British rock record.

Pedalboard

In the order a real board is wired. Wah first, before the amp, so the amp distorts what the filter did — an auto-wah, swept by how hard the note was picked. Phaser after the lead amp, chorus and tremolo after the clean one, the spring tank last. The spring chirps downward on every transient, because springs are dispersive.

The drums

Sampled from Karoryfer's Big Rusty Drums, a vintage Polish kit recorded close and overhead: four velocity layers, three takes of each, so a repeated hit is not the same file twice and a soft hit is a different recording rather than a loud one turned down. One deep snare supplies the hit, the ghost, the rimshot and the cross-stick. The mics are mixed in the library's own proportions, every voice is gated to a rock length, and the toms are tuned apart so a fill travels.

Turn Sampled kit off for the synthesized one — a kick that is shell, punch and beater click; a snare that is a tuned head and a set of wires. It also stands in for any drum whose samples cannot be reached, one drum at a time. The two are calibrated voice by voice, so switching changes the sound of the drums and not the balance of the band. Either kit runs into a bus compressor and a short parallel room, before the fader ride, so it glues the kit without levelling the quiet sections back up.

Chaos Engine

• ρ Lorenz — the pitch attractor behind the hook and the solo. Low circles one box shape, ~28 is the butterfly, high runs the neck.
• Chaos ↔ Taste — 0 lets the learned policy pick every note, 1 hands it to the attractor.
• Temperature — low is greedy and repetitive, high explores.
• r Logistic — the drummer's ornaments. Below 3.57 bars repeat; toward 4.0 no two are alike.

The policy's reward table is tuned for rock: the fourth, fifth, minor third and ♭7 score highest, and the major seventh scores close to nothing, because a rock lead does not play it.

The changes

Each style draws mostly from its own pool of progressions and the rest from the vocabulary every band of the period shared, so two styles at one seed are related without being the same. The signatures: metal's ♭II, a half-step above the tonic; progressive's dorian ♮6, heard as a major IV under a minor tonic; grunge's flat side and its unresolved tritone; British rock's mixolydian ♭VII; stoner's refusal to move at all, because there the riff is the harmony. Punk gets three chords and the speed does the arranging.

Every pool also carries changes borrowed from jazz: the minor ii–V–i with a ♭9 on the dominant, the tritone substitute, the backdoor ♭VII7, secondary dominants, the diminished seventh neoclassical metal runs on, and the descent in major thirds. The ♯9 does the work in the groove, rap-metal and stoner pools. The distorted guitars still play power chords through all of it; the colour lands on the clean guitar, the keys, the bass note and the notes the solo chooses.

Riffs and solos

Two riffs per song — one for the verses, one for the choruses — played verbatim, because a riff that changed every bar would be an improvisation nobody could hum. The solo is composed differently: faster note values, more licence to wander, and pinned to the minor pentatonic rather than to the chord, which is why a guitarist can solo over a whole song in one shape. The ♭5 appears only as a short passing tone.

Solos also break into two-handed tapping at about fourteen notes a second: the picking hand taps high on the neck, pulls off to a note the other hand holds, and hammers back on. A run crosses the changes rather than sitting on one shape — the tapped shape follows whichever chord is sounding and moves to the nearest note of the next, which is voice leading. Five shapes, and the shred styles reach for it often: some solos are led by it for several unbroken bars, and some have none of it at all. It is played rather than quantised — the pull-off comes fractionally early, the hammer-on late, and the last note is held and shaken.

Fretboard & Riff Sequencer

Both play the rig live through whichever channel the Voice selector names, so a chug you sequence goes through the same amp and cabinet the song does. Palm mute chokes the note the way a picking hand does — shorter, darker, picked nearer the bridge.

Seeds

Every song comes from a seed. Each press rolls a new one; close the lock beside it (or type a seed) to keep it — the same seed rebuilds the identical song, so you can change the amps against an arrangement you already like.

Master & exports

The desk rather than the rig: a plate reverb, the drum room, a tempo-synced tape delay (darker and slightly detuned on every repeat), a tone sweep across the mix, and a mix-bus compressor gluing the band together.

MP3 and WAV render offline — faster than realtime and sample-deterministic, so the file is the mix you set up rather than a recording of playback. Either asks how to write it out:

• Single file — the whole mix, with your levels and mutes applied.
• Separate instruments — one file per part, named after the seed. Same event list and lead-in for each, so the stems line up sample-accurately in a DAW, each carrying only its own cabinet, spring and delay tails.

MIDI writes one track per part with the drums on channel 10.

Credits & licence

Created by: Wilfredo Bigol Jr

Contact

Licence

The music is yours. All generated and exported audio is released under the Creative Commons CC0 1.0 Universal Public Domain Dedication. You may copy, modify, distribute and perform the work, even for commercial purposes, all without asking permission.

The engine is not. The Chaos Engine — the generative system behind this app — is the intellectual property of Wilfredo Bigol Jr. Reproducing, distributing or modifying it, in whole or in part, without permission is prohibited.

The drums are Karoryfer Big Rusty Drums, with the tambourine and cowbell from the Versilian Community Sample Library, and the electric guitar is Karoryfer Emilyguitar — all CC0 1.0. The bass and the acoustic guitar are not recordings at all — they are generated by a four-operator FM engine shared with the sibling 1981 FM Synth Studio. Everything ships with the app, so it runs with no network; samples/CREDITS.md records what was done to each.

MP3 encoding by LAME (lamejs 1.2.1), used under the LGPL-3.0 and kept as a separate, replaceable file.