Weights

Random pitch CV for the Workshop Computer, with the randomness guided by a Music Thing 8mu: its faders are probabilities. Two voices wander at once. At every step each draws a note of the scale, an octave, a gate length and a ratchet, each weighted by a page of faders, and plays them as a 1V/oct pitch, a gate and an envelope.

Built from the same template as WaveSeq and CVSeq: the same 8mu handling and pickup, USB modes, knob pickup, web editor, 8mu sharing between editor windows, and presets. It works with an 8mu alone, no computer needed.

The 8mu

The four buttons choose the page. Each button has four pages: press it again for its second page (voice 2's), again for its envelope page, again for voice 2's envelope page, and again to go back to the first.

Faders are relative probabilities. A fader at zero is never chosen, and one twice as high as another is chosen twice as often. With a first page all at zero, every choice is equally likely. (The bottom two positions of a fader, out of 127, count as zero, as faders don't always read exactly 0.)

Two voices. Both voices use the first and third pages. The second and fourth pages belong to voice 2, once they are used:

• While every fader on a voice 2 page is at zero, voice 2 draws from the matching page of voice 1 as well. You get two different lines with the same probabilities.
• Raise any fader on a voice 2 page and voice 2 uses that page, independently of voice 1.

Each kind of page decides this on its own: voice 2 can have its own notes while sharing the octaves, gates and ratchets, for example. Pull every fader on a second page back to zero to share again.

Envelope pages. Each note's envelope can stray from the knobs. The ATTACK and DECAY faders are a spread from a quarter of the knob's time (fader 1) to four times it (fader 8), with faders 4 and 5 a little either side (÷4, ÷2.7, ÷1.8, ÷1.2, ×1.2, ×1.8, ×2.7, ×4), kept within each setting's range. PLUCK goes from half the knob's range less pluck to half more (−50%, −36%, −21%, −7%, +7%, +21%, +36%, +50%). DEPTH is the envelope's peak: fader 1 an eighth of full (about 0.6 V) up to fader 8 full (5 V), for accents. The faders are probabilities as on every page, but an envelope page all at zero is off: the knobs' envelope exactly, at full depth. They are drawn with the note: once a step, and again at every hit of a new-note ratchet (faders 5-8 on RATCHETS), so a held-note ratchet repeats its envelope.

Notes. One fader per note of the scale, from the root up. A scale with fewer than eight notes ignores the extra faders. No scale has more than eight, so every note has a fader of its own.

Gates. Fader 1 is a rest: no gate and no envelope, though the pitch still moves. Faders 2-7 hold the gate for 1/7 to 6/7 of the time to the next hit. Fader 8 is tied: the gate stays up into the next note, which arrives without a new envelope (with slew on, it glides in). A gate always falls at least a millisecond before the next hit, so every new note gets a fresh rising edge.

Ratchets. Faders 1-4 hit 1×, 1.5×, 2× and 3× a step, each hit on the step's note, octave and gate. Faders 5-8 hit 1.5×, 2×, 3× and 4×, and every hit draws a new note, octave and gate. 1.5× is two hits, the second two thirds of the way through the step, so it pushes against the beat.

Pickup. After a page change a fader does nothing until it reaches (or passes) the value already stored, so changing page never changes the probabilities by accident.

LEDs. For 3 seconds after a fader or button is touched, the LEDs show the page's probabilities, the likeliest fully lit. After that they show what is playing:

On a first page that voice 2 shares, voice 2's choice shows dimly too.

Without an 8mu the card plays its default probabilities, still under the panel controls.

Panel

Every knob has a dead zone at each end, so the whole range of every setting can be reached even if a knob doesn't quite read its ends. The six settings each keep their value: after the switch moves, a knob does nothing until it's turned to (or past) its new setting's value. LED 6 (bottom right) blinks fast while a knob is waiting. At power-up the knobs take over straight away for the switch's position; the others start at Major, ×1 and no slew (centre), or a 300 ms decay, the shortest attack and pluckiness halfway.

Scales (37), in order round Main: Major, Natural minor, Harmonic minor, Melodic minor, Dorian, Phrygian, Lydian, Mixolydian, Locrian, Major pentatonic, Minor pentatonic, Blues, Whole tone, Diminished, Bebop dominant, Hirajoshi, Fifths, Octaves, Raga Bhairav, Raga Todi, Raga Marwa, Raga Malkauns, Raga Hindol, Maqam Rast, Maqam Bayati, Maqam Hijaz, Maqam Saba, Just major, Just minor, Harmonic series, Subharmonic series, Slendro, Pelog, Pelog selisir, 5-TET, 7-TET, 8-TET. All have their root at C (0 V), and none has more than eight notes, so each note has its own fader. The ragas are in just intonation (pure ratios, as they're usually sung): Bhairav and Todi with their flat second and sixth, Todi's low minor third (32/27) and sharp fourth, Marwa (six notes, no fifth), pentatonic Malkauns and Hindol. Rast, Bayati and Saba have their neutral (quarter-tone) steps; Hijaz is in equal semitones. The harmonic series is harmonics 8-15 folded into an octave; the subharmonic series is its mirror, subharmonics 16 down to 9. Gamelan tunings vary from one orchestra to the next; these are typical values. 8-TET divides the octave into eight equal 150-cent steps. When the scale changes the Computer's six LEDs show its number (1-37) in binary for a second, LED 1 the lowest bit.

The other inputs are unused. LEDs: the top four show the 8mu's page (lit for a first page, blinking slowly for a second, fast for a third and faster still for a fourth), or with no 8mu or editor, LEDs 1 and 2 show the gates; the bottom two follow the envelopes.

Three ways to use it

The card picks its mode once, at power-up, so after plugging a computer in, power-cycle the module. This needs Computer Rev 1.1 hardware; older boards are always a USB device.

Web editor

Open web/index.html in Chrome or Edge, with the computer plugged into the Computer's USB socket and the module power-cycled, and press Connect card & 8mu. It's a single file and needs no network. The page reads the faders from the card, then sends every edit as it's made.

• Now playing. The scale, clock, slew and envelope settings live from the card (a dotted outline while a knob is waiting to pick up), and each voice's note, voltage, gate, ratchet and envelope. Internal tempo sets the card's own clock, 40-300 BPM, used while nothing is plugged into Pulse In 1 (it follows Main when that sets it with the switch held down).
• Faders. The sixteen pages, as on the 8mu, with each fader's meaning in the current scale (note names, or cents for microtonal scales) and its share of the probability. A second page says whether voice 2 is sharing the first, with buttons to copy the first page onto it or clear it to share again. A row of dots shows the 8mu's LEDs as the card would light them.
• Pitch. The last eight seconds of both voices.
• Presets. Six built in, plus your own, kept in the browser: save, update, rename, delete, export and import as JSON. A preset is all sixteen pages and the internal tempo (the scale, clock and envelopes stay on the knobs). Loading one sends it to the card.
• 8mu on the computer. Its faders and buttons drive the page with the same pickup as the card, and the page lights its LEDs as the card would. Several 8mus all drive it together. With another Workshop editor open too (CVSeq, WaveSeq), one 8mu drives whichever editor you last clicked in; the header shows 8mu: here or 8mu: elsewhere.

Without a card the page still edits and keeps presets, and plays a simulation of the card at the internal tempo.

The protocol is documented in sysex.h. The card and the editor must be the same version (protocol 3). Presets saved before the envelope pages load with them off.

Building

mkdir build && cd build
cmake .. && make

Needs the Pico SDK. EightMU.h and ComputerCard.h are copied from Demonstrations+HelloWorlds/PicoSDK/ComputerCard.

The engine (weights.h) and the 8mu handling (ui.h) have no hardware in them, and test/run_tests.sh builds and runs their tests, and a test of the whole card against stand-ins for the Computer, on a PC with any C++17 compiler.

Not yet

• Fader settings aren't saved on the card, so they're lost at power-off. Keep them as presets in the web editor and send them again after power-up.

Written with AI help (Claude Code).