Beltfor Alchemy Lab

What it does

A voice goes in on J1. What comes out on J9 and J10 is that voice corrected to a key and scale, at a speed from transparent to robot, with up to four harmony voices that walk the scale from whatever note you sing, a stereo doubler, and a formant control that changes the size of the singer without changing the pitch.

It is the same C engine as Belt for the Ableton Move, recompiled for the Alchemy Lab: six pots on two pages, three settings knobs, three buttons, six CV jacks that read independently of the pots, and 102 LEDs. Fixed latency of 24 milliseconds, with the dry path delayed to match so the wet and dry stay in phase. It also brings firmware swapping from the SD card, so this module can carry several firmwares and change between them from a settings page.

The panel

Every hole sits where it sits on an Alchemy Lab. The coordinates come from the SDK's own KiCad front-panel template, not from a photo, and the drawing is generated from them. The PLAY page is what you reach for while singing; the SETUP page, the four voices, appears while B3 is held.

Turn a knob or press a button to see what it does.

Singing through it

1  Bring a voice up to line level and patch it to J1. J2 too if you have it;
   the engine listens to the two summed and keeps the dry path stereo.
2  Set KEY and SCALE. The ring around KEY lights the scale's notes;
   sing, and your note appears on it, green when you are on one.
3  RETUNE is the speed. 0 is the robot. Start around a quarter of the way up.
4  Hold B3. P1 and P2 are the first two voices' intervals:
   a third and a fifth above out of the box. Let go.
5  Bring up HARMONY. Sing a scale.

Hold B1 for HARD: correction instant and total, whatever RETUNE and AMOUNT say, for as long as you hold. Tap B1 to latch it; tap again to release. The B1 light goes white while HARD is on and is the tuner's colour otherwise. B2 is the harmonies' button: tap to latch the mute; hold it 0.6 s for HOLD, the voices kept on their notes. Both buttons stand down while B3 is held, so reaching for Settings never fires either.

FORMANT resizes the singer without moving the pitch: left is bigger, right is smaller, the centre is you. Half an octave each way. AMOUNT is how far towards the scale note the correction goes; at 100 it lands on it. The three engine parameters that did not fit the panel, FLEX, HUMANIZE and WET, are knobs in the Settings pages, and they are the ones to reach for if it sounds too corrected: FLEX leaves singing that is far from any scale note alone, HUMANIZE keeps your vibrato through the correction and gives each harmony voice a slow wander of its own.

What the voices sing

A harmony voice does not add a fixed interval. It takes your corrected note, finds its degree in the scale, and walks that many degrees up or down the scale, so a third above C in C major is E, and a third above D is F. This is the engine's own arithmetic on the page: pick a key, a scale and four intervals, then drag the note you are singing.

Chromatic is the exception: with no scale to walk, the intervals become fixed semitones, a third is four of them, a fifth seven. Octaves are always twelve semitones, whatever the scale. Unison is a voice on your own note, which with HUMANIZE up and SPREAD wide is a second singer. The engine's targets carry half of your vibrato; the doubler is a pair of streams ten cents either side of the corrected lead, panned hard.

Each voice also has a static detune of a few cents and a slow random wander, both scaled by HUMANIZE, so four voices do not sound like one singer copied. With HUMANIZE at zero they lock to the grid and sound like a keyboard, which is sometimes what you want.

The second page, and the settings

Belt has sixteen parameters and the panel has six pots. The four voices and the doubler live on the SETUP page, held on B3; the three you set once and leave, FLEX, HUMANIZE and WET, are knobs in the Settings pages.

SETUP — hold B3

Opening the page to look cannot change anything. The SDK's pot catch runs on the way in and on the way out: a SETUP pot only takes effect once you turn it through the value it already holds, and letting go of B3 hands the pots back to the PLAY page without jumping anything. Both pages, SETUP and PLAY, are saved to flash five seconds after you stop moving them, with hands off the buttons, and come back at the next power-up.

At boot the PLAY page keeps its saved values, and the pots catch them. The six pots are shared by every firmware on the card, so after a switch in the picker they point wherever the last firmware left them; adopting them would overwrite Belt's settings with Mark's or Smack's. So a pot does nothing until you turn it through the value Belt saved, then takes over. Only the first boot after flashing, with nothing saved, adopts the pots as they sit.

First boot after flashing seeds the SETUP page with a third and a fifth above, the other two voices off, no doubler, spread 70%.

Settings — hold B2 + B3 for two seconds

Settings pages at a glance

B1 steps to the next page when you let go of it and is lit orange while Settings is open; a press of B2 or B3 leaves. FLEX, HUMANIZE and WET apply when Settings closes, and so do the Chord page's P1 to P5; BRIGHTNESS, OCTAVE and the whole Sources page as you turn (the chord sources keep running while Settings is open); USB PORT at the next power-up. Every Settings value except the picker's three is saved in Belt's preset slot 14 about five seconds after Settings closes; USB PORT is also written to /alchemy/usb.cfg on the card, shared by every firmware on it. The panel drawing has all four pages: pick Settings 1 to Settings 4 above it and tap any pot, button or jack.

Leaving Settings counts as a change: brightness, FLEX, HUMANIZE and WET are saved with the pages five seconds later. The USB port setting goes to the card the moment Settings closes. On module 2 the Settings chord opens, but more slowly with audio patched than in Mark; the likely reason, not yet measured, is the seven voices leaving the control loop less time.

What it remembers

Belt keeps its working state, both pages, the Settings knobs and the last session's CPU readings, in preset slot 14, counting from zero (15 in HostLink's list). Each firmware in the family has a slot of its own: Smack 12, Mark 13, Belt 14, Relay 15. Picking Mark in the picker and coming back later finds Belt as you left it. Slots 0 to 11 are for presets saved by hand; saving one by hand into 12 to 15 overwrites that firmware's working state.

Builds before this one autosaved into slot 0. At boot Belt still reads slot 0 once, if it holds Belt's own settings (the schema guard refuses any other firmware's), so a module carries its settings over; once slot 14 holds anything, it wins. Switching away and back has been confirmed on the hardware in Mark and Smack; Belt's own round trip, and a cold power-cycle, are next on the test list.

The chords-all build (beta) also keeps the chord sequencer's progression and the CV inputs' learned offsets in slot 14. Its Settings pages and saved extras are a new shape, so its first power-up starts from Belt's first-boot defaults, once. HOLD, and which Launchpad page is up, are never saved.

Reading the rings

The SDK draws every pot's value on its ring in the colour of the page you are on. On top of that the firmware adds the tuner, three button lights and a report at boot.

The boot report

Seven pitch-shifting voices on a 480 MHz Cortex-M7 was the open question. You cannot watch a status light while singing, so the module measures its own worst-case load and shows it on the P1 ring for the first two and a half seconds of the next power-up.

A single dim blue LED means it has not measured anything yet. Green is under 75%, amber under 90%, red means it did not fit.

What the rack said. The first build on module 2 recorded a 100% worst block, and the control loop starved: the Settings chord stopped opening reliably. Two of the three known fallbacks fixed it. The engine is now compiled with faster float maths (its own file only; the SDK and libDaisy keep strict arithmetic), and its buffers moved from the external SDRAM into 160 KB of internal D2 SRAM, because seven voices reading the pitch-mark rings at scattered offsets every sample were waiting on SDRAM. The worst average load since is 63.7%, with no red on B2 and no clicks. The third fallback, fewer voices on by default, has not been needed. The worst single block, what this ring shows, has not been written down since the fix; the average is read over HostLink.

Every knob under CV

The Alchemy Lab reads pots and jacks separately and the SDK adds them in software, per page and per pot. Five jacks modulate the PLAY page; the sixth is a gate.

A slow LFO into J8 is the obvious first patch: the singer's size breathing while the pitch stays put. A sequencer into J3 makes HARD rhythmic. J4 steps the key from a sequencer, which changes what every harmony voice sings on the spot.

J3 is read raw in the audio callback, not through the SDK's 20 Hz pot filter, which would round a gate's edges into a swell. It fires on the magnitude of the swing away from the jack's calibrated zero, 1.5 V to assert and 0.5 V to release, so a gate counts whichever way the front end signs it. On module 2 a clock into J3 flashes B1 white in time; at fast rates the light stops matching, because the LEDs redraw at about 60 Hz while J3 is read every 2.7 ms, one 128-sample block. A pulse shorter than a block can fall between two reads, so give it 5 ms or more.

J4 to J8 have not been tried on the hardware yet. The mapping above is what the source sets up.

The alternatives in the table are Settings choices: J4 to J7 become volt-per-octave chord pitches with Chords from = CV, J3 the chord clock, J8 the HOLD gate. They are beta, in the chords section.

Chords: Hide and Seek, HOLD and chord sources

Out of the box the four harmony voices walk the scale from what you sing. They can also sing notes you hold: on the Launchpad, on a USB keyboard, from Belt's own chord sequencer, from CV on J4 to J7, or from another Lab over the rear header. Every one of those becomes the same held-note message to the engine, so they all behave alike. And HOLD keeps the voices on the notes they are on while you sing on.

Hide and Seek in 30 seconds

1  Hold B2 + B3 two seconds for Settings. Tap B1 to step to page 3, Chord.
   P1 MIDI NOTES stays on Harmony, the default. Turn P2 LEAD to 0. Press B3 to leave.
2  Hold B3 and turn P1 and P2, VOICE 1 and 2, fully left to Off
   (3 and 4 are off already).
3  Hold a chord: the Launchpad's PLAY page (top button 3), a USB keyboard, or
   Settings page 4 Chords from = Seq for Belt's own progression.
4  Sing. Each held note is your voice moved to that pitch, up to four at once.

It is a harmonizer, not a filter-bank vocoder: the engine finds your pitch and re-pitches your own voice, keeping its formants, which is why the words stay clear. LEAD at 0 takes out the corrected lead, the dry voice and the doubler together, so only the held notes sound. With LEAD up, the chord sits under your lead.

What a held note does

Notes 0, 1 and 2 are the engine's control notes (momentary HARD, doubler and harmony mute), not pitches; a USB keyboard's are skipped for that reason. A release always goes through, even with Settings open, so a note cannot stick because Settings was up when you let go.

HOLD

HOLD is on while the latch or the J8 gate is. B2 is blue while HOLD is on (dim blue if the harmonies are also muted), the Launchpad's harmony-voice pads turn blue, top 4 is bright blue, the XL's upper button 3 bright green. HOLD is never saved: a power-up always starts with the voices following you. B2 no longer mutes while it is held, because the 0.6 s of silence before a HOLD would drop the very voices being held; the Launchpad and XL MUTE buttons keep their hold-to-mute.

The Launchpad's PLAY page

In PLAY, rows 1 to 6 are three octaves of keyboard, two rows each with the sharps above the naturals as on the KEY page: rows 1–2 are C5 to B5, rows 3–4 C4 to B4, rows 5–6 C3 to B3. Every pad held is a held note at velocity 100 (the Mini MK3 has no velocity). Held pads are green, the key's root blue, in-scale notes dim blue. Rows 7 and 8 stay the harmony voices and the tuner. Leaving PLAY lets go of every held pad. Top 3 is dim green in KEY and bright in PLAY. Every power-up starts in KEY.

Where else held notes come from

The Launchpad's PLAY pads and a USB keyboard always play. On top of them, one more source at a time, chosen on the last Settings page. Changing it lets go of every note the old source held, and only those.

The Launchpad's CHORDS page

The playing chord is shown only while Chords from is Seq. The progression is saved with Belt's slot; key and scale come from the panel, triads or sevenths and the tempo from the Sources page. Changes are heard at once: the voices on notes the old and new chord share are left alone, and only the notes that change move.

CV chords

With Chords from on CV, Sources P6 CV INS chooses how the jacks are read. 0 V is the Octave's C (C3 out of the box) and each volt an octave up, either way.

A chord changes only once the voltage has clearly left the one it is on and stayed away 3 ms. In Chord mode that means another scale note is nearer than the held one by more than about 25 mV; per jack, the jack has gone more than half a semitone plus a little, about 54 mV. So a voltage parked on a boundary does not flicker, and a jump lands once, not on every step on the way. A change of KEY or SCALE re-snaps the chord at once. With J3 on Chord clock the jacks are read only on a J3 edge: sample and hold.

Calibration

Rounding leaves little room, ±41.7 mV to a semitone, and an Alchemy Lab's J3 to J6 are rated about ±50 mV as outputs. So the sending module's error is learned once, per input, and taken off. What the sender plays while you learn depends on the mode:

With the reference coming in, open Settings, page 4, and turn CV CAL to Learn. A quarter of a second later each input's offset, what it reads minus what it should, is stored in Belt's slot. Turn it back to On and leave Settings: for three seconds B3 shows the result, green every input the mode learns, amber some, red none. An input more than 0.25 V off is taken for a wrong patch, not an error, and keeps its old offset, so in the four-jack modes learn with all four patched. Only a turn into Learn learns; a slot saved with Learn showing does not relearn at power-up. Off reads the jacks raw.

Two Labs: MIDI over the rear header

Pin numbers are the positions on the SDK's header drawing. With Chords from on MIDI, Belt holds the notes that arrive, on any channel; a note-on at velocity 0 counts as a note-off. MIDI clock, start and stop are read and ignored. Belt only listens: its UART runs receive-only with no transmit pin, so Belt never drives its own pin 8, and a cable crossed both ways still works. The bytes are read in the control loop, not in the interrupt. With any other source, rear-header MIDI is read and dropped.

The sender is the Alchemy Lab sequencer (seq-alchemy) with its Chord out setting on MIDI or MIDI + clock: it sends the chord playing as three or four notes around middle C, the notes that leave switched off before the new ones come on, and shared notes left held. That exact byte stream, C, Am, F, G and a stop, has been run through Belt's receiver and engine on the Mac: Belt held the sender's notes every millisecond, and nothing after the stop.

USB-MIDI keyboards

With the USB port in Launchpad mode, a USB-MIDI device that is not a Launchpad Mini MK3 or a Launch Control XL is taken as a keyboard: any class-compliant one, an Arturia KeyStep 32 for instance, no set-up. Its notes are held notes, as on the PLAY pads, and its sustain pedal holds them; every channel and cable is merged. It plays with whatever Chords from says. Key presses are ignored while Settings is open; releases and the pedal never are.

If the host does not come up, the /lpdiag.txt report on the card has a keys: line: which slots are keyboards, their USB IDs, how many notes each holds, and the notes received and dropped since power-up.

Firmware on the card

Hermetic's firmware treats the SD card as a place for presets and files. This one also reads .bin files from it, and can write one into the module's firmware slot, verify it, and reboot into it, from a settings page, with the module on and in the rack. Smack and Mark for the Alchemy Lab carry the same page, so any of the three can hand the module to any other.

The gesture

USB and controllers

The front USB-C does one thing per power-up. By default it talks to the Mac: the web programmer, presets, the card. Switched over, the Lab becomes the USB host for a Launchpad Mini MK3, a Launch Control XL, a Haute42 gamepad and USB-MIDI keyboards, or a two-in, two-out USB audio interface.

The USB port setting

Hold B2 + B3 for two seconds to open Settings, tap B1 to the Firmware page, and turn P6, the bottom-right pot of Settings page 2, the Firmware page, next to the picker. It is a three-way selector:

What B2 says about the port

If the host does not come up, a report of what it saw is written to the card as /lpdiag.txt about 17 seconds after power-up, for reading back in Mac mode.

Hooking up controllers

While Settings is open the controllers do nothing: the pads, buttons and knobs are ignored and any held effect lets go. Knobs and faders move the pages' stored values, so what they set is saved like a pot move, and the panel pot then has to catch the new value before it takes over again.

Launchpad Mini MK3

Launch Control XL

Haute42

Hold a button and its effect plays; let go and it returns to dry. The last button pressed wins. Let it go while another is still held and that one takes over; with nothing held it is back to dry. The effects are a performance-effects stage on Belt's stereo output, after the voices. Belt has no tempo of its own, so the effects run at 120 BPM.

The stutters, reverse, tape stop and half speed play from a two-second record of the output that is always running, so they have something to work on the moment you press. Changes fade over about 1.5 ms, so a punch in or out does not click. The same stage and the same map are in Mark. They run in Launchpad mode only.

Download it

For the Alchemy Lab V2 only. This is a beta: it has been heard on one module, and the CV inputs and a cold boot are still to be tried.

Download belt_alchemy.bin  All releases and notes

• Install it through the front USB-C as below. To switch firmware from the card, also get Mark and Smack, and put all three .bin files in /alchemy on the microSD card.
• Belt keeps its working state in preset slot 14. If you saved something in that slot with another firmware, it is overwritten the first time Belt saves. Move it to another slot first.
• Custom firmware: Hermetic Modular did not write or test it, so please report problems here with an issue, not to them.

To build it yourself instead (a few minutes with the ARM toolchain installed):

git clone --recurse-submodules 
cd belt-alchemy
make libdaisy        # once
make                 # build/belt_alchemy.bin, ~341 KB
make test            # the engine on its allocation pool, and the 48 kHz tuner proof

Needs arm-none-eabi-gcc, make and dfu-util. libDaisy has nested submodules, so the clone must be recursive or the HAL is missing. make BENCH_USB=1 BUILD_DIR=build-bench builds a variant with HostLink on the Daisy Seed's own micro-USB, for a module out of its case on a bench bootloader; it is not the one to rack.

Installing

The Alchemy Lab's bootloader serves DFU on the front-panel USB-C, so the module stays in the rack. Once this firmware is on it, so does every update after.

make program-dfu

make program-live

How it is built

The engine's C file was not touched. Everything below is the layer between it and the Alchemy SDK, and the handful of decisions that shaped that layer.

Found something, or want it to do something it doesn't? Open an issue. A report from someone singing through it is worth more than anything measurable on a laptop.

Specifications

The rest of them

Belt started on the Ableton Move under Schwung. This is its second body, and one of three firmwares for this module that share the picker, the allocator and the shape of the code.