Plaits24 synthesis models behind four knobs, on the Alchemy Lab

This is Plaits, Emilie Gillet's macro-oscillator for Mutable Instruments, compiled for the Alchemy Lab. All 24 models are here: the eight added in Plaits 1.2, the eight pitched classics, and the eight noise, physical-model and drum models. The DSP is plaits::Voice from the eurorack repository, byte for byte. The firmware around it does what Plaits' own ui.cc and settings.cc did, on six pots, three buttons and ten jacks.

Untriggered, the model drones. Once a trigger arrives, on J3 or from B1, Plaits' low-pass gate closes between hits and its internal decay envelope runs, shaped by DECAY and LPG COLOUR.

The original has four big knobs, three attenuverters, two model buttons and a set of hidden settings reached by holding a button and turning a knob. Here the performance controls sit on the PLAY page. The attenuverters and the hidden settings move to a SETUP page you hold B3 to see. A single MODEL selector replaces the two model buttons, and B2 jumps between banks.

Out on J9, the model's auxiliary output on J10. The inputs J1 and J2 are unused: they are AC-coupled, and Plaits' FM input needs DC.

B1 · STRIKE. A press is a trigger, and holding it holds the gate, just as a gate on J3 would. It strikes the drums and physical models, clocks the Chiptune arpeggiator, and opens the low-pass gate. It is ORed with J3.

B2 · BANK. A tap jumps MODEL to the same position in the next bank: model 9 goes to 17, 17 to 1, 1 to 9. The stored value moves, so the ring follows and the change is saved, and the MODEL pot has to catch its new value before it takes over again.

B3 · SETUP. Hold it to show the SETUP page on the pots. Let go and you're back on PLAY. B2 stands down while B3 is held, so the Settings chord (B2 + B3 for two seconds) never jumps a bank on the way in.

A real Plaits sends a probe signal through its normalled jacks to tell a patched cable from an empty jack. The Alchemy Lab has no probe, so the firmware decides each "patched" flag another way.

TRIGGER is a Settings switch (Settings 1, P2):

• Auto (default): drones until the first trigger on J3 or B1 since power-up, then stays gated until the next power-up.
• Drone: never gated.
• Triggered: always gated, silent between hits.

A timeout back to drone was considered and rejected, because the gate would then open by itself after a quiet spell.

LEVEL CV (Settings 1, P5, off by default). When on, J4 drives the low-pass gate directly, replacing the internal envelope: an envelope, or a VCA CV. 0 V on J4 is then silence, which is why it defaults to off.

TIMBRE and MORPH (J6, J7) count as patched while more than 50 mV has been seen there in the last two seconds. The flag only chooses what the attenuverter scales: the CV when patched, the internal envelope when not. A wrong guess is audible but harmless.

FM is never patched: this port has no FM jack. So once the module is triggered, the FM attenuverter sets how far the internal envelope sweeps the pitch. On the Speech model it sets the prosody.

MODEL ring. The three banks as dim stretches of colour, with the playing model's LED bright, as described above.

Other rings. The SDK draws them from the knobs. FREQUENCY, FINE and the attenuverters are bipolar and fan out from a white centre dot, so noon is off. OCTAVE is an 11-dot selector.

B1 is white while the trigger (J3 or B1) is high. Otherwise it glows dim, brighter as the output gets louder.

B2 shows the active bank: orange, green or red. It turns red when the audio callback averages over 80 % CPU.

B3 has a dim Setup tint while you're on PLAY.

P1 at boot. For 2.5 s after power-up, the P1 ring shows the previous session's worst audio-block load. Green is under 75 %, amber is under 90 %, and red is 90 % or more. A single dim blue dot means there is no data yet, as on the first boot after a flash. CPU is the main thing to watch on this build.

The image is plaits_alchemy.bin. Put it in the /alchemy folder on the Lab's SD card, beside any other Alchemy firmware, and pick it from Settings page 2 (the Firmware page):

• P1 FILE picks the file.
• P2 FLASH writes it: turn it below 85 %, then past 95 %. It checks the file, writes it, verifies it and reboots.
• P3 DFU is the same gesture, and reboots into the bootloader instead.

The same picker is in Clouds, Elements and Marbles for Alchemy Lab, and in Mark, Belt and Smack.

Where it saves. Plaits keeps its working state in its own file on the card, /alchemy/state/plaits.st, and saves about five seconds after the last change, when no button is held. Switching to another firmware and back leaves Plaits as you left it. On return, each pot has to catch its saved value before it takes over.

Up to v0.1.0 it used preset slot 11; the first boot of v0.2.0 copies that onto the card. The other Lab firmwares keep their own slots, 1–5 and 12–15, so 6–11 are free for presets you save by hand.

First boot after a flash: Plaits' factory state. MODEL is on Virtual analog, OCTAVE on Free, decay at noon, LPG colour fully VCFA, attenuverters off.

What is proven

• The firmware builds: a 451 KB image, 93 % of the SRAM region. The engine is compiled for size (-Os) to fit. At -O2 it fits too, but leaves only about 13 KB.
• A native test renders all 24 models for one second each, drone and triggered: none is silent and none sits at the rails.
• The slowest model, Particle, took about 2 µs per 12-frame call on a desktop CPU under sanitizers, against a 250 µs budget. That only bounds the problem; it is not a measurement on the Lab.

Not yet tested on the module

• Nothing has been flashed or heard.
• CPU on the Lab's Cortex-M7 at -Os. Watch B2 and the P1 boot readout, especially on the 6-op FM models, Particle and Inharmonic string.
• Output level against a real Plaits. Full scale maps to the codec's full scale, which may be hot or quiet.
• The CV scaling on J4 and J6–J8, taken from Plaits' default calibration.
• Finding models by ear with 24 zones on one pot.

Left out

• User wavetables and DX7 banks sent as audio. The 6-op models use their built-in patches, as on a fresh Plaits.
• The FM jack, the MODEL CV, and Plaits' own calibration procedure. The Lab's factory CV calibration is used instead.
• The Launchpad, USB-audio and controller modes of the other Alchemy firmwares. This build is HostLink only. It never writes the card's USB setting, so the other firmwares keep theirs.