Chord Drone Firmware for the Acid Rain HEX

Custom firmware for my Acid Rain Technology HEX, running Ripsaw, turning it into a four-voice chord drone instrument. One V/oct input plays and transposes a whole chord; the encoder moves through voicings and chords. The synthesis engine, calibration, and settings storage are untouched.

Sound

Background

My live set leans on one specific chord combination, and I needed a simple way to generate those chords with variations. First attempt was Morphagene, recording the chords in by hand. Then I tried writing firmware for a Noise Engineering Librae Legio, whose Legio platform is open for custom development through libDaisy. What I got out of it was not usable enough.

The Ripsaw had been out of my rack for a while. It is already a four-voice oscillator, so it needed less changing than anything else I had.

The modification is specific to that chord combination. Rewriting it for a different set would be easy, but this covers my upcoming live sets.

Patches

Acid Rain publishes firmware for HEX but not source or an SDK, so there is nothing to build against and everything here is patched into the compiled application. The module is an RP2040 with 2 MB of flash.

The patch list below is mine. The firmware it applies to is Acid Rain’s, so you supply that from a dump of your own module.

Dump your own module, check it matches, then apply:

import json, hashlib
img = bytearray(open("hex-full-flash.bin", "rb").read())
man = json.load(open("hex_chord_drone_patches.json"))

assert hashlib.sha256(img).hexdigest() == man["original_full_flash_sha256"]

for p in man["patches"]:
    off, orig = int(p["flash_offset"], 16), bytes.fromhex(p["original_hex"])
    assert img[off:off + len(orig)] == orig, p["name"]
    img[off:off + len(orig)] = bytes.fromhex(p["candidate_hex"])

open("hex-chord-drone.bin", "wb").write(img[:0x18000])

The result should hash to the expected_result value in the manifest. That is the image flashed and read back on my module, with calibration and settings unchanged.

Controls

Control	Behavior
First V/oct input	Transposes all automatically generated chord voices together, one octave per volt
V/oct inputs 2–4	Override their corresponding voices individually
Top encoder, normal mode	Selects five octave voicings of the current chord
Short encoder press	Switches between voicing and chord selection
Long encoder press	Resets to the original suspended chord and first voicing

The former fine-tuning mode became chord selection. Everything else kept its existing behavior.

Chords

Five choices, shown with compact LED labels:

Display	Notes relative to C
S2	C–D–F–G
P5	C–G–C–G
7S	C–D–G–B♭
m7	C–E♭–G–B♭
M7	C–E–G–B

Pitch

With the default chord and voicing, 0 V produces C2–D2–F2–G2, and +1 V raises the whole chord to C3–D3–F3–G3. Fractional voltages transpose continuously. Range is MIDI 12–96.

Each input keeps its own original calibration.

Signal path

V/oct input 1Plays / transposes

→

Chord + voicingSelected with the encoder

→

Four voicesChord drone

V/oct inputs 2–4 override their corresponding voices individually. The synthesis engine, calibration, and settings storage are unchanged.