Corrode

A TB-303 acid synthesizer voice for the Electro-Smith Daisy Patch Init

Corrode brings the iconic acid sound of the Roland TB-303 to a 10HP Eurorack module. Built on Robin Schmidt's Open303 DSP engine and optimized for the ARM Cortex-M7 processor on the Daisy Patch Init. No built-in sequencer: drive it from your favorite external sequencer via CV/Gate or USB MIDI.

Inspired by AcidusVersio by Blue Nautilus, which proved the Open303 engine runs on the Daisy platform. Corrode adapts the concept for the patch.Init()'s different control layout and adds modern C++ improvements, performance optimizations, and expanded CV integration for a deeply patchable Eurorack acid voice.

Features

• Authentic TB-303 sound — Diode ladder filter with feedback highpass, exponential envelopes, accent sweep circuit with capacitor accumulation, exponential slide/glide
• Sawtooth and square waveforms — Hardware toggle switch for instant waveform selection
• Dual-mode knob system — 4 knobs × 2 pages = 8 parameters, switched via button press
• Full CV/Gate integration — V/Oct pitch, filter cutoff CV, dedicated accent gate input, envelope mod CV
• USB MIDI — Complete note, velocity, and CC control over all parameters
• External audio input — Run external signals through the 303 filter
• Envelope follower output — CV output tracks the filter envelope for patching to other modules
• Optimized for ARM — Modern C++17, single-precision DSP, fast math approximations, stable anti-aliasing

Hardware Requirements

• Daisy Patch Init (10HP Eurorack module with Daisy Patch Submodule)
• Eurorack case with ±12V power supply (90mA on +12V, 5mA on -12V)
• External sequencer: hardware (Arturia Keystep, Torso T-1, modular sequencer) or DAW via USB MIDI
• USB Micro-B cable (for flashing firmware and USB MIDI)

Processor Specs (Daisy Patch Submodule)

Spec	Value
CPU	ARM Cortex-M7 @ 480 MHz
FPU	Hardware double-precision (FPv5-D16)
SDRAM	64 MB
Flash	8 MB QSPI
Audio Codec	PCM3060, 24-bit, 48 kHz
Latency	~1 ms (48-sample blocks)

Control Mapping

Physical Controls Overview

Control	Type	Count
Knobs (CTRL 1–4)	Potentiometer (0–3.3 V)	4
CV Inputs (CV 1–4)	Bipolar (±5 V)	4
Gate Inputs	Digital, active high	2
Gate Outputs	Digital GPIO	2
CV Output	12-bit DAC (0–5 V)	1
Toggle Switch	2-position	1
Button	Momentary	1
LED	User-controllable	1
Audio In / Out	Stereo (L/R)	1 each

Knob Assignments (Dual-Mode System)

Press the button to switch between Mode A and Mode B. The LED indicates the current mode: steady = Mode A, blinking = Mode B.

Mode A — Tone (Primary Sound Shaping)

Knob	Parameter	Range	Notes
CTRL 1	Cutoff Frequency	50 Hz – 12 kHz	The 303's signature control
CTRL 2	Resonance	0 – 100%	Diode ladder squelch; does not self-oscillate (authentic behavior)
CTRL 3	Envelope Mod	0 – 100%	Depth of filter envelope sweep
CTRL 4	Decay	30 ms – 3 s	Filter envelope decay time

Mode B — Character (Expression & Dynamics)

Knob	Parameter	Range	Notes
CTRL 1	Accent Level	0 – 100%	Accent strength (filter + VCA boost)
CTRL 2	Slide Time	10 ms – 200 ms	Exponential glide between slid notes
CTRL 3	Drive / Overdrive	Clean – saturated	Soft clipping / distortion amount
CTRL 4	Tuning	±1 semitone	Fine-tune oscillator pitch

Toggle Switch

Position	Function
Up	Sawtooth waveform
Down	Square waveform

CV Inputs

Jack	Function	Voltage	Notes
CV 1	V/Oct Pitch	±5 V (1V/Oct)	Primary pitch input from sequencer
CV 2	Cutoff CV	±5 V	Additive modulation of filter cutoff
CV 3	Envelope Mod CV	±5 V	Modulate envelope depth dynamically
CV 4	Accent CV	±5 V	Continuous accent amount; alternative to Gate 2 accent trigger

Gate Inputs

Jack	Function	Notes
Gate In 1	Note Gate	Rising edge triggers note-on; falling edge triggers note-off
Gate In 2	Accent / Slide Gate	High + Gate 1 rising edge = accented note. High while Gate 1 already high (legato) = slide to next note

Outputs

Jack	Function	Notes
Audio Out L	Mono synth output	Main 303 voice
Audio Out R	Mono synth output (duplicate)	Identical to L for easy stereo patching
CV Out	Filter envelope follower	0–5 V envelope output
Gate Out 1	Accent echo	Mirrors accent state
Gate Out 2	Note echo	Mirrors gate state

Audio Input (Filter Mode)

Jack	Function
Audio In L	External audio routed through the 303 filter, envelope, and VCA. When patched, replaces the internal oscillator.

USB MIDI

All parameters are controllable via MIDI CC messages over USB. MIDI note-on/off messages trigger the synth voice with velocity mapped to accent amount.

MIDI CC	Parameter
CC 74	Cutoff Frequency
CC 71	Resonance
CC 70	Envelope Mod
CC 75	Decay
CC 76	Accent Level
CC 5	Slide Time (Portamento)
CC 77	Drive
CC 94	Tuning

MIDI notes trigger the oscillator pitch directly (bypassing CV 1 when MIDI is active). Velocity > 100 triggers accent behavior.

Architecture Overview

┌─────────────────────────────────────────────────────────────┐
│                   CORRODE — Signal Flow                     │
│                                                             │
│  CV 1 (V/Oct) ──→ ┌──────────┐                             │
│  MIDI Note ──────→ │ Pitch    │──→ ┌─────────────────────┐ │
│  Gate In 1 ──────→ │ + Gate   │    │ Open303 DSP Engine  │ │
│  Gate In 2 ──────→ │ + Accent │    │                     │ │
│                    └──────────┘    │  Oscillator (BLEP)  │ │
│                                    │    ↓ Saw / Square   │ │
│  Audio In ──────────────────────→  │  [OR External Audio]│ │
│                                    │    ↓                │ │
│  CTRL 1/CV 2 ──→ Cutoff ────────→ │  Diode Ladder LPF  │ │
│  CTRL 2 ──────→ Resonance ──────→ │  (4-pole + FB HPF)  │ │
│  CTRL 3/CV 3 ──→ Env Mod ───────→ │    ↑ Filter Env     │ │
│  CTRL 4 ──────→ Decay ──────────→ │    ↓                │ │
│  Accent ──────→ Accent Circuit ──→ │  VCA Envelope       │ │
│  Slide ───────→ Glide Circuit ───→ │    ↓                │ │
│  Drive ───────→ Soft Clip ───────→ │  Output Stage       │ │
│                                    └─────────┬───────────┘ │
│                                              ↓             │
│                                    ┌─────────────────────┐ │
│                                    │ Audio Out L / R     │ │
│                                    │ CV Out (Env Follow) │ │
│                                    │ Gate Out 1/2        │ │
│                                    └─────────────────────┘ │
└─────────────────────────────────────────────────────────────┘

DSP Engine Components

The Open303 engine provides these core DSP blocks, each modeling a specific circuit in the original TB-303:

• BlepOscillator — Band-limited sawtooth and square waves via polyBLEP anti-aliasing. The 303's square wave has a distinctive non-50% duty cycle.
• TeeBeeFilter — 4 cascaded one-pole lowpass stages (24 dB/oct) with tanh saturation in the feedback path and a highpass filter (~120 Hz) in the resonance loop. The filter does not self-oscillate at maximum resonance, matching authentic 303 behavior.
• AnalogEnvelope — RC-circuit modeled exponential attack/decay for both filter (MEG) and amplitude (VEG). Accent forces the filter decay to minimum.
• Accent Circuit — Simultaneously boosts filter cutoff, increases VCA level, and shortens filter decay. The accent sweep circuit uses a capacitor that accumulates charge across consecutive accented notes.
• Slide Circuit — Exponential portamento via RC lowpass on the pitch CV. Slide also extends the gate (legato), preventing envelope retrigger.

Open303 Engine Modifications

See docs/open303_modifications.md for the full changelog. Key changes:

1. float conversion — All DSP from double to float for ~2× throughput on Cortex-M7
2. Anti-aliasing filter replacement — Elliptic quarter-band filter (numerically unstable in SP) replaced with 4th-order Butterworth
3. Fast tanh() approximation — Padé rational approximation: x * (27 + x²) / (27 + 9x²)
4. Fast exp() approximation — Optimized exponential for envelope coefficient calculations
5. Lookup tables — Pre-computed pitch-to-frequency table (2048 entries, linear interpolation)
6. Modern C++17 — constexpr, std::array, enum class, RAII, no raw pointers
7. Removed unused components — FourierTransformer, AcidSequencer, VST wrapper stripped
8. Block processing — Inner loops restructured for Cortex-M7 cache behavior
9. External audio input — Audio input routing through the filter/VCA chain
10. Drive stage — Soft-clipping distortion inspired by Devil Fish modifications

Build Instructions

Prerequisites

1. Install the Daisy Toolchain (ARM GCC, Make, OpenOCD)
2. Clone with submodules:

   git clone --recursive https://github.com/yourusername/corrode.git
   cd corrode

3. Build libDaisy and DaisySP:

   cd lib/libDaisy && make && cd ../..
   cd lib/DaisySP && make && cd ../..

Build & Flash

make                    # Build firmware
make program-dfu        # Flash via USB (hold BOOT button, then RESET)

To enter DFU mode: hold the BOOT button on the Daisy Patch Submodule, press and release RESET, then release BOOT.

Development

make clean              # Clean build artifacts
make DEBUG=1            # Build with debug symbols

License

GPL v3 — See LICENSE for details.

Acknowledgments

• Robin Schmidt — Open303 DSP engine
• Blue Nautilus — AcidusVersio, proving the Open303-on-Daisy concept
• Electro-Smith — Daisy platform, libDaisy, and DaisySP
• midilab — JC-303, modern Open303 maintenance
• Tim Stinchcombe — TB-303 diode ladder filter analysis
• Robin Whittle — Devil Fish modifications and accent circuit documentation