Fr330hfr33

Fr330hfr33 is a hardware-tested acid bass instrument for the Music Thing Modular Workshop Computer. It combines a saw or square oscillator, resonant diode-style filter, accent, glide, distortion, USB MIDI, and a persistent 16-step sequencer.

Current release

Version 0.9.1:

uf2/Fr330hfr33-v0.9.1.uf2

SHA-256:

aff2cc66575f2d9bfd661f6b7106e63f905338911620f1fbb5e1067afaf63efa

The release has passed functional, USB disconnect/reconnect, persistence, extended-running, and processor-stress hardware tests.

Playing the card

The three knobs retain the same jobs in all modes:

• Main: filter cutoff
• X: resonance
• Y: envelope decay and glide time

Main has a wide performance sweep. The lower range is dark and plucky, while the upper half continues opening progressively rather than becoming fully bright around noon. The filter can be switched between 24 dB/octave four-pole and 18 dB/octave three-pole responses.

X moves from gentle body and nasal emphasis into strong resonance and self-oscillation. Y moves from short, immediate notes to longer envelopes and more pronounced slides.

Switch up — CV and MIDI

• CV In 1: 1V/oct pitch
• Pulse In 1: gate
• Pulse In 2 high: manual glide
• USB MIDI notes take priority while held
• MIDI velocity 112–127 triggers accent

Overlapping MIDI notes create 303-style legato slides without retriggering the envelope. The most recently played held note has priority.

Accent makes the note louder and brighter. Closely spaced accents accumulate filter-sweep energy, so a run of accents can rise higher than an isolated accent.

In USB host mode, MIDI CC1 controls cutoff. The Main knob uses pickup: it resumes control only after crossing the current CC1 position, avoiding a jump. CC1 does not control cutoff in USB device mode.

If USB MIDI is interrupted, held MIDI state is cleared and the gate closes. USB reconnection or a new physical gate restores normal control.

Switch middle — sequencer

The sequencer can run as a scale-aware generator or as an editable pattern of 1–16 steps. It uses the internal tempo unless it receives an external clock on Pulse In 2; optional MIDI clock advances it every 12 MIDI clock ticks.

Editable patterns have separate lanes for:

• Note
• Octave
• Accent
• Gate length
• Tie

Patterns support Initial Step, forward, reverse, and pendulum playback. Individual lanes can be randomized, with note choices taken from the selected scale.

Four pattern slots and the general editor settings persist across power cycles. In sequencer mode LED 3 identifies the active slot:

• One pulse: slot 1
• Two pulses: slot 2
• Three pulses: slot 3
• Four pulses: slot 4

The Acidness control coordinates melodic movement, octave jumps, gate length, accent density, and slides. Acidify Pattern applies that character to the editable pattern while keeping the result fixed until edited again.

Switch down — battery pull

Holding the switch down simulates pulling the batteries from a running TB-303. Gate, pitch, filter, resonance, and both audio outputs collapse through an audible dying tail. Releasing the switch restores power smoothly.

Sound options

The Web MIDI editor provides:

• Saw or square oscillator
• 24 dB or 18 dB diode-style filter
• Distortion off, RAT-style hard clipping, or Tube Screamer-style soft clipping
• Distortion amount and tone
• Scale, root, base octave, range, accent probability, gate length, and glide
• Internal tempo, MIDI channel, and MIDI clock sync
• Pattern lanes, playback direction, Initial Step, pendulum mode, and slots
• Acidness and Acidify Pattern

RAT and Tube Screamer modes contrast most clearly with the saw. The square is already strongly rail-shaped, so the two clipping styles can sound similar, although their tone controls remain effective.

The editor is split into Sound and Sequencer pages. Connection controls, status, and Apply remain at the top of both pages.

Connections and indicators

Outputs:

• Audio Out 1: enveloped, filtered, and optionally distorted voice
• Audio Out 2: phase-aligned raw selected oscillator
• CV Out 1: calibrated 1V/oct pitch
• Pulse Out 1: current gate
• CV Out 2 and Pulse Out 2: unused

LEDs:

• LEDs 1, 3, and 5: CV/MIDI, sequencer, and battery-pull modes
• LED 2: gate
• LED 4: accent
• LED 6: note brightness; fully lit in 18 dB filter mode

Web MIDI editor

Launch the hosted Fr330hfr33 editor

Use Chrome or Edge with Web MIDI and SysEx enabled. Connect with a USB-C data cable, close other applications using the MIDI port, select Fr330hfr33, and press Apply.

To serve the editor locally:

python3 -m http.server 5173 --directory web_config

Then open:

http://localhost:5173/Fr330hfr33.html

Building

Prerequisites:

• CMake
• Arm GNU toolchain (arm-none-eabi-gcc)
• Raspberry Pi Pico SDK

Set the Pico SDK path and build:

export PICO_SDK_PATH=/path/to/pico-sdk
cmake -S . -B build -DCMAKE_BUILD_TYPE=Release
cmake --build build -j

The generated firmware is:

build/Fr330hfr33.uf2

If pico-sdk and 303bass are sibling folders, setting PICO_SDK_PATH is optional.

The Web MIDI SysEx format is documented in protocol.md.