85 PlantHolder

By Phil Miller

Originally a companion for the Instruo Pocket Scion, now a generally fun stereo reverb that's also a USB MIDI host. Plug in the Scion or any USB MIDI device — the Computer will power it — and break MIDI channels 1 and 2 out as v/Oct + gate (route voices in the Scion companion app, or your device's MIDI Settings). The reverb is modelled on the Fairfield Circuitry Placeholder (EB) — three cross-coupled delay lines with a tone-tilted feedback path. DECAY self-oscillates around 2–3 o'clock, highly interactive with Size, Tone, LPF, and Mix. The name nods to the Scion's plant-biosonification heritage and to the Householder reflection matrix at the heart of the Placeholder's feedback delay network. Requires a Rev1_1 board (Q2 2025 or later) for USB host mode.

DownloadPlantHolder 0.2.0

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Panel

Controls

Main knob

Mix
Dry/wet balance. Held at its stored value while the MOD page is latched

X knob

Size
Delay-line length, 12–160 ms

Y knob

Ratio
Scales the length of delay lines 2 and 3 relative to line 1

Switch

Size / Ratio: X and Y set reverb SIZE and the length ratio between delay lines

Decay / Tone: X and Y set reverb DECAY and the TONE tilt

Mod Page: Hold Down for 1 s to latch the MOD page (LED 5). Main becomes the wet hi-cut and X/Y become MOD depth and type, until you hold Down again. While Down is held unlatched, the knobs keep their Middle meanings.

Inputs & Outputs

Inputs

Outputs

Inputs

Audio 1

Audio Left
Left channel audio input

Audio 2

Audio Right
Right channel audio input

CV 1

Decay CV
Bipolar trim added to the DECAY knob; full scale covers the whole range

CV 2

Size CV
Bipolar trim added to the SIZE knob; full scale covers the whole 12–160 ms range

Outputs

Audio 1

Reverb Left
Left channel reverb output

Audio 2

Reverb Right
Right channel reverb output

CV 1

Pitch Ch 1
v/Oct for MIDI channel 1, A4 = 0 V, with ±2-semitone pitch bend

CV 2

Pitch Ch 2
v/Oct for MIDI channel 2, A4 = 0 V, with ±2-semitone pitch bend

Pulse 1

Gate Ch 1
Gate for MIDI channel 1, last-note priority

Pulse 2

Gate Ch 2
Gate for MIDI channel 2, last-note priority

LEDs

Host Status
Solid when a USB MIDI device is mounted. Blinks when the board is older than Rev1_1 and cannot run USB host mode

MIDI Activity
Flashes on incoming note and pitch-bend messages

Gate Ch 1

Gate Ch 2

Mod Latched
Lit while the MOD page is latched

Controls

Main knob

Mix
Dry/wet balance. Held at its stored value while the MOD page is latched

X knob

Decay
Feedback amount, from a single reflection up into self-oscillation around 2–3 o'clock. Highly interactive with Size, Tone, LPF, and Mix

Y knob

Tone
Shelving tilt around 500 Hz, with a detent at noon. CW cuts lows and lifts highs. Sits inside the feedback path as well as on the wet output, so it doubles as a damping control

Switch

Size / Ratio: X and Y set reverb SIZE and the length ratio between delay lines

Decay / Tone: X and Y set reverb DECAY and the TONE tilt

Inputs & Outputs

Inputs

Outputs

Inputs

Audio 1

Audio Left
Left channel audio input

Audio 2

Audio Right
Right channel audio input

CV 1

Decay CV
Bipolar trim added to the DECAY knob; full scale covers the whole range

CV 2

Size CV
Bipolar trim added to the SIZE knob; full scale covers the whole 12–160 ms range

Outputs

Audio 1

Reverb Left
Left channel reverb output

Audio 2

Reverb Right
Right channel reverb output

CV 1

Pitch Ch 1
v/Oct for MIDI channel 1, A4 = 0 V, with ±2-semitone pitch bend

CV 2

Pitch Ch 2
v/Oct for MIDI channel 2, A4 = 0 V, with ±2-semitone pitch bend

Pulse 1

Gate Ch 1
Gate for MIDI channel 1, last-note priority

Pulse 2

Gate Ch 2
Gate for MIDI channel 2, last-note priority

LEDs

Host Status
Solid when a USB MIDI device is mounted. Blinks when the board is older than Rev1_1 and cannot run USB host mode

MIDI Activity
Flashes on incoming note and pitch-bend messages

Gate Ch 1

Gate Ch 2

Mod Latched
Lit while the MOD page is latched

Controls

Main knob

Wet Hi-Cut
1st-order low-pass on the wet output, sweeping 1 kHz to fully open. This is the pedal's F switch, which lives in its output section. MIX keeps whatever value it had before the page was latched

X knob

Mod Depth
Depth of the delay-time modulation, up to about 4 % of the delay length. Deliberately subtle, as on the pedal

Y knob

Mod Type
None / cyclical / random / both, in four steps across the knob

Switch

Size / Ratio: X and Y set reverb SIZE and the length ratio between delay lines

Decay / Tone: X and Y set reverb DECAY and the TONE tilt

Inputs & Outputs

Inputs

Outputs

Inputs

Audio 1

Audio Left
Left channel audio input

Audio 2

Audio Right
Right channel audio input

CV 1

Decay CV
Bipolar trim added to the DECAY knob; full scale covers the whole range

CV 2

Size CV
Bipolar trim added to the SIZE knob; full scale covers the whole 12–160 ms range

Outputs

Audio 1

Reverb Left
Left channel reverb output

Audio 2

Reverb Right
Right channel reverb output

CV 1

Pitch Ch 1
v/Oct for MIDI channel 1, A4 = 0 V, with ±2-semitone pitch bend

CV 2

Pitch Ch 2
v/Oct for MIDI channel 2, A4 = 0 V, with ±2-semitone pitch bend

Pulse 1

Gate Ch 1
Gate for MIDI channel 1, last-note priority

Pulse 2

Gate Ch 2
Gate for MIDI channel 2, last-note priority

LEDs

Host Status
Solid when a USB MIDI device is mounted. Blinks when the board is older than Rev1_1 and cannot run USB host mode

MIDI Activity
Flashes on incoming note and pitch-bend messages

Gate Ch 1

Gate Ch 2

Mod Latched
Lit while the MOD page is latched

README

PlantHolder — Computer Card

PlantHolder is a Workshop Computer card originally intended as a companion for the Instruo Pocket Scion. The name is a nod to two ideas: the Scion’s origins in plant biosonification, and the Householder-reflection feedback matrix at the heart of the Fairfield Circuitry Placeholder reverb this card emulates.

While it was originally designed for the Scion, it's just a generally fun reverb that's also a MIDI host. Use it with any USB MIDI device you like!

The reverb will self-oscillate at around 2–3 o'clock on the decay knob. This is highly interactive with Size, Tone, LPF, and overall Mix.

Features

USB MIDI host — plug the Pocket Scion or any USB MIDI device into the Computer’s USB port. The Computer's USB port will power the Scion, which is a nice bonus!
2-channel CV/Gate — MIDI channels 1 and 2 (route voices in the Scion companion app, or your device's MIDI Settings); v/Oct + gate with ±2-semitone pitch bend.
Reverb — three cross-coupled delay lines with a tone-tilted feedback path (Placeholder EB, 12–160 ms). DECAY runs from a single reflection up into self-oscillation.
CV — CV In 1 → DECAY, CV In 2 → SIZE.

Board requirement

USB host mode requires Rev1_1 or newer (Q2 2025+). On an older board the USB host is not started and LED 0 blinks slowly; on a Rev1_1 board LED 0 is off until the USB device is mounted.

Hardware connections

Computer jack	Role
Audio In 1/2	L/R audio input
Audio Out 1/2	L/R reverb output
CV Out 1 / Pulse Out 1	v/Oct + gate — MIDI ch 1
CV Out 2 / Pulse Out 2	v/Oct + gate — MIDI ch 2
CV In 1	DECAY (bipolar trim) — controls the reverb's decay time
CV In 2	SIZE (bipolar trim) — controls the reverb's size

Controls

Switch Up — SIZE / RATIO

Knob	Function
Main	MIX
X	SIZE (12–160 ms)
Y	RATIO

Switch Middle — DECAY / TONE

Knob	Function
Main	MIX
X	DECAY
Y	TONE

Switch Down (hold > 1 s) — MOD (LED 5)

Toggle with a 1 s hold on Down.

Knob	Function
Main	Wet hi-cut (1 kHz → open)
X	MOD depth
Y	MOD type — none / cyclical / random / both

Reverb topology

Per the EB Specifications:

mono in ──[HPF]──► 3 delay lines (SIZE / RATIO)
                     ├─ nested self-feedback, out of phase
                     └─ 4th path: Σ ──[TONE tilt]──► in phase
                    both scaled by DECAY, so self cancels
                              └──► Σ/3 ──[TONE]──[HI-CUT]──► stereo wet

All four feedback paths share the DECAY gain, so each line's own contribution cancels and what remains is pure cross-feedback — "every delay line feeds back only to every other delay line". The in-phase mode therefore has a gain of 2 × DECAY, which crosses unity around three quarters up the knob: that is the pedal's documented onset of self-oscillation at 2–3 o'clock. Past it the card sings, bounded by a soft limiter rather than running away.

TONE sits inside that fourth path as well as on the wet output, so it acts as a damping control — which is why oscillation is easier to provoke at either extreme of TONE than at noon.

Implementation: placeholder_reverb.h (PlaceholderReverb).

Known limitation: delay-line length is slewed one sample per sample with no fractional interpolation, so large SIZE sweeps pitch-shift audibly and deep MOD is grainy. Fine for slow moves; not a smooth-scrub delay.

LEDs

LED	Indication
0	Solid: Scion mounted. Blinking: board older than Rev1_1
1	MIDI activity
2	Gate ch 1
3	Gate ch 2
4	Unused
5	MOD page latched

Building

cd releases/85_plant_holder
cmake -S . -B build -DPICO_SDK_PATH=/path/to/pico-sdk
cmake --build build

Or, from inside the repo's dev container (.devcontainer/), just make — the harness stages the firmware into UF2/.

Flash UF2/plant_holder.uf2.

License

MIT — see LICENSE.

Credits

Fairfield Circuitry Placeholder (EB) — reverb topology.
rppicomidi/usb_midi_host.
ComputerCard — Chris Johnson.

Verify your download

Confirm the file you downloaded really is that new firmware.

macOS / Linux (Terminal)

shasum -a 256 firmware.uf2

Linux also has sha256sum firmware.uf2.

Windows (PowerShell)

Get-FileHash firmware.uf2 -Algorithm SHA256

Compare the result to the SHA256 on the website — it should match exactly.