81_West_Coast_LPG

Dual vactrol-emulating low-pass gate for the Music Thing Workshop System Computer.

A low-pass gate (LPG) is the classic West-Coast building block: a combined VCA and low-pass filter, controlled together by a vactrol (an LED bonded to a photoresistor). The vactrol's slow, nonlinear light response gives the LPG its signature fast attack / slow exponential "plong" - strike it with a trigger and you get a naturally percussive, plucked decay; feed it CV and it behaves as a smooth combined VCA/VCF.

This card provides two independent LPGs, one per column of jacks.

Status: 0.1 The continuous CV hold and the VCA/VCF-only switch positions are only lightly tested - please open an issue/PR with fixes. The DSP is plain integer ComputerCard code (see src/vactrol.cpp).

Quick start - the gate starts closed

A vactrol low-pass gate makes no sound on its own. Each channel's gate sits shut until something opens it, so if you patch audio in and hear silence - or a very faint, muffled signal - nothing is broken; you just haven't opened the gate yet. Open it one of two ways:

Ping it - send a trigger/clock to Pulse In. Each pulse strikes the vactrol with a fast attack and slow decay (the percussive "plong"). This is the classic use; set the switch to Middle (LPG).
Hold it open - send a positive CV to CV In to keep the gate open continuously, turning the channel into a plain VCA/filter. It takes a healthy voltage: roughly +6 V opens it fully; a couple of volts only cracks it open (and stays quiet).

Two things that catch people out:

Match the column. Left jacks = channel 1, right jacks = channel 2, and the two channels are completely independent. Your audio, your ping, and your CV must all be in the same column to interact - audio in the left input is unaffected by a pulse in the right.
Switch = Up (VCF only) is the quietest "is it broken?" case. That mode has no amplitude gate, but the filter cutoff sits at its ~30 Hz floor until the envelope opens it, so an un-pinged channel passes only a faint, muffled rumble. Use Middle (LPG) to hear the full effect.

10-second sanity check: unplug the audio inputs and send a clock to Pulse In. With nothing patched to audio, the channel self-pings an internal noise burst, so you'll hear a plucky drum/marimba hit on every pulse - proof the card is alive and the gate is working.

Panel / I/O

Left column of jacks = channel 1, right column = channel 2.

Control	Function
Audio In 1 / 2	Channel input. If nothing is plugged in, the channel self-pings (see below).
Audio Out 1 / 2	Channel output (the gated/filtered signal).
Pulse In 1 / 2	Ping / strike the vactrol - gives the percussive plong envelope.
CV In 1 / 2	Continuous gate level - positive CV opens the gate (~+6 V = fully open; a few volts only partially opens it). Combined with the ping via `max()`, so triggers and a CV "hold" coexist.
CV Out 1 / 2	The vactrol "glow" envelope as a 0..+6 V CV - use it to modulate other modules.
Knob X	Channel 1 decay / response time (~10 ms … 3 s).
Knob Y	Channel 2 decay / response time.
Main knob	Resonance / "colour" of both filters (low = soft, high = pingy / near self-oscillating).
Switch	Per-card mode: Down = VCA only, Middle = LPG (VCA + VCF), Up = VCF only.
LEDs	Each column glows with that channel's envelope (the vactrol's light).

Patch ideas

Plucked / bonk voice - patch an oscillator into Audio In, a clock into Pulse In, set the switch to Middle (LPG). Each clock pulse plucks the note. Knob X/Y sets how long it rings; Main knob adds resonant "ping".
Self-pinging percussion - plug nothing into the audio inputs and send a clock to Pulse In. The card excites the channel with a short internal noise burst, so a resonant filter rings into a tom/marimba/zap. The switch mode changes the flavour: VCA only → noisy hat/thump, LPG/VCF → resonant pitched hit. Main knob = pitch/resonance, Knob X/Y = decay.
Envelope follower-ish / VCA duty - send a CV (e.g. an LFO or another envelope) to CV In to use a channel as a plain dynamic VCA/filter, and tap the smoothed vactrol envelope back out of CV Out.

How it works (brief)

Each channel models the vactrol as a one-pole follower with asymmetric time constants: a fast (~3 ms) attack and a slow, knob-controlled exponential decay. That conductance value (env) simultaneously drives the VCA gain and the cutoff of a resonant state-variable low-pass filter (cutoff sweeps exponentially ~30 Hz … 12 kHz with the envelope). Filter coefficients are refreshed at a decimated control rate to keep the per-sample audio path cheap; the SVF uses 64-bit internals so high resonance can't overflow.

Build

Requires the RPi Pico SDK with PICO_SDK_PATH set (same as the other ComputerCard cards):

cd releases/81_West_Coast_LPG/src
mkdir -p build && cd build
cmake ..
make

This produces vactrol.uf2. Flash it to the Workshop System Computer in the usual way: hold BOOTSEL, then copy the .uf2 onto the mounted RPI-RP2 drive.

macOS tip: if the RPI-RP2 drive ejects the instant you start copying (in Finder or from the terminal with cp) and the firmware never lands, flash over USB instead - it bypasses the mass-storage drive entirely:
brew install picotool
# board in BOOTSEL mode:
picotool load -v -x vactrol.uf2

Built with Chris Johnson's ComputerCard library.

Workshop Computer Program Cards

West Coast LPG

README