Component
Low-pass gate
v0.5.108
Low-pass gate: filter and volume together, with the after-glow of a vactrol.
Software v0.5.108
Overview
Low-pass gate: filter and volume together, with the after-glow of a vactrol.
A low-pass gate in the style of the Buchla 292: filter and amplifier in one, driven by a vactrol. In Both mode the same control sets brightness and volume together, so a note that dies away also gets duller, like a struck piece of wood or a plucked string.
Send a trigger to Ping and you get a short, natural "bonk" without any envelope: the classic West Coast percussion sound. Or patch an envelope or LFO into CV to open the gate slowly; it still closes with the lazy vactrol tail. Env gives that vactrol state as a CV for use elsewhere.
Controls
| Control | Range | Default | What it does |
|---|---|---|---|
| Open | 0 … 1 | 0 | How far the gate is open without a hit. |
| Decay | 0.02 … 4 s | 0.25 | How long the vactrol glows on: the tail of each note. |
| Mode | LP / Both / VCA | Both | LP: only the filter follows the vactrol; Both: filter and volume; VCA: only the volume. |
| Res | 0 … 1 | 0.1 | Filter peak, from none to a peak that rings along. |
| Level | 0 … 1 | 0.9 | Output level. |
Ports
| Port | Direction | Signal | What it carries |
|---|---|---|---|
| CV | in | cv | Opens the gate, 0 … 1, added to Open. |
| Ping | in | gate | Ping: a trigger flashes the vactrol for a short, natural hit. |
| In | in | audio | The audio to be gated. |
| Out | out | audio | The gated audio. |
| Env | out | cv | The vactrol state as CV, 0 … 1. |
How it works
A vactrol is a small lamp against a light-dependent resistor. It reacts quickly to light but recovers slowly in the dark, and more slowly the darker it gets. The model follows that: the vactrol state rises in about 1.5 ms and falls with a time constant that grows as it closes, scaled by Decay. A ping flashes the lamp well past fully open.
The state drives a two-pole state-variable low-pass filter that sweeps exponentially from 20 Hz (closed) to 20 kHz (open), and an amplifier. Mode picks what follows the vactrol: LP (only the filter), Both (filter and volume) or VCA (only the volume). Res sets the filter peak, from none (Q 0.6) to a peak that rings along (Q 6).
Runs on
- Teensy 4.1 (the MusicBrain Brain): about 3.5 % CPU per instance (one note, default settings), measured on firmware 0.5.108.
- The browser: the editor's simulator runs the same C++ code as WebAssembly, so what you hear while patching is what the Brain plays.
Try it
In the editor's example set: 🌊 West Coast (Turing + Folder + LPG), 🎛️ Buchla-stem (Complex + LPG), 🧪 Buchla voice, 🌊 West Coast.
Editor: https://editor.musicbrain.nl
Sources & licences
- Modelled on the behaviour of the Buchla 292 low-pass gate; our own model, not a circuit simulation.
- Own kernel
mmb_dsp/lpg.h, shared by the Brain and the browser, MIT licence.
Versions
- 0.5.95: first version, with ping, three modes and the Env output.