qbrt

qbrt · filters · schema v2

Stereo state-variable filter with vactrol-style ping input.

A stereo resonant filter with a 'ping' excitation input — the project's big-knob VCF, also used as a pluck/drum resonator. Each channel runs a state-variable filter whose mode crossfades continuously between low-pass and band-pass, with a big resonance (Q) control that can sing right up to self-oscillation. The twist is the PING input: a trigger fires a short vactrol-shaped impulse into the filter, so it rings at its cutoff frequency for a moment and then settles — patch a drum sequencer into PING and sweep CUTOFF and you get kick/tom-style pluck-resonator sounds with no oscillator at all. Ignore PING and it's an ordinary stereo filter you patch audio through. (This is the resonator RIOTGIRLS uses internally.)

the faceplate

7 inputs · 2 outputs · 4 params

about this module

Hover a control, or open the patch panel and hover a jack.

inputs

id	cable	what it does
`L`	audio	Left audio input — the signal fed through the left filter channel. audio signal
`R`	audio	Right audio input — the signal fed through the right filter channel. audio signal
`ping`	gate	Ping trigger: each rising edge fires a short vactrol-shaped excitation impulse into both channels, making the filter ring at its cutoff frequency (a pluck). Drive it from a clock or drum sequencer for resonator-style drum hits; the PING DECAY control sets how long each ring lasts. gate / trigger
`cutoff`	cv	CV that displaces the CUTOFF (center frequency) around the knob, log-scaled so ±1 sweeps about ±5 octaves — patch an envelope or LFO here for filter sweeps, or to pitch the ping resonance. control voltage (CV); modulates cutoff (multiplicative ≈ octaves — ±1 CV spans the param’s log range)
`resonance`	cv	CV that displaces the RESONANCE (Q) around the knob, so a modulator can push the filter toward or away from self-oscillation. control voltage (CV); modulates resonance (additive offset — ±1 CV sweeps the full range, centered on the knob)
`mode`	cv	CV that picks the filter MODE (low-pass vs. band-pass) in discrete steps — useful for switching response under gate control. control voltage (CV); modulates mode (integer buckets — CV selects a discrete step)
`pingDecay`	cv	CV that displaces the PING DECAY (ring length) around the knob, log-scaled — modulate it to make ping hits longer or shorter per trigger. control voltage (CV); modulates pingDecay (multiplicative ≈ octaves — ±1 CV spans the param’s log range)

outputs

id	cable	what it does
`L`	audio	Left filtered output — the left channel after the resonant filter (and any ping ring). audio signal
`R`	audio	Right filtered output — the right channel after the resonant filter (and any ping ring). audio signal

params

id	label	range	default	curve
`cutoff`	Cut	20..20000Hz	1000	log
`resonance`	Res	0..0.99	0.7	linear
`mode`	Mode	0..1	0	linear
`pingDecay`	Ping	0.005..0.5s	0.15	log

controls

control	what it does
Cut	CUTOFF — the filter's center/corner frequency (20 Hz–20 kHz); also the pitch the filter rings at when PING fires.
Mode	MODE — crossfades the filter response continuously from low-pass (0) to band-pass (1); band-pass narrows the passband around CUTOFF for a more vocal/ringing tone.
Ping	PING DECAY — how long the ring lasts after each PING trigger (5 ms–0.5 s); short for clicky percussive plucks, long for sustained resonant tones.
Res	RESONANCE — the filter Q: higher values sharpen the peak and emphasize the ring, approaching self-oscillation near the top.

source

qbrt.ts on GitHub.