filter

filter · filters · schema v1

Multi-mode resonant filter (low / band / high). CV inputs sum into cutoff and resonance.

A multi-mode state-variable filter applying subtractive synthesis' bread-and-butter filtering to an audio signal. Three continuously-selectable modes (lowpass/bandpass/highpass) shape the spectral response; cutoff frequency and resonance (Q/emphasis) are both continuously modulatable via CV inputs for dynamic sweep effects. The cutoff CV input maps -1..+1 to ±5 octaves around the knob position via the Faust DSP itself, enabling smooth audio-rate modulation without requiring a separate CV scale stage. Typical voice structure: VCO → Filter → VCA, with an envelope or LFO patched into the cutoff and resonance CV inputs for expressive timbral shaping.

the faceplate

3 inputs · 1 outputs · 3 params

inputs

id	cable	what it does
`audio`	audio	The signal to be filtered — typically an oscillator output or other harmonically-rich source. The filter operates on this audio-rate signal. audio signal
`cutoff`	cv	CV input that modulates the cutoff frequency parameter at audio rate. A -1..+1 signal sweeps the frequency symmetrically around your fader setting by ±5 octaves (20 Hz to 20 kHz musical range), mapped internally by the Faust DSP source — no external scaling needed. Typical patch: envelope or LFO output → here, for dynamic sweep effects. control voltage (CV); modulates cutoff (summed directly (the destination DSP scales it))
`res`	cv	CV input that sums into the resonance (Q) parameter at audio rate, emphasizing energy near the cutoff frequency. Linear scaling over the 0..0.99 range — patch a slow LFO or envelope here to add vocal-like formant sweeps or self-oscillation dynamics. control voltage (CV)

outputs

id	cable	what it does
`audio`	audio	The filtered output signal, shaped by the selected mode, cutoff frequency, and resonance amount. Patch into a VCA, VCF, or the next stage of your voice chain. audio signal

params

id	label	range	default	curve
`cutoff`	Cutoff	20..20000Hz	1000	log
`resonance`	Res	0..0.99	0.1	linear
`mode`	Mode	0..2	0	discrete

controls

control	what it does
Cutoff	The corner / center frequency of the filter, ranging 20 Hz (sub-bass territory) to 20 kHz (silence / presence peak). Log fader so the travel lives where your ear cares most — most musical action clusters in the mid-range knob positions.
Mode	Discrete 3-position filter type selector (0 = LP, 1 = BP, 2 = HP). Lowpass (LP) lets low frequencies pass and rolls off above the cutoff — the textbook subtractive-synthesis filter, sculpting bright oscillators into warm, mellow tones by removing highs. Bandpass (BP) lets frequencies near the cutoff pass through and attenuates both below and above — useful for isolating a narrow spectral slice or creating resonant peaks for plucks and bells. Highpass (HP) lets high frequencies pass and attenuates below the cutoff — removes bass / rumble and brings out brightness, useful for thinning out sources or creating thin, nasal tones.
Res	Filter Q / peak emphasis, 0..0.99 — 0 is flat (no resonance, just frequency rolloff), raising it peaks the response near the cutoff (boosts that region, adds character), and at high values it can self-oscillate (the filter rings at cutoff indefinitely with no input signal). Resonance interacts with mode: highpass resonance can sculpt upper-midrange sheen, lowpass resonance can warm up oscillators by emphasizing fundamentals.

source

filter.ts on GitHub.