Cagire/docs/engine_filters.md

Filters

Filters attenuate frequencies above or below a cutoff point.

Lowpass Filter

The lowpass filter (lpf) attenuates frequencies above the cutoff.

saw 1000 lpf .               ( cut above 1000 Hz )
saw 500 lpf 0.8 lpq .        ( with resonance )

Parameter	Range	Description
lpf	Hz	Cutoff frequency
lpq	0-1	Resonance (peak at cutoff)

Highpass Filter

The highpass filter (hpf) attenuates frequencies below the cutoff.

kick 200 hpf .               ( cut below 200 Hz )
pad 400 hpf 0.3 hpq .        ( with resonance )

Parameter	Range	Description
hpf	Hz	Cutoff frequency
hpq	0-1	Resonance

Bandpass Filter

The bandpass filter (bpf) attenuates frequencies outside a band around the center frequency.

noise 1000 bpf 0.7 bpq .     ( narrow band around 1000 Hz )

Parameter	Range	Description
bpf	Hz	Center frequency
bpq	0-1	Resonance (narrower band)

Filter Slope

The ftype parameter sets the filter slope (rolloff steepness).

Value	Slope
1	12 dB/octave
2	24 dB/octave (default)
3	48 dB/octave

saw 800 lpf 3 ftype .        ( 48 dB/oct lowpass )

Filter Envelope

Filters can be modulated by an ADSR envelope. The envelope multiplies the base cutoff:

final_cutoff = lpf + (lpe × envelope × lpf)

When the envelope is at 1.0 and lpe is 1.0, the cutoff doubles. When the envelope is at 0, the cutoff equals lpf.

saw 200 lpf 2 lpe 0.01 lpa 0.3 lpd .   ( cutoff sweeps from 600 Hz down to 200 Hz )

Parameter	Description
lpe	Envelope depth (multiplier, 1.0 = double cutoff at peak)
lpa	Attack time in seconds
lpd	Decay time in seconds
lps	Sustain level (0-1)
lpr	Release time in seconds

The same pattern works for highpass (hpe, hpa, etc.) and bandpass (bpe, bpa, etc.).

Ladder Filters

Ladder filters use a different algorithm (Moog-style) with self-oscillation at high resonance.

saw 800 llpf 0.7 llpq .      ( ladder lowpass )
saw 300 lhpf 0.5 lhpq .      ( ladder highpass )
saw 1000 lbpf 0.8 lbpq .     ( ladder bandpass )

Parameter	Range	Description
llpf	Hz	Ladder lowpass cutoff
llpq	0-1	Ladder lowpass resonance
lhpf	Hz	Ladder highpass cutoff
lhpq	0-1	Ladder highpass resonance
lbpf	Hz	Ladder bandpass cutoff
lbpq	0-1	Ladder bandpass resonance

Ladder filters share the lowpass envelope parameters (lpe, lpa, etc.).

EQ

The 3-band EQ applies shelf and peak filters at fixed frequencies.

kick 3 eqlo -2 eqhi .        ( +3 dB at 200 Hz, -2 dB at 5000 Hz )
snare 2 eqmid .              ( +2 dB at 1000 Hz )

Parameter	Frequency	Type
eqlo	200 Hz	Low shelf (dB)
eqmid	1000 Hz	Peak (dB)
eqhi	5000 Hz	High shelf (dB)

Tilt EQ

Tilt EQ applies a high shelf at 800 Hz with up to ±6 dB gain.

pad -0.5 tilt .              ( -3 dB above 800 Hz )
hat 0.5 tilt .               ( +3 dB above 800 Hz )

Parameter	Range	Description
tilt	-1 to 1	High shelf gain (-1 = -6 dB, 0 = flat, 1 = +6 dB)