Filter — `GDVP_NODE_FILTER` ✅

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A TPT-SVF — Topology-Preserving Transform State-Variable Filter with Zero-Delay Feedback. It is unconditionally stable across the entire Nyquist range, including self-oscillation, and offers four response types and a cascaded slope engine from 6 to 24 dB/oct. Source: gdvp_dsp_filter.c, payload gdvp_node_filter_t in gdvp_nodes.h.

Signal & ports

Audio in (port 0): the signal to filter.
Control in (port 1): modulation of cutoff — the classic envelope-to-cutoff or LFO-to-cutoff routing in stock patches wires here.
Output (port 0): filtered audio.

Parameters

Parameter	Param ID / patch key	Range	Meaning
Mode	patch `mode` / `GDVP_PARAM_FILTER_TYPE_CV` (NRPN 2/1)	0–3	`0`=Low-pass, `1`=Band-pass, `2`=High-pass, `3`=Notch
Cutoff	`GDVP_PARAM_FILTER_CUTOFF_CV` (CC 74) / patch `cutoff`	0–16383	Corner frequency; perceptually mapped (see cutoff curve)
Resonance	`GDVP_PARAM_FILTER_RESONANCE_CV` (CC 71) / patch `resonance`	0–16383	Emphasis at cutoff; high values self-oscillate
Slope	`GDVP_PARAM_FILTER_SLOPE` / patch `slope`	0–3	`0`=6, `1`=12, `2`=18, `3`=24 dB/oct (see slope)
Drive	`GDVP_PARAM_FILTER_DRIVE_CV` (NRPN 2/0)	0–16383	Pre-gain into the saturation stage
Saturation type	(node `saturation_type`)	enum	Saturation curve applied in the filter path

Filter envelope parameters (when a dedicated filter envelope is used)

The canonical map also defines a full filter-envelope set routed via CC: Attack (CC 75), Decay (CC 76), Sustain (CC 77), Release (CC 78) — GDVP_PARAM_FILTER_ATTACK_CV … _RELEASE_CV. These drive an Envelope node wired into the filter's port 1.

Behaviour notes

Perceptual cutoff curve

The linear 14-bit cutoff CV is reshaped through an x³ polynomial before indexing the coefficient LUT (gdvp_cv_to_filter_index). Without this, a linear knob spends ~95% of its travel in the muffled low end and ~5% in a bright sliver; the cube law redistributes resolution so the sweep sounds smooth and musical across the whole range.

Cascaded slope engine

The base SVF is one 12 dB/oct section. To reach steeper slopes the filter cascades a second section: its first integrators (s1,s2) live inside the 28-byte node, and the second section's state (s3,s4) spills into a voice-indexed static bank keyed by (voice_id, logical filter node). This keeps the node byte-exact and the bank sized to the true voice bound (128 voices × patch nodes) with zero dynamic allocation. On a cold-strike voice allocation the section-2 state is zeroed alongside s1/s2. Practically: slopes are clean and per-voice independent; resonance and modulation are preserved across the cascade.

Zero-Delay Feedback and stability

The trapezoidal integrators use 64-bit intermediate products and Q16.16 state with explicit saturation limits (±32767.99), so the filter stays stable even at extreme resonance and cutoff, including self-oscillation. There are two unswitched hot paths: static coefficients, and a per-sample FM-modulated coefficient path when cutoff is being modulated at audio rate.

OSAC — exact coefficients at both rates

When the filter runs oversampled (128-sample blocks), it does not approximate by halving the coefficient. Instead gdvp_lut_resolve_g_ptr(frames) selects the mathematically exact g = tan(π·fc/fs) LUT for the active rate (64 or 128). This is the "Oversampling Accuracy Correction" (OSAC) referenced throughout the codebase. See Glossary: OSAC.

Bifurcated cutoff/resonance

Patch/UI cutoff and resonance write the base fields (base_cutoff_q16, base_resonance_q16); MIDI/LFO modulation writes separate bipolar offset fields (mod_cutoff_q16, mod_resonance_q16, centred at 8192). The DSP merges both at audio rate, so live modulation never overwrites the knob position. See Parameters: bifurcation.

Modulate cutoff from an Envelope (port 1) or LFO for sweeps.
The acid_squelch / lfo_filter_sweep examples are filter-forward patches (library).