aether_nodes/
moog_ladder.rs

1//! Moog ladder filter — the filter that defined synthesizer sound.
2//!
3//! Huovilainen model: accurate self-oscillation, stable at high resonance,
4//! correct at audio-rate cutoff modulation.
5//!
6//! Param layout:
7//!   0 = cutoff     (Hz, 20 – 20000)
8//!   1 = resonance  (0.0 – 4.0, self-oscillates above ~3.8)
9//!   2 = drive      (0.0 – 1.0, input saturation)
10
11use aether_core::{node::DspNode, param::ParamBlock, state::StateBlob, BUFFER_SIZE, MAX_INPUTS};
12use std::f32::consts::PI;
13
14#[derive(Clone, Copy, Default)]
15struct LadderState {
16    stage: [f32; 4],
17    #[allow(dead_code)]
18    stage_tanh: [f32; 4],
19    delay: [f32; 6],
20}
21
22pub struct MoogLadder {
23    state: LadderState,
24    /// Thermal voltage constant — used in the Huovilainen nonlinear model.
25    /// Kept as a field so it can be tuned per-instance if needed.
26    #[allow(dead_code)]
27    thermal: f32,
28}
29
30impl MoogLadder {
31    pub fn new() -> Self {
32        Self {
33            state: LadderState::default(),
34            thermal: 0.000_025, // thermal voltage
35        }
36    }
37
38    #[inline(always)]
39    fn process_sample(&mut self, input: f32, cutoff: f32, resonance: f32, drive: f32, sr: f32) -> f32 {
40        let f = cutoff / (sr * 0.5);
41        let f = f.clamp(0.0, 1.0);
42
43        // Huovilainen's nonlinear ladder
44        let fc = f * PI;
45        let fc2 = fc * fc;
46        let fc3 = fc2 * fc;
47
48        let fcr = 1.8730 * fc3 + 0.4955 * fc2 - 0.6490 * fc + 0.9988;
49        let acr = -3.9364 * fc2 + 1.8409 * fc + 0.9968;
50
51        let f2 = (2.0 / 1.3) * f;
52        let res4 = resonance * acr;
53
54        // Input with drive saturation
55        let inp = (input * (1.0 + drive * 3.0)).tanh();
56
57        // Four-stage ladder with feedback
58        let inp_sub = inp - res4 * self.state.delay[5];
59
60        // Stage 1
61        let t1 = self.state.stage[0] * fcr;
62        let t2 = self.state.delay[0] * fcr;
63        self.state.stage[0] = inp_sub * f2 - t1;
64        self.state.delay[0] = self.state.stage[0] + t1;
65        let out1 = self.state.delay[0].tanh();
66
67        // Stage 2
68        let t1 = self.state.stage[1] * fcr;
69        let t2_2 = self.state.delay[1] * fcr;
70        self.state.stage[1] = out1 * f2 - t1;
71        self.state.delay[1] = self.state.stage[1] + t1;
72        let out2 = self.state.delay[1].tanh();
73
74        // Stage 3
75        let t1 = self.state.stage[2] * fcr;
76        let _t2_3 = self.state.delay[2] * fcr;
77        self.state.stage[2] = out2 * f2 - t1;
78        self.state.delay[2] = self.state.stage[2] + t1;
79        let out3 = self.state.delay[2].tanh();
80
81        // Stage 4
82        let t1 = self.state.stage[3] * fcr;
83        let _t2_4 = self.state.delay[3] * fcr;
84        self.state.stage[3] = out3 * f2 - t1;
85        self.state.delay[3] = self.state.stage[3] + t1;
86        let out4 = self.state.delay[3];
87
88        // Feedback
89        self.state.delay[5] = (self.state.delay[4] + out4) * 0.5;
90        self.state.delay[4] = out4;
91
92        // Suppress unused variable warnings
93        let _ = (t2, t2_2, fc3);
94
95        out4
96    }
97}
98
99impl Default for MoogLadder {
100    fn default() -> Self { Self::new() }
101}
102
103impl DspNode for MoogLadder {
104    fn process(
105        &mut self,
106        inputs: &[Option<&[f32; BUFFER_SIZE]>; MAX_INPUTS],
107        output: &mut [f32; BUFFER_SIZE],
108        params: &mut ParamBlock,
109        sample_rate: f32,
110    ) {
111        let silence = [0.0f32; BUFFER_SIZE];
112        let input = inputs[0].unwrap_or(&silence);
113
114        for (i, out) in output.iter_mut().enumerate() {
115            let cutoff    = params.get(0).current.clamp(20.0, sample_rate * 0.45);
116            let resonance = params.get(1).current.clamp(0.0, 4.0);
117            let drive     = params.get(2).current.clamp(0.0, 1.0);
118            *out = self.process_sample(input[i], cutoff, resonance, drive, sample_rate);
119            params.tick_all();
120        }
121    }
122
123    fn capture_state(&self) -> StateBlob { StateBlob::EMPTY }
124    fn type_name(&self) -> &'static str { "MoogLadder" }
125}
aether_nodes/moog_ladder.rs

aether_nodes/
moog_ladder.rs
