Stage 3: 3-band LR4 crossover + per-band compressors into the 'All' channel

Splits the input into low/mid/high with a Linkwitz-Riley 24 dB/oct crossover,
compresses each band, sums them, then runs the sum through a fourth 'All'
compressor. Bypassing the three bands collapses the plugin to a simple full-band
comp driven by 'All' (the crossover sums flat in magnitude).

- src/dsp/biquad.rs: generic RBJ biquad (Transposed Direct Form II), LP/HP/AP
- src/dsp/crossover.rs: 3-band LR4 filterbank; lower band all-pass-compensated at
  the higher crossover so the bands sum to flat magnitude (an all-pass, not a
  bit-exact null — that only holds for linear-phase FIR). Mirrors nih-plug's
  crossover plugin design.
- src/lib.rs: 4 Compressor instances (low/mid/high/all) + Crossover; params
  restructured to 4 nested CompressorParams (id_prefix low/mid/high/all) plus
  global crossover_low_hz/crossover_high_hz/look_ahead_ms; 4-column lo|mid|hi|all
  egui UI; latency = two series stages (bands + all), constant, reported once
- 10 unit tests (adds biquad LP/AP magnitude, crossover flat-magnitude
  reconstruction, band-split sanity)
- README: Stage 3 marked done; corrected the 'sum flat' expectation to flat
  magnitude (IIR LR sums to an all-pass, not a time-domain null)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -59,7 +59,7 @@ a first-class mode, not an afterthought.
 ### Crossover Filterbank
 - Linkwitz-Riley 4th-order (LR4) filters at each crossover frequency
 - LR4 = two cascaded biquads (Butterworth LP or HP)
-- Bands sum phase-coherently back to flat
+- Bands sum phase-coherently to flat **magnitude** (the sum is an all-pass; lower bands get an all-pass at each later crossover to match phase — not a bit-exact time-domain null)
 - Crossover frequencies are user-adjustable parameters
 ### Per-Band Compressor
 - Level detection: switchable peak / RMS (RMS window currently hardcoded small; can be exposed later)
@@ -114,9 +114,9 @@ src/
   dsp/
     mod.rs          # ✅ module declarations
     compressor.rs   # ✅ full-band comp: peak/RMS detector, gain computer, ballistics, look-ahead delay
-    crossover.rs    #    (planned) LR4 filterbank (biquad chains)
+    crossover.rs    # ✅ LR4 3-band filterbank with all-pass phase compensation
+    biquad.rs       # ✅ generic biquad (Transposed Direct Form II)
     limiter.rs      #    (planned) output true-peak brickwall limiter
-    biquad.rs       #    (planned) generic biquad (Direct Form II transposed)
     delay.rs        #    (planned) look-ahead delay (currently lives inside compressor.rs)
     oversampler.rs  #    (planned) 4x oversampler for true-peak detection
   editor/
@@ -170,10 +170,12 @@ is essential — without it FL silently skips a plugin it has seen before.)
  
 Work through these stages in order — each stage produces a loadable, audible plugin.
 
-**Status (2026-06-15):** Stages 1–2 are complete. Look-ahead + latency reporting (from Stage 4)
-and a basic slider UI (from Stage 5) were pulled forward and already work. **Next: Stage 3 —
-crossover filterbank.** DSP currently lives in `src/dsp/compressor.rs`; params and the egui
-editor are still inline in `src/lib.rs` (not yet split into `params.rs` / `editor/`).
+**Status (2026-06-17):** Stages 1–3 complete — full-band compressor, peak/RMS detection, and now
+the 3-band LR4 crossover feeding per-band compressors summed into the 'All' channel (4 reusable
+`Compressor` instances). Look-ahead + latency (Stage 4) and a basic 4-column UI (Stage 5) are in.
+**Next: Stage 4 — output brickwall limiter + oversampler.** DSP is in `src/dsp/`
+(`biquad.rs`, `crossover.rs`, `compressor.rs`); params and the egui editor are still inline in
+`src/lib.rs` (not yet split into `params.rs` / `editor/`).
  
 ### Stage 1 — Skeleton plugin ✅
 - [x] NIH-plug "passthrough" compiling and loading in DAW
@@ -186,17 +188,17 @@ editor are still inline in `src/lib.rs` (not yet split into `params.rs` / `edito
 - [x] Implement gain computer (threshold, ratio, soft knee)
 - [x] Implement attack/release envelope (smooth decoupled peak detector)
 - [x] Wire into `process()`; covered by unit tests (static curve, knee continuity, steady state, RMS, constant latency)
-### Stage 3 — Crossover filterbank  ⬅ next
-- [ ] Implement LR4 LP and HP biquad chains in `crossover.rs`
-- [ ] Verify bands sum flat (null test: sum vs dry should be silence)
-- [ ] Add per-band bypass; with all bands bypassed, output must null against dry (proves the "simple comp" mode path)
-- [ ] Apply per-band compressor to each band
-- [ ] Sum bands back together
-- [ ] Run the summed signal through the 'All' channel comp/lim (reuse the per-band compressor) before output
-### Stage 4 — Look-ahead + brickwall limiter  *(look-ahead + latency done early)*
-- [x] Look-ahead delay (circular buffer) — currently inside `compressor.rs`, no separate `delay.rs` yet
+### Stage 3 — Crossover filterbank ✅
+- [x] Implement LR4 LP/HP biquad chains in `crossover.rs` (+ generic `biquad.rs`, Transposed Direct Form II)
+- [x] Verify bands sum flat — for IIR LR4 the sum is an **all-pass** (flat *magnitude*, phase-shifted), not a bit-exact null; lower bands get an all-pass at each later crossover to phase-match. Tested via `bands_sum_to_flat_magnitude`
+- [x] Per-band bypass — a bypassed band passes its delayed dry band; with all three bypassed the 'All' channel sees the flat-magnitude reconstruction = the simple-comp mode
+- [x] Apply per-band compressor to each band
+- [x] Sum bands back together
+- [x] Run the summed signal through the 'All' channel compressor before output
+### Stage 4 — Output brickwall limiter + oversampler  ⬅ next  *(look-ahead + latency already done)*
+- [x] Look-ahead delay (circular buffer) — inside `compressor.rs`, no separate `delay.rs`
 - [x] Wire look-ahead: detector reads N samples ahead of the VCA
-- [x] Report latency — via `context.set_latency_samples()`, reported once as a constant (see Latency note)
+- [x] Report latency — `context.set_latency_samples()` once; now the constant two-stage total (bands + 'All')
 - [ ] Implement `oversampler.rs` (4x, use a polyphase FIR or windowed sinc)
 - [ ] Implement brickwall output limiter with true-peak detection
 ### Stage 5 — Basic egui UI  *(basic version done early)*

src/dsp/biquad.rs

@@ -0,0 +1,148 @@
+//! Generic second-order IIR biquad, Transposed Direct Form II.
+//!
+//! Coefficient formulas are the RBJ Audio EQ Cookbook
+//! (<https://www.w3.org/TR/audio-eq-cookbook/>), prenormalised by `a0`. Scalar `f32`; we run
+//! one filter per channel rather than SIMD to match the rest of the per-channel DSP.
+
+use std::f32::consts;
+
+/// Butterworth Q (= 1/√2). Two cascaded Butterworth sections make a 4th-order Linkwitz-Riley.
+pub const NEUTRAL_Q: f32 = consts::FRAC_1_SQRT_2;
+
+/// Prenormalised biquad coefficients `[b0, b1, b2, a1, a2]` (already divided by `a0`).
+#[derive(Clone, Copy)]
+pub struct BiquadCoefficients {
+    b0: f32,
+    b1: f32,
+    b2: f32,
+    a1: f32,
+    a2: f32,
+}
+
+impl Default for BiquadCoefficients {
+    fn default() -> Self {
+        Self::identity()
+    }
+}
+
+impl BiquadCoefficients {
+    /// Passes the signal through unchanged.
+    pub fn identity() -> Self {
+        Self { b0: 1.0, b1: 0.0, b2: 0.0, a1: 0.0, a2: 0.0 }
+    }
+
+    pub fn lowpass(sample_rate: f32, frequency: f32, q: f32) -> Self {
+        let (cos_w0, alpha) = Self::omega(sample_rate, frequency, q);
+        let a0 = 1.0 + alpha;
+        Self {
+            b0: ((1.0 - cos_w0) / 2.0) / a0,
+            b1: (1.0 - cos_w0) / a0,
+            b2: ((1.0 - cos_w0) / 2.0) / a0,
+            a1: (-2.0 * cos_w0) / a0,
+            a2: (1.0 - alpha) / a0,
+        }
+    }
+
+    pub fn highpass(sample_rate: f32, frequency: f32, q: f32) -> Self {
+        let (cos_w0, alpha) = Self::omega(sample_rate, frequency, q);
+        let a0 = 1.0 + alpha;
+        Self {
+            b0: ((1.0 + cos_w0) / 2.0) / a0,
+            b1: -(1.0 + cos_w0) / a0,
+            b2: ((1.0 + cos_w0) / 2.0) / a0,
+            a1: (-2.0 * cos_w0) / a0,
+            a2: (1.0 - alpha) / a0,
+        }
+    }
+
+    pub fn allpass(sample_rate: f32, frequency: f32, q: f32) -> Self {
+        let (cos_w0, alpha) = Self::omega(sample_rate, frequency, q);
+        let a0 = 1.0 + alpha;
+        Self {
+            b0: (1.0 - alpha) / a0,
+            b1: (-2.0 * cos_w0) / a0,
+            b2: (1.0 + alpha) / a0,
+            a1: (-2.0 * cos_w0) / a0,
+            a2: (1.0 - alpha) / a0,
+        }
+    }
+
+    /// Shared intermediate terms: `(cos ω0, α)`.
+    fn omega(sample_rate: f32, frequency: f32, q: f32) -> (f32, f32) {
+        let w0 = consts::TAU * (frequency / sample_rate);
+        (w0.cos(), w0.sin() / (2.0 * q))
+    }
+}
+
+/// A biquad filter holding its two state variables.
+#[derive(Clone, Copy, Default)]
+pub struct Biquad {
+    coefficients: BiquadCoefficients,
+    s1: f32,
+    s2: f32,
+}
+
+impl Biquad {
+    /// Replace the coefficients (keeps the state — fine for smooth coefficient changes).
+    pub fn set_coefficients(&mut self, coefficients: BiquadCoefficients) {
+        self.coefficients = coefficients;
+    }
+
+    /// Process one sample (Transposed Direct Form II).
+    #[inline]
+    pub fn process(&mut self, x: f32) -> f32 {
+        let c = &self.coefficients;
+        let y = c.b0 * x + self.s1;
+        self.s1 = c.b1 * x - c.a1 * y + self.s2;
+        self.s2 = c.b2 * x - c.a2 * y;
+        y
+    }
+
+    /// Clear the filter state.
+    pub fn reset(&mut self) {
+        self.s1 = 0.0;
+        self.s2 = 0.0;
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const SR: f32 = 48_000.0;
+
+    fn magnitude_at(mut coeffs_filter: Biquad, freq: f32) -> f32 {
+        use std::f32::consts::TAU;
+        let n = 16_000usize;
+        let mut acc = 0.0f64;
+        for i in 0..n {
+            let x = (TAU * freq * i as f32 / SR).sin();
+            let y = coeffs_filter.process(x);
+            if i >= n - 8_000 {
+                acc += (y * y) as f64;
+            }
+        }
+        // RMS of a unit sine is 1/√2; divide it out to get the magnitude response.
+        ((acc / 8_000.0).sqrt() as f32) * std::f32::consts::SQRT_2
+    }
+
+    #[test]
+    fn lowpass_passes_dc_blocks_highs() {
+        let lp = {
+            let mut b = Biquad::default();
+            b.set_coefficients(BiquadCoefficients::lowpass(SR, 1_000.0, NEUTRAL_Q));
+            b
+        };
+        assert!((magnitude_at(lp, 100.0) - 1.0).abs() < 0.05); // ~passband
+        assert!(magnitude_at(lp, 12_000.0) < 0.05); // ~stopband
+    }
+
+    #[test]
+    fn allpass_is_unity_magnitude() {
+        for &f in &[100.0, 1_000.0, 8_000.0] {
+            let mut b = Biquad::default();
+            b.set_coefficients(BiquadCoefficients::allpass(SR, 2_000.0, NEUTRAL_Q));
+            assert!((magnitude_at(b, f) - 1.0).abs() < 0.02, "allpass not flat at {f} Hz");
+        }
+    }
+}

src/dsp/crossover.rs

@@ -0,0 +1,207 @@
+//! 3-band Linkwitz-Riley (LR4, 24 dB/oct) crossover filterbank.
+//!
+//! Each crossover splits into a low-passed band output and a high-passed remainder that feeds
+//! the next crossover. Because higher bands pass through more filters, lower bands are phase-
+//! compensated with an all-pass at every *later* crossover frequency so the three bands sum back
+//! to flat **magnitude** (the sum is an all-pass of the input — phase-shifted, not bit-identical,
+//! which is inherent to IIR Linkwitz-Riley). Approach mirrors NIH-plug's `crossover` plugin.
+//!
+//! For 3 bands there are two crossovers (low/mid at `f_lo`, mid/high at `f_hi`); only the low
+//! band needs compensation (one all-pass at `f_hi`).
+
+use super::biquad::{Biquad, BiquadCoefficients, NEUTRAL_Q};
+
+/// Mono/stereo only, matching the plugin's audio layouts.
+const MAX_CHANNELS: usize = 2;
+
+/// One channel's worth of filter state for the 3-band split.
+#[derive(Clone, Copy, Default)]
+struct BandSplitter {
+    lp_lo: [Biquad; 2], // LR4 low-pass at f_lo (two cascaded Butterworth)
+    hp_lo: [Biquad; 2], // LR4 high-pass at f_lo
+    lp_hi: [Biquad; 2], // LR4 low-pass at f_hi
+    hp_hi: [Biquad; 2], // LR4 high-pass at f_hi
+    ap_low: Biquad,     // all-pass at f_hi, phase-compensates the low band
+}
+
+impl BandSplitter {
+    /// Split one sample into `[low, mid, high]`.
+    fn split(&mut self, x: f32) -> [f32; 3] {
+        // Crossover at f_lo: low-passed band + high-passed remainder.
+        let mut lp = x;
+        for f in &mut self.lp_lo {
+            lp = f.process(lp);
+        }
+        let mut hp = x;
+        for f in &mut self.hp_lo {
+            hp = f.process(hp);
+        }
+
+        // Low band is phase-compensated for the f_hi crossover the upper bands pass through.
+        let low = self.ap_low.process(lp);
+
+        // Crossover at f_hi splits the remainder into mid + high.
+        let mut mid = hp;
+        for f in &mut self.lp_hi {
+            mid = f.process(mid);
+        }
+        let mut high = hp;
+        for f in &mut self.hp_hi {
+            high = f.process(high);
+        }
+
+        [low, mid, high]
+    }
+
+    fn set_coefficients(
+        &mut self,
+        lp_lo: BiquadCoefficients,
+        hp_lo: BiquadCoefficients,
+        lp_hi: BiquadCoefficients,
+        hp_hi: BiquadCoefficients,
+        ap_low: BiquadCoefficients,
+    ) {
+        for f in &mut self.lp_lo {
+            f.set_coefficients(lp_lo);
+        }
+        for f in &mut self.hp_lo {
+            f.set_coefficients(hp_lo);
+        }
+        for f in &mut self.lp_hi {
+            f.set_coefficients(lp_hi);
+        }
+        for f in &mut self.hp_hi {
+            f.set_coefficients(hp_hi);
+        }
+        self.ap_low.set_coefficients(ap_low);
+    }
+
+    fn reset(&mut self) {
+        for f in self
+            .lp_lo
+            .iter_mut()
+            .chain(&mut self.hp_lo)
+            .chain(&mut self.lp_hi)
+            .chain(&mut self.hp_hi)
+        {
+            f.reset();
+        }
+        self.ap_low.reset();
+    }
+}
+
+pub struct Crossover {
+    channels: usize,
+    splitters: [BandSplitter; MAX_CHANNELS],
+}
+
+impl Default for Crossover {
+    fn default() -> Self {
+        Self {
+            channels: 2,
+            splitters: [BandSplitter::default(); MAX_CHANNELS],
+        }
+    }
+}
+
+impl Crossover {
+    pub fn new() -> Self {
+        Self::default()
+    }
+
+    /// Set the active channel count and clear state. Call from `initialize()`.
+    pub fn prepare(&mut self, channels: usize) {
+        self.channels = channels.clamp(1, MAX_CHANNELS);
+        self.reset();
+    }
+
+    /// Recompute and apply crossover coefficients. Cheap enough to call once per block.
+    /// Frequencies are clamped to a valid range and forced monotonic (`f_lo <= f_hi`).
+    pub fn update(&mut self, sample_rate: f32, low_hz: f32, high_hz: f32) {
+        let max_hz = sample_rate * 0.49;
+        let f_lo = low_hz.clamp(20.0, max_hz);
+        let f_hi = high_hz.clamp(f_lo, max_hz);
+
+        let lp_lo = BiquadCoefficients::lowpass(sample_rate, f_lo, NEUTRAL_Q);
+        let hp_lo = BiquadCoefficients::highpass(sample_rate, f_lo, NEUTRAL_Q);
+        let lp_hi = BiquadCoefficients::lowpass(sample_rate, f_hi, NEUTRAL_Q);
+        let hp_hi = BiquadCoefficients::highpass(sample_rate, f_hi, NEUTRAL_Q);
+        let ap_low = BiquadCoefficients::allpass(sample_rate, f_hi, NEUTRAL_Q);
+
+        for s in &mut self.splitters {
+            s.set_coefficients(lp_lo, hp_lo, lp_hi, hp_hi, ap_low);
+        }
+    }
+
+    pub fn reset(&mut self) {
+        for s in &mut self.splitters {
+            s.reset();
+        }
+    }
+
+    /// Split one sample of `channel` into `[low, mid, high]`.
+    #[inline]
+    pub fn split(&mut self, channel: usize, x: f32) -> [f32; 3] {
+        self.splitters[channel].split(x)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::f32::consts::TAU;
+
+    const SR: f32 = 48_000.0;
+
+    #[test]
+    fn bands_sum_to_flat_magnitude() {
+        // LR4 bands sum to an all-pass: the magnitude is flat at every frequency (including the
+        // crossovers), even though the time-domain signal is phase-shifted (so it is NOT a
+        // bit-exact null — that only holds for linear-phase FIR crossovers).
+        let mut xo = Crossover::new();
+        xo.prepare(1);
+        xo.update(SR, 200.0, 2_500.0);
+
+        for &f in &[50.0, 200.0, 1_000.0, 2_500.0, 9_000.0] {
+            xo.reset();
+            let n = 24_000usize;
+            let (mut in_acc, mut out_acc) = (0.0f64, 0.0f64);
+            for i in 0..n {
+                let x = (TAU * f * i as f32 / SR).sin();
+                let [lo, mid, hi] = xo.split(0, x);
+                let y = lo + mid + hi;
+                if i >= n - 8_000 {
+                    in_acc += (x * x) as f64;
+                    out_acc += (y * y) as f64;
+                }
+            }
+            let ratio = (out_acc / in_acc).sqrt() as f32;
+            assert!(
+                (ratio - 1.0).abs() < 0.06,
+                "reconstruction not flat at {f} Hz: {ratio}"
+            );
+        }
+    }
+
+    #[test]
+    fn bands_are_actually_split() {
+        // Sanity: the low band should keep lows and reject highs; the high band vice versa.
+        fn band_energy(band: usize, freq: f32) -> f64 {
+            let mut xo = Crossover::new();
+            xo.prepare(1);
+            xo.update(SR, 200.0, 2_500.0);
+            let n = 24_000usize;
+            let mut acc = 0.0f64;
+            for i in 0..n {
+                let x = (TAU * freq * i as f32 / SR).sin();
+                let bands = xo.split(0, x);
+                if i >= n - 8_000 {
+                    acc += (bands[band] * bands[band]) as f64;
+                }
+            }
+            acc
+        }
+        assert!(band_energy(0, 50.0) > band_energy(0, 9_000.0) * 100.0); // low band: lows >> highs
+        assert!(band_energy(2, 9_000.0) > band_energy(2, 50.0) * 100.0); // high band: highs >> lows
+    }
+}

src/dsp/mod.rs

@@ -4,4 +4,6 @@
 //! per band and for the 'All' aggregate channel — see README.md). Later stages add the
 //! crossover filterbank, output limiter, and oversampler alongside it.
 
+pub mod biquad;
 pub mod compressor;
+pub mod crossover;

src/lib.rs

@@ -4,8 +4,15 @@ use std::sync::Arc;
 
 mod dsp;
 use dsp::compressor::{Compressor, CompressorSettings, MAX_LOOKAHEAD_MS};
+use dsp::crossover::Crossover;
 
-/// Level-detection mode for the compressor's detector.
+/// Band indices into the compressor array: low, mid, high, then the 'All' aggregate channel.
+const LOW: usize = 0;
+const MID: usize = 1;
+const HIGH: usize = 2;
+const ALL: usize = 3;
+
+/// Level-detection mode for a compressor's detector.
 #[derive(Enum, PartialEq, Clone, Copy)]
 enum DetectionMode {
     #[id = "peak"]
@@ -16,14 +23,17 @@ enum DetectionMode {
     Rms,
 }
 
-/// Codename 206 — Stage 2: a single full-band compressor with look-ahead.
+/// Codename 206 — Stage 3: 3-band crossover + per-band compressors summed into an 'All' channel.
 ///
-/// The `CompressorParams` struct is `#[nested]` so the exact same controls + DSP can be
-/// reused for the three bands and the 'All' aggregate channel in later stages.
+/// Signal: input → LR4 crossover → {low, mid, high} each through their own compressor → sum →
+/// 'All' compressor → output. Bypassing low+mid+high collapses it to a plain full-band comp
+/// driven by the 'All' channel (the crossover sums flat).
 struct Codename206 {
     params: Arc<Codename206Params>,
     sample_rate: f32,
-    comp: Compressor,
+    crossover: Crossover,
+    /// Compressors indexed by [`LOW`], [`MID`], [`HIGH`], [`ALL`].
+    comps: [Compressor; 4],
 }
 
 #[derive(Params)]
@@ -31,15 +41,24 @@ struct Codename206Params {
     #[persist = "editor-state"]
     editor_state: Arc<EguiState>,
 
-    /// Look-ahead time: how far ahead the detector reads so gain reduction can lead
-    /// transients. The reported latency is constant (the max look-ahead) regardless of this
-    /// value, so it is safe to adjust during playback.
+    /// Low/Mid crossover frequency.
+    #[id = "xover_lo"]
+    pub crossover_low_hz: FloatParam,
+    /// Mid/High crossover frequency.
+    #[id = "xover_hi"]
+    pub crossover_high_hz: FloatParam,
+    /// Global look-ahead time (constant reported latency — safe to adjust during playback).
     #[id = "lookahead"]
     pub look_ahead_ms: FloatParam,
 
-    /// The full-band compressor controls (reused per band + 'All' channel later).
-    #[nested(group = "Compressor")]
-    pub comp: CompressorParams,
+    #[nested(id_prefix = "low", group = "Low")]
+    pub low: CompressorParams,
+    #[nested(id_prefix = "mid", group = "Mid")]
+    pub mid: CompressorParams,
+    #[nested(id_prefix = "high", group = "High")]
+    pub high: CompressorParams,
+    #[nested(id_prefix = "all", group = "All")]
+    pub all: CompressorParams,
 }
 
 #[derive(Params)]
@@ -67,7 +86,8 @@ impl Default for Codename206 {
         Self {
             params: Arc::new(Codename206Params::default()),
             sample_rate: 48_000.0,
-            comp: Compressor::new(),
+            crossover: Crossover::new(),
+            comps: [Compressor::new(), Compressor::new(), Compressor::new(), Compressor::new()],
         }
     }
 }
@@ -75,7 +95,23 @@ impl Default for Codename206 {
 impl Default for Codename206Params {
     fn default() -> Self {
         Self {
-            editor_state: EguiState::from_size(360, 360),
+            editor_state: EguiState::from_size(760, 520),
+
+            crossover_low_hz: FloatParam::new(
+                "Crossover Lo/Mid",
+                200.0,
+                FloatRange::Skewed { min: 30.0, max: 1_000.0, factor: FloatRange::skew_factor(-1.0) },
+            )
+            .with_value_to_string(formatters::v2s_f32_hz_then_khz(0))
+            .with_string_to_value(formatters::s2v_f32_hz_then_khz()),
+
+            crossover_high_hz: FloatParam::new(
+                "Crossover Mid/Hi",
+                2_500.0,
+                FloatRange::Skewed { min: 500.0, max: 18_000.0, factor: FloatRange::skew_factor(-1.0) },
+            )
+            .with_value_to_string(formatters::v2s_f32_hz_then_khz(0))
+            .with_string_to_value(formatters::s2v_f32_hz_then_khz()),
 
             look_ahead_ms: FloatParam::new(
                 "Look-ahead",
@@ -85,7 +121,10 @@ impl Default for Codename206Params {
             .with_unit(" ms")
             .with_value_to_string(formatters::v2s_f32_rounded(2)),
 
-            comp: CompressorParams::default(),
+            low: CompressorParams::default(),
+            mid: CompressorParams::default(),
+            high: CompressorParams::default(),
+            all: CompressorParams::default(),
         }
     }
 }
@@ -113,13 +152,9 @@ impl Default for CompressorParams {
                 s.split(':').next().and_then(|x| x.trim().parse::<f32>().ok())
             })),
 
-            knee_db: FloatParam::new(
-                "Knee",
-                6.0,
-                FloatRange::Linear { min: 0.0, max: 24.0 },
-            )
-            .with_unit(" dB")
-            .with_value_to_string(formatters::v2s_f32_rounded(1)),
+            knee_db: FloatParam::new("Knee", 6.0, FloatRange::Linear { min: 0.0, max: 24.0 })
+                .with_unit(" dB")
+                .with_value_to_string(formatters::v2s_f32_rounded(1)),
 
             attack_ms: FloatParam::new(
                 "Attack",
@@ -132,28 +167,37 @@ impl Default for CompressorParams {
             release_ms: FloatParam::new(
                 "Release",
                 100.0,
-                FloatRange::Skewed { min: 1.0, max: 1000.0, factor: FloatRange::skew_factor(-2.0) },
+                FloatRange::Skewed { min: 1.0, max: 1_000.0, factor: FloatRange::skew_factor(-2.0) },
             )
             .with_unit(" ms")
             .with_value_to_string(formatters::v2s_f32_rounded(1)),
 
-            makeup_db: FloatParam::new(
-                "Makeup",
-                0.0,
-                FloatRange::Linear { min: -12.0, max: 24.0 },
-            )
-            // Applied per sample, so smooth it to avoid zipper noise.
-            .with_smoother(SmoothingStyle::Linear(20.0))
-            .with_unit(" dB")
-            .with_value_to_string(formatters::v2s_f32_rounded(1)),
+            makeup_db: FloatParam::new("Makeup", 0.0, FloatRange::Linear { min: -12.0, max: 24.0 })
+                .with_smoother(SmoothingStyle::Linear(20.0))
+                .with_unit(" dB")
+                .with_value_to_string(formatters::v2s_f32_rounded(1)),
 
             bypass: BoolParam::new("Bypass", false),
         }
     }
 }
 
+/// Build the per-block compressor settings for one channel's params (makeup filled per sample).
+fn build_settings(p: &CompressorParams, lookahead: usize, sample_rate: f32) -> CompressorSettings {
+    CompressorSettings {
+        threshold_db: p.threshold_db.value(),
+        ratio: p.ratio.value(),
+        knee_db: p.knee_db.value(),
+        attack_coef: Compressor::time_to_coef(p.attack_ms.value(), sample_rate),
+        release_coef: Compressor::time_to_coef(p.release_ms.value(), sample_rate),
+        makeup_db: 0.0,
+        lookahead_samples: lookahead,
+        use_rms: p.detection.value() == DetectionMode::Rms,
+        bypass: p.bypass.value(),
+    }
+}
+
 impl Codename206 {
-    /// Look-ahead in samples for the current parameter value and sample rate.
     fn lookahead_samples(&self) -> usize {
         (self.params.look_ahead_ms.value() * 0.001 * self.sample_rate).round() as usize
     }
@@ -199,37 +243,41 @@ impl Plugin for Codename206 {
             (),
             |_, _| {},
             move |egui_ctx, setter, _state| {
+                // One column of controls for a single compressor channel.
+                let band_col = |ui: &mut egui::Ui, title: &str, p: &CompressorParams| {
+                    ui.strong(title);
+                    ui.add(widgets::ParamSlider::for_param(&p.detection, setter));
+                    ui.label("Threshold");
+                    ui.add(widgets::ParamSlider::for_param(&p.threshold_db, setter));
+                    ui.label("Ratio");
+                    ui.add(widgets::ParamSlider::for_param(&p.ratio, setter));
+                    ui.label("Knee");
+                    ui.add(widgets::ParamSlider::for_param(&p.knee_db, setter));
+                    ui.label("Attack");
+                    ui.add(widgets::ParamSlider::for_param(&p.attack_ms, setter));
+                    ui.label("Release");
+                    ui.add(widgets::ParamSlider::for_param(&p.release_ms, setter));
+                    ui.label("Makeup");
+                    ui.add(widgets::ParamSlider::for_param(&p.makeup_db, setter));
+                    ui.add(widgets::ParamSlider::for_param(&p.bypass, setter));
+                };
+
                 egui::CentralPanel::default().show(egui_ctx, |ui| {
                     ui.heading(Self::NAME);
-                    ui.separator();
-                    egui::Grid::new("params").num_columns(2).show(ui, |ui| {
-                        ui.label("Detection");
-                        ui.add(widgets::ParamSlider::for_param(&params.comp.detection, setter));
-                        ui.end_row();
-                        ui.label("Threshold");
-                        ui.add(widgets::ParamSlider::for_param(&params.comp.threshold_db, setter));
-                        ui.end_row();
-                        ui.label("Ratio");
-                        ui.add(widgets::ParamSlider::for_param(&params.comp.ratio, setter));
-                        ui.end_row();
-                        ui.label("Knee");
-                        ui.add(widgets::ParamSlider::for_param(&params.comp.knee_db, setter));
-                        ui.end_row();
-                        ui.label("Attack");
-                        ui.add(widgets::ParamSlider::for_param(&params.comp.attack_ms, setter));
-                        ui.end_row();
-                        ui.label("Release");
-                        ui.add(widgets::ParamSlider::for_param(&params.comp.release_ms, setter));
-                        ui.end_row();
-                        ui.label("Makeup");
-                        ui.add(widgets::ParamSlider::for_param(&params.comp.makeup_db, setter));
-                        ui.end_row();
+                    ui.horizontal(|ui| {
+                        ui.label("Xover Lo/Mid");
+                        ui.add(widgets::ParamSlider::for_param(&params.crossover_low_hz, setter));
+                        ui.label("Xover Mid/Hi");
+                        ui.add(widgets::ParamSlider::for_param(&params.crossover_high_hz, setter));
                         ui.label("Look-ahead");
                         ui.add(widgets::ParamSlider::for_param(&params.look_ahead_ms, setter));
-                        ui.end_row();
-                        ui.label("Bypass");
-                        ui.add(widgets::ParamSlider::for_param(&params.comp.bypass, setter));
-                        ui.end_row();
+                    });
+                    ui.separator();
+                    ui.columns(4, |cols| {
+                        band_col(&mut cols[0], "LOW", &params.low);
+                        band_col(&mut cols[1], "MID", &params.mid);
+                        band_col(&mut cols[2], "HIGH", &params.high);
+                        band_col(&mut cols[3], "ALL", &params.all);
                     });
                 });
             },
@@ -247,16 +295,29 @@ impl Plugin for Codename206 {
             .main_output_channels
             .map(NonZeroU32::get)
             .unwrap_or(2) as usize;
-        self.comp.prepare(self.sample_rate, channels, MAX_LOOKAHEAD_MS);
 
-        // Latency is constant (the fixed audio delay) and reported exactly once, so changing
-        // the look-ahead knob during playback never renegotiates latency with the host.
-        context.set_latency_samples(self.comp.latency());
+        for comp in &mut self.comps {
+            comp.prepare(self.sample_rate, channels, MAX_LOOKAHEAD_MS);
+        }
+        self.crossover.prepare(channels);
+        self.crossover.update(
+            self.sample_rate,
+            self.params.crossover_low_hz.value(),
+            self.params.crossover_high_hz.value(),
+        );
+
+        // Two compressor stages in series (bands → 'All'), each with the same fixed look-ahead
+        // delay. Reported once as a constant; see the look-ahead note in the compressor module.
+        let total_latency = self.comps[LOW].latency() + self.comps[ALL].latency();
+        context.set_latency_samples(total_latency);
         true
     }
 
     fn reset(&mut self) {
-        self.comp.reset();
+        self.crossover.reset();
+        for comp in &mut self.comps {
+            comp.reset();
+        }
     }
 
     fn process(
@@ -265,33 +326,58 @@ impl Plugin for Codename206 {
         _aux: &mut AuxiliaryBuffers,
         _context: &mut impl ProcessContext<Self>,
     ) -> ProcessStatus {
-        // Look-ahead is a detector-tap offset within a fixed delay; it never changes latency.
         let lookahead = self.lookahead_samples();
 
-        // Block-rate compressor settings (these change slowly; makeup is smoothed per sample).
-        let c = &self.params.comp;
-        let mut set = CompressorSettings {
-            threshold_db: c.threshold_db.value(),
-            ratio: c.ratio.value(),
-            knee_db: c.knee_db.value(),
-            attack_coef: Compressor::time_to_coef(c.attack_ms.value(), self.sample_rate),
-            release_coef: Compressor::time_to_coef(c.release_ms.value(), self.sample_rate),
-            makeup_db: 0.0,
-            lookahead_samples: lookahead,
-            use_rms: c.detection.value() == DetectionMode::Rms,
-            bypass: c.bypass.value(),
-        };
+        // Crossover coefficients track the frequency params (recomputed per block — cheap).
+        self.crossover.update(
+            self.sample_rate,
+            self.params.crossover_low_hz.value(),
+            self.params.crossover_high_hz.value(),
+        );
+
+        // Block-rate settings for the three bands + the 'All' channel.
+        let band_params = [&self.params.low, &self.params.mid, &self.params.high];
+        let mut band_set = [
+            build_settings(&self.params.low, lookahead, self.sample_rate),
+            build_settings(&self.params.mid, lookahead, self.sample_rate),
+            build_settings(&self.params.high, lookahead, self.sample_rate),
+        ];
+        let mut all_set = build_settings(&self.params.all, lookahead, self.sample_rate);
 
         let mut in_frame = [0.0f32; 2];
+        let mut band_in = [[0.0f32; 2]; 3];
+        let mut band_out = [[0.0f32; 2]; 3];
+        let mut summed = [0.0f32; 2];
         let mut out_frame = [0.0f32; 2];
-        for mut frame in buffer.iter_samples() {
-            set.makeup_db = c.makeup_db.smoothed.next();
 
+        for mut frame in buffer.iter_samples() {
             let n = frame.len().min(2);
             for ch in 0..n {
                 in_frame[ch] = *frame.get_mut(ch).unwrap();
             }
-            self.comp.process(&in_frame[..n], &mut out_frame[..n], &set);
+
+            // Split each channel into low/mid/high.
+            for ch in 0..n {
+                let [lo, mid, hi] = self.crossover.split(ch, in_frame[ch]);
+                band_in[LOW][ch] = lo;
+                band_in[MID][ch] = mid;
+                band_in[HIGH][ch] = hi;
+            }
+
+            // Compress each band (per-sample smoothed makeup), then sum.
+            summed[..n].fill(0.0);
+            for b in 0..3 {
+                band_set[b].makeup_db = band_params[b].makeup_db.smoothed.next();
+                self.comps[b].process(&band_in[b][..n], &mut band_out[b][..n], &band_set[b]);
+                for ch in 0..n {
+                    summed[ch] += band_out[b][ch];
+                }
+            }
+
+            // 'All' aggregate channel over the summed bands.
+            all_set.makeup_db = self.params.all.makeup_db.smoothed.next();
+            self.comps[ALL].process(&summed[..n], &mut out_frame[..n], &all_set);
+
             for ch in 0..n {
                 *frame.get_mut(ch).unwrap() = out_frame[ch];
             }
@@ -304,7 +390,7 @@ impl Plugin for Codename206 {
 impl ClapPlugin for Codename206 {
     const CLAP_ID: &'static str = "com.mikkeli.codename-206";
     const CLAP_DESCRIPTION: Option<&'static str> =
-        Some("Multiband compressor/limiter (stage 2: full-band compressor)");
+        Some("Multiband compressor/limiter (stage 3: 3-band + 'All' channel)");
     const CLAP_MANUAL_URL: Option<&'static str> = Some(Self::URL);
     const CLAP_SUPPORT_URL: Option<&'static str> = None;
     const CLAP_FEATURES: &'static [ClapFeature] = &[