Kahawai Crossfeed DSP — Design Specification

Status: spec only — not implemented. Written against main at 2a6997c (2026-09-29). Scope: a new DspStage for headphone crossfeed with a 4-option preset pulldown plus custom mode. No implementation authorized.

1. Goal

Give headphone listeners relief from "super stereo" — the hard left/right split that makes some recordings (especially early stereo) fatiguing over long sessions. The stage blends a low-passed, slightly delayed copy of each channel into the other, approximating what each ear would hear from real speakers.

Non-goals: HRTF/speaker virtualization (a separate, larger project), room simulation, multichannel downmixing.

2. Algorithm

2.1 BS2B topology — Bauer, Chu Moy, and Meier presets

Per channel, in place on interleaved f32:

out_L = in_L + feed * LP_tau(in_R)
out_R = in_R + feed * LP_tau(in_L)

• LP: first-order low-pass at the preset's cutoff frequency.
• tau: a fixed interaural delay of a few hundred microseconds (head-width scale). Not user-exposed in v1; reported via latency_frames().
• feed: the preset's feed level, converted from dB to linear.

This is the topology every BS2B-based implementation (foobar2000, EasyEffects, Roon, Neutron) uses; the three presets differ only in two numbers.

2.2 Linkwitz preset

Siegfried Linkwitz's December 1971 Audio article ("Improved Headphone Listening") publishes an RC crossfeed network whose response curves (Fig. 2) are the reference. Chu Moy's well-known variant is itself a tweak of this circuit, so the Linkwitz preset is the ancestor of two of the other three.

Implementation: derive the analog prototype's transfer function from the published schematic and digitize it with the bilinear transform inside prepare(). If the schematic's component values prove ambiguous, fall back to fitting the BS2B topology (cutoff + feed + delay) to the article's published response curves, and say so in the code comment. Either way the preset must be labeled in the UI as "digitized from the published 1971 circuit" — never imply it is the analog circuit.

2.3 Preset table

Cutoff/feed values for the first three are BS2B's published presets, confirmed against the EasyEffects docs, the bs2b-crossfeed-macos project, and bs2b's own release notes. Selecting a preset fills the cutoff/feed fields with its values (visible to the user); editing either field flips the pulldown to Custom — the same convention BS2B-based UIs use.

3. Settings model

New file crates/kahawai-player-core/src/crossfeed.rs, mirroring the AnalogSettings pattern (analog.rs:132):

/// Which classic circuit this stage imitates.
pub enum CrossfeedPreset { Bauer, ChuMoy, Meier, Linkwitz, Custom }
 
/// User-facing settings; persisted in `engine-settings.json`.
#[serde(default)]
pub struct CrossfeedSettings {
    pub enabled: bool,          // default false
    pub preset: CrossfeedPreset, // default Bauer
    /// Low-pass cutoff of the crossfed path, Hz. From preset; editable in Custom.
    pub cutoff_hz: f32,         // default 700.0
    /// Feed level of the crossfed path, dB. From preset; editable in Custom.
    pub feed_db: f32,           // default 4.5
}

• impl Default, plus clamped() rejecting non-finite values and pulling ranges into bounds (same contract as AnalogSettings::clamped).
• DspSettings (engine.rs) gains #[serde(default)] pub crossfeed: CrossfeedSettings. Off by default, so Auto bit-perfect and native DSD behavior on a fresh install are unchanged.

4. DspStage implementation

pub struct CrossfeedStage implements DspStage (dsp.rs:218):

• prepare(sample_rate) — (re)design the low-pass biquad (or the Linkwitz bilinear-transform biquads) at the new rate. Crossfeed's psychoacoustic constants are defined by head geometry, not sample rate, so the design scales while the perception stays put. This is what makes the stage cheap and correct on high-resolution audio with no special-casing.
• process(&mut [f32], channels) — stereo only. Any other channel count passes through untouched (never invent behavior for layouts the presets weren't designed for).
• latency_frames() — the delay-line length in frames (sub-millisecond); 0 when bypassed. Feeds the same latency accounting the look-ahead limiter already uses.
• reset() — clear delay lines and filter state (track change, seek).
• Bit-transparency: when enabled is false, return before touching the buffer — the same early-return bypass AnalogStage::process uses.
• Enable/disable ramps over ~15 ms (EQ_RAMP_SECONDS in dsp.rs) so toggling never clicks.

Cost: ~10 multiply-adds per sample per channel plus a delay line of dozens of samples. At 192 kHz stereo this is single-digit millions of MAC/s — the lightest stage in the chain by a wide margin, far below the oversampled analog stage.

5. Engine integration

• New field crossfeed: CrossfeedStage on the engine struct, with set_crossfeed() mirroring set_analog() (engine.rs:1131): no-op when settings are unchanged, live-applied otherwise.
• Chain position: first, before EQ. Rationale: crossfeed simulates the speaker acoustics; EQ corrects the headphone's own response, so it should correct the signal as the ear will actually receive it. This amends the fixed-order comment at engine.rs:1670.
• Never invoked on the DoP / bit-perfect path — same exclusion the analog stage documents (analog.rs module docs: "the DoP and bit-perfect paths never call it").
• signalPath.ts gains the crossfeed entry so the signal-path display stays truthful.

6. Tauri API and frontend

• New set_crossfeed(settings) command mirroring set_analog (player/ui/src/tauri.ts:242); DspSettings in types.ts extended.
• types.ts: CROSSFEED_PRESET_INFO: Record<CrossfeedPreset, { label, blurb }> mirroring FLAVOUR_INFO (types.ts:212). Suggested blurbs (plain, honest, no superlatives):
  • Bauer — "The 1961 original via BS2B. Virtual speakers at 30 degrees."
  • Chu Moy — "A DIY-era tweak of Linkwitz's circuit. A touch more present."
  • Jan Meier — "From Jan Meier's Corda headphone amps. The strongest classic."
  • Linkwitz — "Linkwitz's 1971 circuit, digitized. The ancestor of the others."
• New CrossfeedSection.vue in the Sound settings, following AnalogSection.vue: a UiSelect pulldown with the four presets plus Custom, the blurb line under it, cutoff/feed sliders visible in Custom mode, and a single enable toggle. No A/B slot machinery in v1 — a bypass toggle is enough to hear what the stage does (the analog store's A/B pattern is reusable later if listeners ask for it).

7. Tests (kahawai-player-core)

• Bypass is bit-transparent: enabled: false → output bytes identical to input.
• Preset table: each named preset resolves to its published cutoff/feed.
• Reference check: output matches a direct scalar implementation of §2.1 on a synthetic stereo signal (not just "it changed something").
• Non-stereo passthrough: 1- and 6-channel buffers untouched.
• Sample-rate independence: the −3 dB point of the crossfed path measures the same at 44.1 kHz and 192 kHz.
• latency_frames() equals the delay-line length; 0 when bypassed.
• clamped() rejects NaN/infinite; no NaN/inf in output on silence or denormals.
• Linkwitz preset: frequency response matches the 1971 article's curves within a stated tolerance (tolerance documented in the test, not hand-waved).

8. Docs

• New plan doc docs/v1/Crossfeed.md (sibling to Analog Emulation) recording the preset sources and the Linkwitz digitization method.
• In-app strings follow the Editorial Style Guide: no claim the UI can't verify ("digitized from," not "identical to").

9. Phases

• Phase 1 (this spec): 4 presets + custom, stereo only, fixed first-in-chain position, bypass toggle.
• Phase 2 (optional, not spec'd): SPL Phonitor-style angle/center parameterization; A/B slot comparison; Linkwitz response refinement against measured hardware.