Sheen — Master-End Polish Coat

Sheen is the eighth module in the chassis but it’s not a slot. It sits pinned at the master end of the chain (post-Punch, pre-master-gain) and is always on at factory defaults — so the plugin sounds finished out of the box. The only front-panel surface is the brushed-brass “API Bus Channel Strip” brand plate in the chassis header. Click it to flip the chassis to the Sheen back view and tune the polish.

Why hidden?

Real consoles don’t have an “amount of console-ness” knob. The polish coat is a property of the chassis, not a control surface. Most users never need to flip the panel — they just hear that the plugin sounds expensive. Advanced users discover the brass plate and gain the full five-slider control. Both audiences are served without compromise.

Signal flow

[in] → BODY (low shelf) → PRESENCE (peak) → AIR (high shelf)
     → WARMTH (Sonnox Inflator polynomial @ 2× oversample)
     → WIDTH (M/S side-only HPF + shelf)
     → [out]

Stage order matters. EQ stages run first so their tonal contour is what the warmth shaper sees. Warmth runs before width so harmonic content goes through the M/S processing rather than being added on top of it (Clariphonic / Vitamin convention — adds depth instead of just spreading).

The five stages

Stage	Algorithm	Range	Factory default
BODY	Low shelf @ 100 Hz, Q=0.707	-2.0 to +3.0 dB	+1.0 dB
PRESENCE	Peak EQ @ 3 kHz, Q=1.0	-3.0 to +3.0 dB	0.0 dB (transparent)
AIR	High shelf @ 14 kHz, Q=0.5	0.0 to +4.0 dB	+1.8 dB
WARMTH	Sonnox Inflator polynomial @ Curve=0, 2× oversampled	0–100% mix	20%
WIDTH	M/S side-only: HPF @ 150 Hz + shelf @ 500 Hz	0–100%	50% (= +12.5% sides above 500 Hz)

BODY

A gentle low shelf at 100 Hz with conservative Q. At the +1.0 dB factory default it adds subtle solidity to bass and lower-midrange content without crossing into mud territory. Shared territory with Slate Thickness, Pultec 100 Hz, Vitamin LO band, Pensado factory low-shelf preset, and Wells ToneCentric low-mid weighting — all polish plugins converge here.

PRESENCE

A peak EQ at 3 kHz with Q=1.0. Defaults to transparent (0 dB) — the slider is purely discoverable. Move it up for vocal forwardness and snare crack; move it down for the AR-1 / Pensado “smile” cut.

AIR

A low-Q high shelf at 14 kHz. Low Q matters here — high-Q air shelves create a resonant peak just below the corner that sounds glassy. At the +1.8 dB factory default, this is the strongest cross-product agreement in the polish-plugin space (Maag AIR band, Pultec EQP-1A, Slate Shimmer, Vitamin HI, Ozone air band, Soothe2 “Fresh Air” all converge in the 12–15 kHz / +1.5–2 dB region). Note: the console-bus heritage research found no measured air bump in any classic master bus (SSL G, Neve 33609, API 2500, Studer, Trident are all flat to 20 kHz). The air shelf is therefore a perceptual-compensation design choice, not hardware emulation.

WARMTH

The Sonnox Inflator transfer function at Curve=0, applied wet/dry at 20% factory mix:

f(x) = 1.5·x − 0.0625·x² − 0.375·x³ − 0.0625·x⁴

This is the public-domain reverse-engineered polynomial from RCJacH’s open-source JSFX (which nulls Sonnox at every Curve setting). Curve=0 produces a balanced even+odd harmonic mix that resembles a soft tube saturator. Patent-free, mathematically nailed down, and the most-loved harmonic generator in the polish-plugin category.

The stage runs at 2× oversampling (linear-interp upsample → polynomial → average + IIR downsample) to push the worst-case alias of the quartic term above 22 kHz at 44.1 kHz host rate.

At the 20% default mix on a typical -10 dBFS signal, WARMTH adds ~+0.83 dB perceived loudness with negligible measurable IMD. Push to 100% for ~+3.5 dB low-level gain plus pronounced soft-saturation character.

WIDTH

A frequency-dependent M/S side-channel processor:

Below 150 Hz: side gain forced to 0 (mono lows protect bass)
150–500 Hz: side passes through neutrally
Above 500 Hz: side gets +0…+1.94 dB shelf depending on slider position

Implementation: M/S encode → HPF the side at 150 Hz (kills mono-bass) → high-shelf the side at 500 Hz with gain proportional to slider → M/S decode. The mid channel passes through unchanged. Held intentionally subtle — width slamming sounds gimmicky.

Factory defaults: where do they come from?

Every factory value traces back to a citation. v1.0 development included three parallel deep-research reports:

Classic console-bus measurements — SSL G-Series, Neve 33609, API 2500, Studer 169, Trident A-Range. Measured tonal signatures at unity (no compression engaged), harmonic profiles, and crosstalk characteristics.
Polish-plugin teardowns — Slate Revival, Kush AR-1, Sonnox Inflator, Maag EQ4, Soundtoys Decapitator Tone, Acustica Pensado, Waves Vitamin, Greg Wells ToneCentric, iZotope Ozone Vintage Tape, PSP Vintage Warmer 2, Brainworx bx_console N/E/G. Stage configurations, default knob positions, and consensus polish recipes.
Tape and transformer harmonic profiles — Studer A800 / Ampex ATR-102 / MCI JH series with 456 / GP9 tape; Jensen JT-11-DM, Lundahl LL1538/LL1517, Carnhill VTB1148, UTC A-20, Sowter 9120. Measured harmonic spectra at -18 / -12 / -6 dBFS reference levels.

The full synthesis lives in docs/adr/0006-sheen-pinned-master-end-polish.md in the repository, with per-stage citations.

UX

Brass plate — replaces the static “API Bus Channel Strip” label with a brushed-brass surface. Hover brightens it; click flips the chassis to the Sheen back view. While the back view is open, the plate stays glowing so you know how to flip back.

Back view layout — header with ← STRIP VIEW button + SHEEN brass wordmark; master-bypass strip with MASTER BYPASS toggle and ↺ RESTORE FACTORY button on the right; five vertical slider columns (BODY / PRESENCE / AIR / WARMTH / WIDTH) with per-stage bypass below each slider.

Mutual exclusion — the Sheen back view and the Dynamic EQ back view are mutually exclusive. Opening one auto-closes the other. Esc closes any open back view.

RESTORE FACTORY — re-writes every Sheen parameter to its factory default in one event-frame batch. Implemented as RawParamEvent::SetParameterNormalized writes so the host sees the change as automation, not as model-only state mutation.

Auto-gain interaction

Sheen is excluded from global_auto_gain compensation. Auto-comp on a polish stage defeats its purpose — the +1 LU of perceived loudness from WARMTH and the RMS gain from BODY/AIR are the point, not artifacts to compensate. The chassis output gain stays under the master gain slider; users who want to trim Sheen’s contribution can do so manually.

Compatibility

Sessions saved before v1.0 load with Sheen ON at factory defaults — playback will subtly differ from pre-1.0 behavior. For bit-identical playback of a pre-1.0 session, flip the back-panel master Sheen bypass.

11 new automation params (5 stage values + 5 stage bypasses + 1 master Sheen bypass) — none reuse existing param IDs.

Parameters

ID	Type	Range	Default
`sheen_bypass`	Bool	true/false	false (Sheen ON)
`sheen_body_db`	Float	-2.0..=3.0 dB	+1.0
`sheen_body_bypass`	Bool	true/false	false
`sheen_presence_db`	Float	-3.0..=3.0 dB	0.0
`sheen_presence_bypass`	Bool	true/false	false
`sheen_air_db`	Float	0.0..=4.0 dB	+1.8
`sheen_air_bypass`	Bool	true/false	false
`sheen_warmth`	Float	0.0..=1.0	0.20
`sheen_warmth_bypass`	Bool	true/false	false
`sheen_width`	Float	0.0..=1.0	0.50
`sheen_width_bypass`	Bool	true/false	false

All FloatParams use SmoothingStyle::Linear(5.0) for click-free transitions. Cached in the audio thread once per buffer; filter coefficients regenerate only when the cached value actually changes.