# Codename 206

*Called 206 because the Peugeot 206 has a 'maxi' variant. You'll know this is a Maximizer knockoff if you can follow that trail of thoughts.*  
Multiband Compressor / Limiter VST3 — Project Plan  

## Overview
 
A VST3 multiband compressor/limiter with a custom gain curve display, inspired by FL Studio's Maximizer.  
Built with **Rust** + **NIH-plug** (VST3 + CLAP output) + **egui** for the UI.  
 
**Goals:**  
- 3-band (configurable crossover points) compressor/limiter  
- An 'All' aggregate channel: a 4th comp/lim stack on the summed bands, so bypassing all bands turns the plugin into a simple full-band compressor (mirrors FL's Maximizer)  
- Look-ahead brickwall output limiter with true-peak detection  
- Real-time gain reduction metering per band  
- Custom gain curve visualiser  
- Fully resizable vector UI  
---
 
## Tech Stack
 
| Layer | Choice |
|---|---|
| Language | Rust (stable) |
| Plugin framework | [NIH-plug](https://github.com/robbert-vdh/nih-plug) |
| Plugin formats | VST3, CLAP |
| UI framework | egui (via `nih_plug_egui`) |
| Build tooling | `cargo xtask bundle` |
 
---
 
## Signal Flow
 
```
Input
  └─ Crossover filterbank (Linkwitz-Riley LR4 @ each crossover freq)
       ├─ Band 1 (low)   → pre-gain → look-ahead delay → compressor VCA → makeup ─┐ (dry/wet mix)
       ├─ Band 2 (mid)   → pre-gain → look-ahead delay → compressor VCA → makeup ─┤ (dry/wet mix)
       └─ Band 3 (high)  → pre-gain → look-ahead delay → compressor VCA → makeup ─┤ (dry/wet mix)
                                                                                  │
       Sum of bands ◄─────────────────────────────────────────────────────------┘
         └─ 'All' channel → pre-gain → look-ahead delay → compressor VCA → makeup
              └─ output brickwall limiter (true-peak, 4x oversampled) → output
```
 
The detector for each band reads `look_ahead_ms` ahead of the VCA, so gain reduction is already ramping when the transient arrives.

**The 'All' aggregate channel** (mirrors FL's Maximizer): the three bands are summed and the
result passes through a *fourth*, full-band compressor/limiter stack before the output limiter.
Because the LR4 filterbank sums phase-coherently flat, **bypassing all three bands leaves the
summed signal identical to the input** — so the plugin collapses into a plain single-band
compressor/limiter driven entirely by the 'All' channel. That makes "multiband off = simple comp"
a first-class mode, not an afterthought.
 
---
 
## DSP Architecture
 
### Crossover Filterbank
- Linkwitz-Riley 4th-order (LR4) filters at each crossover frequency
- LR4 = two cascaded biquads (Butterworth LP or HP)
- 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
- **Pre-gain (drive)**: scales the band *before* the detector, so it pushes harder into compression and feeds the sum/limiter hotter — a mild "compressed semi-distortion" without a dedicated saturator. Applied in the wiring (the compressor itself is untouched). Pairs with makeup for full input/output gain-staging
- Level detection: switchable peak / RMS (RMS window currently hardcoded small; can be exposed later)
- Gain computer: threshold, ratio, soft knee
- Attack / release envelopes (logarithmic ballistics)
- Makeup gain per band (−24…+24 dB — attenuates as well as boosts)
- Look-ahead: circular delay buffer on the audio path; detector reads ahead
### 'All' Aggregate Channel
- Structurally **identical to a per-band compressor** — reuse the same comp/lim code/params, just fed the summed signal instead of a filtered band
- Runs after the three bands are summed, before the output brickwall limiter
- Bands have a per-channel dry/wet **mix** (parallel compression); at 0% (or all three dry) the (phase-coherent) crossover sum equals the dry input, so the 'All' channel alone acts as a full-band comp/lim
- Has its own look-ahead; the plugin reports a single **constant** total latency (the fixed band + 'All' look-ahead), set once — see Latency below
### Output Limiter
- Brickwall, ceiling = 0 dBFS or user-defined (`output_ceiling`). Look-ahead + sliding-max peak detection + a ceiling clamp guarantee the output never exceeds the ceiling
- Short attack (≤ 0.1 ms), auto-release (release time user-set)
- **True-peak**: 4× polyphase oversampling estimates the inter-sample peak (detection only — the upsampled signal is discarded); the limiter targets a 0.3 dB margin under the ceiling to cover the 4× residual
### Latency
- Reported via `context.set_latency_samples()` in `initialize()` — **never** from `process()`; renegotiating latency mid-stream crashes some hosts (FL included)
- Reported latency is a **constant** (the max look-ahead); the look-ahead control only moves the detector tap within that fixed delay
- All bands use equal delay to preserve phase alignment
---
 
## Parameters
 
### Global
- `output_ceiling` — brickwall ceiling (dBFS, default 0.0)
- `limiter_release_ms` — output limiter release time
- `look_ahead_ms` — look-ahead time (0–5 ms). Reported latency is **constant** (the max look-ahead); the knob only moves the detector tap within that fixed delay, so it is safe to adjust during playback (changing reported latency mid-stream crashes some hosts, FL included)
- `crossover_low_hz` — low/mid crossover frequency
- `crossover_high_hz` — mid/high crossover frequency

> **Crossover automation caveat:** the lo ≤ hi limit is enforced in the **editor only** (the two
> are independent params). Host automation writes them directly, so it can drive lo past hi and
> momentarily invert the mid band. The DSP clamps to a monotonic split so it won't break audio,
> but FL's automation can misbehave once inverted. Not fixed by design — just don't automate the
> two across each other.
### Per-Channel Compressor (× 4: low, mid, high, **all** — one `#[nested]` params struct reused)
- `pre_gain_db` — drive into the compressor (−24…+36 dB, smoothed)
- `detection` — peak / RMS level detection
- `low_slope` — low-level shaper slope at the silence floor (1 = unity, >1 fans up/boost, <1 fans down/cut). **Serial**: reshapes the level *before* the threshold, so a boost can lift quiet material up into compression
- `low_curve` — bends the low shaper toward a bounded saturation (0% = straight line) so the serial composition doesn't run away
- `threshold_db`
- `ratio` — 1.0 (off) to ∞ (limiting)
- `knee_db` — soft knee width
- `attack_ms`
- `release_ms`
- `makeup_db` — makeup gain (−24…+24 dB)
- `mix` — per-channel dry/wet mix (parallel compression); 0% = dry (a clean bypass), 100% = fully processed. Bands at 0% → simple full-band comp via the 'all' channel

The 'all' channel uses the same struct so its UI and DSP are identical to a band; it just sits after the band sum.
---
 
## Project Structure
 
Target layout (✅ = exists today; the rest is planned):

```
src/
  lib.rs            # ✅ Plugin trait + DSP wiring + process()
  params.rs         # ✅ Params structs, defaults, build_settings()
  editor.rs         # ✅ egui editor: meter panel + rolling plot (drawn via Painter) + slider columns
  meters.rs         # ✅ lock-free Meters (atomics): decayed bar values + raw plot feed
  dsp/
    mod.rs          # ✅ module declarations
    compressor.rs   # ✅ full-band comp: peak/RMS detector, gain computer, ballistics, look-ahead delay
    crossover.rs    # ✅ LR4 3-band filterbank with all-pass phase compensation
    biquad.rs       # ✅ generic biquad (Transposed Direct Form II)
    limiter.rs      # ✅ look-ahead brickwall limiter (true-peak via oversampler)
    oversampler.rs  # ✅ 4x polyphase oversampler for true-peak detection (detection-only)
```

The editor lives in an `editor/` module — one file per visualiser widget (each owns its GUI
state), with `mod.rs` as the aggregator/layout. Drawn directly with egui's `Painter`.

```
src/
  editor/
    mod.rs            # aggregator: create(), EditorState, layout, placeholder slider columns
    meter.rs          # |L | GR | R| level + gain-reduction bars + per-channel ceiling lamp
    plot.rs           # rolling in/out/GR scope (200 Hz ring feed) + ceiling-hit markers
    crossover.rs      # log-freq strip with draggable crossover handles + number boxes
    gain_curve.rs     # static gain-curve display (out vs in) for the selected channel
```

Remaining UI work: replace the placeholder per-channel slider columns in `mod.rs` with the real
layout.

Deferred until the redesign — no need to split prematurely while the layout is still a placeholder.
 
---
 
## Build Steps
 
The project is already scaffolded (NIH-plug + nih_plug_egui, pinned to a fixed git rev in
`Cargo.toml`). You do **not** need the Steinberg VST3 SDK — NIH-plug bundles its own bindings.

**Prerequisites (Windows):**
- Rust stable (`rustup` — `winget install Rustlang.Rustup`)
- Visual Studio 2022 with the "Desktop development with C++" workload (provides the MSVC linker)

```powershell
# Build + bundle the VST3 and CLAP
cargo xtask bundle codename_206 --release
# Output: target\bundled\Codename 206.vst3  and  Codename 206.clap
```

### Deployment

FL Studio scans `C:\Program Files\Common Files\VST3` by default, **ignores directory junctions**
(so a symlinked bundle is invisible to its scanner), and caches failed scans. So deployment must
copy a *real* bundle into a folder FL scans, then FL must be told to rescan failed plugins.

Use the provided script (no need to remember the details):

```powershell
.\deploy.ps1            # build, then copy to the global VST3/CLAP folders (one UAC prompt)
.\deploy.ps1 -SkipBuild # reinstall the last build without rebuilding
.\deploy.ps1 -User      # copy to %LOCALAPPDATA%\Programs\Common\VST3 instead (no admin) — best for a dev loop
```

`deploy.bat` is a double-click wrapper around the same script.

After deploying, in FL Studio: **Options → Manage plugins → tick "Rescan previously failed
plugins" → Find installed plugins**, then search for **Codename 206**. (The rescan-failed step
is essential — without it FL silently skips a plugin it has seen before.)

> **Known issue (deferred):** the **CLAP** build shows its name/vendor/type correctly in FL, but
> the **VST3** still displays stale/missing metadata there. Suspected cause is FL caching the VST3
> by its unchanged `VST3_CLASS_ID`. Likely fix is to regenerate that class ID (and/or clear FL's
> plugin DB); low priority for now — use the CLAP build meanwhile.
 
---
 
## Implementation Order
 
Work through these stages in order — each stage produces a loadable, audible plugin.

**Status (2026-06-25):** Stages 1–4 done — the full signal chain works: 3-band LR4 crossover →
per-band pre-gain + compressors (peak/RMS) → per-channel dry/wet mix → 'All' channel → **true-peak
brickwall limiter** (4× oversampled detection). `lib.rs` is split into `params.rs`, `meters.rs`, and
an `editor/` widget module. Stage 6 visualisers are essentially complete: per-channel **|L | GR | R|
meters** + **per-channel ceiling lamps**, a **rolling in/out/GR plot** (200 Hz ring feed, flow-speed,
ceiling-hit markers), **draggable crossover handles**, and a **static gain-curve display**. **Next:
replace the placeholder slider columns with the real UI layout.**
 
### Stage 1 — Skeleton plugin ✅
- [x] NIH-plug "passthrough" compiling and loading in DAW
- [ ] `Params` struct with all parameters declared *(partial — compressor + look-ahead params done; global `input_gain`/`output_ceiling` and crossover params pending)*
- [x] `process()` passes audio through untouched *(since superseded by the compressor)*
- [x] Verify plugin loads and parameters appear in DAW *(verified in FL Studio)*
### Stage 2 — Single-band (full-band) compressor ✅
- [ ] Implement `biquad.rs` — generic biquad, Direct Form II transposed *(deferred to Stage 3 — not needed for the full-band comp)*
- [x] Level detector — switchable **peak / RMS** (RMS window hardcoded for now)
- [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 ✅
- [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 ✅
- [x] Look-ahead delay (circular buffer) — inside `compressor.rs` and `limiter.rs`, no separate `delay.rs`
- [x] Wire look-ahead: detector reads N samples ahead of the VCA
- [x] Report latency — `context.set_latency_samples()` once; constant three-stage total (bands + 'All' + limiter)
- [x] Brickwall output limiter (`limiter.rs`): look-ahead + sliding-max + ceiling clamp guarantee
- [x] `oversampler.rs` — 4× polyphase windowed-sinc, detection-only (returns the inter-sample max)
- [x] True-peak limiting: limiter peak = max(sample, inter-sample); targets a 0.3 dB margin under the ceiling for the 4× residual
### Stage 5 — Basic egui UI  *(basic version done early)*
- [x] Add `nih_plug_egui` editor
- [x] Sliders for all current parameters (`ParamSlider` grid)
- [x] Per-channel dry/wet mix (parallel compression; replaced the bypass toggle)
- [x] Confirm UI controls update DSP in real time
### Stage 6 — Custom visualisations
- [x] Per-channel level meters (output level, `|L | GR | R|` cluster)
- [x] Per-channel gain-reduction meters (vertical bars) + latching ceiling lamp
- [x] Rolling in/out/gain-reduction plot (per-channel tabs, flow-speed selector)
- [x] Static gain-curve display (out vs in; includes pre-gain + makeup) for the selected channel
- [x] Draggable crossover handles on a log-frequency display (with number boxes)
- [ ] Replace the placeholder slider columns with the real UI
---
 
## Key Implementation Notes
 
### No allocations in `process()`
Rust's borrow checker will help, but be explicit. All buffers (delay lines, filter states)
must be pre-allocated in `initialize()`. Use `assert_process_allocs` feature flag during
development to catch violations.
 
### Denormal flushing
Handled by the framework — no plugin code needed. NIH-plug wraps `process()` and `reset()` in
`process_wrapper`, which enables the CPU's **Flush-To-Zero** mode for the duration via its
`ScopedFtz` guard (x86 `MXCSR` bit 15 / AArch64 `FPCR` bit 24, set with inline asm and restored
on drop). FTZ has a fixed threshold at the normal/subnormal boundary (~−759 dB for f32), so the
decaying envelope/RMS tails and all the IIR filter state are flushed to zero automatically,
far below audibility. We therefore do **not** set the register ourselves or flush values in code.
(Note: NIH-plug sets FTZ but not DAZ; for our feed-forward IIR work FTZ on results is sufficient.)
 
### Parameter smoothing
NIH-plug provides `Smoother` — use it for all gain/threshold params to avoid zipper noise.
 
### Thread safety
Params are atomics. The editor and audio thread communicate only through params and a shared
`Arc<Meters>` (`meters.rs`) — never a mutex on the audio path. Two lock-free feeds, both gated on
the editor being open:
- **Bar meters** — decayed atomic scalars, one store per block; the editor reads them each frame.
- **Scrolling plot** — a single-producer/single-consumer `ScopeRing` of buckets clocked at
  ~200 Hz, so the plot's horizontal resolution is decoupled from the ~60 fps repaint. The editor
  drains all new buckets each frame. The scope is **transport-gated** (advances only while playing)
  so it freezes rather than scrolling silence when the host is stopped/paused.

Never pass DSP state to the UI directly.
 
### VST3 licensing
You must accept Steinberg's VST3 SDK licence before distributing VST3 binaries.
NIH-plug's VST3 bindings are GPLv3; if you distribute, the plugin must also be GPLv3
(or you need a commercial Steinberg licence). CLAP has no such restriction.
 
---
 
## Reference Material
 
- [NIH-plug repo](https://github.com/robbert-vdh/nih-plug) — read the `plugins/` examples first
- [NIH-plug docs](https://nih-plug.robbertvanderhelm.nl/)
- [Cookiecutter template](https://github.com/robbert-vdh/nih-plug-template)
- [egui docs](https://docs.rs/egui)
- Zölzer, *DAFX: Digital Audio Effects* — biquad filter cookbook
- Giannoulis et al., "Digital Dynamic Range Compressor Design" (JAES 2012) — compressor ballistics reference
- AES paper on true-peak limiting / inter-sample peaks (ITU-R BS.1770)