# 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)   → look-ahead delay → compressor VCA → gain stage ─┐ (bypassable)
       ├─ Band 2 (mid)   → look-ahead delay → compressor VCA → gain stage ─┤ (bypassable)
       └─ Band 3 (high)  → look-ahead delay → compressor VCA → gain stage ─┤ (bypassable)
                                                                           │
       Sum of bands ◄──────────────────────────────────────────────------┘
         └─ 'All' channel → look-ahead delay → compressor VCA → gain stage
              └─ 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
- 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
- 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 are individually bypassable; with all three bypassed 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
- True-peak brickwall (ceiling = 0 dBFS or user-defined)
- 4x oversampling for inter-sample peak detection
- Short attack (≤ 0.1 ms), auto-release
### 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
- `input_gain` — pre-gain before filterbank (dB)
- `output_ceiling` — brickwall ceiling (dBFS, default 0.0)
- `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
### Per-Channel Compressor (× 4: low, mid, high, **all** — one `#[nested]` params struct reused)
- `detection` — peak / RMS level detection
- `threshold_db`
- `ratio` — 1.0 (off) to ∞ (limiting)
- `attack_ms`
- `release_ms`
- `knee_db` — soft knee width
- `makeup_gain_db`
- `bypass` — per-channel bypass (bypassing low+mid+high = 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 + Params + egui editor (all inline for now)
  params.rs         #    (planned) split Params out of lib.rs
  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      #    (planned) output true-peak brickwall limiter
    delay.rs        #    (planned) look-ahead delay (currently lives inside compressor.rs)
    oversampler.rs  #    (planned) 4x oversampler for true-peak detection
  editor/
    mod.rs          #    (planned) egui editor split out of lib.rs
    widgets/
      gain_curve.rs #    (planned) custom egui Widget: gain curve display
      band_meter.rs #    (planned) per-band gain reduction meter
      level_meter.rs#    (planned) input/output level meter
```
 
---
 
## 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.)
 
---
 
## Implementation Order
 
Work through these stages in order — each stage produces a loadable, audible plugin.

**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
- [ ] `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  ⬅ 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 — `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)*
- [x] Add `nih_plug_egui` editor
- [x] Sliders for all current parameters (`ParamSlider` grid)
- [ ] Per-band bypass toggles *(partial — single-band bypass present; per-band arrives with Stage 3)*
- [x] Confirm UI controls update DSP in real time
### Stage 6 — Custom visualisations
- [ ] `level_meter.rs` — input/output RMS + peak meters
- [ ] `band_meter.rs` — per-band gain reduction meters (vertical bars)
- [ ] `gain_curve.rs` — static gain curve display per band (threshold/ratio/knee)
- [ ] Draggable crossover handles on a frequency display
---
 
## 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
The compressor flushes its envelope/RMS state to zero in code once it decays below audibility
(`flush_denormal` in `compressor.rs`). The hardware `_MM_SET_FLUSH_ZERO_MODE` intrinsic is now
deprecated and the matching DAZ helper isn't exposed by `std::arch`, so a global hardware FTZ/DAZ
(via inline asm on the audio thread) is deferred until the IIR crossover/limiter filters land,
where it matters more.
 
### 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
`Arc<Mutex<...>>` meter data. 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)