Codename 206

Called 206 because the Peugeot 206 has a 'maxi' variant. Subdued lineage to the FL Studio plugin 'maximiser'
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
• Look-ahead brickwall output limiter with true-peak detection
• Real-time gain reduction metering per band
• Custom gain curve visualiser
• Fully resizable vector UI

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Tech Stack

Layer	Choice
Language	Rust (stable)
Plugin framework	NIH-plug
Plugin formats	VST3, CLAP
UI framework	egui (via nih_plug_egui)
Build tooling	cargo xtask bundle

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Signal Flow

Input
  └─ Crossover filterbank (Linkwitz-Riley LR4 @ each crossover freq)
       ├─ Band 1 (low)   → look-ahead delay → compressor VCA → gain stage
       ├─ Band 2 (mid)   → look-ahead delay → compressor VCA → gain stage
       └─ Band 3 (high)  → look-ahead delay → compressor VCA → gain stage
            └─ Sum → 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.

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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 back to flat
• Crossover frequencies are user-adjustable parameters

Per-Band Compressor

• Level detection: switchable RMS / peak, with configurable window
• 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

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

• Look-ahead duration must be reported via Plugin::latency() for DAW compensation
• All bands use equal delay to preserve phase alignment

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Parameters

Global

• input_gain — pre-gain before filterbank (dB)
• output_ceiling — brickwall ceiling (dBFS, default 0.0)
• look_ahead_ms — look-ahead time (0–10 ms)
• crossover_low_hz — low/mid crossover frequency
• crossover_high_hz — mid/high crossover frequency

Per Band (× 3, use a #[nested] params struct)

• threshold_db
• ratio — 1.0 (off) to ∞ (limiting)
• attack_ms
• release_ms
• knee_db — soft knee width
• makeup_gain_db
• bypass — per-band bypass

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Project Structure

src/
  lib.rs            # Plugin entry point, implements Plugin trait
  params.rs         # Params struct with NIH-plug #[id] attributes
  dsp/
    mod.rs
    crossover.rs    # LR4 filterbank (biquad chains)
    compressor.rs   # Per-band compressor + look-ahead
    limiter.rs      # Output true-peak brickwall limiter
    biquad.rs       # Generic biquad filter (Direct Form II transposed)
    delay.rs        # Circular buffer for look-ahead delay lines
    oversampler.rs  # 4x oversampler for true-peak detection
  editor/
    mod.rs          # egui editor setup via nih_plug_egui
    widgets/
      gain_curve.rs # Custom egui Widget: gain curve display
      band_meter.rs # Per-band gain reduction meter
      level_meter.rs# Input/output level meter

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Build Steps

# Install Rust (if not already)
curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh
 
# Clone NIH-plug cookiecutter or start fresh
cargo new --lib my_maximizer
cd my_maximizer
 
# Add dependencies to Cargo.toml:
# nih-plug = { git = "https://github.com/robbert-vdh/nih-plug", features = ["assert_process_allocs"] }
# nih-plug-egui = { git = "https://github.com/robbert-vdh/nih-plug" }
 
# Build and bundle
cargo xtask bundle my_maximizer --release
# Output: target/bundled/my_maximizer.vst3

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Implementation Order

Work through these stages in order — each stage produces a loadable, audible plugin.

Stage 1 — Skeleton plugin

• NIH-plug "passthrough" compiling and loading in DAW
• Params struct with all parameters declared (no DSP yet)
• process() passes audio through untouched
• Verify plugin loads and parameters appear in DAW

Stage 2 — Single-band compressor (no look-ahead, no UI)

• Implement biquad.rs — generic biquad, Direct Form II transposed
• Implement basic RMS level detector
• Implement gain computer (threshold, ratio, knee)
• Implement attack/release envelope on gain reduction
• Wire into process(), test with a sine sweep

Stage 3 — Crossover filterbank

• Implement LR4 LP and HP biquad chains in crossover.rs
• Verify bands sum flat (null test: sum vs dry should be silence)
• Apply per-band compressor to each band
• Sum bands back to output

Stage 4 — Look-ahead + brickwall limiter

• Implement delay.rs circular buffer
• Wire look-ahead: detector reads N samples ahead of VCA
• Report latency via Plugin::latency()
• Implement oversampler.rs (4x, use a polyphase FIR or windowed sinc)
• Implement brickwall output limiter with true-peak detection

Stage 5 — Basic egui UI

• Add nih_plug_egui editor
• Knobs / sliders for all parameters
• Per-band bypass toggles
• 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

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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

Add #[cfg(target_arch = "x86_64")] std::arch::x86_64::_MM_SET_FLUSH_ZERO_MODE(...) in initialize(), or add a small DC offset (1e-25) to filter inputs.

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.

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Reference Material

• NIH-plug repo — read the plugins/ examples first
• NIH-plug docs
• Cookiecutter template
• egui docs
• 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)