InstaShadow

Free, open-source dual-stage mastering compressor plugin inspired by the Shadow Hills Mastering Compressor, built with JUCE. Available as VST3, AU and LV2.

What Is This?

InstaShadow is a dual-stage mastering compressor that combines an optical compressor and a discrete VCA compressor in series, followed by a switchable output transformer saturation section. The design is inspired by the Shadow Hills Mastering Compressor — a legendary hardware unit used in professional mastering studios worldwide.

The optical stage provides smooth, program-dependent compression with a natural two-stage release, while the VCA stage offers precise, fast compression with selectable ratio, attack, and release settings. The transformer section adds subtle harmonic coloration with three distinct characters.

Download

Latest Release: v1.0

Windows

File	Description
InstaShadow-VST3-Win64.zip	VST3 plugin — copy to C:\Program Files\Common Files\VST3\

macOS (Universal Binary: Apple Silicon + Intel)

File	Description
InstaShadow-VST3-macOS.zip	VST3 plugin — copy to ~/Library/Audio/Plug-Ins/VST3/
InstaShadow-AU-macOS.zip	Audio Unit — copy to ~/Library/Audio/Plug-Ins/Components/

Linux (x64, built on Ubuntu 22.04)

File	Description
InstaShadow-VST3-Linux-x64.zip	VST3 plugin — copy to ~/.vst3/
InstaShadow-LV2-Linux-x64.zip	LV2 plugin — copy to ~/.lv2/

macOS note: Builds are Universal Binary (Apple Silicon + Intel). Not code-signed — after copying the plugin, remove the quarantine flag in Terminal:

xattr -cr ~/Library/Audio/Plug-Ins/VST3/InstaShadow.vst3
xattr -cr ~/Library/Audio/Plug-Ins/Components/InstaShadow.component

Signal Flow

Input → Sidechain HPF → Optical Compressor (T4B) → VCA Compressor → Transformer → Output

Each stage can be independently bypassed. The sidechain HPF prevents low-frequency energy from triggering excessive compression (pumping on bass-heavy material).

Features

Optical Compressor — Port-Hamiltonian T4B Model

The optical stage physically models the electro-optical attenuator (T4B) found in classic hardware compressors like the LA-2A. Rather than using simplified envelope followers, InstaShadow implements a Port-Hamiltonian energy-based model:

• EL panel modeled as a capacitive energy store — the audio signal charges the panel, which emits light proportional to stored energy
• CdS photoresistor modeled as a nonlinear dissipator — resistance follows the gamma curve R = k · L^(-γ) where γ ≈ 0.7
• Implicit trapezoidal integration with Newton-Raphson iteration (3-5 iterations per sample) for numerical stability
• 2x oversampling for the implicit solver
• CdS memory effect — the photoresistor "remembers" past illumination, creating a natural two-stage release:
  • Fast phase (~60 ms): first 50-80% of gain reduction releases quickly
  • Slow phase (0.5-5 s): remaining recovery depends on how long and how hard the signal was compressed
• Fixed 2:1 ratio and soft knee emerge naturally from the physics — not explicitly coded
• Program-dependent attack (~10 ms average) — reacts differently to transients vs. sustained signals

Control	Range	Default
Threshold	-40 to 0 dB	-20 dB
Gain (makeup)	0 to 20 dB	0 dB
Sidechain HPF	20 to 500 Hz	90 Hz
Bypass	On/Off	Off

Discrete VCA Compressor

A feed-forward VCA compressor with precise, repeatable compression characteristics:

• Soft-knee gain computer (6 dB knee width) for transparent threshold behavior
• 7 ratio settings: 1.2:1, 2:1, 3:1, 4:1, 6:1, 10:1, Flood (20:1)
• 6 attack presets: 0.1 ms, 0.5 ms, 1 ms, 5 ms, 10 ms, 30 ms
• 6 release presets: 100 ms, 250 ms, 500 ms, 800 ms, 1.2 s, Dual
• Dual release mode mimics the optical stage's two-stage behavior within the VCA:
  • Fast release envelope (~60 ms) handles the initial recovery
  • Slow release envelope (~2 s) handles the tail
  • The deeper (more compressed) of the two envelopes is used at any given moment

Control	Range	Default
Threshold	-40 to 0 dB	-20 dB
Gain (makeup)	0 to 20 dB	0 dB
Ratio	1.2:1 — Flood	2:1
Attack	0.1 ms — 30 ms	1 ms
Release	100 ms — Dual	500 ms
Bypass	On/Off	Off

Output Transformer Saturation

Three switchable transformer types add subtle harmonic coloration, modeled with 4x oversampled waveshaping:

Type	Character	Harmonics	Drive	Wet Mix
Nickel	Transparent, clean	Minimal	1.05	30%
Iron	Warm, musical	Even-order (2nd)	1.15	50%
Steel	Aggressive, present	Even + odd (2nd + 3rd)	1.3	60%

• Waveshaping: tanh(drive · x) / drive — preserves unity gain at low levels
• Even harmonics via x · |x| (asymmetric warmth)
• Odd harmonics via x³ (edge and presence)
• Dry/wet blending keeps the effect subtle and musical
• Iron adds a +0.2 dB low shelf at 110 Hz
• Steel adds a +0.4 dB low shelf at 40 Hz
• 4x oversampling (JUCE polyphase IIR) prevents aliasing artifacts

Metering

• Analog-style needle VU meters (L/R) with ballistic needle movement — cream-colored face, scale markings from -20 to +3 dB, red zone above 0 dB
• Optical GR meter — horizontal bar showing optical stage gain reduction
• Discrete GR meter — horizontal bar showing VCA stage gain reduction
• Output VU meter — vertical stereo bar meter in the output section

Global Controls

Control	Description
Stereo Link	Links L/R sidechain for matched stereo compression. Off = dual-mono
Bypass	Global bypass — passes audio unprocessed
Output Gain	-12 to +12 dB final output level

GUI

• Layout inspired by the original Shadow Hills Mastering Compressor hardware
• Optical controls on the left, discrete controls on the right, meters in the center
• Transformer and output controls at the bottom center
• Dark modern UI with InstaDrums/InstaGrain visual style
• 3D metal knobs with multi-layer glow effects (orange for main controls, blue for sidechain HPF)
• Analog needle VU meters with inertial needle movement
• Carbon fiber background texture
• Rajdhani custom font (embedded)
• Fully resizable window (800×500 — 1400×900) with proportional scaling
• State save/restore — all settings recalled with DAW session

How It Works

The Port-Hamiltonian Approach

Traditional plugin compressors use simplified envelope followers with fixed attack/release time constants. This misses the complex, program-dependent behavior of real optical compressors.

InstaShadow uses a Port-Hamiltonian formulation — an energy-based modeling framework from mathematical physics. The system is described by two coupled energy ports:

1. Port 1 (EL panel): A capacitive energy store with Hamiltonian H = q²/(2C). The audio signal drives charge into the capacitor, which converts electrical energy to light.

2. Port 2 (CdS cell): A nonlinear dissipative element whose resistance depends on illumination via a gamma curve. A separate "memory" state variable tracks accumulated illumination history, creating the characteristic two-stage release.

The coupled system is solved using implicit trapezoidal integration — a symplectic integrator that preserves the energy structure of the Hamiltonian. Newton-Raphson iteration (3-5 steps per sample) resolves the implicit equation at each time step. 2x oversampling ensures numerical stability.

This approach naturally produces:

• Program-dependent attack and release (emerges from the physics)
• Soft-knee compression (emerges from the nonlinear CdS gamma curve)
• Approximately 2:1 ratio (emerges from the voltage divider topology)
• Two-stage release with memory effect (emerges from the CdS illumination history)

None of these behaviors are explicitly programmed — they are consequences of the physical model.

Building

Requirements

• CMake 3.22+
• JUCE framework (cloned to ../JUCE relative to project)

Windows

• Visual Studio 2022 Build Tools (C++ workload)

macOS

• Xcode 14+

Linux (Ubuntu 22.04+)

sudo apt-get install build-essential cmake git libasound2-dev \
  libfreetype6-dev libx11-dev libxrandr-dev libxcursor-dev \
  libxinerama-dev libwebkit2gtk-4.1-dev libcurl4-openssl-dev

Build Steps

git clone https://github.com/juce-framework/JUCE.git ../JUCE
cmake -B build -G "Visual Studio 17 2022" -A x64    # Windows
cmake -B build -G Xcode                              # macOS
cmake -B build -DCMAKE_BUILD_TYPE=Release             # Linux
cmake --build build --config Release

Output:

• VST3: build/InstaShadow_artefacts/Release/VST3/InstaShadow.vst3
• AU: build/InstaShadow_artefacts/Release/AU/InstaShadow.component (macOS)
• LV2: build/InstaShadow_artefacts/Release/LV2/InstaShadow.lv2

Tech Stack

• Language: C++17
• Framework: JUCE 8
• Build: CMake + MSVC / Xcode / GCC
• Optical DSP: Custom Port-Hamiltonian solver (implicit trapezoidal + Newton-Raphson)
• VCA DSP: Custom feed-forward compressor with soft-knee gain computer
• Transformer DSP: Custom waveshaping with juce::dsp::Oversampling (4x polyphase IIR)
• Filters: juce::dsp::IIR (sidechain HPF, tonestack EQ)
• Font: Rajdhani (SIL Open Font License)

License

GPL-3.0