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COLOSSUS · 16

An 8-voice polyphonic dual-VCO module for VCV Rack 2, inspired by the TipTop Audio Triax 8.

Part of the Synth-etic Intelligence module collection.

Overview

COLOSSUS · 16 packs 16 oscillators into a single 30 HP module — two independent VCOs per voice across 8 voices of polyphony. It was designed from the ground up around the idea that a dual oscillator should be more than two oscillators sitting next to each other: VCO1 and VCO2 talk to each other through through-zero FM, hard sync, and XOR ring modulation, producing the kind of rich, evolving timbres that normally require multiple modules and careful patching.

Every harmonically-rich waveform (sawtooth, square, triangle, XOR) is antialiased with MinBLEP correction. The entire DSP engine runs on SIMD float_4 processing, keeping CPU usage well under 2% at full 8-voice polyphony.

Features

  • 8-Voice Polyphony — VCV polyphonic cables, up to 16 channels
  • Dual VCO per Voice — 16 oscillators total, each with 4 mixable waveforms
  • Through-Zero FM — VCO1 → VCO2 linear FM that stays in tune at unison
  • Hard Sync — Bidirectional between VCO1 ↔ VCO2, MinBLEP antialiased
  • XOR Ring Modulation — Pulse wave multiplication with full MinBLEP tracking of all 4 edge types
  • Sub-Oscillator — One octave below VCO1, switchable square/sine, with dedicated output
  • PWM — Per-VCO pulse width modulation with polyphonic CV and attenuverter
  • 6 Waveform Volume CVs — Polyphonic per-voice volume automation (SAW1, SQR1, SUB, XOR, SQR2, SAW2)
  • Per-Voice Outputs — Individual audio and gate outputs for voices 1–8
  • Soft Clipping — tanh() saturation at ±3V prevents harsh digital distortion
  • SIMD Optimized — float_4 processing, ~1.6% CPU at 8 voices (48 kHz)

Panel Layout

┌─────────────────────────────────────────────────────────┐
│  COLOSSUS · 16                SYNTH-ETIC INTELLIGENCE   │
│                                                         │
│  ┌─VCO 1──────┐  ┌─GLOBAL──┐  ┌─VCO 2──────┐          │
│  │ DET OCT FM │  │  SYNC1  │  │ FM  OCT FIN │          │
│  │ SUB TRI SIN│VIB│  SYNC2  │VIB│ SIN TRI XOR│          │
│  │ PWM SQR SAW│  │  V/OCT  │  │ SAW SQR PWM│          │
│  │ [sw] [sub] │  │  GATE   │  │  [cv] [cv] │          │
│  │ cv  cv  cv │  │  AUDIO  │  │  cv  cv  cv│          │
│  └────────────┘  └─────────┘  └────────────┘          │
│  PWM1 │ GT1 GT2 GT3 GT4 │MIX│ GT5 GT6 GT7 GT8        │
│       │ V1  V2  V3  V4  │MIX│ V5  V6  V7  V8         │
└─────────────────────────────────────────────────────────┘

Controls Reference

VCO1 (Left Section)

Control Type Range Default Description
Detune Knob 0–50 cents 0 Detune VCO1 against VCO2 for thickness and beating
Octave Snap knob 16' 8' 4' 2' 1' 4' (0V) Octave selection in organ footage notation
FM Source Snap knob Sin/Tri/Saw/Sqr/Sub Sine Which VCO1 waveform modulates VCO2 via FM
Sub Level Knob 0–10 0 (off) Sub-oscillator volume (one octave below VCO1)
Triangle Knob 0–10 0 VCO1 triangle wave volume
Sine Knob 0–10 1 VCO1 sine wave volume
Vibrato Knob 0–100% 0% VCO1 vibrato depth (~5.5 Hz sine LFO)
PWM Knob 0–100% 50% VCO1 square wave pulse width
Square Knob 0–10 1 VCO1 square wave volume
Sawtooth Knob 0–10 0 VCO1 sawtooth wave volume
Sub Wave Toggle Sqr / Sin Square Sub-oscillator waveform selection

VCO2 (Right Section)

Control Type Range Default Description
FM Amt Knob 0–10 0 (off) Through-zero FM depth (VCO1 → VCO2)
Octave Snap knob 16' 8' 4' 2' 4' (0V) Octave selection
Fine Knob 0–10 0 Fine tuning: 0–5 = 0–1 semitone (smooth), 5–10 = +2–12 semitones (FM ratios)
Vibrato Knob 0–100% 0% VCO2 vibrato depth
Sine Knob 0–10 0 VCO2 sine wave volume
Triangle Knob 0–10 0 VCO2 triangle wave volume
XOR Knob 0–10 0 (off) XOR ring modulation output volume
Sawtooth Knob 0–10 0 VCO2 sawtooth wave volume
Square Knob 0–10 1 VCO2 square wave volume
PWM Knob 0–100% 50% VCO2 square wave pulse width

Global (Center Section)

Control Type Description
Sync 1 3-way switch VCO1 sync: Hard / Off / Soft (syncs VCO1 to VCO2)
Sync 2 3-way switch VCO2 sync: Hard / Off / Soft (syncs VCO2 to VCO1)

Inputs

Jack Polyphonic Description
V/Oct Yes (up to 16 ch) Pitch input — drives polyphony channel count
Gate Yes Gate input
PWM1 CV Yes VCO1 pulse width CV with attenuverter (±5V × att = ±0.5 PWM)
PWM2 CV Yes VCO2 pulse width CV with attenuverter
FM CV Yes FM depth CV (auto-detects mono/poly)
SUB CV Yes Sub-oscillator volume CV (0–10V, replaces knob when patched)
SQR1 CV Yes VCO1 square volume CV
SAW1 CV Yes VCO1 sawtooth volume CV
SAW2 CV Yes VCO2 sawtooth volume CV
SQR2 CV Yes VCO2 square volume CV
XOR CV Yes XOR output volume CV

Outputs

Jack Polyphonic Description
Audio Yes (8 ch) Main polyphonic output (VCO1 + VCO2 mix)
Mix No (mono) Mono sum of all voices
Sub Yes (8 ch) Dedicated sub-oscillator output
Voice 1–8 No (mono each) Individual per-voice audio outputs
Gate 1–8 No (mono each) Individual per-voice gate pass-through
Gate Mix No (mono) OR of all gates

Patch Ideas

Classic Detuned Unison

Set VCO1 Sine to 10, VCO2 Sine to 10, Detune to ~30%. Play a chord. Instant thick analog pad.

Through-Zero FM Bell

Set both VCOs to the same octave. Turn FM Amt to ~3. Use Fine knob on VCO2 to dial in harmonic ratios (the 5–10 range gives stepped semitones ideal for FM). The FM stays perfectly in tune — you get waveshaping, not detuning.

Sync Lead

Enable Sync 2 → Hard. Raise VCO2 Sawtooth. Sweep VCO2 Fine knob for the classic hard sync lead sound. The MinBLEP antialiasing keeps it clean even at high pitches.

XOR Ring Modulation

Turn up Square on both VCOs. Raise XOR volume. Detune VCO2 slightly with Fine. Sweep PWM on either VCO to morph the XOR character — both VCOs' pulse widths shape the ring modulation output.

Per-Voice Volume Animation

Patch an 8-channel random source into SAW1 CV. Each voice gets independent sawtooth volume, creating shimmering polyphonic textures. CV replaces the knob when patched (0–10V maps directly to 0–10 volume).

Technical Details

DSP Architecture

  • VcoEngine struct encapsulates all per-oscillator state (phase, MinBLEP buffers, delta tracking)
  • Two VcoEngine instances per module (VCO1, VCO2), processed in SIMD groups of 4 voices
  • Phase accumulation with simd::clamp(freq * sampleTime, 0, 0.49) prevents aliasing at extreme frequencies
  • V/Oct conversion uses dsp::exp2_taylor5() for fast, accurate pitch tracking

Antialiasing

All waveforms with discontinuities use subsample-accurate MinBLEP correction:

  • Sawtooth — MinBLEP on phase wrap (1 edge type)
  • Square — MinBLEP on phase wrap + PWM threshold crossing (2 edge types)
  • XOR — MinBLEP tracking all 4 edge types: VCO1 wrap, VCO1 PWM fall, VCO2 wrap, VCO2 PWM fall
  • Triangle — Direct phase calculation with MinBLEP correction
  • Sine — Inherently band-limited, no correction needed

MinBLEP impulse uses 16 zero crossings × 16× oversampling, stored in a shared static lookup table.

FM Implementation

Through-zero linear FM is applied after exponential pitch conversion:

freq2 = baseFreq2 + (fmModulator * fmDepth * baseFreq2)

This keeps both VCOs in tune at unison (FM Amount = 0 → perfect unison). At higher FM depths, the frequency can go negative (through-zero), clamped to a minimum of 0.1 Hz to prevent phase reversal crashes.

Output Stage

  1. Per-voice waveform mix (all volumes are SIMD float_4)
  2. DC blocking via dsp::TRCFilter at 10 Hz cutoff
  3. Soft clipping: 3.0 × tanh(signal / 3.0) — saturates smoothly at ±3V
  4. Output scaled to ±2V
  5. NaN/Inf sanitization

CV Behavior

Waveform volume CV inputs follow the CV-replaces-knob pattern (not additive):

  • When a CV cable is patched, the knob value is ignored
  • 0–10V maps directly to 0–10 volume (1:1, no scaling)
  • Polyphonic CV gives per-voice volume control

Building from Source

Requirements

  • VCV Rack SDK 2.x (download)
  • C++11 compiler (GCC 7+, Clang, or MSVC)
  • GNU Make

Build

git clone https://github.com/trek-e/Colossus16.git
cd Colossus16

# Point to your Rack SDK
export RACK_DIR=/path/to/Rack-SDK

# Build
make -j$(nproc)

# Install to VCV Rack plugins directory
make install

Cross-Platform Notes

The Makefile auto-detects the platform and installs to the correct plugins directory:

Platform Plugins Directory
macOS ARM ~/Library/Application Support/Rack2/plugins-mac-arm64/
macOS Intel ~/Library/Application Support/Rack2/plugins-mac-x64/
Windows %LOCALAPPDATA%/Rack2/plugins-win-x64/
Linux ~/.Rack2/plugins-lin-x64/

Other Synth-etic Intelligence Modules

Module Description
Hurricane 8 8-voice polyphonic PPG-style wavetable oscillator with mip-mapped band-limited playback
HydraQuartet VCF-OB 16-voice polyphonic Oberheim-style multimode filter with 12dB SEM / 24dB OB-X modes

License

GPL-3.0-or-later — See LICENSE for details.

Author

Synth-etic Intelligence syntheticint@thepainterofsilence.com

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8-voice polyphonic dual-VCO module for VCV Rack with through-zero FM

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GPL-3.0 license
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