HydraQuartet VCF-OB

8-voice polyphonic Oberheim-style multimode filter for VCV Rack 2.x

By Synth-etic Intelligence

Description

HydraQuartet VCF-OB is a polyphonic multimode filter module inspired by classic Oberheim synthesizer filters. It brings the distinctive Oberheim filter sound — musical, characterful, and versatile — to VCV Rack's modular environment.

Features

• 8-voice polyphonic processing - Supports 1-8 channels on polyphonic cables
• Oberheim SEM-style 12dB/oct state-variable filter - Smooth, musical character
• Four simultaneous filter modes - Lowpass, Highpass, Bandpass, and Notch outputs
• Comprehensive CV control - Cutoff and Resonance with dedicated CV inputs
• Cutoff attenuverter - Bipolar CV control for expressive modulation
• Drive control - Adds saturation character for warmth and grit
• Part of HydraQuartet series - Matches the visual style and workflow of HydraQuartet VCO

Building from Source

Prerequisites

• VCV Rack SDK 2.x (download from https://vcvrack.com/downloads/)
• Make (included with Xcode Command Line Tools on Mac)
• C++11 compatible compiler
• Git
• Inkscape (optional, for panel editing)

Mac Installation

1. Install dependencies via Homebrew:

brew install git wget cmake autoconf automake libtool jq python zstd pkg-config

2. Download and extract VCV Rack SDK 2.x from https://vcvrack.com/downloads/

3. Set RACK_DIR environment variable (add to ~/.zshrc or ~/.bashrc):

export RACK_DIR=~/Rack-SDK

4. Clone the repository:

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

5. Build the plugin:

make

6. Install to VCV Rack plugins folder:

make install

Troubleshooting

Error: "No rule to make target 'plugin.mk'"

This means RACK_DIR is not set correctly. Verify with:

echo $RACK_DIR

The path should point to your Rack SDK installation directory. Restart your terminal after setting the environment variable.

Usage

Controls

• Audio Input (top left) - Connect your audio source here. Accepts mono or polyphonic cables (1-8 channels).

• Cutoff Section

  • Cutoff Knob - Main filter cutoff frequency control
  • Cutoff CV Input - CV modulation input for cutoff frequency
  • CV Attenuverter - Bipolar control for CV amount (-100% to +100%)

• Resonance Section

  • Resonance Knob - Filter resonance/emphasis control
  • Resonance CV Input - CV modulation input for resonance

• Drive Control - Adds saturation character before the filter for additional harmonic content

• Filter Outputs (bottom section)

  • LP Output - Lowpass filtered signal
  • HP Output - Highpass filtered signal
  • BP Output - Bandpass filtered signal
  • Notch Output - Notch (band-reject) filtered signal

Polyphonic Operation

HydraQuartetVCF supports up to 8 channels of polyphonic processing. When you connect a polyphonic cable to the audio input, the module automatically processes each channel independently with the same filter settings. All outputs will match the number of input channels.

Workflow Tips

1. Start with the Cutoff knob at 12 o'clock (middle position)
2. Adjust Resonance to taste - higher values add emphasis at the cutoff frequency
3. Use the CV Attenuverter to fine-tune modulation depth from envelopes or LFOs
4. Experiment with Drive for additional harmonic richness
5. All four filter outputs are available simultaneously - try mixing them for unique tonal colors

Current Status

Version: 0.5.0 (beta)

Development Phase: Phase 1 - Foundation Complete

This is an early beta release. The core infrastructure is complete, with DSP implementation in progress.

• ✅ Phase 1: Foundation - Plugin compiles, loads in VCV Rack, panel design complete
• 🚧 Phase 2: DSP implementation in progress
• ⏳ Future phases: OB-X style 24dB filter, CV mode switching, per-voice control

Known Limitations

• DSP filter implementation is stubbed (no audio processing yet)
• Self-oscillation not yet implemented
• Filter type switching (SEM vs OB-X) not yet available

Reporting Issues

Found a bug or have a feature request? Please report it on the GitHub issues page: https://github.com/trek-e/HydraQuartetVCF/issues

License

This plugin is licensed under GPL-3.0-or-later.

The VCV Rack Component Library graphics used in this plugin are licensed under CC BY-NC 4.0 (Creative Commons Attribution-NonCommercial). This means the plugin is for non-commercial use. For commercial licensing, custom component graphics would be required.

See LICENSE.txt for the complete GPL-3.0 license text.

Credits

• Inspired by classic Oberheim SEM and OB-X filters - The iconic sound that defined 1970s-80s analog synthesis
• Built with VCV Rack SDK - The open-source virtual modular synthesizer platform
• Developed by Synth-etic Intelligence - Part of the HydraQuartet series

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HydraQuartet VCF-OB - Bringing classic Oberheim filter character to VCV Rack