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A Modular FM Delay Processor for Creative Audio Production

This is in the general category of educational tools for students who want to experiment with aspects of Frequency Modulation, also as an imagination expander.

Formerly known as: Voltage Phase Module (VPM)
Audio Demo: Piano Self-Modulation + Sine Waves

TODO: Re-write and try using the builtin JUCE interpolated delay

Think of FM Engine as just the FM part of an FM synthesizer. Except it allows the oft dreamt of applying FM to any two sources, using one as the carrier and one as the modulator. Smooth sine shaped modulators sound better, but it can be used in as many ways as you can imagine. Obviously if you use modulators that contain high frequencies or noise it will just result in noise, so there's a built in low pass filter. However, try feeding it two different sine synths, or playing with a single sine synth in mode 1 and play a chord.

• 🔄 Modular FM Processing - Pure frequency modulation without oscillators
• 🎯 Audio-Rate Time Modulation - Delay time modulated by sidechain amplitude
• 📈 Advanced Oversampling - JUCE built-in with planned SIMD optimization
• 🎛️ Flexible Routing - Three algorithms supporting mono to full stereo processing
• ⚡ Low Latency - Precise PDC (Plugin Delay Compensation) support
• 🔊 Multiple Interpolation - Linear and Lagrange cubic interpolation

The core delay modulation follows these equations:

out_L = DelayL(carrier_L, maxDelayLength × lowpass(atan(clipper(modulator_L + 1) × 0.5)))
out_R = DelayR(carrier_R, maxDelayLength × lowpass(atan(clipper(modulator_R + 1) × 0.5)))

• JUCE Framework 8.07 or higher
• C++17 compatible compiler
• CMake 3.15+ (for CMake builds) or Projucer

1. Open FM_Engine.jucer in Projucer
2. Configure your target IDE/build system
3. Export and build in your IDE

mkdir build
cd build
cmake .. -DCMAKE_BUILD_TYPE=Release
make -j$(nproc)

1. Copy the built VST3 to your plugin directory:
   • Windows: C:\Program Files\Common Files\VST3\
   • macOS: ~/Library/Audio/Plug-Ins/VST3/
   • Linux: ~/.vst3/

1. Self-Modulating Instruments

   • Route piano → Algorithm 0 → Use velocity as modulator
   • Creates dynamic FM textures based on playing dynamics

2. Sidechain FM Effects

   • Main: Vocal, Sidechain: Kick drum
   • Algorithm 2 for stereo sidechain modulation
   • Rhythmic vocal texturing

3. Stereo Image Manipulation

   • Algorithm 2 with independent L/R sidechain sources
   • Creates complex stereo movement and spatial effects

• Modulation Depth: Automate for dramatic build-ups
• Max Delay Range: Switch between ranges for different effects
• Algorithm: Real-time routing changes for arrangement dynamics
• Lowpass Cutoff: Filter sweeps on the modulator signal

The InterpolatedDelay class features:

• Circular Buffer: Efficient memory usage with wraparound
• Lagrange Interpolation: 3rd-order polynomial for smooth delays
• Oversampling Support: Handles 2x oversampled processing
• PDC Integration: Bipolar modulation for advanced timing control

The routeSample() function implements three distinct algorithms:

// Algorithm 0: Mono Carrier/Modulator
carrier = {L, L}, modulator = {R, R}

// Algorithm 1: Mono Mix
carrier = {(L+R)/2, (L+R)/2}, modulator = {(SC_L+SC_R)/2, (SC_L+SC_R)/2}

// Algorithm 2: Full Stereo  
carrier = {L, R}, modulator = {SC_L, SC_R}

sequenceDiagram
    participant Input
    participant Router
    participant Oversampler
    participant DelayEngine
    participant Output
    
    Input->>Router: Audio Block
    Router->>Oversampler: Routed Channels
    Oversampler->>Oversampler: Upsample 2x
    Oversampler->>DelayEngine: Process at 2x Rate
    DelayEngine->>DelayEngine: FM Processing
    DelayEngine->>Oversampler: Processed Audio
    Oversampler->>Oversampler: Downsample
    Oversampler->>Output: Final Audio

No Sidechain Input

• Ensure your DAW supports sidechain routing
• Check that the sidechain bus is enabled in your DAW
• Verify Algorithm 1 or 2 is selected for sidechain processing

High CPU Usage

• Disable oversampling if not needed
• Use Linear interpolation instead of Lagrange
• Reduce the maximum delay range

Artifacts/Clicking

• Enable oversampling for complex program material
• Adjust the lowpass cutoff to filter high-frequency modulation
• Check that input levels aren't clipping the modulator

FM Engine evolved from the original Voltage Phase Module concept, first documented in a 2014 Bedroom Producers Blog article. The project has undergone significant development:

• 2014: Initial phase modulation concept
• 2024: Realization that the effect is actually frequency modulation
• 2025: Complete rewrite with JUCE 8.07 and modern architecture

The name change from "Voltage Phase Module" to "FM Engine" reflects the correct understanding of the underlying DSP principles.

This is a proprietary plugin developed by AmateurTools DSP. The source code is provided for educational purposes only.

Copyright © 2025 AmateurTools DSP (Josh Gura)
All rights reserved.

This software is provided for personal use only.
You may not copy, modify, distribute, sublicense, or sell any part 
of this software, in whole or in part, without explicit, prior 
written permission from the copyright holder.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, 
EXPRESS OR IMPLIED.

For technical support or questions about FM Engine:

• Email: [Contact AmateurTools DSP]
• Audio Demo: SoundCloud Demo
• Original Article: Bedroom Producers Blog

Built with ❤️ using JUCE 8.07