AMT6001 — ADSR Synthesizer with Turbo Effects

University of Plymouth · Audio & Music Technology · Programming Portfolio
Student: Harry Gray · October 2025

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Overview

A polyphonic ADSR synthesizer VST3/AU plugin built with the JUCE framework in C++. Features a real-time oscilloscope, ADSR envelope visualizer, harmonic control, bit-crush distortion, and a "Turbo" reverb mode - all controllable via MIDI keyboard or mouse input.

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Features

Feature	Description
8-Voice Polyphony	Supports up to 8 simultaneous MIDI voices
ADSR Envelope	Real-time graphical envelope with Attack, Decay, Sustain, Release controls
Harmonic Control	Adjust fundamental and 2nd harmonic amplitudes independently
Detune	Shifts harmonics off-axis for chorus-like thickening
Bit-Crush Distortion	Variable bit-depth reduction for lo-fi distortion textures
Turbo Mode	One-click reverb applied to the master output
Oscilloscope	Live waveform display at 60fps
MIDI Keyboard	On-screen virtual keyboard for standalone use
DAW Ready	VST3 & AU — tested in FL Studio 2024 and validated with Pluginval

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Project Structure

/Source
├── PluginProcessor.cpp/h   # Audio engine — MIDI rendering, effects, parameter management
├── PluginEditor.cpp/h      # GUI logic — layout, user interaction, parameter binding
├── BassNote.h              # Synthesiser voice — ADSR, harmonics, bit-crush
├── BassSound.h             # SynthesiserSound definition (JUCE requirement)
├── ADSRGraph.h             # Real-time ADSR envelope visualiser
└── Osc.h                   # Real-time oscilloscope (60fps, 512-point buffer)

Signal Flow

MIDI Input → Parameter Change → Process Effects → GUI Feedback → Audio Output

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Technical Details

OOP Design

• Encapsulation — Private member variables in ADSRGraph.h accessed only via setters (setAttack(), setDecay(), etc.)
• Inheritance — BassNote extends juce::SynthesiserVoice; editor implements juce::Slider::Listener and juce::Button::Listener
• Polymorphism — Virtual function overrides (canPlaySound, startNote, renderNextBlock) enable runtime voice dispatch
• Abstraction — Osc.h exposes a minimal interface (pushBuffer, clear) hiding internal buffer management

Key JUCE Modules Used

juce_core · juce_audio_basics · juce_audio_processors · juce_gui_basics · juce_graphics · juce_audio_utils · juce_dsp

Known Limitations

• 8-voice polyphony cap (voice stealing on 9+ simultaneous notes)
• Bit-crush only distortion (no tube/wave-shaping saturation)
• No anti-aliasing / oversampling — aliasing artefacts possible at high frequencies
• Fixed window size (may not scale correctly on high-DPI displays)
• Oscilloscope limited to 512 points (~11.6ms window at 44.1kHz)

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Bug Fixes & Notable Decisions

Vector overflow — Pre-allocated audioPoints to 512 elements with a wrap-around write pointer to prevent std::vector out-of-bounds errors in Osc.h.

Phase discontinuity (clicking/popping) — Replaced if with while for phase accumulation wrapping, guaranteeing phase stays within [0, 2π).

Logic error (ADSR not updating) — synth.getVoice(1) was hardcoded as 1 instead of loop variable i, meaning only the first voice ever updated.

MIDI not producing audio — keyboardState.processNextMidiBuffer() was missing from processBlock, so the MIDI buffer was never forwarded to the synth.

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Testing

Method	Result
DAW (FL Studio 2024)	No crashes under rapid parameter changes and complex MIDI input
Waveform Analysis (iZotope RX)	Output frequency within ±0.5Hz of expected (C6 = 1046.5Hz measured vs 1047Hz expected)
Pluginval	ALL TESTS PASSED

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Building

1. Download and install JUCE
2. Open AMT6001.jucer in Projucer
3. Set your IDE exporter (tested with Visual Studio 2022)
4. Build in Release mode
5. Copy the generated .vst3 file to your DAW's plugin directory (e.g. C:\Program Files\Common Files\VST3\)

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Future Development

• Additional oscillator shapes (square, triangle, wavetable)
• LFO modulation
• Anti-aliasing / oversampling
• Preset system
• More saturation/distortion stages
• Logarithmic MIDI velocity response
• Resizable / HiDPI-aware GUI

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References

Key resources: JUCE Documentation · Pluginval · JUCE Forums

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University of Plymouth · AMT6001 · 2025