Physical Audio Tetrad

The Physical Audio Tetrad is a cutting-edge software synthesizer that stands out in the crowded field of virtual instruments by forging a unique connection between physical modeling, granular synthesis, and experimental sound sculpture. Developed in collaboration with composer Gadi Sassoon, Tetrad is a digital twin of his unique, installation-based synth setup that uses slashed metal plates and custom synthesis processes. This powerful hybrid instrument is engineered to create complex, evolving, and often metallic soundscapes that defy the typical harmonic limitations of traditional synthesizers.

The Quad-Bus Signal Architecture

The core of Tetrad’s immense sonic complexity lies in its namesake four-part signal chain. The instrument routes four separate signal paths (hence "Tetrad") through a cascade of unique processors, ensuring that every sound is rich with independent movement and dimension:

1. Quad Oscillators: The sound begins with four oscillators per voice, offering a choice of five tuning modes, including basic, chordal, and highly experimental microtonal/free modes. This allows the user to build complex harmonic structures right from the source, further shaped by integrated Frequency Modulation (FM) controls.

2. Quad AM Processing: The four signals then pass through an Amplitude Modulation (AM) module. This processor can be set to rhythmic rates, allowing it to function as a pseudo-sequencer that rapidly modulates the level of each of the four buses independently, adding complex, tempo-synced rhythmic patterns or aggressive, high-rate textural distortion.

3. Quad Granulator: Following the AM stage, the signal is fed into a powerful Granulator. This splits the audio into tiny "grains" that can be mangled, rearranged, and layered. Crucially, the granular parameters (such as grain size, pitch, and feedback) can be applied either globally or individually per bus, creating four completely unique, layered textures from the original signal.

Slashed Plates and Spatialization

The final stage is the unique element that gives Tetrad its metallic, sculptural sound—the Quad Plate Resonators.

• Physical Modeling: Tetrad utilizes pure physical modeling to simulate four resonating metal plates, which are mathematically "slashed" to give them distinct, non-linear modes of vibration. The signal drives these virtual plates, generating complex, ringing metallic tones.

• Graphical Positioning: Engineers can graphically position and excite these four plates on a virtual soundstage, adjusting parameters like decay, preamp drive, and the amount of "metal cuts" per plate. This not only shapes the timbre but also provides an inherent, three-dimensional spatial quality to the sound.

Modulation and Expression

To manage and animate this complexity, Tetrad features a comprehensive Modulation Matrix. With over 40 assignable destinations, eight multi-wave LFOs, and envelope followers, the user can route intricate control signals to virtually every parameter in the engine. Furthermore, the instrument offers full MPE (MIDI Polyphonic Expression) compatibility, allowing performers to control pitch, timbre, and pressure for each individual note, transforming Tetrad into a highly expressive, nuanced performance tool for experimental composition.