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FILTRONIX: ZDF Circuit-Modeled Filter

1. FILTRONIX: Circuit-Modeled Filter

FILTRONIX is a “ZDF Circuit-Modeled Filter” that faithfully reconstructs the legendary analog synthesizer filter circuits in the digital domain.
It is currently available as a free VST plugin, so I highly encourage you to give it a try.
At the moment, it is Windows-only, but if there is enough positive feedback, I plan to develop a Mac version as well.
While it currently features only a low-pass filter, multi-mode filter support is planned for future updates.
Additionally, since the circuit modeling is quite CPU-intensive, further optimization is on the roadmap.

The download link is located at the bottom of this page.

2. Core Technologies

Several key technologies have been implemented to capture that authentic analog feel.

Circuit Modeling

Mathematical Description of Circuit Behavior: The physical behavior of individual components such as capacitors, resistors, and transistors is formulated as complex differential equations. By calculating these tens of thousands of times per second, the plugin recreates the unique “drive” and “distortion” that occurs when electricity flows through physical circuits, as well as the aggressive “self-oscillation” that happens near the physical limits.

ZDF (Zero-Delay Feedback) Algorithm

Standard digital filters often produce a harsh, digital ringing when resonance is pushed in the high frequencies. This issue is resolved using the ZDF algorithm, a technique favored by industry-leading analog modeling developers like u-he.

In conventional digital filters, a “1-sample delay” occurs in the feedback loop calculation, making it difficult to reproduce the smoothness and extreme oscillation characteristic of analog hardware. FILTRONIX employs Zero-Delay Feedback (ZDF) topology to solve this mathematical paradox. This ensures an analog-like “glue” and stability without breaking down, even when modulating the cutoff and resonance at high speeds.

4x Oversampling and 33-Tap FIR Filter

To drastically eliminate aliasing noise, the plugin implements 4x Oversampling, running the internal processing at four times the base sample rate. For the crucial downsampling stage, it uses a high-quality 33-tap FIR polyphase filter, maintaining crystal-clear audio quality while keeping group delay to an absolute minimum (accurately phase-compensated by 4 samples).

Pursuit of Non-Linearity: ADAA and Fast Math LUT

The true magic of analog circuits lies in their “non-linearity”—how the sound characteristics change depending on the input level. FILTRONIX features a tanh saturation stage utilizing ADAA (Anti-Derivative Anti-Aliasing) technology. Furthermore, a Fast Math LUT (Look-Up Table) built with 65,536-step Cubic Hermite interpolation reproduces extreme, screaming circuit saturation while keeping CPU load as low as possible.

3. Four Filter Models

FILTRONIX models four historic circuits, each with its own distinct character.

• M-Filter (Transistor Ladder): Models the ladder filter of a legendary American monophonic synthesizer. It features a sharp, pronounced resonant peak and the classic, musical low-end roll-off as resonance is increased.

• S-Filter (Classic SVF): Based on the State Variable Filter (SVF) from an iconic ’70s synthesizer module. It retains a thick low-end even at high resonance settings, delivering razor-sharp tones right on the edge of self-oscillation.

• P-Filter (Polyphonic OTA): Recreates the OTA cascade filter of the legendary 5-voice polyphonic synthesizer that defined the ’80s music scene. It delivers a consistently fat and punchy sound.

• W-Filter (British CMOS): Models the unique CMOS circuit of a cult-classic, yellow-and-black British synthesizer. It is characterized by the inherent instability of CMOS chips and a sudden, aggressive self-oscillation.

Technical Specifications

filtronix_1.0.0.0

Download via Gumroad You can get it for free by entering $0 in the “Name a fair price” box. Any optional donations to support my development are always highly appreciated!