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Ladder Filter Machine (4-Pole)

The 4-Pole Ladder Filter Machine is one of the two filter options available per voice in the DSP engine (FilterSection in private/src/SquiggleBoy.hpp).

Structure

This filter machine consists of two 4-pole filters in series:

1. Low-Pass Filter: A 4-pole transistor ladder filter emulation (LadderFilterLP).
2. High-Pass Filter: A 4-pole linear State Variable Filter (SVF) configured as a high-pass (LinearSVF4PoleHighPass).

The audio signal from the source machine passes through the low-pass ladder filter first, and the output is then processed by the high-pass SVF.

Features

• Cutoff Tracking: The HP cutoff frequency tracks the base frequency of the voice’s oscillator (m_vcoBaseFreq) multiplied by m_hpCutoffFactor. The LP cutoff is computed relative to the HP cutoff: vcoBaseFreq * hpCutoff * lpCutoff, so the LP is always at or below the HP.
• Resonance: Both filters feature adjustable resonance (m_lpResonance and m_hpResonance).
• Saturation: The ladder filter includes an internal saturation stage, controlled by m_saturationGain, which drives the filter into non-linear distortion.
• Sample Rate Reduction: After the filtering stages, the signal passes through a sample rate reducer (SampleRateReducer), adding digital aliasing and grit.

Related

• DSP Overview
• SVF Filter Machine (2-Pole)

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