VA Filters

modules/yup_dsp/base/yup_AnalogFilterCoefficients.h

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+/*
+  ==============================================================================
+
+   This file is part of the YUP library.
+   Copyright (c) 2026 - kunitoki@gmail.com
+
+   YUP is an open source library subject to open-source licensing.
+
+   The code included in this file is provided under the terms of the ISC license
+   http://www.isc.org/downloads/software-support-policy/isc-license. Permission
+   to use, copy, modify, and/or distribute this software for any purpose with or
+   without fee is hereby granted provided that the above copyright notice and
+   this permission notice appear in all copies.
+
+   YUP IS PROVIDED "AS IS" WITHOUT ANY WARRANTY, AND ALL WARRANTIES, WHETHER
+   EXPRESSED OR IMPLIED, INCLUDING MERCHANTABILITY AND FITNESS FOR PURPOSE, ARE
+   DISCLAIMED.
+
+  ==============================================================================
+*/
+
+#pragma once
+
+namespace yup
+{
+
+//==============================================================================
+/**
+    Coefficients for the saturator used by analog-model filters.
+
+    The coefficients describe a symmetric atan waveshaper. A drive of zero keeps
+    the processor transparent.
+*/
+template <typename CoeffType = double>
+struct AnalogSaturatorCoefficients
+{
+    CoeffType drive = static_cast<CoeffType> (0);
+    CoeffType preScale = static_cast<CoeffType> (1);
+    CoeffType postScale = static_cast<CoeffType> (1);
+};
+
+//==============================================================================
+/**
+    Coefficients for a trapezoidal-integrator two-pole state-variable filter.
+
+    The output gains select low-pass, high-pass, band-pass, peak, or notch
+    responses without changing the integrator state update.
+*/
+template <typename CoeffType = double>
+struct AnalogTwoPoleCoefficients
+{
+    CoeffType g = static_cast<CoeffType> (0);
+    CoeffType h = static_cast<CoeffType> (1);
+    CoeffType r2 = static_cast<CoeffType> (0);
+    CoeffType gainCorrection = static_cast<CoeffType> (1);
+    CoeffType lowOut = static_cast<CoeffType> (1);
+    CoeffType bandOut = static_cast<CoeffType> (0);
+    CoeffType highOut = static_cast<CoeffType> (0);
+};
+
+//==============================================================================
+/**
+    Coefficients for one trapezoidal one-pole section used inside ladder models.
+*/
+template <typename CoeffType = double>
+struct AnalogOnePoleCoefficients
+{
+    CoeffType alpha = static_cast<CoeffType> (0);
+    CoeffType beta = static_cast<CoeffType> (1);
+};
+
+//==============================================================================
+/**
+    Coefficients for a Korg35-inspired analog-model filter.
+*/
+template <typename CoeffType = double>
+struct AnalogKorg35Coefficients
+{
+    std::array<AnalogOnePoleCoefficients<CoeffType>, 3> poles;
+    CoeffType alpha0 = static_cast<CoeffType> (1);
+    CoeffType feedback = static_cast<CoeffType> (0);
+    CoeffType gainCorrection = static_cast<CoeffType> (1);
+};
+
+//==============================================================================
+/**
+    Moog ladder output mode.
+*/
+enum class AnalogMoogLadderMode
+{
+    lowpass24,
+    highpass24,
+    lowpass18,
+    highpass18,
+    lowpass12,
+    highpass12,
+    lowpass6,
+    highpass6,
+    bandpass12,
+    bandpass6
+};
+
+//==============================================================================
+/**
+    Coefficients for a four-pole Moog ladder-style analog-model filter.
+*/
+template <typename CoeffType = double>
+struct AnalogMoogLadderCoefficients
+{
+    std::array<AnalogOnePoleCoefficients<CoeffType>, 4> poles;
+    std::array<CoeffType, 5> outputs {
+        static_cast<CoeffType> (0),
+        static_cast<CoeffType> (0),
+        static_cast<CoeffType> (0),
+        static_cast<CoeffType> (0),
+        static_cast<CoeffType> (1)
+    };
+    CoeffType alpha0 = static_cast<CoeffType> (1);
+    CoeffType feedback = static_cast<CoeffType> (0);
+    CoeffType gainCorrection = static_cast<CoeffType> (1);
+};
+
+//==============================================================================
+/**
+    Coefficients for a Roland diode-ladder-inspired low-pass filter.
+*/
+template <typename CoeffType = double>
+struct AnalogRolandDiodeCoefficients
+{
+    CoeffType cutoff = static_cast<CoeffType> (0);
+    CoeffType feedback = static_cast<CoeffType> (0);
+    CoeffType gainCorrection = static_cast<CoeffType> (1);
+    CoeffType a = static_cast<CoeffType> (0);
+    CoeffType a2 = static_cast<CoeffType> (0);
+    CoeffType aInv = static_cast<CoeffType> (1);
+    CoeffType b = static_cast<CoeffType> (1);
+    CoeffType b2 = static_cast<CoeffType> (1);
+    CoeffType c = static_cast<CoeffType> (1);
+    CoeffType g0 = static_cast<CoeffType> (0);
+    CoeffType g = static_cast<CoeffType> (0);
+    CoeffType fg = static_cast<CoeffType> (1);
+    CoeffType highpassA = static_cast<CoeffType> (0);
+    CoeffType highpassB = static_cast<CoeffType> (1);
+};
+
+//==============================================================================
+/**
+    Coefficients for the vowel/formant analog-model filter.
+*/
+template <typename CoeffType = double>
+struct AnalogVowelCoefficients
+{
+    std::array<AnalogTwoPoleCoefficients<CoeffType>, 3> formants;
+    CoeffType gainCompensation = static_cast<CoeffType> (1);
+};
+
+} // namespace yup

modules/yup_dsp/base/yup_AnalogPoles.h

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+/*
+  ==============================================================================
+
+   This file is part of the YUP library.
+   Copyright (c) 2026 - kunitoki@gmail.com
+
+   YUP is an open source library subject to open-source licensing.
+
+   The code included in this file is provided under the terms of the ISC license
+   http://www.isc.org/downloads/software-support-policy/isc-license. Permission
+   to use, copy, modify, and/or distribute this software for any purpose with or
+   without fee is hereby granted provided that the above copyright notice and
+   this permission notice appear in all copies.
+
+   YUP IS PROVIDED "AS IS" WITHOUT ANY WARRANTY, AND ALL WARRANTIES, WHETHER
+   EXPRESSED OR IMPLIED, INCLUDING MERCHANTABILITY AND FITNESS FOR PURPOSE, ARE
+   DISCLAIMED.
+
+  ==============================================================================
+*/
+
+#pragma once
+
+namespace yup
+{
+
+//==============================================================================
+/** State for a one-pole analog-model filter.
+
+    The state is designed to be used with the coefficients defined in AnalogOnePoleCoefficients.
+*/
+template <typename CoeffType>
+struct AnalogOnePoleState
+{
+    CoeffType z1 = static_cast<CoeffType> (0);
+
+    void reset() noexcept
+    {
+        z1 = static_cast<CoeffType> (0);
+    }
+
+    CoeffType processLowPass (CoeffType input, const AnalogOnePoleCoefficients<CoeffType>& coefficients) noexcept
+    {
+        const auto v = (input - z1) * coefficients.alpha;
+        const auto output = v + z1;
+        z1 = output + v;
+
+        return output;
+    }
+
+    CoeffType processHighPass (CoeffType input, const AnalogOnePoleCoefficients<CoeffType>& coefficients) noexcept
+    {
+        return input - processLowPass (input, coefficients);
+    }
+
+    CoeffType getFeedbackOutput (const AnalogOnePoleCoefficients<CoeffType>& coefficients) const noexcept
+    {
+        return coefficients.beta * z1;
+    }
+};
+
+//==============================================================================
+/** State for a two-pole analog-model filter.
+
+    The state is designed to be used with the coefficients defined in AnalogTwoPoleCoefficients.
+*/
+template <typename CoeffType>
+struct AnalogTwoPoleState
+{
+    CoeffType s1 = static_cast<CoeffType> (0);
+    CoeffType s2 = static_cast<CoeffType> (0);
+
+    void reset() noexcept
+    {
+        s1 = static_cast<CoeffType> (0);
+        s2 = static_cast<CoeffType> (0);
+    }
+};
+
+//==============================================================================
+/** Computes the low-pass output of a one-pole analog-model filter.
+
+    @param input The input sample to process
+    @param state The current state of the filter
+    @param coefficients The coefficients defining the filter behavior
+
+    @return The low-pass output sample
+*/
+template <typename CoeffType>
+CoeffType getAnalogOnePoleLowPassOutput (CoeffType input, CoeffType state, const AnalogOnePoleCoefficients<CoeffType>& coefficients) noexcept
+{
+    return coefficients.alpha * input + (static_cast<CoeffType> (1) - coefficients.alpha) * state;
+}
+
+/** Computes the high-pass output of a one-pole analog-model filter.
+
+    @param input The input sample to process
+    @param state The current state of the filter
+    @param coefficients The coefficients defining the filter behavior
+
+    @return The high-pass output sample
+*/
+template <typename CoeffType>
+CoeffType getAnalogOnePoleHighPassOutput (CoeffType input, CoeffType state, const AnalogOnePoleCoefficients<CoeffType>& coefficients) noexcept
+{
+    return (static_cast<CoeffType> (1) - coefficients.alpha) * (input - state);
+}
+
+/** Computes the next state of a one-pole analog-model filter.
+
+    @param input The input sample to process
+    @param state The current state of the filter
+    @param coefficients The coefficients defining the filter behavior
+
+    @return The next state of the filter
+*/
+template <typename CoeffType>
+CoeffType getAnalogOnePoleNextState (CoeffType input, CoeffType state, const AnalogOnePoleCoefficients<CoeffType>& coefficients) noexcept
+{
+    return static_cast<CoeffType> (2) * coefficients.alpha * input
+         + (static_cast<CoeffType> (1) - static_cast<CoeffType> (2) * coefficients.alpha) * state;
+}
+
+//==============================================================================
+/** Processes a single sample through a two-pole analog-model filter.
+
+    This function implements the processing for a trapezoidal-integrator two-pole state-variable filter.
+    The output is a combination of low-pass, high-pass, and band-pass responses based on the coefficients provided.
+
+    @param input The input sample to process
+    @param coefficients The coefficients defining the filter behavior
+    @param state The state of the filter, which will be updated during processing
+
+    @return The processed output sample
+ */
+template <typename SampleType, typename CoeffType>
+SampleType processAnalogTwoPole (
+    SampleType input,
+    const AnalogTwoPoleCoefficients<CoeffType>& coefficients,
+    AnalogTwoPoleState<CoeffType>& state) noexcept
+{
+    const auto inputValue = static_cast<CoeffType> (input);
+    const auto highpass = (inputValue - coefficients.r2 * state.s1 - coefficients.g * state.s1 - state.s2) * coefficients.h;
+    const auto bandpass = coefficients.g * highpass + state.s1;
+    const auto bandpassFeedback = clipAnalogResonance (bandpass);
+    const auto lowpass = coefficients.g * bandpass + state.s2;
+
+    state.s1 = coefficients.g * highpass + bandpassFeedback;
+    state.s2 = coefficients.g * bandpassFeedback + lowpass;
+
+    const auto output = (coefficients.lowOut * lowpass + coefficients.bandOut * bandpass + coefficients.highOut * highpass)
+                      * coefficients.gainCorrection;
+
+    return static_cast<SampleType> (output);
+}
+
+//==============================================================================
+/** Computes the complex frequency response of a two-pole analog-model filter.
+
+    @param coefficients The coefficients defining the filter behavior
+    @param frequency The frequency at which to evaluate the response
+    @param sampleRate The sample rate of the system
+
+    @return The complex frequency response
+ */
+template <typename CoeffType>
+Complex<CoeffType> getAnalogTwoPoleComplexResponse (
+    const AnalogTwoPoleCoefficients<CoeffType>& coefficients,
+    CoeffType frequency,
+    double sampleRate) noexcept
+{
+    using ComplexType = Complex<CoeffType>;
+
+    const auto highpassState1 = -coefficients.h * (coefficients.r2 + coefficients.g);
+    const auto highpassState2 = -coefficients.h;
+    const auto highpassInput = coefficients.h;
+
+    const auto bandpassState1 = coefficients.g * highpassState1 + static_cast<CoeffType> (1);
+    const auto bandpassState2 = coefficients.g * highpassState2;
+    const auto bandpassInput = coefficients.g * highpassInput;
+
+    const auto lowpassState1 = coefficients.g * bandpassState1;
+    const auto lowpassState2 = coefficients.g * bandpassState2 + static_cast<CoeffType> (1);
+    const auto lowpassInput = coefficients.g * bandpassInput;
+
+    const auto state11 = coefficients.g * highpassState1 + bandpassState1;
+    const auto state12 = coefficients.g * highpassState2 + bandpassState2;
+    const auto state1Input = coefficients.g * highpassInput + bandpassInput;
+
+    const auto state21 = coefficients.g * bandpassState1 + lowpassState1;
+    const auto state22 = coefficients.g * bandpassState2 + lowpassState2;
+    const auto state2Input = coefficients.g * bandpassInput + lowpassInput;
+
+    const auto outputState1 = (coefficients.lowOut * lowpassState1 + coefficients.bandOut * bandpassState1 + coefficients.highOut * highpassState1)
+                            * coefficients.gainCorrection;
+    const auto outputState2 = (coefficients.lowOut * lowpassState2 + coefficients.bandOut * bandpassState2 + coefficients.highOut * highpassState2)
+                            * coefficients.gainCorrection;
+    const auto outputInput = (coefficients.lowOut * lowpassInput + coefficients.bandOut * bandpassInput + coefficients.highOut * highpassInput)
+                           * coefficients.gainCorrection;
+
+    const auto z = polar (static_cast<CoeffType> (1), frequencyToAngular (frequency, static_cast<CoeffType> (sampleRate)));
+    const auto determinant = (z - state11) * (z - state22) - state12 * state21;
+    const auto responseState1 = ((z - state22) * state1Input + state12 * state2Input) / determinant;
+    const auto responseState2 = (state21 * state1Input + (z - state11) * state2Input) / determinant;
+
+    return ComplexType (outputInput) + outputState1 * responseState1 + outputState2 * responseState2;
+}
+
+} // namespace yup