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Reworked filters
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‎modules/yup_dsp/base/yup_AnalogPoles.h‎

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

‎modules/yup_dsp/base/yup_AnalogSaturator.h‎

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/*
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==============================================================================
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This file is part of the YUP library.
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Copyright (c) 2026 - kunitoki@gmail.com
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YUP is an open source library subject to open-source licensing.
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The code included in this file is provided under the terms of the ISC license
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http://www.isc.org/downloads/software-support-policy/isc-license. Permission
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to use, copy, modify, and/or distribute this software for any purpose with or
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without fee is hereby granted provided that the above copyright notice and
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this permission notice appear in all copies.
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YUP IS PROVIDED "AS IS" WITHOUT ANY WARRANTY, AND ALL WARRANTIES, WHETHER
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EXPRESSED OR IMPLIED, INCLUDING MERCHANTABILITY AND FITNESS FOR PURPOSE, ARE
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DISCLAIMED.
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==============================================================================
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*/
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#pragma once
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namespace yup
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{
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//==============================================================================
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/** Clips the input value to the range [-4, 4] using a hyperbolic tangent function.
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This function is used to limit the feedback in analog-model filters to prevent
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instability while preserving the character of the resonance.
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@param input The input value to clip
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@return The clipped output value
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*/
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template <typename CoeffType>
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CoeffType clipAnalogResonance (CoeffType input) noexcept
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{
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return std::tanh (input * static_cast<CoeffType> (0.25)) * static_cast<CoeffType> (4);
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}
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/** Analog saturator structure.
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This structure implements a simple symmetric atan-based waveshaper for analog
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saturation effects. The drive parameter controls the amount of saturation, while
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preScale and postScale allow for adjusting the input and output levels to
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achieve the desired tonal characteristics.
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*/
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template <typename SampleType, typename CoeffType>
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struct AnalogSaturator
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{
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void setCoefficients (const AnalogSaturatorCoefficients<CoeffType>& newCoefficients) noexcept
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{
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coefficients = newCoefficients;
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}
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SampleType process (SampleType input) const noexcept
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{
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if (coefficients.drive <= static_cast<CoeffType> (0))
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return input;
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return static_cast<SampleType> (
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std::atan (static_cast<CoeffType> (input) * coefficients.preScale) * coefficients.postScale);
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}
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AnalogSaturatorCoefficients<CoeffType> coefficients;
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};
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} // namespace yup

‎modules/yup_dsp/designers/yup_FilterDesigner.cpp‎

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template <typename CoeffType>
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int FilterDesigner<CoeffType>::designButterworth (
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std::vector<BiquadCoefficients<CoeffType>>& coefficients,
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FilterModeType filterMode,
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int order,
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CoeffType frequency,
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CoeffType frequency2,
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double sampleRate,
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std::vector<BiquadCoefficients<CoeffType>>& coefficients) noexcept
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double sampleRate) noexcept
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{
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// Validate inputs
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jassert (order >= 2 && order <= 16);
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template <typename CoeffType>
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int FilterDesigner<CoeffType>::designLinkwitzRiley (
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std::vector<BiquadCoefficients<CoeffType>>& lowCoeffs,
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std::vector<BiquadCoefficients<CoeffType>>& highCoeffs,
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int order,
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CoeffType crossoverFreq,
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double sampleRate,
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std::vector<BiquadCoefficients<CoeffType>>& lowCoeffs,
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std::vector<BiquadCoefficients<CoeffType>>& highCoeffs) noexcept
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double sampleRate) noexcept
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{
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jassert (order >= 2 && order <= 16);
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jassert ((order & 1) == 0); // Must be even

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