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/*******************************************************************************
* Copyright 2009-2016 Jörg Müller
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
******************************************************************************/
#include "fx/ButterworthCalculator.h"
#include <cmath>
#define BWPB41 0.76536686473
#define BWPB42 1.84775906502
AUD_NAMESPACE_BEGIN
ButterworthCalculator::ButterworthCalculator(float frequency) :
m_frequency(frequency)
{
}
void ButterworthCalculator::recalculateCoefficients(SampleRate rate, std::vector<float> &b, std::vector<float> &a)
{
float omega = 2 * std::tan(m_frequency * M_PI / rate);
float o2 = omega * omega;
float o4 = o2 * o2;
float x1 = o2 + 2.0f * (float)BWPB41 * omega + 4.0f;
float x2 = o2 + 2.0f * (float)BWPB42 * omega + 4.0f;
float y1 = o2 - 2.0f * (float)BWPB41 * omega + 4.0f;
float y2 = o2 - 2.0f * (float)BWPB42 * omega + 4.0f;
float o228 = 2.0f * o2 - 8.0f;
float norm = x1 * x2;
a.push_back(1);
a.push_back((x1 + x2) * o228 / norm);
a.push_back((x1 * y2 + x2 * y1 + o228 * o228) / norm);
a.push_back((y1 + y2) * o228 / norm);
a.push_back(y1 * y2 / norm);
b.push_back(o4 / norm);
b.push_back(4 * o4 / norm);
b.push_back(6 * o4 / norm);
b.push_back(b[1]);
b.push_back(b[0]);
}
AUD_NAMESPACE_END
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