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diff --git a/extern/audaspace/include/fx/ButterworthCalculator.h b/extern/audaspace/include/fx/ButterworthCalculator.h
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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.
+ ******************************************************************************/
+
+#pragma once
+
+/**
+ * @file ButterworthCalculator.h
+ * @ingroup fx
+ * The ButterworthCalculator class.
+ */
+
+#include "fx/IDynamicIIRFilterCalculator.h"
+
+AUD_NAMESPACE_BEGIN
+
+/**
+ * The ButterworthCalculator class calculates fourth order Butterworth low pass
+ * filter coefficients for a dynamic DynamicIIRFilter.
+ */
+class AUD_LOCAL ButterworthCalculator : public IDynamicIIRFilterCalculator
+{
+private:
+	/**
+	 * The attack value in seconds.
+	 */
+	const float m_frequency;
+
+	// delete copy constructor and operator=
+	ButterworthCalculator(const ButterworthCalculator&) = delete;
+	ButterworthCalculator& operator=(const ButterworthCalculator&) = delete;
+
+public:
+	/**
+	 * Creates a ButterworthCalculator object.
+	 * @param frequency The cutoff frequency.
+	 */
+	ButterworthCalculator(float frequency);
+
+	virtual void recalculateCoefficients(SampleRate rate, std::vector<float> &b, std::vector<float> &a);
+};
+
+AUD_NAMESPACE_END