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audio/softsynth/mt32/srchelper/srctools/include/IIR2xResampler.h

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+/* Copyright (C) 2015-2022 Sergey V. Mikayev
+ *
+ *  This program is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU Lesser General Public License as published by
+ *  the Free Software Foundation, either version 2.1 of the License, or
+ *  (at your option) any later version.
+ *
+ *  This program is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU Lesser General Public License for more details.
+ *
+ *  You should have received a copy of the GNU Lesser General Public License
+ *  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef SRCTOOLS_IIR_2X_RESAMPLER_H
+#define SRCTOOLS_IIR_2X_RESAMPLER_H
+
+#include "ResamplerStage.h"
+
+namespace SRCTools {
+
+static const unsigned int IIR_RESAMPER_CHANNEL_COUNT = 2;
+static const unsigned int IIR_SECTION_ORDER = 2;
+
+typedef FloatSample IIRCoefficient;
+typedef FloatSample BufferedSample;
+
+typedef BufferedSample SectionBuffer[IIR_SECTION_ORDER];
+
+// Non-trivial coefficients of a 2nd-order section of a parallel bank
+// (zero-order numerator coefficient is always zero, zero-order denominator coefficient is always unity)
+struct IIRSection {
+	IIRCoefficient num1;
+	IIRCoefficient num2;
+	IIRCoefficient den1;
+	IIRCoefficient den2;
+};
+
+class IIRResampler : public ResamplerStage {
+public:
+	enum Quality {
+		// Used when providing custom IIR filter coefficients.
+		CUSTOM,
+		// Use fast elliptic filter with symmetric ripple: N=8, Ap=As=-99 dB, fp=0.125, fs = 0.25 (in terms of sample rate)
+		FAST,
+		// Use average elliptic filter with symmetric ripple: N=12, Ap=As=-106 dB, fp=0.193, fs = 0.25 (in terms of sample rate)
+		GOOD,
+		// Use sharp elliptic filter with symmetric ripple: N=18, Ap=As=-106 dB, fp=0.238, fs = 0.25 (in terms of sample rate)
+		BEST
+	};
+
+	// Returns the retained fraction of the passband for the given standard quality value
+	static double getPassbandFractionForQuality(Quality quality);
+
+protected:
+	explicit IIRResampler(const Quality quality);
+	explicit IIRResampler(const unsigned int useSectionsCount, const IIRCoefficient useFIR, const IIRSection useSections[]);
+	~IIRResampler();
+
+	const struct Constants {
+		// Coefficient of the 0-order FIR part
+		IIRCoefficient fir;
+		// 2nd-order sections that comprise a parallel bank
+		const IIRSection *sections;
+		// Number of 2nd-order sections
+		unsigned int sectionsCount;
+		// Delay line per channel per section
+		SectionBuffer *buffer;
+
+		Constants(const unsigned int useSectionsCount, const IIRCoefficient useFIR, const IIRSection useSections[], const Quality quality);
+	} constants;
+}; // class IIRResampler
+
+class IIR2xInterpolator : public IIRResampler {
+public:
+	explicit IIR2xInterpolator(const Quality quality);
+	explicit IIR2xInterpolator(const unsigned int useSectionsCount, const IIRCoefficient useFIR, const IIRSection useSections[]);
+
+	void process(const FloatSample *&inSamples, unsigned int &inLength, FloatSample *&outSamples, unsigned int &outLength);
+	unsigned int estimateInLength(const unsigned int outLength) const;
+
+private:
+	FloatSample lastInputSamples[IIR_RESAMPER_CHANNEL_COUNT];
+	unsigned int phase;
+};
+
+class IIR2xDecimator : public IIRResampler {
+public:
+	explicit IIR2xDecimator(const Quality quality);
+	explicit IIR2xDecimator(const unsigned int useSectionsCount, const IIRCoefficient useFIR, const IIRSection useSections[]);
+
+	void process(const FloatSample *&inSamples, unsigned int &inLength, FloatSample *&outSamples, unsigned int &outLength);
+	unsigned int estimateInLength(const unsigned int outLength) const;
+};
+
+} // namespace SRCTools
+
+#endif // SRCTOOLS_IIR_2X_RESAMPLER_H