IIR2xResampler.h

/* 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