lowpass.h

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#ifndef INCLUDE_LOWPASS_H
#define INCLUDE_LOWPASS_H

#define _USE_MATH_DEFINES
#include <math.h>
#include "dsp/dsptypes.h"

#undef M_PI
#define M_PI        3.14159265358979323846

template <class Type> class Lowpass {
public:
    Lowpass() : m_ptr(0) { }

    void create(int nTaps, double sampleRate, double cutoff)
    {
        double wc = 2.0 * M_PI * cutoff;
        double Wc = wc / sampleRate;
        int i;

        // check constraints
        if(!(nTaps & 1)) {
            qDebug("Lowpass filter has to have an odd number of taps");
            nTaps++;
        }

        // make room
        m_samples.resize(nTaps);
        for(int i = 0; i < nTaps; i++)
            m_samples[i] = 0;
        m_ptr = 0;
        m_taps.resize(nTaps / 2 + 1);

        // generate Sinc filter core
        for(i = 0; i < nTaps / 2 + 1; i++) {
            if(i == (nTaps - 1) / 2)
                m_taps[i] = Wc / M_PI;
            else
                m_taps[i] = sin(((double)i - ((double)nTaps - 1.0) / 2.0) * Wc) / (((double)i - ((double)nTaps - 1.0) / 2.0) * M_PI);
        }

        // apply Hamming window
        for(i = 0; i < nTaps / 2 + 1; i++)
            m_taps[i] *= 0.54 + 0.46 * cos((2.0 * M_PI * ((double)i - ((double)nTaps - 1.0) / 2.0)) / (double)nTaps);

        // normalize
        Real sum = 0;
        for(i = 0; i < (int)m_taps.size() - 1; i++)
            sum += m_taps[i] * 2;
        sum += m_taps[i];
        for(i = 0; i < (int)m_taps.size(); i++)
            m_taps[i] /= sum;
    }

    Type filter(Type sample)
    {
        Type acc = 0;
        int a = m_ptr;
        int b = a - 1;
        int i, n_taps, size;

        m_samples[m_ptr] = sample;
        size = m_samples.size(); // Valgrind optim (2)

        while (b < 0)
        {
            b += size;
        }

        n_taps = m_taps.size() - 1; // Valgrind optim

        for (i = 0; i < n_taps; i++)
        {
            acc += (m_samples[a] + m_samples[b]) * m_taps[i];
            a++;

            while (a >= size)
            {
                a -= size;
            }

            b--;

            while(b < 0)
            {
                b += size;
            }
        }

        acc += m_samples[a] * m_taps[i];
        m_ptr++;

        while(m_ptr >= size)
        {
            m_ptr -= size;
        }

        return acc;
    }

private:
    std::vector<Real> m_taps;
    std::vector<Type> m_samples;
    int m_ptr;
};

#endif // INCLUDE_LOWPASS_H