single_pole_iir.h

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/* -*- c++ -*- */
/*
 * Copyright 2002,2006,2012 Free Software Foundation, Inc.
 *
 * This file is part of GNU Radio
 *
 * GNU Radio is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 3, or (at your option)
 * any later version.
 *
 * GNU Radio 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 General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with GNU Radio; see the file COPYING.  If not, write to
 * the Free Software Foundation, Inc., 51 Franklin Street,
 * Boston, MA 02110-1301, USA.
 */

#ifndef INCLUDED_SINGLE_POLE_IIR_H
#define INCLUDED_SINGLE_POLE_IIR_H

#include <gnuradio/filter/api.h>
#include <stdexcept>
#include <gnuradio/gr_complex.h>

namespace gr {
  namespace filter {

    /*!
     * \brief class template for single pole IIR filter
     */
    template<class o_type, class i_type, class tap_type>
    class single_pole_iir
    {
    public:
      /*!
       * \brief construct new single pole IIR with given alpha
       *
       * computes y(i) = (1-alpha) * y(i-1) + alpha * x(i)
       */
      single_pole_iir(tap_type alpha = 1.0)
      {
        d_prev_output = 0;
        set_taps(alpha);
      }

      /*!
       * \brief compute a single output value.
       * \returns the filtered input value.
       */
      o_type filter(const i_type input);

      /*!
       * \brief compute an array of N output values.
       * \p input must have n valid entries.
       */
      void filterN(o_type output[], const i_type input[], unsigned long n);

      /*!
       * \brief install \p alpha as the current taps.
       */
      void set_taps(tap_type alpha)
      {
        if(alpha < 0 || alpha > 1)
          throw std::out_of_range("Alpha must be in [0, 1]\n");

        d_alpha = alpha;
        d_one_minus_alpha = 1.0 - alpha;
      }

      //! reset state to zero
      void reset()
      {
        d_prev_output = 0;
      }

      o_type prev_output() const { return d_prev_output; }

    protected:
      tap_type  d_alpha;
      tap_type  d_one_minus_alpha;
      o_type    d_prev_output;
    };

    //
    // general case.  We may want to specialize this
    //
    template<class o_type, class i_type, class tap_type>
    o_type
    single_pole_iir<o_type, i_type, tap_type>::filter(const i_type input)
    {
      o_type    output;

      output = d_alpha * input + d_one_minus_alpha * d_prev_output;
      d_prev_output = output;

      return (o_type) output;
    }


    template<class o_type, class i_type, class tap_type>
    void
    single_pole_iir<o_type, i_type, tap_type>::filterN(o_type output[],
                                                       const i_type input[],
                                                       unsigned long n)
    {
      for(unsigned i = 0; i < n; i++)
        output[i] = filter(input[i]);
    }


    //
    // Specialized case for gr_complex output and double taps
    // We need to have a gr_complexd type for the calculations and prev_output variable (in stead of double)

    template<class i_type>
    class single_pole_iir<gr_complex, i_type, double>
    {
    public:
      /*!
       * \brief construct new single pole IIR with given alpha
       *
       * computes y(i) = (1-alpha) * y(i-1) + alpha * x(i)
       */
      single_pole_iir(double alpha = 1.0)
      {
        d_prev_output = 0;
        set_taps(alpha);
      }

      /*!
       * \brief compute a single output value.
       * \returns the filtered input value.
       */
      gr_complex filter(const i_type input);

      /*!
       * \brief compute an array of N output values.
       * \p input must have n valid entries.
       */
      void filterN(gr_complex output[], const i_type input[], unsigned long n);

      /*!
       * \brief install \p alpha as the current taps.
       */
      void set_taps(double alpha)
      {
        if(alpha < 0 || alpha > 1)
          throw std::out_of_range("Alpha must be in [0, 1]\n");

        d_alpha = alpha;
        d_one_minus_alpha = 1.0 - alpha;
      }

      //! reset state to zero
      void reset()
      {
        d_prev_output = 0;
      }

      gr_complexd prev_output() const { return d_prev_output; }

    protected:
      double    d_alpha;
      double    d_one_minus_alpha;
      gr_complexd       d_prev_output;
    };

    template< class i_type>
    gr_complex
    single_pole_iir<gr_complex, i_type, double>::filter(const i_type input)
    {
      gr_complexd       output;

      output = d_alpha * (gr_complexd)input + d_one_minus_alpha * d_prev_output;
      d_prev_output = output;

      return (gr_complex) output;
    }

    //Do we need to specialize this, although it is the same as the general case?

    template<class i_type>
    void
    single_pole_iir<gr_complex, i_type, double>::filterN(gr_complex output[],
                                                         const i_type input[],
                                                         unsigned long n)
    {
      for(unsigned i = 0; i < n; i++)
        output[i] = filter(input[i]);
    }

  } /* namespace filter */
} /* namespace gr */

#endif /* INCLUDED_SINGLE_POLE_IIR_H */