iir_filter.h

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/* -*- c++ -*- */
/*
 * Copyright 2002,2012 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_IIR_FILTER_H
#define INCLUDED_IIR_FILTER_H

#include <filter/api.h>
#include <vector>
#include <stdexcept>

namespace gr {
  namespace filter {
    namespace kernel {

      /*!
       * \brief base class template for Infinite Impulse Response filter (IIR)
       */
      template<class i_type, class o_type, class tap_type>
      class iir_filter
      {
      public:
        /*!
         * \brief Construct an IIR with the given taps.
         *
         * This filter uses the Direct Form I implementation, where
         * \p fftaps contains the feed-forward taps, and \p fbtaps the feedback ones.
         *
         * \p fftaps and \p fbtaps must have equal numbers of taps
         *
         * The input and output satisfy a difference equation of the form

         \f[
         y[n] - \sum_{k=1}^{M} a_k y[n-k] = \sum_{k=0}^{N} b_k x[n-k]
         \f]

         * with the corresponding rational system function

         \f[
         H(z) = \frac{\sum_{k=0}^{N} b_k z^{-k}}{1 - \sum_{k=1}^{M} a_k z^{-k}}
         \f]

         * Note that some texts define the system function with a + in
         * the denominator. If you're using that convention, you'll
         * need to negate the feedback taps.
         */
        iir_filter(const std::vector<tap_type>& fftaps,
                   const std::vector<tap_type>& fbtaps) throw (std::invalid_argument)
        {
          set_taps(fftaps, fbtaps);
        }

        iir_filter() : d_latest_n(0),d_latest_m(0) { }

        ~iir_filter() {}

        /*!
         * \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 filter_n(o_type output[], const i_type input[], long n);

        /*!
         * \return number of taps in filter.
         */
        unsigned ntaps_ff() const { return d_fftaps.size(); }
        unsigned ntaps_fb() const { return d_fbtaps.size(); }

        /*!
         * \brief install new taps.
         */
        void set_taps(const std::vector<tap_type> &fftaps,
                      const std::vector<tap_type> &fbtaps) throw (std::invalid_argument)
        {
          d_latest_n = 0;
          d_latest_m = 0;
          d_fftaps = fftaps;
          d_fbtaps = fbtaps;

          int n = fftaps.size();
          int m = fbtaps.size();
          d_prev_input.resize(2 * n);
          d_prev_output.resize(2 * m);

          for(int i = 0; i < 2 * n; i++) {
            d_prev_input[i] = 0;
          }
          for(int i = 0; i < 2 * m; i++) {
            d_prev_output[i] = 0;
          }
        }

      protected:
        std::vector<tap_type>   d_fftaps;
        std::vector<tap_type>   d_fbtaps;
        int                     d_latest_n;
        int                     d_latest_m;
        std::vector<tap_type>   d_prev_output;
        std::vector<i_type>     d_prev_input;
      };

      //
      // general case.  We may want to specialize this
      //
      template<class i_type, class o_type, class tap_type>
      o_type
      iir_filter<i_type, o_type, tap_type>::filter(const i_type input)
      {
        tap_type acc;
        unsigned i = 0;
        unsigned n = ntaps_ff();
        unsigned m = ntaps_fb();

        if(n == 0)
          return (o_type)0;

        int latest_n = d_latest_n;
        int latest_m = d_latest_m;

        acc = d_fftaps[0] * input;
        for(i = 1; i < n; i ++)
          acc += (d_fftaps[i] * d_prev_input[latest_n + i]);
        for(i = 1; i < m; i ++)
          acc += (d_fbtaps[i] * d_prev_output[latest_m + i]);

        // store the values twice to avoid having to handle wrap-around in the loop
        d_prev_output[latest_m] = acc;
        d_prev_output[latest_m+m] = acc;
        d_prev_input[latest_n] = input;
        d_prev_input[latest_n+n] = input;

        latest_n--;
        latest_m--;
        if(latest_n < 0)
          latest_n += n;
        if(latest_m < 0)
          latest_m += m;

        d_latest_m = latest_m;
        d_latest_n = latest_n;
        return (o_type)acc;
      }

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

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

#endif /* INCLUDED_IIR_FILTER_H */