rational_resampler.h

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/* -*- c++ -*- */
/*
 * Copyright 2005,2012,2018 Free Software Foundation, Inc.
 *
 * This file is part of GNU Radio
 *
 * SPDX-License-Identifier: GPL-3.0-or-later
 *
 */
 
#ifndef RATIONAL_RESAMPLER_H
#define RATIONAL_RESAMPLER_H
 
#include <gnuradio/block.h>
#include <gnuradio/filter/api.h>
 
 
namespace gr {
namespace filter {
 
/*!
 * \brief Rational Resampling Polyphase FIR filter with IN_T
 * input, OUT_T output and TAP_T taps.
 * \ingroup resamplers_blk
 *
 * Make a rational resampling FIR filter. If the input signal is
 * at rate fs, then the output signal will be at a rate of \p
 * interpolation * fs / \p decimation.
 *
 * The interpolation and decimation rates should be kept as
 * small as possible, and generally should be relatively prime
 * to help reduce complexity in memory and computation.
 *
 * The set of \p taps supplied to this filterbank should be
 * designed around the resampling amount and must avoid aliasing
 * (when interpolation/decimation < 1) and images (when
 * interpolation/decimation > 1).
 *
 * As with any filter, the behavior of the filter taps (or
 * coefficients) is determined by the highest sampling rate that
 * the filter will ever see. In the case of a resampler that
 * increases the sampling rate, the highest sampling rate observed
 * is \p interpolation since in the filterbank, the number of
 * filter arms is equal to \p interpolation. When the resampler
 * decreases the sampling rate (decimation > interpolation), then
 * the highest rate is the input sample rate of the original
 * signal. We must create a filter based around this value to
 * reduce any aliasing that may occur from out-of-band signals.
 *
 * Another way to think about how to create the filter taps is
 * that the filter is effectively applied after interpolation and
 * before decimation. And yet another way to think of it is that
 * the taps should be a LPF that is at least as narrow as the
 * narrower of the required anti-image postfilter or anti-alias
 * prefilter.
 */
template <class IN_T, class OUT_T, class TAP_T>
class FILTER_API rational_resampler : virtual public block
{
public:
    typedef std::shared_ptr<rational_resampler<IN_T, OUT_T, TAP_T>> sptr;
 
    /*!
     * Make a rational resampling FIR filter.
     *
     * \param interpolation The integer interpolation rate of the filter
     * \param decimation The integer decimation rate of the filter
     * \param taps The filter taps to control images and aliases
     * \param fractional_bw The fractional bandwidth of the filter (0 to 0.5)
     */
    static sptr make(unsigned interpolation,
                     unsigned decimation,
                     const std::vector<TAP_T>& taps = std::vector<TAP_T>(),
                     float fractional_bw = 0.0);
 
    virtual unsigned interpolation() const = 0;
    virtual unsigned decimation() const = 0;
 
    virtual void set_taps(const std::vector<TAP_T>& taps) = 0;
    virtual std::vector<TAP_T> taps() const = 0;
};
typedef rational_resampler<gr_complex, gr_complex, gr_complex> rational_resampler_ccc;
typedef rational_resampler<gr_complex, gr_complex, float> rational_resampler_ccf;
typedef rational_resampler<float, gr_complex, gr_complex> rational_resampler_fcc;
typedef rational_resampler<float, float, float> rational_resampler_fff;
typedef rational_resampler<float, std::int16_t, float> rational_resampler_fsf;
typedef rational_resampler<std::int16_t, gr_complex, gr_complex> rational_resampler_scc;
 
} /* namespace filter */
} /* namespace gr */
 
#endif /* RATIONAL_RESAMPLER_H */