pfb_arb_resampler_fff.h

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/* -*- c++ -*- */
/*
 * Copyright 2009-2012 Free Software Foundation, Inc.
 *
 * This file is part of GNU Radio
 *
 * SPDX-License-Identifier: GPL-3.0-or-later
 *
 */
 
 
#ifndef INCLUDED_PFB_ARB_RESAMPLER_FFF_H
#define INCLUDED_PFB_ARB_RESAMPLER_FFF_H
 
#include <gnuradio/block.h>
#include <gnuradio/filter/api.h>
 
namespace gr {
namespace filter {
 
/*!
 * \brief Polyphase filterbank arbitrary resampler with
 *        float input, float output and float taps
 * \ingroup resamplers_blk
 *
 * \details
 * This block takes in a signal stream and performs arbitrary
 * resampling. The resampling rate can be any real number
 * <EM>r</EM>. The resampling is done by constructing <EM>N</EM>
 * filters where <EM>N</EM> is the interpolation rate.  We then
 * calculate <EM>D</EM> where <EM>D = floor(N/r)</EM>.
 *
 * Using <EM>N</EM> and <EM>D</EM>, we can perform rational
 * resampling where <EM>N/D</EM> is a rational number close to the
 * input rate <EM>r</EM> where we have <EM>N</EM> filters and we
 * cycle through them as a polyphase filterbank with a stride of
 * <EM>D</EM> so that <EM>i+1 = (i + D) % N</EM>.
 *
 * To get the arbitrary rate, we want to interpolate between two
 * points. For each value out, we take an output from the current
 * filter, <EM>i</EM>, and the next filter <EM>i+1</EM> and then
 * linearly interpolate between the two based on the real
 * resampling rate we want.
 *
 * The linear interpolation only provides us with an approximation
 * to the real sampling rate specified. The error is a
 * quantization error between the two filters we used as our
 * interpolation points.  To this end, the number of filters,
 * <EM>N</EM>, used determines the quantization error; the larger
 * <EM>N</EM>, the smaller the noise. You can design for a
 * specified noise floor by setting the filter size (parameters
 * <EM>filter_size</EM>). The size defaults to 32 filters, which
 * is about as good as most implementations need.
 *
 * The trick with designing this filter is in how to specify the
 * taps of the prototype filter. Like the PFB interpolator, the
 * taps are specified using the interpolated filter rate. In this
 * case, that rate is the input sample rate multiplied by the
 * number of filters in the filterbank, which is also the
 * interpolation rate. All other values should be relative to this
 * rate.
 *
 * For example, for a 32-filter arbitrary resampler and using the
 * GNU Radio's firdes utility to build the filter, we build a
 * low-pass filter with a sampling rate of <EM>fs</EM>, a 3-dB
 * bandwidth of <EM>BW</EM> and a transition bandwidth of
 * <EM>TB</EM>. We can also specify the out-of-band attenuation to
 * use, <EM>ATT</EM>, and the filter window function (a
 * Blackman-harris window in this case). The first input is the
 * gain of the filter, which we specify here as the interpolation
 * rate (<EM>32</EM>).
 *
 *   <B><EM>self._taps = filter.firdes.low_pass_2(32, 32*fs, BW, TB,
 *      attenuation_dB=ATT, window=fft.window.WIN_BLACKMAN_hARRIS)</EM></B>
 *
 * The theory behind this block can be found in Chapter 7.5 of the
 * following book:
 *
 *   <B><EM>f. harris, "Multirate Signal Processing for Communication
 *      Systems", Upper Saddle River, NJ: Prentice Hall, Inc. 2004.</EM></B>
 */
 
class FILTER_API pfb_arb_resampler_fff : virtual public block
{
public:
    // gr::filter::pfb_arb_resampler_fff::sptr
    typedef std::shared_ptr<pfb_arb_resampler_fff> sptr;
 
    /*!
     * Build the polyphase filterbank arbitrary resampler.
     * \param rate  (float) Specifies the resampling rate to use
     * \param taps  (vector/list of floats) The prototype filter to populate the
     *filterbank. The taps should be generated at the filter_size sampling rate. \param
     *filter_size (unsigned int) The number of filters in the filter bank. This is
     *directly related to quantization noise introduced during the resampling. Defaults to
     *32 filters.
     */
    static sptr
    make(float rate, const std::vector<float>& taps, unsigned int filter_size = 32);
 
    /*!
     * Resets the filterbank's filter taps with the new prototype filter
     * \param taps    (vector/list of floats) The prototype filter to populate the
     * filterbank.
     */
    virtual void set_taps(const std::vector<float>& taps) = 0;
 
    /*!
     * Return a vector<vector<>> of the filterbank taps
     */
    virtual std::vector<std::vector<float>> taps() const = 0;
 
    /*!
     * Print all of the filterbank taps to screen.
     */
    virtual void print_taps() = 0;
 
    /*!
     * Sets the resampling rate of the block.
     */
    virtual void set_rate(float rate) = 0;
 
    /*!
     * Sets the current phase offset in radians (0 to 2pi).
     */
    virtual void set_phase(float ph) = 0;
 
    /*!
     * Gets the current phase of the resampler in radians (2 to 2pi).
     */
    virtual float phase() const = 0;
 
    /*!
     * Gets the number of taps per filter.
     */
    virtual unsigned int taps_per_filter() const = 0;
 
    /*!
     * Gets the interpolation rate of the filter.
     */
    virtual unsigned int interpolation_rate() const = 0;
 
    /*!
     * Gets the decimation rate of the filter.
     */
    virtual unsigned int decimation_rate() const = 0;
 
    /*!
     * Gets the fractional rate of the filter.
     */
    virtual float fractional_rate() const = 0;
 
    /*!
     * Get the group delay of the filter.
     */
    virtual int group_delay() const = 0;
 
    /*!
     * Calculates the phase offset expected by a sine wave of
     * frequency \p freq and sampling rate \p fs (assuming input
     * sine wave has 0 degree phase).
     */
    virtual float phase_offset(float freq, float fs) = 0;
};
 
} /* namespace filter */
} /* namespace gr */
 
#endif /* INCLUDED_PFB_ARB_RESAMPLER_FFF_H */