cvsd_encode_sb.h

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/* -*- c++ -*- */
/*
 * Copyright 2007,2013 Free Software Foundation, Inc.
 *
 * This file is part of GNU Radio
 *
 * SPDX-License-Identifier: GPL-3.0-or-later
 *
 */
#ifndef INCLUDED_VOCODER_CVSD_ENCODER_SB_H
#define INCLUDED_VOCODER_CVSD_ENCODER_SB_H
 
#include <gnuradio/sync_decimator.h>
#include <gnuradio/vocoder/api.h>
 
namespace gr {
namespace vocoder {
 
/*!
 * \brief This block performs CVSD audio encoding.  Its design and
 * implementation is modeled after the CVSD encoder/decoder
 * specifications defined in the Bluetooth standard.
 * \ingroup audio_blk
 *
 * \details
 * CVSD is a method for encoding speech that seeks to reduce the
 * bandwidth required for digital voice transmission. CVSD takes
 * advantage of strong correlation between samples, quantizing the
 * difference in amplitude between two consecutive samples. This
 * difference requires fewer quantization levels as compared to
 * other methods that quantize the actual amplitude level,
 * reducing the bandwidth. CVSD employs a two level quantizer
 * (one bit) and an adaptive algorithm that allows for continuous
 * step size adjustment.
 *
 * The coder can represent low amplitude signals with accuracy
 * without sacrificing performance on large amplitude signals, a
 * trade off that occurs in some non-adaptive modulations.
 *
 * The CVSD encoder effectively provides 8-to-1 compression. More
 * specifically, each incoming audio sample is compared to an
 * internal reference value. If the input is greater or equal to
 * the reference, the encoder outputs a "1" bit. If the input is
 * less than the reference, the encoder outputs a "0" bit.  The
 * reference value is then updated accordingly based on the
 * frequency of outputted "1" or "0" bits. By grouping 8 outputs
 * bits together, the encoder essentially produce one output byte
 * for every 8 input audio samples.
 *
 * This encoder requires that input audio samples are 2-byte short
 * signed integers. The result bandwidth conversion, therefore,
 * is 16 input bytes of raw audio data to 1 output byte of encoded
 * audio data.
 *
 * The CVSD encoder module must be prefixed by an up-converter to
 * over-sample the audio data prior to encoding. The Bluetooth
 * standard specifically calls for a 1-to-8 interpolating
 * up-converter.  While this reduces the overall compression of
 * the codec, this is required so that the encoder can accurately
 * compute the slope between adjacent audio samples and correctly
 * update its internal reference value.
 *
 * References:
 *
 * 1.  Continuously Variable Slope Delta Modulation (CVSD) A Tutorial,
 *     Available: http://www.eetkorea.com/ARTICLES/2003AUG/A/2003AUG29_NTEK_RFD_AN02.PDF.
 *
 * 2.  Specification of The Bluetooth System
 *     Available: http://grouper.ieee.org/groups/802/15/Bluetooth/core_10_b.pdf.
 *
 * 3.  McGarrity, S., Bluetooth Full Duplex Voice and Data Transmission. 2002.
 *     Bluetooth Voice Simulink Model, Available:
 *     http://www.mathworks.com/company/newsletters/digest/nov01/bluetooth.html
 */
class VOCODER_API cvsd_encode_sb : virtual public sync_decimator
{
public:
    // gr::vocoder::cvsd_encode_sb::sptr
    typedef std::shared_ptr<cvsd_encode_sb> sptr;
 
    /*!
     * \brief Constructor parameters to initialize the CVSD encoder.
     * The default values are modeled after the Bluetooth standard and
     * should not be changed except by an advanced user
     *
     * \param min_step      Minimum step size used to update the internal reference.
     * Default: "10" \param max_step      Maximum step size used to update the internal
     * reference.  Default: "1280" \param step_decay    Decay factor applied to step size
     * when there is not a run of J output 1s or 0s. Default: "0.9990234375"  (i.e.
     * 1-1/1024) \param accum_decay   Decay factor applied to the internal reference
     * during every iteration of the codec. Default: "0.96875"  (i.e. 1-1/32) \param K
     * Size of shift register; the number of output bits remembered by codec (must be <=
     * to 32). Default: "32" \param J             Number of bits in the shift register
     * that are equal; i.e. the size of a run of 1s, 0s. Default: "4" \param pos_accum_max
     * Maximum integer value allowed for the internal reference. Default: "32767" (2^15 -
     * 1 or MAXSHORT) \param neg_accum_max Minimum integer value allowed for the internal
     * reference. Default: "-32767" (-2^15 + 1 or MINSHORT+1)
     */
    static sptr make(short min_step = 10,
                     short max_step = 1280,
                     double step_decay = 0.9990234375,
                     double accum_decay = 0.96875,
                     int K = 32,
                     int J = 4,
                     short pos_accum_max = 32767,
                     short neg_accum_max = -32767);
 
    virtual short min_step() = 0;
    virtual short max_step() = 0;
    virtual double step_decay() = 0;
    virtual double accum_decay() = 0;
    virtual int K() = 0;
    virtual int J() = 0;
    virtual short pos_accum_max() = 0;
    virtual short neg_accum_max() = 0;
};
 
} /* namespace vocoder */
} /* namespace gr */
 
#endif /* INCLUDED_VOCODER_CVSD_ENCODE_SB_H */