cvsd_decode_bs.h

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/* -*- c++ -*- */
/*
 * Copyright 2007,2011,2013 Free Software Foundation, Inc.
 *
 * This file is part of GNU Radio
 *
 * SPDX-License-Identifier: GPL-3.0-or-later
 *
 */
 
#ifndef INCLUDED_VOCODER_CVSD_DECODE_BS_H
#define INCLUDED_VOCODER_CVSD_DECODE_BS_H
 
#include <gnuradio/sync_interpolator.h>
#include <gnuradio/vocoder/api.h>
 
namespace gr {
namespace vocoder {
 
/*!
 * \brief This block performs CVSD audio decoding.  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 decoder effectively provides 1-to-8 decompression.
 * More specifically, for each incoming input bit, the decoder
 * outputs one audio sample.  If the input is a "1" bit, the
 * internal reference is increased appropriately and then
 * outputted as the next estimated audio sample.  If the input is
 * a "0" bit, the internal reference is decreased appropriately
 * and then likewise outputted as the next estimated audio sample.
 * Grouping 8 input bits together, the encoder essentially
 * produces 8 output audio samples for everyone one input byte.
 *
 * This decoder requires that output audio samples are 2-byte
 * short signed integers.  The result bandwidth conversion,
 * therefore, is 1 byte of encoded audio data to 16 output bytes
 * of raw audio data.
 *
 * The CVSD decoder module must be post-fixed by a down-converter
 * to under-sample the audio data after decoding.  The Bluetooth
 * standard specifically calls for a 8-to-1 decimating
 * down-converter.  This is required so that so that output
 * sampling rate equals the original input sampling rate present
 * before the encoder.  In all cases, the output down-converter
 * rate must be the inverse of the input up-converter rate before
 * the CVSD encoder.
 *
 * 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_decode_bs : virtual public sync_interpolator
{
public:
    // gr::vocoder::cvsd_decode_bs::sptr
    typedef std::shared_ptr<cvsd_decode_bs> sptr;
 
    /*!
     * \brief Constructor parameters to initialize the CVSD decoder.
     * 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_DECODE_BS_H */