cvsd_decode_bs.h

Go to the documentation of this file.

/* -*- c++ -*- */
/*
 * Copyright 2007,2011,2013 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_VOCODER_CVSD_DECODE_BS_H
#define INCLUDED_VOCODER_CVSD_DECODE_BS_H

#include <gnuradio/vocoder/api.h>
#include <gnuradio/sync_interpolator.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 boost::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 interation 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 */