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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 */
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