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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 */
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