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1 /*
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2 * SpanDSP - a series of DSP components for telephony
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3 *
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4 * v27ter_tx.h - ITU V.27ter modem transmit part
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5 *
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6 * Written by Steve Underwood <steveu@coppice.org>
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7 *
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8 * Copyright (C) 2003 Steve Underwood
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9 *
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10 * All rights reserved.
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11 *
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12 * This program is free software; you can redistribute it and/or modify
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13 * it under the terms of the GNU General Public License version 2, as
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14 * published by the Free Software Foundation.
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15 *
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16 * This program is distributed in the hope that it will be useful,
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17 * but WITHOUT ANY WARRANTY; without even the implied warranty of
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18 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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19 * GNU General Public License for more details.
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20 *
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21 * You should have received a copy of the GNU General Public License
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22 * along with this program; if not, write to the Free Software
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23 * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
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24 *
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25 * $Id: v27ter_tx.h,v 1.25 2006/10/24 13:45:28 steveu Exp $
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26 */
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27
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28 /*! \file */
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29
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30 #if !defined(_V27TER_TX_H_)
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31 #define _V27TER_TX_H_
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32
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33 /*! \page v27ter_tx_page The V.27ter transmitter
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34 \section v27ter_tx_page_sec_1 What does it do?
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35 The V.27ter transmitter implements the transmit side of a V.27ter modem. This
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36 can operate at data rates of 4800 and 2400 bits/s. The audio output is a stream
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37 of 16 bit samples, at 8000 samples/second. The transmit and receive side of
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38 V.27ter modems operate independantly. V.27ter is used for FAX transmission,
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39 where it provides the standard 4800 and 2400 bits/s rates.
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40
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41 \section v27ter_tx_page_sec_2 How does it work?
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42 V.27ter uses DPSK modulation. A common method of producing a DPSK modulated
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43 signal is to use a sampling rate which is a multiple of the baud rate. The raw
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44 signal is then a series of complex pulses, each an integer number of samples
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45 long. These can be shaped, using a suitable complex filter, and multiplied by a
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46 complex carrier signal to produce the final DPSK signal for transmission.
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47
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48 The pulse shaping filter for V.27ter is defined in the spec. It is a root raised
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49 cosine filter with 50% excess bandwidth.
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50
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51 The sampling rate for our transmitter is defined by the channel - 8000 samples/s.
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52 This is a multiple of the baud rate at 4800 bits/s (8-PSK at 1600 baud, 5 samples per
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53 symbol), but not at 2400 bits/s (4-PSK at 1200 baud, 20/3 samples per symbol). The baud
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54 interval is actually 20/3 sample periods at 2400bis/s. A symmetric FIR is used to
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55 apply root raised cosine filtering in the 4800bits/s mode. In the 2400bits/s mode
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56 a polyphase FIR filter is used. This consists of 20 sets of coefficients, offering
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57 zero to 19/20ths of a baud phase shift as well as root raised cosine filtering.
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58 The appropriate coefficient set is chosen for each signal sample generated.
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59
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60 The carrier is generated using the DDS method. Using 2 second order resonators,
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61 started in quadrature, might be more efficient, as it would have less impact on
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62 the processor cache than a table lookup approach. However, the DDS approach
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63 suits the receiver better, so then same signal generator is also used for the
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64 transmitter.
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65 */
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66
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67 #define V27TER_TX_FILTER_STEPS 9
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68
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69 /*!
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70 V.27ter modem transmit side descriptor. This defines the working state for a
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71 single instance of a V.27ter modem transmitter.
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72 */
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73 typedef struct
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74 {
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75 /*! \brief The bit rate of the modem. Valid values are 2400 and 4800. */
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76 int bit_rate;
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77 /*! \brief The callback function used to get the next bit to be transmitted. */
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78 get_bit_func_t get_bit;
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79 /*! \brief A user specified opaque pointer passed to the callback function. */
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80 void *user_data;
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81
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82 /*! \brief The gain factor needed to achieve the specified output power at 2400bps. */
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83 float gain_2400;
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84 /*! \brief The gain factor needed to achieve the specified output power at 4800bps. */
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85 float gain_4800;
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86
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87 /*! \brief The route raised cosine (RRC) pulse shaping filter buffer. */
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88 complexf_t rrc_filter[2*V27TER_TX_FILTER_STEPS];
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89 /*! \brief Current offset into the RRC pulse shaping filter buffer. */
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90 int rrc_filter_step;
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91
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92 /*! \brief The register for the training and data scrambler. */
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93 unsigned int scramble_reg;
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94 /*! \brief A counter for the number of consecutive bits of repeating pattern through
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95 the scrambler. */
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96 int scrambler_pattern_count;
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97 /*! \brief TRUE if transmitting the training sequence, or shutting down transmission.
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98 FALSE if transmitting user data. */
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99 int in_training;
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100 /*! \brief A counter used to track progress through sending the training sequence. */
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101 int training_step;
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102
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103 /*! \brief The current phase of the carrier (i.e. the DDS parameter). */
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104 uint32_t carrier_phase;
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105 /*! \brief The update rate for the phase of the carrier (i.e. the DDS increment). */
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106 int32_t carrier_phase_rate;
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107 /*! \brief The current fractional phase of the baud timing. */
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108 int baud_phase;
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109 /*! \brief The code number for the current position in the constellation. */
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110 int constellation_state;
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111 /*! \brief The get_bit function in use at any instant. */
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112 get_bit_func_t current_get_bit;
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113 /*! \brief Error and flow logging control */
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114 logging_state_t logging;
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115 } v27ter_tx_state_t;
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116
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117 #ifdef __cplusplus
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118 extern "C" {
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119 #endif
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120
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121 /*! Adjust a V.27ter modem transmit context's power output.
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122 \brief Adjust a V.27ter modem transmit context's output power.
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123 \param s The modem context.
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124 \param power The power level, in dBm0 */
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125 void v27ter_tx_power(v27ter_tx_state_t *s, float power);
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126
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127 /*! Initialise a V.27ter modem transmit context.
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128 \brief Initialise a V.27ter modem transmit context.
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129 \param s The modem context.
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130 \param rate The bit rate of the modem. Valid values are 2400 and 4800.
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131 \param tep TRUE is the optional TEP tone is to be transmitted.
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132 \param get_bit The callback routine used to get the data to be transmitted.
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133 \param user_data An opaque pointer.
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134 \return A pointer to the modem context, or NULL if there was a problem. */
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135 v27ter_tx_state_t *v27ter_tx_init(v27ter_tx_state_t *s, int rate, int tep, get_bit_func_t get_bit, void *user_data);
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136
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137 /*! Reinitialise an existing V.27ter modem transmit context, so it may be reused.
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138 \brief Reinitialise an existing V.27ter modem transmit context.
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139 \param s The modem context.
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140 \param rate The bit rate of the modem. Valid values are 2400 and 4800.
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141 \param tep TRUE is the optional TEP tone is to be transmitted.
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142 \return 0 for OK, -1 for bad parameter */
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143 int v27ter_tx_restart(v27ter_tx_state_t *s, int rate, int tep);
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144
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145 /*! Release a V.27ter modem transmit context.
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146 \brief Release a V.27ter modem transmit context.
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147 \param s The modem context.
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148 \return 0 for OK */
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149 int v27ter_tx_release(v27ter_tx_state_t *s);
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150
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151 /*! Change the get_bit function associated with a V.27ter modem transmit context.
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152 \brief Change the get_bit function associated with a V.27ter modem transmit context.
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153 \param s The modem context.
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154 \param get_bit The callback routine used to get the data to be transmitted.
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155 \param user_data An opaque pointer. */
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156 void v27ter_tx_set_get_bit(v27ter_tx_state_t *s, get_bit_func_t get_bit, void *user_data);
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157
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158 /*! Generate a block of V.27ter modem audio samples.
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159 \brief Generate a block of V.27ter modem audio samples.
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160 \param s The modem context.
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161 \param amp The audio sample buffer.
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162 \param len The number of samples to be generated.
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163 \return The number of samples actually generated.
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164 */
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165 int v27ter_tx(v27ter_tx_state_t *s, int16_t amp[], int len);
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166
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167 #ifdef __cplusplus
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168 }
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169 #endif
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170
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171 #endif
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172 /*- End of file ------------------------------------------------------------*/
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