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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 * v17tx.h - ITU V.17 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) 2004 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: v17tx.h,v 1.26 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(_V17TX_H_)
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31 #define _V17TX_H_
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32
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33 /*! \page v17tx_page The V.17 transmitter
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34 \section v17tx_page_sec_1 What does it do?
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35 The V.17 transmitter implements the transmit side of a V.17 modem. This can
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36 operate at data rates of 14400, 12000, 9600 and 7200 bits/second. The audio
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37 output is a stream of 16 bit samples, at 8000 samples/second. The transmit and
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38 receive side of V.17 modems operate independantly. V.17 is mostly used for FAX
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39 transmission, where it provides the standard 14400 bits/second rate.
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40
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41 \section v17tx_page_sec_2 How does it work?
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42 V.17 uses QAM modulation and trellis coding. The data to be transmitted is
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43 scrambled, to whiten it. The least significant 2 bits of each symbol are then
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44 differentially encoded, using a simple lookup approach. The resulting 2 bits are
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45 convolutionally encoded, producing 3 bits. The extra bit is the redundant bit
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46 of the trellis code. The other bits of the symbol pass by the differential
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47 and convolutional coding unchanged. The resulting bits define the constellation
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48 point to be transmitted for the symbol. The redundant bit doubles the size of the
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49 constellation, and so increases the error rate for detecting individual symbols
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50 at the receiver. However, when a number of successive symbols are processed at
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51 the receiver, the redundancy actually provides several dB of improved error
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52 performance.
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53
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54 The standard method of producing a QAM modulated signal is to use a sampling
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55 rate which is a multiple of the baud rate. The raw signal is then a series of
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56 complex pulses, each an integer number of samples long. These can be shaped,
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57 using a suitable complex filter, and multiplied by a complex carrier signal
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58 to produce the final QAM signal for transmission.
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59
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60 The pulse shaping filter is only vaguely defined by the V.17 spec. Some of the
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61 other ITU modem specs. fully define the filter, typically specifying a root
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62 raised cosine filter, with 50% excess bandwidth. This is a pity, since it
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63 increases the variability of the received signal. However, the receiver's
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64 adaptive equalizer will compensate for these differences. The current
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65 design uses a root raised cosine filter with 25% excess bandwidth. Greater
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66 excess bandwidth will not allow the tranmitted signal to meet the spectral
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67 requirements.
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68
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69 The sampling rate for our transmitter is defined by the channel - 8000 per
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70 second. This is not a multiple of the baud rate (i.e. 2400 baud). The baud
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71 interval is actually 10/3 sample periods. Instead of using a symmetric
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72 FIR to pulse shape the signal, a polyphase filter is used. This consists of
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73 10 sets of coefficients, offering zero to 9/10ths of a baud phase shift as well
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74 as root raised cosine filtering. The appropriate coefficient set is chosen for
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75 each signal sample generated.
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76
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77 The carrier is generated using the DDS method. Using two second order resonators,
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78 started in quadrature, might be more efficient, as it would have less impact on
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79 the processor cache than a table lookup approach. However, the DDS approach
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80 suits the receiver better, so the same signal generator is also used for the
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81 transmitter.
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82 */
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83
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84 #define V17_TX_FILTER_STEPS 9
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85
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86 /*!
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87 V.17 modem transmit side descriptor. This defines the working state for a
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88 single instance of a V.17 modem transmitter.
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89 */
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90 typedef struct
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91 {
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92 /*! \brief The bit rate of the modem. Valid values are 4800, 7200 and 9600. */
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93 int bit_rate;
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94 /*! \brief The callback function used to get the next bit to be transmitted. */
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95 get_bit_func_t get_bit;
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96 /*! \brief A user specified opaque pointer passed to the callback function. */
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97 void *user_data;
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98
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99 /*! \brief The gain factor needed to achieve the specified output power. */
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100 float gain;
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101
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102 /*! \brief The route raised cosine (RRC) pulse shaping filter buffer. */
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103 complexf_t rrc_filter[2*V17_TX_FILTER_STEPS];
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104 /*! \brief Current offset into the RRC pulse shaping filter buffer. */
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105 int rrc_filter_step;
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106
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107 /*! \brief The current state of the differential encoder. */
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108 int diff;
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109 /*! \brief The current state of the convolutional encoder. */
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110 int convolution;
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111
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112 /*! \brief The register for the data scrambler. */
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113 unsigned int scramble_reg;
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114 /*! \brief TRUE if transmitting the training sequence. FALSE if transmitting user data. */
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115 int in_training;
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116 /*! \brief TRUE if the short training sequence is to be used. */
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117 int short_train;
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118 /*! \brief A counter used to track progress through sending the training sequence. */
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119 int training_step;
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120
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121 /*! \brief The current phase of the carrier (i.e. the DDS parameter). */
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122 uint32_t carrier_phase;
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123 /*! \brief The update rate for the phase of the carrier (i.e. the DDS increment). */
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124 int32_t carrier_phase_rate;
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125 /*! \brief The current fractional phase of the baud timing. */
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126 int baud_phase;
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127 /*! \brief The code number for the current position in the constellation. */
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128 int constellation_state;
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129
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130 /*! \brief A pointer to the constellation currently in use. */
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131 const complexf_t *constellation;
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132 /*! \brief The current number of data bits per symbol. This does not include
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133 the redundant bit. */
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134 int bits_per_symbol;
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135 /*! \brief The get_bit function in use at any instant. */
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136 get_bit_func_t current_get_bit;
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137 /*! \brief Error and flow logging control */
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138 logging_state_t logging;
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139 } v17_tx_state_t;
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140
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141 #ifdef __cplusplus
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142 extern "C" {
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143 #endif
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144
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145 /*! Adjust a V.17 modem transmit context's power output.
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146 \brief Adjust a V.17 modem transmit context's output power.
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147 \param s The modem context.
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148 \param power The power level, in dBm0 */
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149 void v17_tx_power(v17_tx_state_t *s, float power);
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150
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151 /*! Initialise a V.17 modem transmit context. This must be called before the first
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152 use of the context, to initialise its contents.
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153 \brief Initialise a V.17 modem transmit context.
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154 \param s The modem context.
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155 \param rate The bit rate of the modem. Valid values are 7200, 9600, 12000 and 14400.
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156 \param tep TRUE is the optional TEP tone is to be transmitted.
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157 \param get_bit The callback routine used to get the data to be transmitted.
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158 \param user_data An opaque pointer.
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159 \return A pointer to the modem context, or NULL if there was a problem. */
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160 v17_tx_state_t *v17_tx_init(v17_tx_state_t *s, int rate, int tep, get_bit_func_t get_bit, void *user_data);
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161
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162 /*! Reinitialise an existing V.17 modem transmit context, so it may be reused.
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163 \brief Reinitialise an existing V.17 modem transmit context.
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164 \param s The modem context.
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165 \param rate The bit rate of the modem. Valid values are 7200, 9600, 12000 and 14400.
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166 \param tep TRUE is the optional TEP tone is to be transmitted.
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167 \param short_train TRUE if the short training sequence should be used.
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168 \return 0 for OK, -1 for parameter error. */
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169 int v17_tx_restart(v17_tx_state_t *s, int rate, int tep, int short_train);
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170
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171 /*! Release a V.17 modem transmit context.
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172 \brief Release a V.17 modem transmit context.
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173 \param s The modem context.
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174 \return 0 for OK */
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175 int v17_tx_release(v17_tx_state_t *s);
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176
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177 /*! Change the get_bit function associated with a V.17 modem transmit context.
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178 \brief Change the get_bit function associated with a V.17 modem transmit context.
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179 \param s The modem context.
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180 \param get_bit The callback routine used to get the data to be transmitted.
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181 \param user_data An opaque pointer. */
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182 void v17_tx_set_get_bit(v17_tx_state_t *s, get_bit_func_t get_bit, void *user_data);
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183
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184 /*! Generate a block of V.17 modem audio samples.
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185 \brief Generate a block of V.17 modem audio samples.
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186 \param s The modem context.
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187 \param amp The audio sample buffer.
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188 \param len The number of samples to be generated.
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189 \return The number of samples actually generated.
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190 */
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191 int v17_tx(v17_tx_state_t *s, int16_t amp[], int len);
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192
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193 #ifdef __cplusplus
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194 }
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195 #endif
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196
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197 #endif
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198 /*- End of file ------------------------------------------------------------*/
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