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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 * v29tx.h - ITU V.29 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: v29tx.h,v 1.23 2006/10/24 13:22:02 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(_V29TX_H_)
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31 #define _V29TX_H_
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32
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33 /*! \page v29tx_page The V.29 transmitter
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34 \section v29tx_page_sec_1 What does it do?
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35 The V.29 transmitter implements the transmit side of a V.29 modem. This can
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36 operate at data rates of 9600, 7200 and 4800 bits/s. The audio output is a
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37 stream of 16 bit samples, at 8000 samples/second. The transmit and receive side
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38 of V.29 modems operate independantly. V.29 is mostly used for FAX transmission,
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39 where it provides the standard 9600 and 7200 bits/s rates (the 4800 bits/s mode
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40 is not used for FAX).
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41
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42 \section v29tx_page_sec_2 How does it work?
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43 V.29 uses QAM modulation. The standard method of producing a QAM modulated
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44 signal is to use a sampling rate which is a multiple of the baud rate. The raw
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45 signal is then a series of complex pulses, each an integer number of samples
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46 long. These can be shaped, using a suitable complex filter, and multiplied by a
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47 complex carrier signal to produce the final QAM signal for transmission.
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48
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49 The pulse shaping filter is only vaguely defined by the V.29 spec. Some of the
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50 other ITU modem specs. fully define the filter, typically specifying a root
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51 raised cosine filter, with 50% excess bandwidth. This is a pity, since it
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52 increases the variability of the received signal. However, the receiver's
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53 adaptive equalizer will compensate for these differences. The current
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54 design uses a root raised cosine filter with 25% excess bandwidth. Greater
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55 excess bandwidth will not allow the tranmitted signal to meet the spectral
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56 requirements.
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57
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58 The sampling rate for our transmitter is defined by the channel - 8000 per
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59 second. This is not a multiple of the baud rate (i.e. 2400 baud). The baud
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60 interval is actually 10/3 sample periods. Instead of using a symmetric
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61 FIR to pulse shape the signal, a polyphase filter is used. This consists of
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62 10 sets of coefficients, offering zero to 9/10ths of a baud phase shift as well
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63 as root raised cosine filtering. The appropriate coefficient set is chosen for
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64 each signal sample generated.
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65
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66 The carrier is generated using the DDS method. Using two second order resonators,
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67 started in quadrature, might be more efficient, as it would have less impact on
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68 the processor cache than a table lookup approach. However, the DDS approach
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69 suits the receiver better, so the same signal generator is also used for the
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70 transmitter.
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71
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72 The equation defining QAM modulation is:
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73
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74 s(n) = A*cos(2*pi*f*n + phi(n))
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75
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76 where phi(n) is the phase of the information, and A is the amplitude of the information
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77
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78 using the identity
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79
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80 cos(x + y) = cos(x)*cos(y) - sin(x)*sin(y)
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81
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82 we get
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83
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84 s(n) = A {cos(2*pi*f*n)*cos(phi(n)) - sin(2*pi*f*n)*sin(phi(n))}
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85
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86 substituting with the constellation positions
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87
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88 I(n) = A*cos(phi(n))
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89 Q(n) = A*sin(phi(n))
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90
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91 gives
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92
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93 s(n) = I(n)*cos(2*pi*f*n) - Q(n)*sin(2*pi*f*n)
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94
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95 */
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96
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97 #define V29_TX_FILTER_STEPS 9
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98
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99 /*!
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100 V.29 modem transmit side descriptor. This defines the working state for a
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101 single instance of a V.29 modem transmitter.
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102 */
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103 typedef struct
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104 {
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105 /*! \brief The bit rate of the modem. Valid values are 4800, 7200 and 9600. */
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106 int bit_rate;
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107 /*! \brief The callback function used to get the next bit to be transmitted. */
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108 get_bit_func_t get_bit;
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109 /*! \brief A user specified opaque pointer passed to the callback function. */
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110 void *user_data;
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111
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112 /*! \brief Gain required to achieve the specified output power, not allowing
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113 for the size of the current constellation. */
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114 float base_gain;
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115 /*! \brief Gain required to achieve the specified output power, allowing
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116 for the size of the current constellation. */
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117 float gain;
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118
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119 /*! \brief The route raised cosine (RRC) pulse shaping filter buffer. */
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120 complexf_t rrc_filter[2*V29_TX_FILTER_STEPS];
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121 /*! \brief Current offset into the RRC pulse shaping filter buffer. */
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122 int rrc_filter_step;
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123
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124 /*! \brief The register for the data scrambler. */
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125 unsigned int scramble_reg;
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126 /*! \brief The register for the training scrambler. */
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127 uint8_t training_scramble_reg;
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128 /*! \brief TRUE if transmitting the training sequence, or shutting down transmission.
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129 FALSE if transmitting user data. */
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130 int in_training;
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131 /*! \brief A counter used to track progress through sending the training sequence. */
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132 int training_step;
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133 /*! \brief An offset value into the table of training parameters, used to match the
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134 training pattern to the bit rate. */
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135 int training_offset;
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136
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137 /*! \brief The current phase of the carrier (i.e. the DDS parameter). */
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138 uint32_t carrier_phase;
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139 /*! \brief The update rate for the phase of the carrier (i.e. the DDS increment). */
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140 int32_t carrier_phase_rate;
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141 /*! \brief The current fractional phase of the baud timing. */
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142 int baud_phase;
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143 /*! \brief The code number for the current position in the constellation. */
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144 int constellation_state;
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145 /*! \brief The get_bit function in use at any instant. */
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146 get_bit_func_t current_get_bit;
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147 /*! \brief Error and flow logging control */
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148 logging_state_t logging;
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149 } v29_tx_state_t;
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150
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151 #ifdef __cplusplus
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152 extern "C" {
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153 #endif
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154
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155 /*! Adjust a V.29 modem transmit context's power output.
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156 \brief Adjust a V.29 modem transmit context's output power.
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157 \param s The modem context.
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158 \param power The power level, in dBm0 */
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159 void v29_tx_power(v29_tx_state_t *s, float power);
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160
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161 /*! Initialise a V.29 modem transmit context. This must be called before the first
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162 use of the context, to initialise its contents.
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163 \brief Initialise a V.29 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 4800, 7200 and 9600.
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166 \param tep TRUE is the optional TEP tone is to be transmitted.
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167 \param get_bit The callback routine used to get the data to be transmitted.
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168 \param user_data An opaque pointer.
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169 \return A pointer to the modem context, or NULL if there was a problem. */
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170 v29_tx_state_t *v29_tx_init(v29_tx_state_t *s, int rate, int tep, get_bit_func_t get_bit, void *user_data);
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171
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172 /*! Reinitialise an existing V.29 modem transmit context, so it may be reused.
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173 \brief Reinitialise an existing V.29 modem transmit context.
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174 \param s The modem context.
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175 \param rate The bit rate of the modem. Valid values are 4800, 7200 and 9600.
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176 \param tep TRUE is the optional TEP tone is to be transmitted.
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177 \return 0 for OK, -1 for bad parameter */
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178 int v29_tx_restart(v29_tx_state_t *s, int rate, int tep);
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179
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180 /*! Release a V.29 modem transmit context.
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181 \brief Release a V.29 modem transmit context.
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182 \param s The modem context.
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183 \return 0 for OK */
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184 int v29_tx_release(v29_tx_state_t *s);
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185
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186 /*! Change the get_bit function associated with a V.29 modem transmit context.
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187 \brief Change the get_bit function associated with a V.29 modem transmit context.
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188 \param s The modem context.
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189 \param get_bit The callback routine used to get the data to be transmitted.
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190 \param user_data An opaque pointer. */
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191 void v29_tx_set_get_bit(v29_tx_state_t *s, get_bit_func_t get_bit, void *user_data);
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192
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193 /*! Generate a block of V.29 modem audio samples.
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194 \brief Generate a block of V.29 modem audio samples.
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195 \param s The modem context.
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196 \param amp The audio sample buffer.
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197 \param len The number of samples to be generated.
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198 \return The number of samples actually generated.
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199 */
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200 int v29_tx(v29_tx_state_t *s, int16_t *amp, int len);
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201
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202 #ifdef __cplusplus
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203 }
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204 #endif
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205
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206 #endif
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207 /*- End of file ------------------------------------------------------------*/
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