00001 /* 00002 * SpanDSP - a series of DSP components for telephony 00003 * 00004 * v29tx.h - ITU V.29 modem transmit part 00005 * 00006 * Written by Steve Underwood <steveu@coppice.org> 00007 * 00008 * Copyright (C) 2003 Steve Underwood 00009 * 00010 * All rights reserved. 00011 * 00012 * This program is free software; you can redistribute it and/or modify 00013 * it under the terms of the GNU Lesser General Public License version 2.1, 00014 * as published by the Free Software Foundation. 00015 * 00016 * This program is distributed in the hope that it will be useful, 00017 * but WITHOUT ANY WARRANTY; without even the implied warranty of 00018 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 00019 * GNU Lesser General Public License for more details. 00020 * 00021 * You should have received a copy of the GNU Lesser General Public 00022 * License along with this program; if not, write to the Free Software 00023 * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA. 00024 * 00025 * $Id: v29tx.h,v 1.39 2009/04/12 09:12:11 steveu Exp $ 00026 */ 00027 00028 /*! \file */ 00029 00030 #if !defined(_SPANDSP_V29TX_H_) 00031 #define _SPANDSP_V29TX_H_ 00032 00033 /*! \page v29tx_page The V.29 transmitter 00034 \section v29tx_page_sec_1 What does it do? 00035 The V.29 transmitter implements the transmit side of a V.29 modem. This can 00036 operate at data rates of 9600, 7200 and 4800 bits/s. The audio output is a 00037 stream of 16 bit samples, at 8000 samples/second. The transmit and receive side 00038 of V.29 modems operate independantly. V.29 is mostly used for FAX transmission, 00039 where it provides the standard 9600 and 7200 bits/s rates (the 4800 bits/s mode 00040 is not used for FAX). 00041 00042 \section v29tx_page_sec_2 How does it work? 00043 V.29 uses QAM modulation. The standard method of producing a QAM modulated 00044 signal is to use a sampling rate which is a multiple of the baud rate. The raw 00045 signal is then a series of complex pulses, each an integer number of samples 00046 long. These can be shaped, using a suitable complex filter, and multiplied by a 00047 complex carrier signal to produce the final QAM signal for transmission. 00048 00049 The pulse shaping filter is only vaguely defined by the V.29 spec. Some of the 00050 other ITU modem specs. fully define the filter, typically specifying a root 00051 raised cosine filter, with 50% excess bandwidth. This is a pity, since it 00052 increases the variability of the received signal. However, the receiver's 00053 adaptive equalizer will compensate for these differences. The current 00054 design uses a root raised cosine filter with 25% excess bandwidth. Greater 00055 excess bandwidth will not allow the tranmitted signal to meet the spectral 00056 requirements. 00057 00058 The sampling rate for our transmitter is defined by the channel - 8000 per 00059 second. This is not a multiple of the baud rate (i.e. 2400 baud). The baud 00060 interval is actually 10/3 sample periods. Instead of using a symmetric 00061 FIR to pulse shape the signal, a polyphase filter is used. This consists of 00062 10 sets of coefficients, offering zero to 9/10ths of a baud phase shift as well 00063 as root raised cosine filtering. The appropriate coefficient set is chosen for 00064 each signal sample generated. 00065 00066 The carrier is generated using the DDS method. Using two second order resonators, 00067 started in quadrature, might be more efficient, as it would have less impact on 00068 the processor cache than a table lookup approach. However, the DDS approach 00069 suits the receiver better, so the same signal generator is also used for the 00070 transmitter. 00071 00072 The equation defining QAM modulation is: 00073 00074 s(n) = A*cos(2*pi*f*n + phi(n)) 00075 00076 where phi(n) is the phase of the information, and A is the amplitude of the information 00077 00078 using the identity 00079 00080 cos(x + y) = cos(x)*cos(y) - sin(x)*sin(y) 00081 00082 we get 00083 00084 s(n) = A {cos(2*pi*f*n)*cos(phi(n)) - sin(2*pi*f*n)*sin(phi(n))} 00085 00086 substituting with the constellation positions 00087 00088 I(n) = A*cos(phi(n)) 00089 Q(n) = A*sin(phi(n)) 00090 00091 gives 00092 00093 s(n) = I(n)*cos(2*pi*f*n) - Q(n)*sin(2*pi*f*n) 00094 00095 */ 00096 00097 /*! The number of taps in the pulse shaping/bandpass filter */ 00098 #define V29_TX_FILTER_STEPS 9 00099 00100 /*! 00101 V.29 modem transmit side descriptor. This defines the working state for a 00102 single instance of a V.29 modem transmitter. 00103 */ 00104 typedef struct v29_tx_state_s v29_tx_state_t; 00105 00106 #if defined(__cplusplus) 00107 extern "C" 00108 { 00109 #endif 00110 00111 /*! Adjust a V.29 modem transmit context's power output. 00112 \brief Adjust a V.29 modem transmit context's output power. 00113 \param s The modem context. 00114 \param power The power level, in dBm0 */ 00115 SPAN_DECLARE(void) v29_tx_power(v29_tx_state_t *s, float power); 00116 00117 /*! Initialise a V.29 modem transmit context. This must be called before the first 00118 use of the context, to initialise its contents. 00119 \brief Initialise a V.29 modem transmit context. 00120 \param s The modem context. 00121 \param bit_rate The bit rate of the modem. Valid values are 4800, 7200 and 9600. 00122 \param tep TRUE is the optional TEP tone is to be transmitted. 00123 \param get_bit The callback routine used to get the data to be transmitted. 00124 \param user_data An opaque pointer. 00125 \return A pointer to the modem context, or NULL if there was a problem. */ 00126 SPAN_DECLARE(v29_tx_state_t *) v29_tx_init(v29_tx_state_t *s, int bit_rate, int tep, get_bit_func_t get_bit, void *user_data); 00127 00128 /*! Reinitialise an existing V.29 modem transmit context, so it may be reused. 00129 \brief Reinitialise an existing V.29 modem transmit context. 00130 \param s The modem context. 00131 \param bit_rate The bit rate of the modem. Valid values are 4800, 7200 and 9600. 00132 \param tep TRUE is the optional TEP tone is to be transmitted. 00133 \return 0 for OK, -1 for bad parameter */ 00134 SPAN_DECLARE(int) v29_tx_restart(v29_tx_state_t *s, int bit_rate, int tep); 00135 00136 /*! Release a V.29 modem transmit context. 00137 \brief Release a V.29 modem transmit context. 00138 \param s The modem context. 00139 \return 0 for OK */ 00140 SPAN_DECLARE(int) v29_tx_release(v29_tx_state_t *s); 00141 00142 /*! Free a V.29 modem transmit context. 00143 \brief Free a V.29 modem transmit context. 00144 \param s The modem context. 00145 \return 0 for OK */ 00146 SPAN_DECLARE(int) v29_tx_free(v29_tx_state_t *s); 00147 00148 /*! Get the logging context associated with a V.29 modem transmit context. 00149 \brief Get the logging context associated with a V.29 modem transmit context. 00150 \param s The modem context. 00151 \return A pointer to the logging context */ 00152 SPAN_DECLARE(logging_state_t *) v29_tx_get_logging_state(v29_tx_state_t *s); 00153 00154 /*! Change the get_bit function associated with a V.29 modem transmit context. 00155 \brief Change the get_bit function associated with a V.29 modem transmit context. 00156 \param s The modem context. 00157 \param get_bit The callback routine used to get the data to be transmitted. 00158 \param user_data An opaque pointer. */ 00159 SPAN_DECLARE(void) v29_tx_set_get_bit(v29_tx_state_t *s, get_bit_func_t get_bit, void *user_data); 00160 00161 /*! Change the modem status report function associated with a V.29 modem transmit context. 00162 \brief Change the modem status report function associated with a V.29 modem transmit context. 00163 \param s The modem context. 00164 \param handler The callback routine used to report modem status changes. 00165 \param user_data An opaque pointer. */ 00166 SPAN_DECLARE(void) v29_tx_set_modem_status_handler(v29_tx_state_t *s, modem_tx_status_func_t handler, void *user_data); 00167 00168 /*! Generate a block of V.29 modem audio samples. 00169 \brief Generate a block of V.29 modem audio samples. 00170 \param s The modem context. 00171 \param amp The audio sample buffer. 00172 \param len The number of samples to be generated. 00173 \return The number of samples actually generated. 00174 */ 00175 SPAN_DECLARE(int) v29_tx(v29_tx_state_t *s, int16_t amp[], int len); 00176 00177 #if defined(__cplusplus) 00178 } 00179 #endif 00180 00181 #endif 00182 /*- End of file ------------------------------------------------------------*/