00001 /* 00002 * SpanDSP - a series of DSP components for telephony 00003 * 00004 * v27ter_tx.h - ITU V.27ter 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 00026 /*! \file */ 00027 00028 #if !defined(_SPANDSP_V27TER_TX_H_) 00029 #define _SPANDSP_V27TER_TX_H_ 00030 00031 /*! \page v27ter_tx_page The V.27ter transmitter 00032 \section v27ter_tx_page_sec_1 What does it do? 00033 The V.27ter transmitter implements the transmit side of a V.27ter modem. This 00034 can operate at data rates of 4800 and 2400 bits/s. The audio output is a stream 00035 of 16 bit samples, at 8000 samples/second. The transmit and receive side of 00036 V.27ter modems operate independantly. V.27ter is used for FAX transmission, 00037 where it provides the standard 4800 and 2400 bits/s rates. 00038 00039 \section v27ter_tx_page_sec_2 How does it work? 00040 V.27ter uses DPSK modulation. A common method of producing a DPSK modulated 00041 signal is to use a sampling rate which is a multiple of the baud rate. The raw 00042 signal is then a series of complex pulses, each an integer number of samples 00043 long. These can be shaped, using a suitable complex filter, and multiplied by a 00044 complex carrier signal to produce the final DPSK signal for transmission. 00045 00046 The pulse shaping filter for V.27ter is defined in the spec. It is a root raised 00047 cosine filter with 50% excess bandwidth. 00048 00049 The sampling rate for our transmitter is defined by the channel - 8000 samples/s. 00050 This is a multiple of the baud rate at 4800 bits/s (8-PSK at 1600 baud, 5 samples per 00051 symbol), but not at 2400 bits/s (4-PSK at 1200 baud, 20/3 samples per symbol). The baud 00052 interval is actually 20/3 sample periods at 2400bis/s. A symmetric FIR is used to 00053 apply root raised cosine filtering in the 4800bits/s mode. In the 2400bits/s mode 00054 a polyphase FIR filter is used. This consists of 20 sets of coefficients, offering 00055 zero to 19/20ths of a baud phase shift as well as root raised cosine filtering. 00056 The appropriate coefficient set is chosen for each signal sample generated. 00057 00058 The carrier is generated using the DDS method. Using 2 second order resonators, 00059 started in quadrature, might be more efficient, as it would have less impact on 00060 the processor cache than a table lookup approach. However, the DDS approach 00061 suits the receiver better, so then same signal generator is also used for the 00062 transmitter. 00063 */ 00064 00065 /*! 00066 V.27ter modem transmit side descriptor. This defines the working state for a 00067 single instance of a V.27ter modem transmitter. 00068 */ 00069 typedef struct v27ter_tx_state_s v27ter_tx_state_t; 00070 00071 #if defined(__cplusplus) 00072 extern "C" 00073 { 00074 #endif 00075 00076 /*! Adjust a V.27ter modem transmit context's power output. 00077 \brief Adjust a V.27ter modem transmit context's output power. 00078 \param s The modem context. 00079 \param power The power level, in dBm0 */ 00080 SPAN_DECLARE(void) v27ter_tx_power(v27ter_tx_state_t *s, float power); 00081 00082 /*! Initialise a V.27ter modem transmit context. 00083 \brief Initialise a V.27ter modem transmit context. 00084 \param s The modem context. 00085 \param bit_rate The bit rate of the modem. Valid values are 2400 and 4800. 00086 \param tep TRUE is the optional TEP tone is to be transmitted. 00087 \param get_bit The callback routine used to get the data to be transmitted. 00088 \param user_data An opaque pointer. 00089 \return A pointer to the modem context, or NULL if there was a problem. */ 00090 SPAN_DECLARE(v27ter_tx_state_t *) v27ter_tx_init(v27ter_tx_state_t *s, int bit_rate, int tep, get_bit_func_t get_bit, void *user_data); 00091 00092 /*! Reinitialise an existing V.27ter modem transmit context, so it may be reused. 00093 \brief Reinitialise an existing V.27ter modem transmit context. 00094 \param s The modem context. 00095 \param bit_rate The bit rate of the modem. Valid values are 2400 and 4800. 00096 \param tep TRUE is the optional TEP tone is to be transmitted. 00097 \return 0 for OK, -1 for bad parameter */ 00098 SPAN_DECLARE(int) v27ter_tx_restart(v27ter_tx_state_t *s, int bit_rate, int tep); 00099 00100 /*! Release a V.27ter modem transmit context. 00101 \brief Release a V.27ter modem transmit context. 00102 \param s The modem context. 00103 \return 0 for OK */ 00104 SPAN_DECLARE(int) v27ter_tx_release(v27ter_tx_state_t *s); 00105 00106 /*! Free a V.27ter modem transmit context. 00107 \brief Free a V.27ter modem transmit context. 00108 \param s The modem context. 00109 \return 0 for OK */ 00110 SPAN_DECLARE(int) v27ter_tx_free(v27ter_tx_state_t *s); 00111 00112 /*! Get the logging context associated with a V.27ter modem transmit context. 00113 \brief Get the logging context associated with a V.27ter modem transmit context. 00114 \param s The modem context. 00115 \return A pointer to the logging context */ 00116 SPAN_DECLARE(logging_state_t *) v27ter_tx_get_logging_state(v27ter_tx_state_t *s); 00117 00118 /*! Change the get_bit function associated with a V.27ter modem transmit context. 00119 \brief Change the get_bit function associated with a V.27ter modem transmit context. 00120 \param s The modem context. 00121 \param get_bit The callback routine used to get the data to be transmitted. 00122 \param user_data An opaque pointer. */ 00123 SPAN_DECLARE(void) v27ter_tx_set_get_bit(v27ter_tx_state_t *s, get_bit_func_t get_bit, void *user_data); 00124 00125 /*! Change the modem status report function associated with a V.27ter modem transmit context. 00126 \brief Change the modem status report function associated with a V.27ter modem transmit context. 00127 \param s The modem context. 00128 \param handler The callback routine used to report modem status changes. 00129 \param user_data An opaque pointer. */ 00130 SPAN_DECLARE(void) v27ter_tx_set_modem_status_handler(v27ter_tx_state_t *s, modem_tx_status_func_t handler, void *user_data); 00131 00132 /*! Generate a block of V.27ter modem audio samples. 00133 \brief Generate a block of V.27ter modem audio samples. 00134 \param s The modem context. 00135 \param amp The audio sample buffer. 00136 \param len The number of samples to be generated. 00137 \return The number of samples actually generated. 00138 */ 00139 SPAN_DECLARE_NONSTD(int) v27ter_tx(v27ter_tx_state_t *s, int16_t amp[], int len); 00140 00141 #if defined(__cplusplus) 00142 } 00143 #endif 00144 00145 #endif 00146 /*- End of file ------------------------------------------------------------*/