v27ter_tx.h

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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 General Public License version 2, as
00014  * 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 General Public License for more details.
00020  *
00021  * You should have received a copy of the GNU General Public License
00022  * along with this program; if not, write to the Free Software
00023  * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
00024  *
00025  * $Id: v27ter_tx.h,v 1.30 2007/05/12 12:25:39 steveu Exp $
00026  */
00027 
00028 /*! \file */
00029 
00030 #if !defined(_SPANDSP_V27TER_TX_H_)
00031 #define _SPANDSP_V27TER_TX_H_
00032 
00033 /*! \page v27ter_tx_page The V.27ter transmitter
00034 \section v27ter_tx_page_sec_1 What does it do?
00035 The V.27ter transmitter implements the transmit side of a V.27ter modem. This
00036 can operate at data rates of 4800 and 2400 bits/s. The audio output is a stream
00037 of 16 bit samples, at 8000 samples/second. The transmit and receive side of
00038 V.27ter modems operate independantly. V.27ter is used for FAX transmission,
00039 where it provides the standard 4800 and 2400 bits/s rates. 
00040 
00041 \section v27ter_tx_page_sec_2 How does it work?
00042 V.27ter uses DPSK modulation. A common method of producing a DPSK modulated
00043 signal is to use a sampling rate which is a multiple of the baud rate. The raw
00044 signal is then a series of complex pulses, each an integer number of samples
00045 long. These can be shaped, using a suitable complex filter, and multiplied by a
00046 complex carrier signal to produce the final DPSK signal for transmission. 
00047 
00048 The pulse shaping filter for V.27ter is defined in the spec. It is a root raised
00049 cosine filter with 50% excess bandwidth. 
00050 
00051 The sampling rate for our transmitter is defined by the channel - 8000 samples/s.
00052 This is a multiple of the baud rate at 4800 bits/s (8-PSK at 1600 baud, 5 samples per
00053 symbol), but not at 2400 bits/s (4-PSK at 1200 baud, 20/3 samples per symbol). The baud
00054 interval is actually 20/3 sample periods at 2400bis/s. A symmetric FIR is used to
00055 apply root raised cosine filtering in the 4800bits/s mode. In the 2400bits/s mode
00056 a polyphase FIR filter is used. This consists of 20 sets of coefficients, offering
00057 zero to 19/20ths of a baud phase shift as well as root raised cosine filtering.
00058 The appropriate coefficient set is chosen for each signal sample generated.
00059 
00060 The carrier is generated using the DDS method. Using 2 second order resonators,
00061 started in quadrature, might be more efficient, as it would have less impact on
00062 the processor cache than a table lookup approach. However, the DDS approach
00063 suits the receiver better, so then same signal generator is also used for the
00064 transmitter.
00065 */
00066 
00067 #define V27TER_TX_FILTER_STEPS      9
00068 
00069 /*!
00070     V.27ter modem transmit side descriptor. This defines the working state for a
00071     single instance of a V.27ter modem transmitter.
00072 */
00073 typedef struct
00074 {
00075     /*! \brief The bit rate of the modem. Valid values are 2400 and 4800. */
00076     int bit_rate;
00077     /*! \brief The callback function used to get the next bit to be transmitted. */
00078     get_bit_func_t get_bit;
00079     /*! \brief A user specified opaque pointer passed to the callback function. */
00080     void *user_data;
00081 
00082 #if defined(USE_FIXED_POINT)
00083     /*! \brief The gain factor needed to achieve the specified output power at 2400bps. */
00084     int32_t gain_2400;
00085     /*! \brief The gain factor needed to achieve the specified output power at 4800bps. */
00086     int32_t gain_4800;
00087 #else
00088     /*! \brief The gain factor needed to achieve the specified output power at 2400bps. */
00089     float gain_2400;
00090     /*! \brief The gain factor needed to achieve the specified output power at 4800bps. */
00091     float gain_4800;
00092 #endif
00093     /*! \brief The route raised cosine (RRC) pulse shaping filter buffer. */
00094 #if defined(USE_FIXED_POINT)
00095     complexi16_t rrc_filter[2*V27TER_TX_FILTER_STEPS];
00096 #else
00097     complexf_t rrc_filter[2*V27TER_TX_FILTER_STEPS];
00098 #endif
00099     /*! \brief Current offset into the RRC pulse shaping filter buffer. */
00100     int rrc_filter_step;
00101     
00102     /*! \brief The register for the training and data scrambler. */
00103     unsigned int scramble_reg;
00104     /*! \brief A counter for the number of consecutive bits of repeating pattern through
00105                the scrambler. */
00106     int scrambler_pattern_count;
00107     /*! \brief TRUE if transmitting the training sequence, or shutting down transmission.
00108                FALSE if transmitting user data. */
00109     int in_training;
00110     /*! \brief A counter used to track progress through sending the training sequence. */
00111     int training_step;
00112 
00113     /*! \brief The current phase of the carrier (i.e. the DDS parameter). */
00114     uint32_t carrier_phase;
00115     /*! \brief The update rate for the phase of the carrier (i.e. the DDS increment). */
00116     int32_t carrier_phase_rate;
00117     /*! \brief The current fractional phase of the baud timing. */
00118     int baud_phase;
00119     /*! \brief The code number for the current position in the constellation. */
00120     int constellation_state;
00121     /*! \brief The get_bit function in use at any instant. */
00122     get_bit_func_t current_get_bit;
00123     /*! \brief Error and flow logging control */
00124     logging_state_t logging;
00125 } v27ter_tx_state_t;
00126 
00127 #if defined(__cplusplus)
00128 extern "C"
00129 {
00130 #endif
00131 
00132 /*! Adjust a V.27ter modem transmit context's power output.
00133     \brief Adjust a V.27ter modem transmit context's output power.
00134     \param s The modem context.
00135     \param power The power level, in dBm0 */
00136 void v27ter_tx_power(v27ter_tx_state_t *s, float power);
00137 
00138 /*! Initialise a V.27ter modem transmit context.
00139     \brief Initialise a V.27ter modem transmit context.
00140     \param s The modem context.
00141     \param rate The bit rate of the modem. Valid values are 2400 and 4800.
00142     \param tep TRUE is the optional TEP tone is to be transmitted.
00143     \param get_bit The callback routine used to get the data to be transmitted.
00144     \param user_data An opaque pointer.
00145     \return A pointer to the modem context, or NULL if there was a problem. */
00146 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);
00147 
00148 /*! Reinitialise an existing V.27ter modem transmit context, so it may be reused.
00149     \brief Reinitialise an existing V.27ter modem transmit context.
00150     \param s The modem context.
00151     \param rate The bit rate of the modem. Valid values are 2400 and 4800.
00152     \param tep TRUE is the optional TEP tone is to be transmitted.
00153     \return 0 for OK, -1 for bad parameter */
00154 int v27ter_tx_restart(v27ter_tx_state_t *s, int rate, int tep);
00155 
00156 /*! Release a V.27ter modem transmit context.
00157     \brief Release a V.27ter modem transmit context.
00158     \param s The modem context.
00159     \return 0 for OK */
00160 int v27ter_tx_release(v27ter_tx_state_t *s);
00161 
00162 /*! Change the get_bit function associated with a V.27ter modem transmit context.
00163     \brief Change the get_bit function associated with a V.27ter modem transmit context.
00164     \param s The modem context.
00165     \param get_bit The callback routine used to get the data to be transmitted.
00166     \param user_data An opaque pointer. */
00167 void v27ter_tx_set_get_bit(v27ter_tx_state_t *s, get_bit_func_t get_bit, void *user_data);
00168 
00169 /*! Generate a block of V.27ter modem audio samples.
00170     \brief Generate a block of V.27ter modem audio samples.
00171     \param s The modem context.
00172     \param amp The audio sample buffer.
00173     \param len The number of samples to be generated.
00174     \return The number of samples actually generated.
00175 */
00176 int v27ter_tx(v27ter_tx_state_t *s, int16_t amp[], int len);
00177 
00178 #if defined(__cplusplus)
00179 }
00180 #endif
00181 
00182 #endif
00183 /*- End of file ------------------------------------------------------------*/

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