v27ter_tx.h

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/*
 * SpanDSP - a series of DSP components for telephony
 *
 * v27ter_tx.h - ITU V.27ter modem transmit part
 *
 * Written by Steve Underwood <steveu@coppice.org>
 *
 * Copyright (C) 2003 Steve Underwood
 *
 * All rights reserved.
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU Lesser General Public License version 2.1,
 * as published by the Free Software Foundation.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this program; if not, write to the Free Software
 * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
 *
 * $Id: v27ter_tx.h,v 1.36 2008/07/16 14:23:48 steveu Exp $
 */

/*! \file */

#if !defined(_SPANDSP_V27TER_TX_H_)
#define _SPANDSP_V27TER_TX_H_

/*! \page v27ter_tx_page The V.27ter transmitter
\section v27ter_tx_page_sec_1 What does it do?
The V.27ter transmitter implements the transmit side of a V.27ter modem. This
can operate at data rates of 4800 and 2400 bits/s. The audio output is a stream
of 16 bit samples, at 8000 samples/second. The transmit and receive side of
V.27ter modems operate independantly. V.27ter is used for FAX transmission,
where it provides the standard 4800 and 2400 bits/s rates. 

\section v27ter_tx_page_sec_2 How does it work?
V.27ter uses DPSK modulation. A common method of producing a DPSK modulated
signal is to use a sampling rate which is a multiple of the baud rate. The raw
signal is then a series of complex pulses, each an integer number of samples
long. These can be shaped, using a suitable complex filter, and multiplied by a
complex carrier signal to produce the final DPSK signal for transmission. 

The pulse shaping filter for V.27ter is defined in the spec. It is a root raised
cosine filter with 50% excess bandwidth. 

The sampling rate for our transmitter is defined by the channel - 8000 samples/s.
This is a multiple of the baud rate at 4800 bits/s (8-PSK at 1600 baud, 5 samples per
symbol), but not at 2400 bits/s (4-PSK at 1200 baud, 20/3 samples per symbol). The baud
interval is actually 20/3 sample periods at 2400bis/s. A symmetric FIR is used to
apply root raised cosine filtering in the 4800bits/s mode. In the 2400bits/s mode
a polyphase FIR filter is used. This consists of 20 sets of coefficients, offering
zero to 19/20ths of a baud phase shift as well as root raised cosine filtering.
The appropriate coefficient set is chosen for each signal sample generated.

The carrier is generated using the DDS method. Using 2 second order resonators,
started in quadrature, might be more efficient, as it would have less impact on
the processor cache than a table lookup approach. However, the DDS approach
suits the receiver better, so then same signal generator is also used for the
transmitter.
*/

#define V27TER_TX_FILTER_STEPS      9

/*!
    V.27ter modem transmit side descriptor. This defines the working state for a
    single instance of a V.27ter modem transmitter.
*/
typedef struct
{
    /*! \brief The bit rate of the modem. Valid values are 2400 and 4800. */
    int bit_rate;
    /*! \brief The callback function used to get the next bit to be transmitted. */
    get_bit_func_t get_bit;
    /*! \brief A user specified opaque pointer passed to the get_bit function. */
    void *get_bit_user_data;

    /*! \brief The callback function used to report modem status changes. */
    modem_tx_status_func_t status_handler;
    /*! \brief A user specified opaque pointer passed to the status function. */
    void *status_user_data;

#if defined(SPANDSP_USE_FIXED_POINT)
    /*! \brief The gain factor needed to achieve the specified output power at 2400bps. */
    int32_t gain_2400;
    /*! \brief The gain factor needed to achieve the specified output power at 4800bps. */
    int32_t gain_4800;
#else
    /*! \brief The gain factor needed to achieve the specified output power at 2400bps. */
    float gain_2400;
    /*! \brief The gain factor needed to achieve the specified output power at 4800bps. */
    float gain_4800;
#endif
    /*! \brief The route raised cosine (RRC) pulse shaping filter buffer. */
#if defined(SPANDSP_USE_FIXED_POINT)
    complexi16_t rrc_filter[2*V27TER_TX_FILTER_STEPS];
#else
    complexf_t rrc_filter[2*V27TER_TX_FILTER_STEPS];
#endif
    /*! \brief Current offset into the RRC pulse shaping filter buffer. */
    int rrc_filter_step;
    
    /*! \brief The register for the training and data scrambler. */
    unsigned int scramble_reg;
    /*! \brief A counter for the number of consecutive bits of repeating pattern through
               the scrambler. */
    int scrambler_pattern_count;
    /*! \brief TRUE if transmitting the training sequence, or shutting down transmission.
               FALSE if transmitting user data. */
    int in_training;
    /*! \brief A counter used to track progress through sending the training sequence. */
    int training_step;

    /*! \brief The current phase of the carrier (i.e. the DDS parameter). */
    uint32_t carrier_phase;
    /*! \brief The update rate for the phase of the carrier (i.e. the DDS increment). */
    int32_t carrier_phase_rate;
    /*! \brief The current fractional phase of the baud timing. */
    int baud_phase;
    /*! \brief The code number for the current position in the constellation. */
    int constellation_state;
    /*! \brief The get_bit function in use at any instant. */
    get_bit_func_t current_get_bit;
    /*! \brief Error and flow logging control */
    logging_state_t logging;
} v27ter_tx_state_t;

#if defined(__cplusplus)
extern "C"
{
#endif

/*! Adjust a V.27ter modem transmit context's power output.
    \brief Adjust a V.27ter modem transmit context's output power.
    \param s The modem context.
    \param power The power level, in dBm0 */
void v27ter_tx_power(v27ter_tx_state_t *s, float power);

/*! Initialise a V.27ter modem transmit context.
    \brief Initialise a V.27ter modem transmit context.
    \param s The modem context.
    \param bit_rate The bit rate of the modem. Valid values are 2400 and 4800.
    \param tep TRUE is the optional TEP tone is to be transmitted.
    \param get_bit The callback routine used to get the data to be transmitted.
    \param user_data An opaque pointer.
    \return A pointer to the modem context, or NULL if there was a problem. */
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);

/*! Reinitialise an existing V.27ter modem transmit context, so it may be reused.
    \brief Reinitialise an existing V.27ter modem transmit context.
    \param s The modem context.
    \param bit_rate The bit rate of the modem. Valid values are 2400 and 4800.
    \param tep TRUE is the optional TEP tone is to be transmitted.
    \return 0 for OK, -1 for bad parameter */
int v27ter_tx_restart(v27ter_tx_state_t *s, int bit_rate, int tep);

/*! Free a V.27ter modem transmit context.
    \brief Free a V.27ter modem transmit context.
    \param s The modem context.
    \return 0 for OK */
int v27ter_tx_free(v27ter_tx_state_t *s);

/*! Change the get_bit function associated with a V.27ter modem transmit context.
    \brief Change the get_bit function associated with a V.27ter modem transmit context.
    \param s The modem context.
    \param get_bit The callback routine used to get the data to be transmitted.
    \param user_data An opaque pointer. */
void v27ter_tx_set_get_bit(v27ter_tx_state_t *s, get_bit_func_t get_bit, void *user_data);

/*! Change the modem status report function associated with a V.27ter modem transmit context.
    \brief Change the modem status report function associated with a V.27ter modem transmit context.
    \param s The modem context.
    \param handler The callback routine used to report modem status changes.
    \param user_data An opaque pointer. */
void v27ter_tx_set_modem_status_handler(v27ter_tx_state_t *s, modem_tx_status_func_t handler, void *user_data);

/*! Generate a block of V.27ter modem audio samples.
    \brief Generate a block of V.27ter modem audio samples.
    \param s The modem context.
    \param amp The audio sample buffer.
    \param len The number of samples to be generated.
    \return The number of samples actually generated.
*/
int v27ter_tx(v27ter_tx_state_t *s, int16_t amp[], int len);

#if defined(__cplusplus)
}
#endif

#endif
/*- End of file ------------------------------------------------------------*/