libspandsp-dev 0.0.6-2.1 / usr / include / spandsp / 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.
 */

/*! \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.
*/

/*!
    V.27ter modem transmit side descriptor. This defines the working state for a
    single instance of a V.27ter modem transmitter.
*/
typedef struct v27ter_tx_state_s 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 */
SPAN_DECLARE(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. */
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);

/*! 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 */
SPAN_DECLARE(int) v27ter_tx_restart(v27ter_tx_state_t *s, int bit_rate, int tep);

/*! Release a V.27ter modem transmit context.
    \brief Release a V.27ter modem transmit context.
    \param s The modem context.
    \return 0 for OK */
SPAN_DECLARE(int) v27ter_tx_release(v27ter_tx_state_t *s);

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

/*! Get the logging context associated with a V.27ter modem transmit context.
    \brief Get the logging context associated with a V.27ter modem transmit context.
    \param s The modem context.
    \return A pointer to the logging context */
SPAN_DECLARE(logging_state_t *) v27ter_tx_get_logging_state(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. */
SPAN_DECLARE(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. */
SPAN_DECLARE(void) v27ter_tx_set_modem_status_handler(v27ter_tx_state_t *s, modem_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.
*/
SPAN_DECLARE_NONSTD(int) v27ter_tx(v27ter_tx_state_t *s, int16_t amp[], int len);

#if defined(__cplusplus)
}
#endif

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

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