147 lines
6.6 KiB
C
147 lines
6.6 KiB
C
/*
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* SpanDSP - a series of DSP components for telephony
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*
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* v27ter_tx.h - ITU V.27ter modem transmit part
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*
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* Written by Steve Underwood <steveu@coppice.org>
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*
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* Copyright (C) 2003 Steve Underwood
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*
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* All rights reserved.
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU Lesser General Public License version 2.1,
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* as published by the Free Software Foundation.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU Lesser General Public License for more details.
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*
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* You should have received a copy of the GNU Lesser General Public
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* License along with this program; if not, write to the Free Software
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* Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
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*/
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/*! \file */
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#if !defined(_SPANDSP_V27TER_TX_H_)
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#define _SPANDSP_V27TER_TX_H_
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/*! \page v27ter_tx_page The V.27ter transmitter
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\section v27ter_tx_page_sec_1 What does it do?
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The V.27ter transmitter implements the transmit side of a V.27ter modem. This
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can operate at data rates of 4800 and 2400 bits/s. The audio output is a stream
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of 16 bit samples, at 8000 samples/second. The transmit and receive side of
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V.27ter modems operate independantly. V.27ter is used for FAX transmission,
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where it provides the standard 4800 and 2400 bits/s rates.
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\section v27ter_tx_page_sec_2 How does it work?
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V.27ter uses DPSK modulation. A common method of producing a DPSK modulated
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signal is to use a sampling rate which is a multiple of the baud rate. The raw
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signal is then a series of complex pulses, each an integer number of samples
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long. These can be shaped, using a suitable complex filter, and multiplied by a
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complex carrier signal to produce the final DPSK signal for transmission.
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The pulse shaping filter for V.27ter is defined in the spec. It is a root raised
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cosine filter with 50% excess bandwidth.
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The sampling rate for our transmitter is defined by the channel - 8000 samples/s.
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This is a multiple of the baud rate at 4800 bits/s (8-PSK at 1600 baud, 5 samples per
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symbol), but not at 2400 bits/s (4-PSK at 1200 baud, 20/3 samples per symbol). The baud
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interval is actually 20/3 sample periods at 2400bis/s. A symmetric FIR is used to
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apply root raised cosine filtering in the 4800bits/s mode. In the 2400bits/s mode
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a polyphase FIR filter is used. This consists of 20 sets of coefficients, offering
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zero to 19/20ths of a baud phase shift as well as root raised cosine filtering.
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The appropriate coefficient set is chosen for each signal sample generated.
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The carrier is generated using the DDS method. Using 2 second order resonators,
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started in quadrature, might be more efficient, as it would have less impact on
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the processor cache than a table lookup approach. However, the DDS approach
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suits the receiver better, so then same signal generator is also used for the
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transmitter.
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*/
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/*!
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V.27ter modem transmit side descriptor. This defines the working state for a
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single instance of a V.27ter modem transmitter.
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*/
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typedef struct v27ter_tx_state_s v27ter_tx_state_t;
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#if defined(__cplusplus)
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extern "C"
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{
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#endif
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/*! Adjust a V.27ter modem transmit context's power output.
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\brief Adjust a V.27ter modem transmit context's output power.
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\param s The modem context.
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\param power The power level, in dBm0 */
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SPAN_DECLARE(void) v27ter_tx_power(v27ter_tx_state_t *s, float power);
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/*! Initialise a V.27ter modem transmit context.
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\brief Initialise a V.27ter modem transmit context.
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\param s The modem context.
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\param bit_rate The bit rate of the modem. Valid values are 2400 and 4800.
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\param tep TRUE is the optional TEP tone is to be transmitted.
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\param get_bit The callback routine used to get the data to be transmitted.
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\param user_data An opaque pointer.
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\return A pointer to the modem context, or NULL if there was a problem. */
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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);
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/*! Reinitialise an existing V.27ter modem transmit context, so it may be reused.
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\brief Reinitialise an existing V.27ter modem transmit context.
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\param s The modem context.
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\param bit_rate The bit rate of the modem. Valid values are 2400 and 4800.
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\param tep TRUE is the optional TEP tone is to be transmitted.
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\return 0 for OK, -1 for bad parameter */
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SPAN_DECLARE(int) v27ter_tx_restart(v27ter_tx_state_t *s, int bit_rate, int tep);
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/*! Release a V.27ter modem transmit context.
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\brief Release a V.27ter modem transmit context.
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\param s The modem context.
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\return 0 for OK */
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SPAN_DECLARE(int) v27ter_tx_release(v27ter_tx_state_t *s);
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/*! Free a V.27ter modem transmit context.
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\brief Free a V.27ter modem transmit context.
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\param s The modem context.
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\return 0 for OK */
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SPAN_DECLARE(int) v27ter_tx_free(v27ter_tx_state_t *s);
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/*! Get the logging context associated with a V.27ter modem transmit context.
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\brief Get the logging context associated with a V.27ter modem transmit context.
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\param s The modem context.
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\return A pointer to the logging context */
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SPAN_DECLARE(logging_state_t *) v27ter_tx_get_logging_state(v27ter_tx_state_t *s);
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/*! Change the get_bit function associated with a V.27ter modem transmit context.
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\brief Change the get_bit function associated with a V.27ter modem transmit context.
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\param s The modem context.
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\param get_bit The callback routine used to get the data to be transmitted.
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\param user_data An opaque pointer. */
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SPAN_DECLARE(void) v27ter_tx_set_get_bit(v27ter_tx_state_t *s, get_bit_func_t get_bit, void *user_data);
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/*! Change the modem status report function associated with a V.27ter modem transmit context.
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\brief Change the modem status report function associated with a V.27ter modem transmit context.
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\param s The modem context.
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\param handler The callback routine used to report modem status changes.
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\param user_data An opaque pointer. */
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SPAN_DECLARE(void) v27ter_tx_set_modem_status_handler(v27ter_tx_state_t *s, modem_status_func_t handler, void *user_data);
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/*! Generate a block of V.27ter modem audio samples.
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\brief Generate a block of V.27ter modem audio samples.
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\param s The modem context.
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\param amp The audio sample buffer.
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\param len The number of samples to be generated.
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\return The number of samples actually generated.
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*/
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SPAN_DECLARE_NONSTD(int) v27ter_tx(v27ter_tx_state_t *s, int16_t amp[], int len);
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#if defined(__cplusplus)
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}
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#endif
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#endif
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/*- End of file ------------------------------------------------------------*/
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