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v17tx.h
Go to the documentation of this file.
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/*
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* SpanDSP - a series of DSP components for telephony
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*
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* v17tx.h - ITU V.17 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) 2004 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_V17TX_H_)
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#define _SPANDSP_V17TX_H_
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/*! \page v17tx_page The V.17 transmitter
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\section v17tx_page_sec_1 What does it do?
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The V.17 transmitter implements the transmit side of a V.17 modem. This can
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operate at data rates of 14400, 12000, 9600 and 7200 bits/second. The audio
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output is a stream of 16 bit samples, at 8000 samples/second. The transmit and
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receive side of V.17 modems operate independantly. V.17 is mostly used for FAX
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transmission, where it provides the standard 14400 bits/second rate.
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\section v17tx_page_sec_2 How does it work?
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V.17 uses QAM modulation and trellis coding. The data to be transmitted is
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scrambled, to whiten it. The least significant 2 bits of each symbol are then
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differentially encoded, using a simple lookup approach. The resulting 2 bits are
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convolutionally encoded, producing 3 bits. The extra bit is the redundant bit
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of the trellis code. The other bits of the symbol pass by the differential
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and convolutional coding unchanged. The resulting bits define the constellation
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point to be transmitted for the symbol. The redundant bit doubles the size of the
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constellation, and so increases the error rate for detecting individual symbols
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at the receiver. However, when a number of successive symbols are processed at
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the receiver, the redundancy actually provides several dB of improved error
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performance.
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The standard method of producing a QAM modulated signal is to use a sampling
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rate which is a multiple of the baud rate. The raw signal is then a series of
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complex pulses, each an integer number of samples long. These can be shaped,
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using a suitable complex filter, and multiplied by a complex carrier signal
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to produce the final QAM signal for transmission.
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The pulse shaping filter is only vaguely defined by the V.17 spec. Some of the
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other ITU modem specs. fully define the filter, typically specifying a root
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raised cosine filter, with 50% excess bandwidth. This is a pity, since it
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increases the variability of the received signal. However, the receiver's
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adaptive equalizer will compensate for these differences. The current
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design uses a root raised cosine filter with 25% excess bandwidth. Greater
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excess bandwidth will not allow the tranmitted signal to meet the spectral
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requirements.
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The sampling rate for our transmitter is defined by the channel - 8000 per
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second. This is not a multiple of the baud rate (i.e. 2400 baud). The baud
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interval is actually 10/3 sample periods. Instead of using a symmetric
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FIR to pulse shape the signal, a polyphase filter is used. This consists of
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10 sets of coefficients, offering zero to 9/10ths of a baud phase shift as well
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as root raised cosine filtering. The appropriate coefficient set is chosen for
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each signal sample generated.
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The carrier is generated using the DDS method. Using two 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 the 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.17 modem transmit side descriptor. This defines the working state for a
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single instance of a V.17 modem transmitter.
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*/
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typedef
struct
v17_tx_state_s
v17_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.17 modem transmit context's power output.
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\brief Adjust a V.17 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
)
v17_tx_power
(
v17_tx_state_t
*s,
float
power);
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/*! Initialise a V.17 modem transmit context. This must be called before the first
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use of the context, to initialise its contents.
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\brief Initialise a V.17 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 7200, 9600, 12000 and 14400.
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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(
v17_tx_state_t
*)
v17_tx_init
(
v17_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.17 modem transmit context, so it may be reused.
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\brief Reinitialise an existing V.17 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 7200, 9600, 12000 and 14400.
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\param tep TRUE is the optional TEP tone is to be transmitted.
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\param short_train TRUE if the short training sequence should be used.
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\return 0 for OK, -1 for parameter error. */
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SPAN_DECLARE(
int
)
v17_tx_restart
(
v17_tx_state_t
*s,
int
bit_rate,
int
tep,
int
short_train
);
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/*! Release a V.17 modem transmit context.
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\brief Release a V.17 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
)
v17_tx_release
(
v17_tx_state_t
*s);
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/*! Free a V.17 modem transmit context.
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\brief Free a V.17 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
)
v17_tx_free
(
v17_tx_state_t
*s);
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/*! Get the logging context associated with a V.17 modem transmit context.
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\brief Get the logging context associated with a V.17 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
*)
v17_tx_get_logging_state
(
v17_tx_state_t
*s);
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/*! Change the get_bit function associated with a V.17 modem transmit context.
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\brief Change the get_bit function associated with a V.17 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
)
v17_tx_set_get_bit
(
v17_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.17 modem transmit context.
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\brief Change the modem status report function associated with a V.17 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
)
v17_tx_set_modem_status_handler
(
v17_tx_state_t
*s,
modem_tx_status_func_t
handler,
void
*user_data);
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/*! Generate a block of V.17 modem audio samples.
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\brief Generate a block of V.17 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
) v17_tx(
v17_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 ------------------------------------------------------------*/
src
spandsp
v17tx.h
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