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v29tx.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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* v29tx.h - ITU V.29 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_V29TX_H_)
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#define _SPANDSP_V29TX_H_
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/*! \page v29tx_page The V.29 transmitter
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\section v29tx_page_sec_1 What does it do?
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The V.29 transmitter implements the transmit side of a V.29 modem. This can
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operate at data rates of 9600, 7200 and 4800 bits/s. The audio output is a
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stream of 16 bit samples, at 8000 samples/second. The transmit and receive side
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of V.29 modems operate independantly. V.29 is mostly used for FAX transmission,
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where it provides the standard 9600 and 7200 bits/s rates (the 4800 bits/s mode
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is not used for FAX).
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\section v29tx_page_sec_2 How does it work?
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V.29 uses QAM modulation. The standard method of producing a QAM 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 QAM signal for transmission.
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The pulse shaping filter is only vaguely defined by the V.29 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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The equation defining QAM modulation is:
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s(n) = A*cos(2*pi*f*n + phi(n))
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where phi(n) is the phase of the information, and A is the amplitude of the information
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using the identity
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cos(x + y) = cos(x)*cos(y) - sin(x)*sin(y)
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we get
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s(n) = A {cos(2*pi*f*n)*cos(phi(n)) - sin(2*pi*f*n)*sin(phi(n))}
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substituting with the constellation positions
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I(n) = A*cos(phi(n))
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Q(n) = A*sin(phi(n))
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gives
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s(n) = I(n)*cos(2*pi*f*n) - Q(n)*sin(2*pi*f*n)
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*/
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/*!
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V.29 modem transmit side descriptor. This defines the working state for a
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single instance of a V.29 modem transmitter.
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*/
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typedef
struct
v29_tx_state_s
v29_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.29 modem transmit context's power output.
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\brief Adjust a V.29 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
)
v29_tx_power
(
v29_tx_state_t
*s,
float
power);
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/*! Initialise a V.29 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.29 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 4800, 7200 and 9600.
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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(
v29_tx_state_t
*)
v29_tx_init
(
v29_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.29 modem transmit context, so it may be reused.
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\brief Reinitialise an existing V.29 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 4800, 7200 and 9600.
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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
)
v29_tx_restart
(
v29_tx_state_t
*s,
int
bit_rate,
int
tep);
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/*! Release a V.29 modem transmit context.
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\brief Release a V.29 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
)
v29_tx_release
(
v29_tx_state_t
*s);
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/*! Free a V.29 modem transmit context.
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\brief Free a V.29 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
)
v29_tx_free
(
v29_tx_state_t
*s);
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/*! Get the logging context associated with a V.29 modem transmit context.
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\brief Get the logging context associated with a V.29 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
*)
v29_tx_get_logging_state
(
v29_tx_state_t
*s);
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/*! Change the get_bit function associated with a V.29 modem transmit context.
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\brief Change the get_bit function associated with a V.29 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
)
v29_tx_set_get_bit
(
v29_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.29 modem transmit context.
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\brief Change the modem status report function associated with a V.29 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
)
v29_tx_set_modem_status_handler
(
v29_tx_state_t
*s,
modem_tx_status_func_t
handler,
void
*user_data);
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/*! Generate a block of V.29 modem audio samples.
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\brief Generate a block of V.29 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
) v29_tx(
v29_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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spandsp
v29tx.h
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