Mercurial > hg > audiostuff
diff spandsp-0.0.6pre17/src/spandsp/v17tx.h @ 4:26cd8f1ef0b1
import spandsp-0.0.6pre17
author | Peter Meerwald <pmeerw@cosy.sbg.ac.at> |
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date | Fri, 25 Jun 2010 15:50:58 +0200 |
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--- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/spandsp-0.0.6pre17/src/spandsp/v17tx.h Fri Jun 25 15:50:58 2010 +0200 @@ -0,0 +1,167 @@ +/* + * SpanDSP - a series of DSP components for telephony + * + * v17tx.h - ITU V.17 modem transmit part + * + * Written by Steve Underwood <steveu@coppice.org> + * + * Copyright (C) 2004 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: v17tx.h,v 1.43.4.1 2009/12/24 16:52:30 steveu Exp $ + */ + +/*! \file */ + +#if !defined(_SPANDSP_V17TX_H_) +#define _SPANDSP_V17TX_H_ + +/*! \page v17tx_page The V.17 transmitter +\section v17tx_page_sec_1 What does it do? +The V.17 transmitter implements the transmit side of a V.17 modem. This can +operate at data rates of 14400, 12000, 9600 and 7200 bits/second. The audio +output is a stream of 16 bit samples, at 8000 samples/second. The transmit and +receive side of V.17 modems operate independantly. V.17 is mostly used for FAX +transmission, where it provides the standard 14400 bits/second rate. + +\section v17tx_page_sec_2 How does it work? +V.17 uses QAM modulation and trellis coding. The data to be transmitted is +scrambled, to whiten it. The least significant 2 bits of each symbol are then +differentially encoded, using a simple lookup approach. The resulting 2 bits are +convolutionally encoded, producing 3 bits. The extra bit is the redundant bit +of the trellis code. The other bits of the symbol pass by the differential +and convolutional coding unchanged. The resulting bits define the constellation +point to be transmitted for the symbol. The redundant bit doubles the size of the +constellation, and so increases the error rate for detecting individual symbols +at the receiver. However, when a number of successive symbols are processed at +the receiver, the redundancy actually provides several dB of improved error +performance. + +The standard method of producing a QAM 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 QAM signal for transmission. + +The pulse shaping filter is only vaguely defined by the V.17 spec. Some of the +other ITU modem specs. fully define the filter, typically specifying a root +raised cosine filter, with 50% excess bandwidth. This is a pity, since it +increases the variability of the received signal. However, the receiver's +adaptive equalizer will compensate for these differences. The current +design uses a root raised cosine filter with 25% excess bandwidth. Greater +excess bandwidth will not allow the tranmitted signal to meet the spectral +requirements. + +The sampling rate for our transmitter is defined by the channel - 8000 per +second. This is not a multiple of the baud rate (i.e. 2400 baud). The baud +interval is actually 10/3 sample periods. Instead of using a symmetric +FIR to pulse shape the signal, a polyphase filter is used. This consists of +10 sets of coefficients, offering zero to 9/10ths 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 two 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 the same signal generator is also used for the +transmitter. +*/ + +/*! + V.17 modem transmit side descriptor. This defines the working state for a + single instance of a V.17 modem transmitter. +*/ +typedef struct v17_tx_state_s v17_tx_state_t; + +#if defined(__cplusplus) +extern "C" +{ +#endif + +/*! Adjust a V.17 modem transmit context's power output. + \brief Adjust a V.17 modem transmit context's output power. + \param s The modem context. + \param power The power level, in dBm0 */ +SPAN_DECLARE(void) v17_tx_power(v17_tx_state_t *s, float power); + +/*! Initialise a V.17 modem transmit context. This must be called before the first + use of the context, to initialise its contents. + \brief Initialise a V.17 modem transmit context. + \param s The modem context. + \param bit_rate The bit rate of the modem. Valid values are 7200, 9600, 12000 and 14400. + \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(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); + +/*! Reinitialise an existing V.17 modem transmit context, so it may be reused. + \brief Reinitialise an existing V.17 modem transmit context. + \param s The modem context. + \param bit_rate The bit rate of the modem. Valid values are 7200, 9600, 12000 and 14400. + \param tep TRUE is the optional TEP tone is to be transmitted. + \param short_train TRUE if the short training sequence should be used. + \return 0 for OK, -1 for parameter error. */ +SPAN_DECLARE(int) v17_tx_restart(v17_tx_state_t *s, int bit_rate, int tep, int short_train); + +/*! Release a V.17 modem transmit context. + \brief Release a V.17 modem transmit context. + \param s The modem context. + \return 0 for OK */ +SPAN_DECLARE(int) v17_tx_release(v17_tx_state_t *s); + +/*! Free a V.17 modem transmit context. + \brief Free a V.17 modem transmit context. + \param s The modem context. + \return 0 for OK */ +SPAN_DECLARE(int) v17_tx_free(v17_tx_state_t *s); + +/*! Get the logging context associated with a V.17 modem transmit context. + \brief Get the logging context associated with a V.17 modem transmit context. + \param s The modem context. + \return A pointer to the logging context */ +SPAN_DECLARE(logging_state_t *) v17_tx_get_logging_state(v17_tx_state_t *s); + +/*! Change the get_bit function associated with a V.17 modem transmit context. + \brief Change the get_bit function associated with a V.17 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) v17_tx_set_get_bit(v17_tx_state_t *s, get_bit_func_t get_bit, void *user_data); + +/*! Change the modem status report function associated with a V.17 modem transmit context. + \brief Change the modem status report function associated with a V.17 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) v17_tx_set_modem_status_handler(v17_tx_state_t *s, modem_tx_status_func_t handler, void *user_data); + +/*! Generate a block of V.17 modem audio samples. + \brief Generate a block of V.17 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) v17_tx(v17_tx_state_t *s, int16_t amp[], int len); + +#if defined(__cplusplus) +} +#endif + +#endif +/*- End of file ------------------------------------------------------------*/