v17tx.h

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00001 /*
00002  * SpanDSP - a series of DSP components for telephony
00003  *
00004  * v17tx.h - ITU V.17 modem transmit part
00005  *
00006  * Written by Steve Underwood <steveu@coppice.org>
00007  *
00008  * Copyright (C) 2004 Steve Underwood
00009  *
00010  * All rights reserved.
00011  *
00012  * This program is free software; you can redistribute it and/or modify
00013  * it under the terms of the GNU General Public License as published by
00014  * the Free Software Foundation; either version 2 of the License, or
00015  * (at your option) any later version.
00016  *
00017  * This program is distributed in the hope that it will be useful,
00018  * but WITHOUT ANY WARRANTY; without even the implied warranty of
00019  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
00020  * GNU General Public License for more details.
00021  *
00022  * You should have received a copy of the GNU General Public License
00023  * along with this program; if not, write to the Free Software
00024  * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
00025  *
00026  * $Id: v17tx.h,v 1.19 2005/12/29 09:54:24 steveu Exp $
00027  */
00028 
00029 /*! \file */
00030 
00031 #if !defined(_V17TX_H_)
00032 #define _V17TX_H_
00033 
00034 /*! \page v17tx_page The V.17 transmitter
00035 \section v17tx_page_sec_1 What does it do?
00036 The V.17 transmitter implements the transmit side of a V.17 modem. This can
00037 operate at data rates of 14400, 12000, 9600 and 7200 bits/second. The audio
00038 output is a stream of 16 bit samples, at 8000 samples/second. The transmit and
00039 receive side of V.17 modems operate independantly. V.17 is mostly used for FAX
00040 transmission, where it provides the standard 14400 bits/second rate. 
00041 
00042 \section v17tx_page_sec_2 How does it work?
00043 V.17 uses QAM modulation and trellis coding. The data to be transmitted is
00044 scrambled, to whiten it. The least significant 2 bits of each symbol are then
00045 differentially encoded, using a simple lookup approach. The resulting 2 bits are
00046 convolutionally encoded, producing 3 bits. The extra bit is the redundant bit
00047 of the trellis code. The other bits of the symbol pass by the differential
00048 and convolutional coding unchanged. The resulting bits define the constellation
00049 point to be transmitted for the symbol. The redundant bit doubles the size of the
00050 constellation, and so increases the error rate for detecting individual symbols
00051 at the receiver. However, when a number of successive symbols are processed at
00052 the receiver, the redundancy actually provides several dB of improved error
00053 performance.
00054 
00055 The standard method of producing a QAM modulated signal is to use a sampling
00056 rate which is a multiple of the baud rate. The raw signal is then a series of
00057 complex pulses, each an integer number of samples long. These can be shaped,
00058 using a suitable complex filter, and multiplied by a complex carrier signal
00059 to produce the final QAM signal for transmission. 
00060 
00061 The sampling rate for our transmitter is defined by the channel - 8000 per
00062 second. This is not a multiple of the baud rate (i.e. 2400 baud). The baud
00063 interval is actually 10/3 sample periods. Generating at the lowest common
00064 multiple of the baud rate and channel sample rate (i.e. 24000 samples/second),
00065 and then decimating to 8000 samples/second, would give good results. However,
00066 this would require considerable computation. A shortcut is to use slightly
00067 shaped pulses, instead of simple square ones. We can achieve the effect of pulse
00068 transitions at the 1/2 and 2/3 sample points by adjusting the first sample of
00069 each new pulse. The adjustment is simple. We need the effect of being 60 degrees
00070 or 120 degrees through a sine wave cycle at the Shannon rate at the sample
00071 point. This simply means we need to step by 0.25 or 0.75 of the actual step size
00072 on the first sample of those pulses which should start at the 1/3 or 2/3 sample
00073 positions. The logic and computation needed for this is much less than the
00074 computation needed for oversampling at 24000 samples/second. 
00075 
00076 The pulse shaping filter is only vaguely defined by the V.17 spec. Some of the
00077 other ITU modem specs. fully define the filter, typically specifying a root
00078 raised cosine filter, with 50% excess bandwidth. This is a pity, since it
00079 increases the variability of the received signal. However, the receiver's
00080 adaptive equalizer will largely compensate for these differences. The current
00081 design uses a root raised cosine filter with 50% excess bandwidth. 
00082 
00083 The carrier is generated using the DDS method. Using two second order resonators,
00084 started in quadrature, might be more efficient, as it would have less impact on
00085 the processor cache than a table lookup approach. However, the DDS approach
00086 suits the receiver better, so the same signal generator is also used for the
00087 transmitter. 
00088 */
00089 
00090 #define V17TX_FILTER_STEPS      27
00091 
00092 /*!
00093     V.17 modem transmit side descriptor. This defines the working state for a
00094     single instance of a V.17 modem transmitter.
00095 */
00096 typedef struct
00097 {
00098     /*! \brief The bit rate of the modem. Valid values are 4800, 7200 and 9600. */
00099     int bit_rate;
00100     /*! \brief The callback function used to get the next bit to be transmitted. */
00101     get_bit_func_t get_bit;
00102     /*! \brief A user specified opaque pointer passed to the callback function. */
00103     void *user_data;
00104 
00105     float gain;
00106 
00107     /*! \brief The route raised cosine (RRC) pulse shaping filter buffer. */
00108     complex_t rrc_filter[2*V17TX_FILTER_STEPS];
00109     /*! \brief Current offset into the RRC pulse shaping filter buffer. */
00110     int rrc_filter_step;
00111     /*! \brief The current constellation position. */
00112     complex_t current_point;
00113 
00114     int diff;
00115     int convolution;
00116 
00117     /*! \brief The register for the data scrambler. */
00118     unsigned int scramble_reg;
00119     /*! \brief TRUE if transmitting the training sequence. FALSE if transmitting user data. */
00120     int in_training;
00121     /*! A counter used to track progress through the optional TEP tone burst */
00122     int tep_step;
00123     /*! \brief TRUE if the short training sequence is to be used. */
00124     int short_train;
00125     /*! \brief A counter used to track progress through sending the training sequence. */
00126     int training_step;
00127 
00128     /*! \brief The current phase of the carrier (i.e. the DDS parameter). */
00129     uint32_t carrier_phase;
00130     /*! \brief The update rate for the phase of the carrier (i.e. the DDS increment). */
00131     int32_t carrier_phase_rate;
00132     /*! \brief The current fractional phase of the baud timing. */
00133     int baud_phase;
00134     /*! \brief The code number for the current position in the constellation. */
00135     int constellation_state;
00136     
00137     /*! \brief A pointer to the constellation currently in use. */
00138     const complex_t *constellation;
00139     /*! \brief The current number of data bits per symbol. This does not include
00140                the redundant bit. */
00141     int bits_per_symbol;
00142     /*! \brief An indicator to mark that we are tidying up to stop transmission. */
00143     int shutdown;
00144     /*! \brief The get_bit function in use at any instant. */
00145     get_bit_func_t current_get_bit;
00146     /*! \brief Error and flow logging control */
00147     logging_state_t logging;
00148 } v17_tx_state_t;
00149 
00150 #ifdef __cplusplus
00151 extern "C" {
00152 #endif
00153 
00154 /*! Adjust a V.17 modem transmit context's power output.
00155     \brief Adjust a V.17 modem transmit context's output power.
00156     \param s The modem context.
00157     \param power The power level, in dBm0 */
00158 void v17_tx_power(v17_tx_state_t *s, float power);
00159 
00160 /*! Initialise a V.17 modem transmit context. This must be called before the first
00161     use of the context, to initialise its contents.
00162     \brief Initialise a V.17 modem transmit context.
00163     \param s The modem context.
00164     \param rate The bit rate of the modem. Valid values are 7200, 9600, 12000 and 14400.
00165     \param tep TRUE is the optional TEP tone is to be transmitted.
00166     \param get_bit The callback routine used to get the data to be transmitted.
00167     \param user_data An opaque pointer.
00168     \return A pointer to the modem context, or NULL if there was a problem. */
00169 v17_tx_state_t *v17_tx_init(v17_tx_state_t *s, int rate, int tep, get_bit_func_t get_bit, void *user_data);
00170 
00171 /*! Reinitialise an existing V.17 modem transmit context, so it may be reused.
00172     \brief Reinitialise an existing V.17 modem transmit context.
00173     \param s The modem context.
00174     \param rate The bit rate of the modem. Valid values are 7200, 9600, 12000 and 14400.
00175     \param tep TRUE is the optional TEP tone is to be transmitted.
00176     \param short_train TRUE if the short training sequence should be used.
00177     \return 0 for OK, -1 for parameter error. */
00178 int v17_tx_restart(v17_tx_state_t *s, int rate, int tep, int short_train);
00179 
00180 /*! Release a V.17 modem transmit context.
00181     \brief Release a V.17 modem transmit context.
00182     \param s The modem context.
00183     \return 0 for OK */
00184 int v17_tx_release(v17_tx_state_t *s);
00185 
00186 /*! Change the get_bit function associated with a V.17 modem transmit context.
00187     \brief Change the get_bit function associated with a V.17 modem transmit context.
00188     \param s The modem context.
00189     \param get_bit The callback routine used to get the data to be transmitted.
00190     \param user_data An opaque pointer. */
00191 void v17_tx_set_get_bit(v17_tx_state_t *s, get_bit_func_t get_bit, void *user_data);
00192 
00193 /*! Generate a block of V.17 modem audio samples.
00194     \brief Generate a block of V.17 modem audio samples.
00195     \param s The modem context.
00196     \param amp The audio sample buffer.
00197     \param len The number of samples to be generated.
00198     \return The number of samples actually generated.
00199 */
00200 int v17_tx(v17_tx_state_t *s, int16_t *amp, int len);
00201 
00202 #ifdef __cplusplus
00203 }
00204 #endif
00205 
00206 #endif
00207 /*- End of file ------------------------------------------------------------*/

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