spandsp 0.0.6
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00001 /* 00002 * SpanDSP - a series of DSP components for telephony 00003 * 00004 * v29rx.h - ITU V.29 modem receive part 00005 * 00006 * Written by Steve Underwood <steveu@coppice.org> 00007 * 00008 * Copyright (C) 2003 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 Lesser General Public License version 2.1, 00014 * as published by the Free Software Foundation. 00015 * 00016 * This program is distributed in the hope that it will be useful, 00017 * but WITHOUT ANY WARRANTY; without even the implied warranty of 00018 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 00019 * GNU Lesser General Public License for more details. 00020 * 00021 * You should have received a copy of the GNU Lesser General Public 00022 * License along with this program; if not, write to the Free Software 00023 * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA. 00024 */ 00025 00026 /*! \file */ 00027 00028 #if !defined(_SPANDSP_V29RX_H_) 00029 #define _SPANDSP_V29RX_H_ 00030 00031 /*! \page v29rx_page The V.29 receiver 00032 \section v29rx_page_sec_1 What does it do? 00033 The V.29 receiver implements the receive side of a V.29 modem. This can operate 00034 at data rates of 9600, 7200 and 4800 bits/s. The audio input is a stream of 16 00035 bit samples, at 8000 samples/second. The transmit and receive side of V.29 00036 modems operate independantly. V.29 is mostly used for FAX transmission, where it 00037 provides the standard 9600 and 7200 bits/s rates (the 4800 bits/s mode is not 00038 used for FAX). 00039 00040 \section v29rx_page_sec_2 How does it work? 00041 V.29 operates at 2400 baud for all three bit rates. It uses 16-QAM modulation for 00042 9600bps, 8-QAM for 7200bps, and 4-PSK for 4800bps. A training sequence is specified 00043 at the start of transmission, which makes the design of a V.29 receiver relatively 00044 straightforward. 00045 00046 The first stage of the training sequence consists of 128 00047 symbols, alternating between two constellation positions. The receiver monitors 00048 the signal power, to sense the possible presence of a valid carrier. When the 00049 alternating signal begins, the power rising above a minimum threshold (-26dBm0) 00050 causes the main receiver computation to begin. The initial measured power is 00051 used to quickly set the gain of the receiver. After this initial settling, the 00052 front end gain is locked, and the adaptive equalizer tracks any subsequent 00053 signal level variation. The signal is oversampled to 24000 samples/second (i.e. 00054 signal, zero, zero, signal, zero, zero, ...) and fed to a complex root raised 00055 cosine pulse shaping filter. This filter has been modified from the conventional 00056 root raised cosine filter, by shifting it up the band, to be centred at the nominal 00057 carrier frequency. This filter interpolates the samples, pulse shapes, and performs 00058 a fractional sample delay at the same time. 48 sets of filter coefficients are used to 00059 achieve a set of finely spaces fractional sample delays, between zero and 00060 one sample. By choosing every fifth sample, and the appropriate set of filter 00061 coefficients, the properly tuned symbol tracker can select data samples at 4800 00062 samples/second from points within 1.125 degrees of the centre and mid-points of 00063 each symbol. The output of the filter is multiplied by a complex carrier, generated 00064 by a DDS. The result is a baseband signal, requiring no further filtering, apart from 00065 an adaptive equalizer. The baseband signal is fed to a T/2 adaptive equalizer. 00066 A band edge component maximisation algorithm is used to tune the sampling, so the samples 00067 fed to the equalizer are close to the mid point and edges of each symbol. Initially 00068 the algorithm is very lightly damped, to ensure the symbol alignment pulls in 00069 quickly. Because the sampling rate will not be precisely the same as the 00070 transmitter's (the spec. says the symbol timing should be within 0.01%), the 00071 receiver constantly evaluates and corrects this sampling throughout its 00072 operation. During the symbol timing maintainence phase, the algorithm uses 00073 a heavier damping. 00074 00075 The carrier is specified as 1700Hz +-1Hz at the transmitter, and 1700 +-7Hz at 00076 the receiver. The receive carrier would only be this inaccurate if the link 00077 includes FDM sections. These are being phased out, but the design must still 00078 allow for the worst case. Using an initial 1700Hz signal for demodulation gives 00079 a worst case rotation rate for the constellation of about one degree per symbol. 00080 Once the symbol timing synchronisation algorithm has been given time to lock to 00081 the symbol timing of the initial alternating pattern, the phase of the demodulated 00082 signal is recorded on two successive symbols - once for each of the constellation 00083 positions. The receiver then tracks the symbol alternations, until a large phase jump 00084 occurs. This signifies the start of the next phase of the training sequence. At this 00085 point the total phase shift between the original recorded symbol phase, and the 00086 symbol phase just before the phase jump occurred is used to provide a coarse 00087 estimation of the rotation rate of the constellation, and it current absolute 00088 angle of rotation. These are used to update the current carrier phase and phase 00089 update rate in the carrier DDS. The working data already in the pulse shaping 00090 filter and equalizer buffers is given a similar step rotation to pull it all 00091 into line. From this point on, a heavily damped integrate and dump approach, 00092 based on the angular difference between each received constellation position and 00093 its expected position, is sufficient to track the carrier, and maintain phase 00094 alignment. A fast rough approximator for the arc-tangent function is adequate 00095 for the estimation of the angular error. 00096 00097 The next phase of the training sequence is a scrambled sequence of two 00098 particular symbols. We train the T/2 adaptive equalizer using this sequence. The 00099 scrambling makes the signal sufficiently diverse to ensure the equalizer 00100 converges to the proper generalised solution. At the end of this sequence, the 00101 equalizer should be sufficiently well adapted that is can correctly resolve the 00102 full QAM constellation. However, the equalizer continues to adapt throughout 00103 operation of the modem, fine tuning on the more complex data patterns of the 00104 full QAM constellation. 00105 00106 In the last phase of the training sequence, the modem enters normal data 00107 operation, with a short defined period of all ones as data. As in most high 00108 speed modems, data in a V.29 modem passes through a scrambler, to whiten the 00109 spectrum of the signal. The transmitter should initialise its data scrambler, 00110 and pass the ones through it. At the end of the ones, real data begins to pass 00111 through the scrambler, and the transmit modem is in normal operation. The 00112 receiver tests that ones are really received, in order to verify the modem 00113 trained correctly. If all is well, the data following the ones is fed to the 00114 application, and the receive modem is up and running. Unfortunately, some 00115 transmit side of some real V.29 modems fail to initialise their scrambler before 00116 sending the ones. This means the first 23 received bits (the length of the 00117 scrambler register) cannot be trusted for the test. The receive modem, 00118 therefore, only tests that bits starting at bit 24 are really ones. 00119 */ 00120 00121 typedef void (*qam_report_handler_t)(void *user_data, const complexf_t *constel, const complexf_t *target, int symbol); 00122 00123 /*! 00124 V.29 modem receive side descriptor. This defines the working state for a 00125 single instance of a V.29 modem receiver. 00126 */ 00127 typedef struct v29_rx_state_s v29_rx_state_t; 00128 00129 #if defined(__cplusplus) 00130 extern "C" 00131 { 00132 #endif 00133 00134 /*! Initialise a V.29 modem receive context. 00135 \brief Initialise a V.29 modem receive context. 00136 \param s The modem context. 00137 \param bit_rate The bit rate of the modem. Valid values are 4800, 7200 and 9600. 00138 \param put_bit The callback routine used to put the received data. 00139 \param user_data An opaque pointer passed to the put_bit routine. 00140 \return A pointer to the modem context, or NULL if there was a problem. */ 00141 SPAN_DECLARE(v29_rx_state_t *) v29_rx_init(v29_rx_state_t *s, int bit_rate, put_bit_func_t put_bit, void *user_data); 00142 00143 /*! Reinitialise an existing V.29 modem receive context. 00144 \brief Reinitialise an existing V.29 modem receive context. 00145 \param s The modem context. 00146 \param bit_rate The bit rate of the modem. Valid values are 4800, 7200 and 9600. 00147 \param old_train TRUE if a previous trained values are to be reused. 00148 \return 0 for OK, -1 for bad parameter */ 00149 SPAN_DECLARE(int) v29_rx_restart(v29_rx_state_t *s, int bit_rate, int old_train); 00150 00151 /*! Release a V.29 modem receive context. 00152 \brief Release a V.29 modem receive context. 00153 \param s The modem context. 00154 \return 0 for OK */ 00155 SPAN_DECLARE(int) v29_rx_release(v29_rx_state_t *s); 00156 00157 /*! Free a V.29 modem receive context. 00158 \brief Free a V.29 modem receive context. 00159 \param s The modem context. 00160 \return 0 for OK */ 00161 SPAN_DECLARE(int) v29_rx_free(v29_rx_state_t *s); 00162 00163 /*! Get the logging context associated with a V.29 modem receive context. 00164 \brief Get the logging context associated with a V.29 modem receive context. 00165 \param s The modem context. 00166 \return A pointer to the logging context */ 00167 SPAN_DECLARE(logging_state_t *) v29_rx_get_logging_state(v29_rx_state_t *s); 00168 00169 /*! Change the put_bit function associated with a V.29 modem receive context. 00170 \brief Change the put_bit function associated with a V.29 modem receive context. 00171 \param s The modem context. 00172 \param put_bit The callback routine used to handle received bits. 00173 \param user_data An opaque pointer. */ 00174 SPAN_DECLARE(void) v29_rx_set_put_bit(v29_rx_state_t *s, put_bit_func_t put_bit, void *user_data); 00175 00176 /*! Change the modem status report function associated with a V.29 modem receive context. 00177 \brief Change the modem status report function associated with a V.29 modem receive context. 00178 \param s The modem context. 00179 \param handler The callback routine used to report modem status changes. 00180 \param user_data An opaque pointer. */ 00181 SPAN_DECLARE(void) v29_rx_set_modem_status_handler(v29_rx_state_t *s, modem_rx_status_func_t handler, void *user_data); 00182 00183 /*! Process a block of received V.29 modem audio samples. 00184 \brief Process a block of received V.29 modem audio samples. 00185 \param s The modem context. 00186 \param amp The audio sample buffer. 00187 \param len The number of samples in the buffer. 00188 \return The number of samples unprocessed. */ 00189 SPAN_DECLARE_NONSTD(int) v29_rx(v29_rx_state_t *s, const int16_t amp[], int len); 00190 00191 /*! Fake processing of a missing block of received V.29 modem audio samples. 00192 (e.g due to packet loss). 00193 \brief Fake processing of a missing block of received V.29 modem audio samples. 00194 \param s The modem context. 00195 \param len The number of samples to fake. 00196 \return The number of samples unprocessed. */ 00197 SPAN_DECLARE_NONSTD(int) v29_rx_fillin(v29_rx_state_t *s, int len); 00198 00199 /*! Get a snapshot of the current equalizer coefficients. 00200 \brief Get a snapshot of the current equalizer coefficients. 00201 \param s The modem context. 00202 \param coeffs The vector of complex coefficients. 00203 \return The number of coefficients in the vector. */ 00204 #if defined(SPANDSP_USE_FIXED_POINT) 00205 SPAN_DECLARE(int) v29_rx_equalizer_state(v29_rx_state_t *s, complexi16_t **coeffs); 00206 #else 00207 SPAN_DECLARE(int) v29_rx_equalizer_state(v29_rx_state_t *s, complexf_t **coeffs); 00208 #endif 00209 00210 /*! Get the current received carrier frequency. 00211 \param s The modem context. 00212 \return The frequency, in Hertz. */ 00213 SPAN_DECLARE(float) v29_rx_carrier_frequency(v29_rx_state_t *s); 00214 00215 /*! Get the current symbol timing correction since startup. 00216 \param s The modem context. 00217 \return The correction. */ 00218 SPAN_DECLARE(float) v29_rx_symbol_timing_correction(v29_rx_state_t *s); 00219 00220 /*! Get the current received signal power. 00221 \param s The modem context. 00222 \return The signal power, in dBm0. */ 00223 SPAN_DECLARE(float) v29_rx_signal_power(v29_rx_state_t *s); 00224 00225 /*! Set the power level at which the carrier detection will cut in 00226 \param s The modem context. 00227 \param cutoff The signal cutoff power, in dBm0. */ 00228 SPAN_DECLARE(void) v29_rx_signal_cutoff(v29_rx_state_t *s, float cutoff); 00229 00230 /*! Set a handler routine to process QAM status reports 00231 \param s The modem context. 00232 \param handler The handler routine. 00233 \param user_data An opaque pointer passed to the handler routine. */ 00234 SPAN_DECLARE(void) v29_rx_set_qam_report_handler(v29_rx_state_t *s, qam_report_handler_t handler, void *user_data); 00235 00236 #if defined(__cplusplus) 00237 } 00238 #endif 00239 00240 #endif 00241 /*- End of file ------------------------------------------------------------*/