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comparison spandsp-0.0.6pre17/src/spandsp/v29rx.h @ 4:26cd8f1ef0b1

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import spandsp-0.0.6pre17
author Peter Meerwald <pmeerw@cosy.sbg.ac.at>
date Fri, 25 Jun 2010 15:50:58 +0200
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3:c6c5a16ce2f2 4:26cd8f1ef0b1
1 /*
2 * SpanDSP - a series of DSP components for telephony
3 *
4 * v29rx.h - ITU V.29 modem receive part
5 *
6 * Written by Steve Underwood <steveu@coppice.org>
7 *
8 * Copyright (C) 2003 Steve Underwood
9 *
10 * All rights reserved.
11 *
12 * This program is free software; you can redistribute it and/or modify
13 * it under the terms of the GNU Lesser General Public License version 2.1,
14 * as published by the Free Software Foundation.
15 *
16 * This program is distributed in the hope that it will be useful,
17 * but WITHOUT ANY WARRANTY; without even the implied warranty of
18 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
19 * GNU Lesser General Public License for more details.
20 *
21 * You should have received a copy of the GNU Lesser General Public
22 * License along with this program; if not, write to the Free Software
23 * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
24 *
25 * $Id: v29rx.h,v 1.72 2009/07/09 13:52:09 steveu Exp $
26 */
27
28 /*! \file */
29
30 #if !defined(_SPANDSP_V29RX_H_)
31 #define _SPANDSP_V29RX_H_
32
33 /*! \page v29rx_page The V.29 receiver
34 \section v29rx_page_sec_1 What does it do?
35 The V.29 receiver implements the receive side of a V.29 modem. This can operate
36 at data rates of 9600, 7200 and 4800 bits/s. The audio input is a stream of 16
37 bit samples, at 8000 samples/second. The transmit and receive side of V.29
38 modems operate independantly. V.29 is mostly used for FAX transmission, where it
39 provides the standard 9600 and 7200 bits/s rates (the 4800 bits/s mode is not
40 used for FAX).
41
42 \section v29rx_page_sec_2 How does it work?
43 V.29 operates at 2400 baud for all three bit rates. It uses 16-QAM modulation for
44 9600bps, 8-QAM for 7200bps, and 4-PSK for 4800bps. A training sequence is specified
45 at the start of transmission, which makes the design of a V.29 receiver relatively
46 straightforward.
47
48 The first stage of the training sequence consists of 128
49 symbols, alternating between two constellation positions. The receiver monitors
50 the signal power, to sense the possible presence of a valid carrier. When the
51 alternating signal begins, the power rising above a minimum threshold (-26dBm0)
52 causes the main receiver computation to begin. The initial measured power is
53 used to quickly set the gain of the receiver. After this initial settling, the
54 front end gain is locked, and the adaptive equalizer tracks any subsequent
55 signal level variation. The signal is oversampled to 24000 samples/second (i.e.
56 signal, zero, zero, signal, zero, zero, ...) and fed to a complex root raised
57 cosine pulse shaping filter. This filter has been modified from the conventional
58 root raised cosine filter, by shifting it up the band, to be centred at the nominal
59 carrier frequency. This filter interpolates the samples, pulse shapes, and performs
60 a fractional sample delay at the same time. 48 sets of filter coefficients are used to
61 achieve a set of finely spaces fractional sample delays, between zero and
62 one sample. By choosing every fifth sample, and the appropriate set of filter
63 coefficients, the properly tuned symbol tracker can select data samples at 4800
64 samples/second from points within 1.125 degrees of the centre and mid-points of
65 each symbol. The output of the filter is multiplied by a complex carrier, generated
66 by a DDS. The result is a baseband signal, requiring no further filtering, apart from
67 an adaptive equalizer. The baseband signal is fed to a T/2 adaptive equalizer.
68 A band edge component maximisation algorithm is used to tune the sampling, so the samples
69 fed to the equalizer are close to the mid point and edges of each symbol. Initially
70 the algorithm is very lightly damped, to ensure the symbol alignment pulls in
71 quickly. Because the sampling rate will not be precisely the same as the
72 transmitter's (the spec. says the symbol timing should be within 0.01%), the
73 receiver constantly evaluates and corrects this sampling throughout its
74 operation. During the symbol timing maintainence phase, the algorithm uses
75 a heavier damping.
76
77 The carrier is specified as 1700Hz +-1Hz at the transmitter, and 1700 +-7Hz at
78 the receiver. The receive carrier would only be this inaccurate if the link
79 includes FDM sections. These are being phased out, but the design must still
80 allow for the worst case. Using an initial 1700Hz signal for demodulation gives
81 a worst case rotation rate for the constellation of about one degree per symbol.
82 Once the symbol timing synchronisation algorithm has been given time to lock to
83 the symbol timing of the initial alternating pattern, the phase of the demodulated
84 signal is recorded on two successive symbols - once for each of the constellation
85 positions. The receiver then tracks the symbol alternations, until a large phase jump
86 occurs. This signifies the start of the next phase of the training sequence. At this
87 point the total phase shift between the original recorded symbol phase, and the
88 symbol phase just before the phase jump occurred is used to provide a coarse
89 estimation of the rotation rate of the constellation, and it current absolute
90 angle of rotation. These are used to update the current carrier phase and phase
91 update rate in the carrier DDS. The working data already in the pulse shaping
92 filter and equalizer buffers is given a similar step rotation to pull it all
93 into line. From this point on, a heavily damped integrate and dump approach,
94 based on the angular difference between each received constellation position and
95 its expected position, is sufficient to track the carrier, and maintain phase
96 alignment. A fast rough approximator for the arc-tangent function is adequate
97 for the estimation of the angular error.
98
99 The next phase of the training sequence is a scrambled sequence of two
100 particular symbols. We train the T/2 adaptive equalizer using this sequence. The
101 scrambling makes the signal sufficiently diverse to ensure the equalizer
102 converges to the proper generalised solution. At the end of this sequence, the
103 equalizer should be sufficiently well adapted that is can correctly resolve the
104 full QAM constellation. However, the equalizer continues to adapt throughout
105 operation of the modem, fine tuning on the more complex data patterns of the
106 full QAM constellation.
107
108 In the last phase of the training sequence, the modem enters normal data
109 operation, with a short defined period of all ones as data. As in most high
110 speed modems, data in a V.29 modem passes through a scrambler, to whiten the
111 spectrum of the signal. The transmitter should initialise its data scrambler,
112 and pass the ones through it. At the end of the ones, real data begins to pass
113 through the scrambler, and the transmit modem is in normal operation. The
114 receiver tests that ones are really received, in order to verify the modem
115 trained correctly. If all is well, the data following the ones is fed to the
116 application, and the receive modem is up and running. Unfortunately, some
117 transmit side of some real V.29 modems fail to initialise their scrambler before
118 sending the ones. This means the first 23 received bits (the length of the
119 scrambler register) cannot be trusted for the test. The receive modem,
120 therefore, only tests that bits starting at bit 24 are really ones.
121 */
122
123 typedef void (*qam_report_handler_t)(void *user_data, const complexf_t *constel, const complexf_t *target, int symbol);
124
125 /*!
126 V.29 modem receive side descriptor. This defines the working state for a
127 single instance of a V.29 modem receiver.
128 */
129 typedef struct v29_rx_state_s v29_rx_state_t;
130
131 #if defined(__cplusplus)
132 extern "C"
133 {
134 #endif
135
136 /*! Initialise a V.29 modem receive context.
137 \brief Initialise a V.29 modem receive context.
138 \param s The modem context.
139 \param bit_rate The bit rate of the modem. Valid values are 4800, 7200 and 9600.
140 \param put_bit The callback routine used to put the received data.
141 \param user_data An opaque pointer passed to the put_bit routine.
142 \return A pointer to the modem context, or NULL if there was a problem. */
143 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);
144
145 /*! Reinitialise an existing V.29 modem receive context.
146 \brief Reinitialise an existing V.29 modem receive context.
147 \param s The modem context.
148 \param bit_rate The bit rate of the modem. Valid values are 4800, 7200 and 9600.
149 \param old_train TRUE if a previous trained values are to be reused.
150 \return 0 for OK, -1 for bad parameter */
151 SPAN_DECLARE(int) v29_rx_restart(v29_rx_state_t *s, int bit_rate, int old_train);
152
153 /*! Release a V.29 modem receive context.
154 \brief Release a V.29 modem receive context.
155 \param s The modem context.
156 \return 0 for OK */
157 SPAN_DECLARE(int) v29_rx_release(v29_rx_state_t *s);
158
159 /*! Free a V.29 modem receive context.
160 \brief Free a V.29 modem receive context.
161 \param s The modem context.
162 \return 0 for OK */
163 SPAN_DECLARE(int) v29_rx_free(v29_rx_state_t *s);
164
165 /*! Get the logging context associated with a V.29 modem receive context.
166 \brief Get the logging context associated with a V.29 modem receive context.
167 \param s The modem context.
168 \return A pointer to the logging context */
169 SPAN_DECLARE(logging_state_t *) v29_rx_get_logging_state(v29_rx_state_t *s);
170
171 /*! Change the put_bit function associated with a V.29 modem receive context.
172 \brief Change the put_bit function associated with a V.29 modem receive context.
173 \param s The modem context.
174 \param put_bit The callback routine used to handle received bits.
175 \param user_data An opaque pointer. */
176 SPAN_DECLARE(void) v29_rx_set_put_bit(v29_rx_state_t *s, put_bit_func_t put_bit, void *user_data);
177
178 /*! Change the modem status report function associated with a V.29 modem receive context.
179 \brief Change the modem status report function associated with a V.29 modem receive context.
180 \param s The modem context.
181 \param handler The callback routine used to report modem status changes.
182 \param user_data An opaque pointer. */
183 SPAN_DECLARE(void) v29_rx_set_modem_status_handler(v29_rx_state_t *s, modem_rx_status_func_t handler, void *user_data);
184
185 /*! Process a block of received V.29 modem audio samples.
186 \brief Process a block of received V.29 modem audio samples.
187 \param s The modem context.
188 \param amp The audio sample buffer.
189 \param len The number of samples in the buffer.
190 \return The number of samples unprocessed. */
191 SPAN_DECLARE_NONSTD(int) v29_rx(v29_rx_state_t *s, const int16_t amp[], int len);
192
193 /*! Fake processing of a missing block of received V.29 modem audio samples.
194 (e.g due to packet loss).
195 \brief Fake processing of a missing block of received V.29 modem audio samples.
196 \param s The modem context.
197 \param len The number of samples to fake.
198 \return The number of samples unprocessed. */
199 SPAN_DECLARE(int) v29_rx_fillin(v29_rx_state_t *s, int len);
200
201 /*! Get a snapshot of the current equalizer coefficients.
202 \brief Get a snapshot of the current equalizer coefficients.
203 \param s The modem context.
204 \param coeffs The vector of complex coefficients.
205 \return The number of coefficients in the vector. */
206 #if defined(SPANDSP_USE_FIXED_POINT)
207 SPAN_DECLARE(int) v29_rx_equalizer_state(v29_rx_state_t *s, complexi16_t **coeffs);
208 #else
209 SPAN_DECLARE(int) v29_rx_equalizer_state(v29_rx_state_t *s, complexf_t **coeffs);
210 #endif
211
212 /*! Get the current received carrier frequency.
213 \param s The modem context.
214 \return The frequency, in Hertz. */
215 SPAN_DECLARE(float) v29_rx_carrier_frequency(v29_rx_state_t *s);
216
217 /*! Get the current symbol timing correction since startup.
218 \param s The modem context.
219 \return The correction. */
220 SPAN_DECLARE(float) v29_rx_symbol_timing_correction(v29_rx_state_t *s);
221
222 /*! Get the current received signal power.
223 \param s The modem context.
224 \return The signal power, in dBm0. */
225 SPAN_DECLARE(float) v29_rx_signal_power(v29_rx_state_t *s);
226
227 /*! Set the power level at which the carrier detection will cut in
228 \param s The modem context.
229 \param cutoff The signal cutoff power, in dBm0. */
230 SPAN_DECLARE(void) v29_rx_signal_cutoff(v29_rx_state_t *s, float cutoff);
231
232 /*! Set a handler routine to process QAM status reports
233 \param s The modem context.
234 \param handler The handler routine.
235 \param user_data An opaque pointer passed to the handler routine. */
236 SPAN_DECLARE(void) v29_rx_set_qam_report_handler(v29_rx_state_t *s, qam_report_handler_t handler, void *user_data);
237
238 #if defined(__cplusplus)
239 }
240 #endif
241
242 #endif
243 /*- End of file ------------------------------------------------------------*/

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