spandsp
0.0.6
Main Page
Related Pages
Classes
Files
File List
File Members
v29rx.h
Go to the documentation of this file.
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
26
/*! \file */
27
28
#if !defined(_SPANDSP_V29RX_H_)
29
#define _SPANDSP_V29RX_H_
30
31
/*! \page v29rx_page The V.29 receiver
32
\section v29rx_page_sec_1 What does it do?
33
The V.29 receiver implements the receive side of a V.29 modem. This can operate
34
at data rates of 9600, 7200 and 4800 bits/s. The audio input is a stream of 16
35
bit samples, at 8000 samples/second. The transmit and receive side of V.29
36
modems operate independantly. V.29 is mostly used for FAX transmission, where it
37
provides the standard 9600 and 7200 bits/s rates (the 4800 bits/s mode is not
38
used for FAX).
39
40
\section v29rx_page_sec_2 How does it work?
41
V.29 operates at 2400 baud for all three bit rates. It uses 16-QAM modulation for
42
9600bps, 8-QAM for 7200bps, and 4-PSK for 4800bps. A training sequence is specified
43
at the start of transmission, which makes the design of a V.29 receiver relatively
44
straightforward.
45
46
The first stage of the training sequence consists of 128
47
symbols, alternating between two constellation positions. The receiver monitors
48
the signal power, to sense the possible presence of a valid carrier. When the
49
alternating signal begins, the power rising above a minimum threshold (-26dBm0)
50
causes the main receiver computation to begin. The initial measured power is
51
used to quickly set the gain of the receiver. After this initial settling, the
52
front end gain is locked, and the adaptive equalizer tracks any subsequent
53
signal level variation. The signal is oversampled to 24000 samples/second (i.e.
54
signal, zero, zero, signal, zero, zero, ...) and fed to a complex root raised
55
cosine pulse shaping filter. This filter has been modified from the conventional
56
root raised cosine filter, by shifting it up the band, to be centred at the nominal
57
carrier frequency. This filter interpolates the samples, pulse shapes, and performs
58
a fractional sample delay at the same time. 48 sets of filter coefficients are used to
59
achieve a set of finely spaces fractional sample delays, between zero and
60
one sample. By choosing every fifth sample, and the appropriate set of filter
61
coefficients, the properly tuned symbol tracker can select data samples at 4800
62
samples/second from points within 1.125 degrees of the centre and mid-points of
63
each symbol. The output of the filter is multiplied by a complex carrier, generated
64
by a DDS. The result is a baseband signal, requiring no further filtering, apart from
65
an adaptive equalizer. The baseband signal is fed to a T/2 adaptive equalizer.
66
A band edge component maximisation algorithm is used to tune the sampling, so the samples
67
fed to the equalizer are close to the mid point and edges of each symbol. Initially
68
the algorithm is very lightly damped, to ensure the symbol alignment pulls in
69
quickly. Because the sampling rate will not be precisely the same as the
70
transmitter's (the spec. says the symbol timing should be within 0.01%), the
71
receiver constantly evaluates and corrects this sampling throughout its
72
operation. During the symbol timing maintainence phase, the algorithm uses
73
a heavier damping.
74
75
The carrier is specified as 1700Hz +-1Hz at the transmitter, and 1700 +-7Hz at
76
the receiver. The receive carrier would only be this inaccurate if the link
77
includes FDM sections. These are being phased out, but the design must still
78
allow for the worst case. Using an initial 1700Hz signal for demodulation gives
79
a worst case rotation rate for the constellation of about one degree per symbol.
80
Once the symbol timing synchronisation algorithm has been given time to lock to
81
the symbol timing of the initial alternating pattern, the phase of the demodulated
82
signal is recorded on two successive symbols - once for each of the constellation
83
positions. The receiver then tracks the symbol alternations, until a large phase jump
84
occurs. This signifies the start of the next phase of the training sequence. At this
85
point the total phase shift between the original recorded symbol phase, and the
86
symbol phase just before the phase jump occurred is used to provide a coarse
87
estimation of the rotation rate of the constellation, and it current absolute
88
angle of rotation. These are used to update the current carrier phase and phase
89
update rate in the carrier DDS. The working data already in the pulse shaping
90
filter and equalizer buffers is given a similar step rotation to pull it all
91
into line. From this point on, a heavily damped integrate and dump approach,
92
based on the angular difference between each received constellation position and
93
its expected position, is sufficient to track the carrier, and maintain phase
94
alignment. A fast rough approximator for the arc-tangent function is adequate
95
for the estimation of the angular error.
96
97
The next phase of the training sequence is a scrambled sequence of two
98
particular symbols. We train the T/2 adaptive equalizer using this sequence. The
99
scrambling makes the signal sufficiently diverse to ensure the equalizer
100
converges to the proper generalised solution. At the end of this sequence, the
101
equalizer should be sufficiently well adapted that is can correctly resolve the
102
full QAM constellation. However, the equalizer continues to adapt throughout
103
operation of the modem, fine tuning on the more complex data patterns of the
104
full QAM constellation.
105
106
In the last phase of the training sequence, the modem enters normal data
107
operation, with a short defined period of all ones as data. As in most high
108
speed modems, data in a V.29 modem passes through a scrambler, to whiten the
109
spectrum of the signal. The transmitter should initialise its data scrambler,
110
and pass the ones through it. At the end of the ones, real data begins to pass
111
through the scrambler, and the transmit modem is in normal operation. The
112
receiver tests that ones are really received, in order to verify the modem
113
trained correctly. If all is well, the data following the ones is fed to the
114
application, and the receive modem is up and running. Unfortunately, some
115
transmit side of some real V.29 modems fail to initialise their scrambler before
116
sending the ones. This means the first 23 received bits (the length of the
117
scrambler register) cannot be trusted for the test. The receive modem,
118
therefore, only tests that bits starting at bit 24 are really ones.
119
*/
120
121
typedef
void (*qam_report_handler_t)(
void
*user_data,
const
complexf_t
*constel,
const
complexf_t
*target,
int
symbol);
122
123
/*!
124
V.29 modem receive side descriptor. This defines the working state for a
125
single instance of a V.29 modem receiver.
126
*/
127
typedef
struct
v29_rx_state_s
v29_rx_state_t
;
128
129
#if defined(__cplusplus)
130
extern
"C"
131
{
132
#endif
133
134
/*! Initialise a V.29 modem receive context.
135
\brief Initialise a V.29 modem receive context.
136
\param s The modem context.
137
\param bit_rate The bit rate of the modem. Valid values are 4800, 7200 and 9600.
138
\param put_bit The callback routine used to put the received data.
139
\param user_data An opaque pointer passed to the put_bit routine.
140
\return A pointer to the modem context, or NULL if there was a problem. */
141
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);
142
143
/*! Reinitialise an existing V.29 modem receive context.
144
\brief Reinitialise an existing V.29 modem receive context.
145
\param s The modem context.
146
\param bit_rate The bit rate of the modem. Valid values are 4800, 7200 and 9600.
147
\param old_train TRUE if a previous trained values are to be reused.
148
\return 0 for OK, -1 for bad parameter */
149
SPAN_DECLARE(
int
)
v29_rx_restart
(
v29_rx_state_t
*s,
int
bit_rate,
int
old_train
);
150
151
/*! Release a V.29 modem receive context.
152
\brief Release a V.29 modem receive context.
153
\param s The modem context.
154
\return 0 for OK */
155
SPAN_DECLARE(
int
)
v29_rx_release
(
v29_rx_state_t
*s);
156
157
/*! Free a V.29 modem receive context.
158
\brief Free a V.29 modem receive context.
159
\param s The modem context.
160
\return 0 for OK */
161
SPAN_DECLARE(
int
)
v29_rx_free
(
v29_rx_state_t
*s);
162
163
/*! Get the logging context associated with a V.29 modem receive context.
164
\brief Get the logging context associated with a V.29 modem receive context.
165
\param s The modem context.
166
\return A pointer to the logging context */
167
SPAN_DECLARE(
logging_state_t
*)
v29_rx_get_logging_state
(
v29_rx_state_t
*s);
168
169
/*! Change the put_bit function associated with a V.29 modem receive context.
170
\brief Change the put_bit function associated with a V.29 modem receive context.
171
\param s The modem context.
172
\param put_bit The callback routine used to handle received bits.
173
\param user_data An opaque pointer. */
174
SPAN_DECLARE(
void
)
v29_rx_set_put_bit
(
v29_rx_state_t
*s,
put_bit_func_t
put_bit,
void
*user_data);
175
176
/*! Change the modem status report function associated with a V.29 modem receive context.
177
\brief Change the modem status report function associated with a V.29 modem receive context.
178
\param s The modem context.
179
\param handler The callback routine used to report modem status changes.
180
\param user_data An opaque pointer. */
181
SPAN_DECLARE(
void
)
v29_rx_set_modem_status_handler
(
v29_rx_state_t
*s,
modem_rx_status_func_t
handler,
void
*user_data);
182
183
/*! Process a block of received V.29 modem audio samples.
184
\brief Process a block of received V.29 modem audio samples.
185
\param s The modem context.
186
\param amp The audio sample buffer.
187
\param len The number of samples in the buffer.
188
\return The number of samples unprocessed. */
189
SPAN_DECLARE_NONSTD
(
int
) v29_rx(
v29_rx_state_t
*s, const int16_t amp[],
int
len);
190
191
/*! Fake processing of a missing block of received V.29 modem audio samples.
192
(e.g due to packet loss).
193
\brief Fake processing of a missing block of received V.29 modem audio samples.
194
\param s The modem context.
195
\param len The number of samples to fake.
196
\return The number of samples unprocessed. */
197
SPAN_DECLARE_NONSTD
(
int
) v29_rx_fillin(
v29_rx_state_t
*s,
int
len);
198
199
/*! Get a snapshot of the current equalizer coefficients.
200
\brief Get a snapshot of the current equalizer coefficients.
201
\param s The modem context.
202
\param coeffs The vector of complex coefficients.
203
\return The number of coefficients in the vector. */
204
#if defined(SPANDSP_USE_FIXED_POINT)
205
SPAN_DECLARE(
int
)
v29_rx_equalizer_state
(
v29_rx_state_t
*s,
complexi16_t
**coeffs);
206
#else
207
SPAN_DECLARE(
int
)
v29_rx_equalizer_state
(
v29_rx_state_t
*s,
complexf_t
**coeffs);
208
#endif
209
210
/*! Get the current received carrier frequency.
211
\param s The modem context.
212
\return The frequency, in Hertz. */
213
SPAN_DECLARE(
float
)
v29_rx_carrier_frequency
(
v29_rx_state_t
*s);
214
215
/*! Get the current symbol timing correction since startup.
216
\param s The modem context.
217
\return The correction. */
218
SPAN_DECLARE(
float
)
v29_rx_symbol_timing_correction
(
v29_rx_state_t
*s);
219
220
/*! Get the current received signal power.
221
\param s The modem context.
222
\return The signal power, in dBm0. */
223
SPAN_DECLARE(
float
)
v29_rx_signal_power
(
v29_rx_state_t
*s);
224
225
/*! Set the power level at which the carrier detection will cut in
226
\param s The modem context.
227
\param cutoff The signal cutoff power, in dBm0. */
228
SPAN_DECLARE(
void
)
v29_rx_signal_cutoff
(
v29_rx_state_t
*s,
float
cutoff);
229
230
/*! Set a handler routine to process QAM status reports
231
\param s The modem context.
232
\param handler The handler routine.
233
\param user_data An opaque pointer passed to the handler routine. */
234
SPAN_DECLARE(
void
)
v29_rx_set_qam_report_handler
(
v29_rx_state_t
*s, qam_report_handler_t handler,
void
*user_data);
235
236
#if defined(__cplusplus)
237
}
238
#endif
239
240
#endif
241
/*- End of file ------------------------------------------------------------*/
src
spandsp
v29rx.h
Generated on Wed Oct 16 2013 09:59:54 for spandsp by
1.8.4