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comparison spandsp-0.0.3/spandsp-0.0.3/src/spandsp/v29tx.h @ 5:f762bf195c4b

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import spandsp-0.0.3
author Peter Meerwald <pmeerw@cosy.sbg.ac.at>
date Fri, 25 Jun 2010 16:00:21 +0200
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4:26cd8f1ef0b1 5:f762bf195c4b
1 /*
2 * SpanDSP - a series of DSP components for telephony
3 *
4 * v29tx.h - ITU V.29 modem transmit 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 General Public License version 2, as
14 * 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 General Public License for more details.
20 *
21 * You should have received a copy of the GNU General Public License
22 * along with this program; if not, write to the Free Software
23 * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
24 *
25 * $Id: v29tx.h,v 1.23 2006/10/24 13:22:02 steveu Exp $
26 */
27
28 /*! \file */
29
30 #if !defined(_V29TX_H_)
31 #define _V29TX_H_
32
33 /*! \page v29tx_page The V.29 transmitter
34 \section v29tx_page_sec_1 What does it do?
35 The V.29 transmitter implements the transmit side of a V.29 modem. This can
36 operate at data rates of 9600, 7200 and 4800 bits/s. The audio output is a
37 stream of 16 bit samples, at 8000 samples/second. The transmit and receive side
38 of V.29 modems operate independantly. V.29 is mostly used for FAX transmission,
39 where it provides the standard 9600 and 7200 bits/s rates (the 4800 bits/s mode
40 is not used for FAX).
41
42 \section v29tx_page_sec_2 How does it work?
43 V.29 uses QAM modulation. The standard method of producing a QAM modulated
44 signal is to use a sampling rate which is a multiple of the baud rate. The raw
45 signal is then a series of complex pulses, each an integer number of samples
46 long. These can be shaped, using a suitable complex filter, and multiplied by a
47 complex carrier signal to produce the final QAM signal for transmission.
48
49 The pulse shaping filter is only vaguely defined by the V.29 spec. Some of the
50 other ITU modem specs. fully define the filter, typically specifying a root
51 raised cosine filter, with 50% excess bandwidth. This is a pity, since it
52 increases the variability of the received signal. However, the receiver's
53 adaptive equalizer will compensate for these differences. The current
54 design uses a root raised cosine filter with 25% excess bandwidth. Greater
55 excess bandwidth will not allow the tranmitted signal to meet the spectral
56 requirements.
57
58 The sampling rate for our transmitter is defined by the channel - 8000 per
59 second. This is not a multiple of the baud rate (i.e. 2400 baud). The baud
60 interval is actually 10/3 sample periods. Instead of using a symmetric
61 FIR to pulse shape the signal, a polyphase filter is used. This consists of
62 10 sets of coefficients, offering zero to 9/10ths of a baud phase shift as well
63 as root raised cosine filtering. The appropriate coefficient set is chosen for
64 each signal sample generated.
65
66 The carrier is generated using the DDS method. Using two second order resonators,
67 started in quadrature, might be more efficient, as it would have less impact on
68 the processor cache than a table lookup approach. However, the DDS approach
69 suits the receiver better, so the same signal generator is also used for the
70 transmitter.
71
72 The equation defining QAM modulation is:
73
74 s(n) = A*cos(2*pi*f*n + phi(n))
75
76 where phi(n) is the phase of the information, and A is the amplitude of the information
77
78 using the identity
79
80 cos(x + y) = cos(x)*cos(y) - sin(x)*sin(y)
81
82 we get
83
84 s(n) = A {cos(2*pi*f*n)*cos(phi(n)) - sin(2*pi*f*n)*sin(phi(n))}
85
86 substituting with the constellation positions
87
88 I(n) = A*cos(phi(n))
89 Q(n) = A*sin(phi(n))
90
91 gives
92
93 s(n) = I(n)*cos(2*pi*f*n) - Q(n)*sin(2*pi*f*n)
94
95 */
96
97 #define V29_TX_FILTER_STEPS 9
98
99 /*!
100 V.29 modem transmit side descriptor. This defines the working state for a
101 single instance of a V.29 modem transmitter.
102 */
103 typedef struct
104 {
105 /*! \brief The bit rate of the modem. Valid values are 4800, 7200 and 9600. */
106 int bit_rate;
107 /*! \brief The callback function used to get the next bit to be transmitted. */
108 get_bit_func_t get_bit;
109 /*! \brief A user specified opaque pointer passed to the callback function. */
110 void *user_data;
111
112 /*! \brief Gain required to achieve the specified output power, not allowing
113 for the size of the current constellation. */
114 float base_gain;
115 /*! \brief Gain required to achieve the specified output power, allowing
116 for the size of the current constellation. */
117 float gain;
118
119 /*! \brief The route raised cosine (RRC) pulse shaping filter buffer. */
120 complexf_t rrc_filter[2*V29_TX_FILTER_STEPS];
121 /*! \brief Current offset into the RRC pulse shaping filter buffer. */
122 int rrc_filter_step;
123
124 /*! \brief The register for the data scrambler. */
125 unsigned int scramble_reg;
126 /*! \brief The register for the training scrambler. */
127 uint8_t training_scramble_reg;
128 /*! \brief TRUE if transmitting the training sequence, or shutting down transmission.
129 FALSE if transmitting user data. */
130 int in_training;
131 /*! \brief A counter used to track progress through sending the training sequence. */
132 int training_step;
133 /*! \brief An offset value into the table of training parameters, used to match the
134 training pattern to the bit rate. */
135 int training_offset;
136
137 /*! \brief The current phase of the carrier (i.e. the DDS parameter). */
138 uint32_t carrier_phase;
139 /*! \brief The update rate for the phase of the carrier (i.e. the DDS increment). */
140 int32_t carrier_phase_rate;
141 /*! \brief The current fractional phase of the baud timing. */
142 int baud_phase;
143 /*! \brief The code number for the current position in the constellation. */
144 int constellation_state;
145 /*! \brief The get_bit function in use at any instant. */
146 get_bit_func_t current_get_bit;
147 /*! \brief Error and flow logging control */
148 logging_state_t logging;
149 } v29_tx_state_t;
150
151 #ifdef __cplusplus
152 extern "C" {
153 #endif
154
155 /*! Adjust a V.29 modem transmit context's power output.
156 \brief Adjust a V.29 modem transmit context's output power.
157 \param s The modem context.
158 \param power The power level, in dBm0 */
159 void v29_tx_power(v29_tx_state_t *s, float power);
160
161 /*! Initialise a V.29 modem transmit context. This must be called before the first
162 use of the context, to initialise its contents.
163 \brief Initialise a V.29 modem transmit context.
164 \param s The modem context.
165 \param rate The bit rate of the modem. Valid values are 4800, 7200 and 9600.
166 \param tep TRUE is the optional TEP tone is to be transmitted.
167 \param get_bit The callback routine used to get the data to be transmitted.
168 \param user_data An opaque pointer.
169 \return A pointer to the modem context, or NULL if there was a problem. */
170 v29_tx_state_t *v29_tx_init(v29_tx_state_t *s, int rate, int tep, get_bit_func_t get_bit, void *user_data);
171
172 /*! Reinitialise an existing V.29 modem transmit context, so it may be reused.
173 \brief Reinitialise an existing V.29 modem transmit context.
174 \param s The modem context.
175 \param rate The bit rate of the modem. Valid values are 4800, 7200 and 9600.
176 \param tep TRUE is the optional TEP tone is to be transmitted.
177 \return 0 for OK, -1 for bad parameter */
178 int v29_tx_restart(v29_tx_state_t *s, int rate, int tep);
179
180 /*! Release a V.29 modem transmit context.
181 \brief Release a V.29 modem transmit context.
182 \param s The modem context.
183 \return 0 for OK */
184 int v29_tx_release(v29_tx_state_t *s);
185
186 /*! Change the get_bit function associated with a V.29 modem transmit context.
187 \brief Change the get_bit function associated with a V.29 modem transmit context.
188 \param s The modem context.
189 \param get_bit The callback routine used to get the data to be transmitted.
190 \param user_data An opaque pointer. */
191 void v29_tx_set_get_bit(v29_tx_state_t *s, get_bit_func_t get_bit, void *user_data);
192
193 /*! Generate a block of V.29 modem audio samples.
194 \brief Generate a block of V.29 modem audio samples.
195 \param s The modem context.
196 \param amp The audio sample buffer.
197 \param len The number of samples to be generated.
198 \return The number of samples actually generated.
199 */
200 int v29_tx(v29_tx_state_t *s, int16_t *amp, int len);
201
202 #ifdef __cplusplus
203 }
204 #endif
205
206 #endif
207 /*- End of file ------------------------------------------------------------*/

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