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comparison spandsp-0.0.6pre17/src/spandsp/v29tx.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 * 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 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: v29tx.h,v 1.41 2009/07/09 13:52:09 steveu Exp $
26 */
27
28 /*! \file */
29
30 #if !defined(_SPANDSP_V29TX_H_)
31 #define _SPANDSP_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 /*!
98 V.29 modem transmit side descriptor. This defines the working state for a
99 single instance of a V.29 modem transmitter.
100 */
101 typedef struct v29_tx_state_s v29_tx_state_t;
102
103 #if defined(__cplusplus)
104 extern "C"
105 {
106 #endif
107
108 /*! Adjust a V.29 modem transmit context's power output.
109 \brief Adjust a V.29 modem transmit context's output power.
110 \param s The modem context.
111 \param power The power level, in dBm0 */
112 SPAN_DECLARE(void) v29_tx_power(v29_tx_state_t *s, float power);
113
114 /*! Initialise a V.29 modem transmit context. This must be called before the first
115 use of the context, to initialise its contents.
116 \brief Initialise a V.29 modem transmit context.
117 \param s The modem context.
118 \param bit_rate The bit rate of the modem. Valid values are 4800, 7200 and 9600.
119 \param tep TRUE is the optional TEP tone is to be transmitted.
120 \param get_bit The callback routine used to get the data to be transmitted.
121 \param user_data An opaque pointer.
122 \return A pointer to the modem context, or NULL if there was a problem. */
123 SPAN_DECLARE(v29_tx_state_t *) v29_tx_init(v29_tx_state_t *s, int bit_rate, int tep, get_bit_func_t get_bit, void *user_data);
124
125 /*! Reinitialise an existing V.29 modem transmit context, so it may be reused.
126 \brief Reinitialise an existing V.29 modem transmit context.
127 \param s The modem context.
128 \param bit_rate The bit rate of the modem. Valid values are 4800, 7200 and 9600.
129 \param tep TRUE is the optional TEP tone is to be transmitted.
130 \return 0 for OK, -1 for bad parameter */
131 SPAN_DECLARE(int) v29_tx_restart(v29_tx_state_t *s, int bit_rate, int tep);
132
133 /*! Release a V.29 modem transmit context.
134 \brief Release a V.29 modem transmit context.
135 \param s The modem context.
136 \return 0 for OK */
137 SPAN_DECLARE(int) v29_tx_release(v29_tx_state_t *s);
138
139 /*! Free a V.29 modem transmit context.
140 \brief Free a V.29 modem transmit context.
141 \param s The modem context.
142 \return 0 for OK */
143 SPAN_DECLARE(int) v29_tx_free(v29_tx_state_t *s);
144
145 /*! Get the logging context associated with a V.29 modem transmit context.
146 \brief Get the logging context associated with a V.29 modem transmit context.
147 \param s The modem context.
148 \return A pointer to the logging context */
149 SPAN_DECLARE(logging_state_t *) v29_tx_get_logging_state(v29_tx_state_t *s);
150
151 /*! Change the get_bit function associated with a V.29 modem transmit context.
152 \brief Change the get_bit function associated with a V.29 modem transmit context.
153 \param s The modem context.
154 \param get_bit The callback routine used to get the data to be transmitted.
155 \param user_data An opaque pointer. */
156 SPAN_DECLARE(void) v29_tx_set_get_bit(v29_tx_state_t *s, get_bit_func_t get_bit, void *user_data);
157
158 /*! Change the modem status report function associated with a V.29 modem transmit context.
159 \brief Change the modem status report function associated with a V.29 modem transmit context.
160 \param s The modem context.
161 \param handler The callback routine used to report modem status changes.
162 \param user_data An opaque pointer. */
163 SPAN_DECLARE(void) v29_tx_set_modem_status_handler(v29_tx_state_t *s, modem_tx_status_func_t handler, void *user_data);
164
165 /*! Generate a block of V.29 modem audio samples.
166 \brief Generate a block of V.29 modem audio samples.
167 \param s The modem context.
168 \param amp The audio sample buffer.
169 \param len The number of samples to be generated.
170 \return The number of samples actually generated.
171 */
172 SPAN_DECLARE_NONSTD(int) v29_tx(v29_tx_state_t *s, int16_t amp[], int len);
173
174 #if defined(__cplusplus)
175 }
176 #endif
177
178 #endif
179 /*- End of file ------------------------------------------------------------*/

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