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comparison spandsp-0.0.6pre17/src/spandsp/v27ter_tx.h @ 4:26cd8f1ef0b1
import spandsp-0.0.6pre17
author | Peter Meerwald <pmeerw@cosy.sbg.ac.at> |
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date | Fri, 25 Jun 2010 15:50:58 +0200 |
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1 /* | |
2 * SpanDSP - a series of DSP components for telephony | |
3 * | |
4 * v27ter_tx.h - ITU V.27ter 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: v27ter_tx.h,v 1.43 2009/07/09 13:52:09 steveu Exp $ | |
26 */ | |
27 | |
28 /*! \file */ | |
29 | |
30 #if !defined(_SPANDSP_V27TER_TX_H_) | |
31 #define _SPANDSP_V27TER_TX_H_ | |
32 | |
33 /*! \page v27ter_tx_page The V.27ter transmitter | |
34 \section v27ter_tx_page_sec_1 What does it do? | |
35 The V.27ter transmitter implements the transmit side of a V.27ter modem. This | |
36 can operate at data rates of 4800 and 2400 bits/s. The audio output is a stream | |
37 of 16 bit samples, at 8000 samples/second. The transmit and receive side of | |
38 V.27ter modems operate independantly. V.27ter is used for FAX transmission, | |
39 where it provides the standard 4800 and 2400 bits/s rates. | |
40 | |
41 \section v27ter_tx_page_sec_2 How does it work? | |
42 V.27ter uses DPSK modulation. A common method of producing a DPSK modulated | |
43 signal is to use a sampling rate which is a multiple of the baud rate. The raw | |
44 signal is then a series of complex pulses, each an integer number of samples | |
45 long. These can be shaped, using a suitable complex filter, and multiplied by a | |
46 complex carrier signal to produce the final DPSK signal for transmission. | |
47 | |
48 The pulse shaping filter for V.27ter is defined in the spec. It is a root raised | |
49 cosine filter with 50% excess bandwidth. | |
50 | |
51 The sampling rate for our transmitter is defined by the channel - 8000 samples/s. | |
52 This is a multiple of the baud rate at 4800 bits/s (8-PSK at 1600 baud, 5 samples per | |
53 symbol), but not at 2400 bits/s (4-PSK at 1200 baud, 20/3 samples per symbol). The baud | |
54 interval is actually 20/3 sample periods at 2400bis/s. A symmetric FIR is used to | |
55 apply root raised cosine filtering in the 4800bits/s mode. In the 2400bits/s mode | |
56 a polyphase FIR filter is used. This consists of 20 sets of coefficients, offering | |
57 zero to 19/20ths of a baud phase shift as well as root raised cosine filtering. | |
58 The appropriate coefficient set is chosen for each signal sample generated. | |
59 | |
60 The carrier is generated using the DDS method. Using 2 second order resonators, | |
61 started in quadrature, might be more efficient, as it would have less impact on | |
62 the processor cache than a table lookup approach. However, the DDS approach | |
63 suits the receiver better, so then same signal generator is also used for the | |
64 transmitter. | |
65 */ | |
66 | |
67 /*! | |
68 V.27ter modem transmit side descriptor. This defines the working state for a | |
69 single instance of a V.27ter modem transmitter. | |
70 */ | |
71 typedef struct v27ter_tx_state_s v27ter_tx_state_t; | |
72 | |
73 #if defined(__cplusplus) | |
74 extern "C" | |
75 { | |
76 #endif | |
77 | |
78 /*! Adjust a V.27ter modem transmit context's power output. | |
79 \brief Adjust a V.27ter modem transmit context's output power. | |
80 \param s The modem context. | |
81 \param power The power level, in dBm0 */ | |
82 SPAN_DECLARE(void) v27ter_tx_power(v27ter_tx_state_t *s, float power); | |
83 | |
84 /*! Initialise a V.27ter modem transmit context. | |
85 \brief Initialise a V.27ter modem transmit context. | |
86 \param s The modem context. | |
87 \param bit_rate The bit rate of the modem. Valid values are 2400 and 4800. | |
88 \param tep TRUE is the optional TEP tone is to be transmitted. | |
89 \param get_bit The callback routine used to get the data to be transmitted. | |
90 \param user_data An opaque pointer. | |
91 \return A pointer to the modem context, or NULL if there was a problem. */ | |
92 SPAN_DECLARE(v27ter_tx_state_t *) v27ter_tx_init(v27ter_tx_state_t *s, int bit_rate, int tep, get_bit_func_t get_bit, void *user_data); | |
93 | |
94 /*! Reinitialise an existing V.27ter modem transmit context, so it may be reused. | |
95 \brief Reinitialise an existing V.27ter modem transmit context. | |
96 \param s The modem context. | |
97 \param bit_rate The bit rate of the modem. Valid values are 2400 and 4800. | |
98 \param tep TRUE is the optional TEP tone is to be transmitted. | |
99 \return 0 for OK, -1 for bad parameter */ | |
100 SPAN_DECLARE(int) v27ter_tx_restart(v27ter_tx_state_t *s, int bit_rate, int tep); | |
101 | |
102 /*! Release a V.27ter modem transmit context. | |
103 \brief Release a V.27ter modem transmit context. | |
104 \param s The modem context. | |
105 \return 0 for OK */ | |
106 SPAN_DECLARE(int) v27ter_tx_release(v27ter_tx_state_t *s); | |
107 | |
108 /*! Free a V.27ter modem transmit context. | |
109 \brief Free a V.27ter modem transmit context. | |
110 \param s The modem context. | |
111 \return 0 for OK */ | |
112 SPAN_DECLARE(int) v27ter_tx_free(v27ter_tx_state_t *s); | |
113 | |
114 /*! Get the logging context associated with a V.27ter modem transmit context. | |
115 \brief Get the logging context associated with a V.27ter modem transmit context. | |
116 \param s The modem context. | |
117 \return A pointer to the logging context */ | |
118 SPAN_DECLARE(logging_state_t *) v27ter_tx_get_logging_state(v27ter_tx_state_t *s); | |
119 | |
120 /*! Change the get_bit function associated with a V.27ter modem transmit context. | |
121 \brief Change the get_bit function associated with a V.27ter modem transmit context. | |
122 \param s The modem context. | |
123 \param get_bit The callback routine used to get the data to be transmitted. | |
124 \param user_data An opaque pointer. */ | |
125 SPAN_DECLARE(void) v27ter_tx_set_get_bit(v27ter_tx_state_t *s, get_bit_func_t get_bit, void *user_data); | |
126 | |
127 /*! Change the modem status report function associated with a V.27ter modem transmit context. | |
128 \brief Change the modem status report function associated with a V.27ter modem transmit context. | |
129 \param s The modem context. | |
130 \param handler The callback routine used to report modem status changes. | |
131 \param user_data An opaque pointer. */ | |
132 SPAN_DECLARE(void) v27ter_tx_set_modem_status_handler(v27ter_tx_state_t *s, modem_tx_status_func_t handler, void *user_data); | |
133 | |
134 /*! Generate a block of V.27ter modem audio samples. | |
135 \brief Generate a block of V.27ter modem audio samples. | |
136 \param s The modem context. | |
137 \param amp The audio sample buffer. | |
138 \param len The number of samples to be generated. | |
139 \return The number of samples actually generated. | |
140 */ | |
141 SPAN_DECLARE_NONSTD(int) v27ter_tx(v27ter_tx_state_t *s, int16_t amp[], int len); | |
142 | |
143 #if defined(__cplusplus) | |
144 } | |
145 #endif | |
146 | |
147 #endif | |
148 /*- End of file ------------------------------------------------------------*/ |