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comparison spandsp-0.0.6pre17/src/spandsp/v27ter_tx.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 * 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 ------------------------------------------------------------*/

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