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comparison spandsp-0.0.6pre17/src/spandsp/t38_non_ecm_buffer.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 * t38_non_ecm_buffer.h - A rate adapting buffer for T.38 non-ECM image
5 * and TCF data
6 *
7 * Written by Steve Underwood <steveu@coppice.org>
8 *
9 * Copyright (C) 2005, 2006, 2007, 2008 Steve Underwood
10 *
11 * All rights reserved.
12 *
13 * This program is free software; you can redistribute it and/or modify
14 * it under the terms of the GNU Lesser General Public License version 2.1,
15 * as published by the Free Software Foundation.
16 *
17 * This program is distributed in the hope that it will be useful,
18 * but WITHOUT ANY WARRANTY; without even the implied warranty of
19 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
20 * GNU Lesser General Public License for more details.
21 *
22 * You should have received a copy of the GNU Lesser General Public
23 * License along with this program; if not, write to the Free Software
24 * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
25 *
26 * $Id: t38_non_ecm_buffer.h,v 1.7.4.1 2009/12/19 06:43:28 steveu Exp $
27 */
28
29 /*! \file */
30
31 #if !defined(_SPANDSP_T38_NON_ECM_BUFFER_H_)
32 #define _SPANDSP_T38_NON_ECM_BUFFER_H_
33
34 /*! \page t38_non_ecm_buffer_page T.38 rate adapting non-ECM image data buffer
35 \section t38_non_ecm_buffer_page_sec_1 What does it do?
36
37 The T.38 rate adapting non-ECM image data buffer is used to buffer TCF and non-ECM
38 FAX image data being gatewayed from a T.38 link to an analogue FAX modem link.
39
40 As well as rate adapting, the buffer has the ability to impose a minimum on the number
41 of bits per row of image data. This allows any row padding zeros to be stripped from
42 the data stream, to minimise the data sent as T.38 packets, and be reinserted before
43 the data is sent to its final destination. Not all T.38 implementations support this
44 feature, so it's use must be negotiated.
45
46 \section t38_non_ecm_buffer_page_sec_2 How does it work?
47
48 When inserting padding bits, whether to ensure a minimum row time or for flow control,
49 it is important the right value is inserted at the right point in the data sequence.
50 If we are in the optional initial period of all ones, we can insert a byte of extra
51 ones at any time. Once we pass that initial stage, TCF and image data need separate
52 handling.
53
54 TCF data is all zeros. Once the period of all zeros has begun it is OK to insert
55 additional bytes of zeros at any point.
56
57 Image data consists of rows, separated by EOL (end of line) markers. Inserting
58 zeros at arbitrary times would corrupt the image. However, it is OK to insert a
59 considerable number of extra zeros just before an EOL. Therefore we track where the
60 EOL markers occur as we fill the buffer. As we empty the buffer stop outputting real
61 data, and start outputting bytes of zero, if we reach this last EOL marker location.
62 The EOL marker is 11 zeros following by 1 (1D mode) or 2 (2D mode) ones. Therefore, it
63 always spills across 2 bytes in the buffer, and there is always a point where we can
64 insert our extra zeros between bytes.
65
66 Padding between the group of successive EOL markers which for the RTC (return to control)
67 marker that ends an image causes some FAX machines not to recognise them as an RTC condition.
68 Therefore, our padding applies special protection so padding never occurs between two
69 successive EOL markers, with no pixel data between them.
70 */
71
72 /*! The buffer length much be a power of two. The chosen length is big enough for
73 over 9s of data at the V.17 14,400bps rate. */
74 #define T38_NON_ECM_TX_BUF_LEN 16384
75
76 /*! \brief A flow controlled non-ECM image data buffer, for buffering T.38 to analogue
77 modem data.
78 */
79 typedef struct t38_non_ecm_buffer_state_s t38_non_ecm_buffer_state_t;
80
81 #if defined(__cplusplus)
82 extern "C"
83 {
84 #endif
85
86 /*! \brief Initialise a T.38 rate adapting non-ECM buffer context.
87 \param s The buffer context.
88 \param mode TRUE for image data mode, or FALSE for TCF mode.
89 \param bits The minimum number of bits per FAX image row.
90 \return A pointer to the buffer context, or NULL if there was a problem. */
91 SPAN_DECLARE(t38_non_ecm_buffer_state_t *) t38_non_ecm_buffer_init(t38_non_ecm_buffer_state_t *s, int mode, int min_row_bits);
92
93 SPAN_DECLARE(int) t38_non_ecm_buffer_release(t38_non_ecm_buffer_state_t *s);
94
95 SPAN_DECLARE(int) t38_non_ecm_buffer_free(t38_non_ecm_buffer_state_t *s);
96
97 /*! \brief Set the mode of a T.38 rate adapting non-ECM buffer context.
98 \param s The buffer context.
99 \param mode TRUE for image data mode, or FALSE for TCF mode.
100 \param bits The minimum number of bits per FAX image row. */
101 SPAN_DECLARE(void) t38_non_ecm_buffer_set_mode(t38_non_ecm_buffer_state_t *s, int mode, int min_row_bits);
102
103 /*! \brief Inject data to T.38 rate adapting non-ECM buffer context.
104 \param s The buffer context.
105 \param buf The data buffer to be injected.
106 \param len The length of the data to be injected. */
107 SPAN_DECLARE(void) t38_non_ecm_buffer_inject(t38_non_ecm_buffer_state_t *s, const uint8_t *buf, int len);
108
109 /*! \brief Inform a T.38 rate adapting non-ECM buffer context that the incoming data has finished,
110 and the contents of the buffer should be played out as quickly as possible.
111 \param s The buffer context. */
112 SPAN_DECLARE(void) t38_non_ecm_buffer_push(t38_non_ecm_buffer_state_t *s);
113
114 /*! \brief Report the input status of a T.38 rate adapting non-ECM buffer context to the specified
115 logging context.
116 \param s The buffer context.
117 \param logging The logging context. */
118 SPAN_DECLARE(void) t38_non_ecm_buffer_report_input_status(t38_non_ecm_buffer_state_t *s, logging_state_t *logging);
119
120 /*! \brief Report the output status of a T.38 rate adapting non-ECM buffer context to the specified
121 logging context.
122 \param s The buffer context.
123 \param logging The logging context. */
124 SPAN_DECLARE(void) t38_non_ecm_buffer_report_output_status(t38_non_ecm_buffer_state_t *s, logging_state_t *logging);
125
126 /*! \brief Get the next bit of data from a T.38 rate adapting non-ECM buffer context.
127 \param user_data The buffer context, cast to a void pointer.
128 \return The next bit, or one of the values indicating a change of modem status. */
129 SPAN_DECLARE_NONSTD(int) t38_non_ecm_buffer_get_bit(void *user_data);
130
131 #if defined(__cplusplus)
132 }
133 #endif
134
135 #endif
136 /*- End of file ------------------------------------------------------------*/

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