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t38_non_ecm_buffer.h
Go to the documentation of this file.
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/*
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* SpanDSP - a series of DSP components for telephony
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*
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* t38_non_ecm_buffer.h - A rate adapting buffer for T.38 non-ECM image
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* and TCF data
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*
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* Written by Steve Underwood <steveu@coppice.org>
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*
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* Copyright (C) 2005, 2006, 2007, 2008 Steve Underwood
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*
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* All rights reserved.
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU Lesser General Public License version 2.1,
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* as published by the Free Software Foundation.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU Lesser General Public License for more details.
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*
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* You should have received a copy of the GNU Lesser General Public
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* License along with this program; if not, write to the Free Software
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* Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
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*/
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/*! \file */
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#if !defined(_SPANDSP_T38_NON_ECM_BUFFER_H_)
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#define _SPANDSP_T38_NON_ECM_BUFFER_H_
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/*! \page t38_non_ecm_buffer_page T.38 rate adapting non-ECM image data buffer
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\section t38_non_ecm_buffer_page_sec_1 What does it do?
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The T.38 rate adapting non-ECM image data buffer is used to buffer TCF and non-ECM
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FAX image data being gatewayed from a T.38 link to an analogue FAX modem link.
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As well as rate adapting, the buffer has the ability to impose a minimum on the number
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of bits per row of image data. This allows any row padding zeros to be stripped from
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the data stream, to minimise the data sent as T.38 packets, and be reinserted before
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the data is sent to its final destination. Not all T.38 implementations support this
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feature, so it's use must be negotiated.
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\section t38_non_ecm_buffer_page_sec_2 How does it work?
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When inserting padding bits, whether to ensure a minimum row time or for flow control,
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it is important the right value is inserted at the right point in the data sequence.
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If we are in the optional initial period of all ones, we can insert a byte of extra
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ones at any time. Once we pass that initial stage, TCF and image data need separate
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handling.
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TCF data is all zeros. Once the period of all zeros has begun it is OK to insert
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additional bytes of zeros at any point.
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Image data consists of rows, separated by EOL (end of line) markers. Inserting
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zeros at arbitrary times would corrupt the image. However, it is OK to insert a
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considerable number of extra zeros just before an EOL. Therefore we track where the
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EOL markers occur as we fill the buffer. As we empty the buffer stop outputting real
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data, and start outputting bytes of zero, if we reach this last EOL marker location.
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The EOL marker is 11 zeros following by 1 (1D mode) or 2 (2D mode) ones. Therefore, it
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always spills across 2 bytes in the buffer, and there is always a point where we can
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insert our extra zeros between bytes.
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Padding between the group of successive EOL markers which for the RTC (return to control)
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marker that ends an image causes some FAX machines not to recognise them as an RTC condition.
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Therefore, our padding applies special protection so padding never occurs between two
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successive EOL markers, with no pixel data between them.
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*/
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/*! The buffer length much be a power of two. The chosen length is big enough for
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over 9s of data at the V.17 14,400bps rate. */
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#define T38_NON_ECM_TX_BUF_LEN 16384
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/*! \brief A flow controlled non-ECM image data buffer, for buffering T.38 to analogue
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modem data.
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*/
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typedef
struct
t38_non_ecm_buffer_state_s
t38_non_ecm_buffer_state_t
;
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#if defined(__cplusplus)
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extern
"C"
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{
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#endif
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/*! \brief Initialise a T.38 rate adapting non-ECM buffer context.
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\param s The buffer context.
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\param mode TRUE for image data mode, or FALSE for TCF mode.
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\param bits The minimum number of bits per FAX image row.
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\return A pointer to the buffer context, or NULL if there was a problem. */
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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);
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SPAN_DECLARE(
int
) t38_non_ecm_buffer_release(
t38_non_ecm_buffer_state_t
*s);
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SPAN_DECLARE(
int
) t38_non_ecm_buffer_free(
t38_non_ecm_buffer_state_t
*s);
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/*! \brief Set the mode of a T.38 rate adapting non-ECM buffer context.
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\param s The buffer context.
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\param mode TRUE for image data mode, or FALSE for TCF mode.
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\param bits The minimum number of bits per FAX image row. */
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SPAN_DECLARE(
void
)
t38_non_ecm_buffer_set_mode
(
t38_non_ecm_buffer_state_t
*s,
int
mode,
int
min_row_bits);
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/*! \brief Inject data to T.38 rate adapting non-ECM buffer context.
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\param s The buffer context.
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\param buf The data buffer to be injected.
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\param len The length of the data to be injected. */
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SPAN_DECLARE(
void
)
t38_non_ecm_buffer_inject
(
t38_non_ecm_buffer_state_t
*s, const uint8_t *buf,
int
len);
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/*! \brief Inform a T.38 rate adapting non-ECM buffer context that the incoming data has finished,
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and the contents of the buffer should be played out as quickly as possible.
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\param s The buffer context. */
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SPAN_DECLARE(
void
)
t38_non_ecm_buffer_push
(
t38_non_ecm_buffer_state_t
*s);
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/*! \brief Report the input status of a T.38 rate adapting non-ECM buffer context to the specified
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logging context.
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\param s The buffer context.
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\param logging The logging context. */
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SPAN_DECLARE(
void
)
t38_non_ecm_buffer_report_input_status
(
t38_non_ecm_buffer_state_t
*s,
logging_state_t
*logging);
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/*! \brief Report the output status of a T.38 rate adapting non-ECM buffer context to the specified
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logging context.
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\param s The buffer context.
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\param logging The logging context. */
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SPAN_DECLARE(
void
)
t38_non_ecm_buffer_report_output_status
(
t38_non_ecm_buffer_state_t
*s,
logging_state_t
*logging);
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/*! \brief Get the next bit of data from a T.38 rate adapting non-ECM buffer context.
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\param user_data The buffer context, cast to a void pointer.
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\return The next bit, or one of the values indicating a change of modem status. */
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SPAN_DECLARE_NONSTD
(
int
) t38_non_ecm_buffer_get_bit(
void
*user_data);
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#if defined(__cplusplus)
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}
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#endif
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#endif
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/*- End of file ------------------------------------------------------------*/
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spandsp
t38_non_ecm_buffer.h
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