242 lines
12 KiB
C
242 lines
12 KiB
C
/*
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* SpanDSP - a series of DSP components for telephony
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*
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* v29rx.h - ITU V.29 modem receive part
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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) 2003 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_V29RX_H_)
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#define _SPANDSP_V29RX_H_
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/*! \page v29rx_page The V.29 receiver
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\section v29rx_page_sec_1 What does it do?
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The V.29 receiver implements the receive side of a V.29 modem. This can operate
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at data rates of 9600, 7200 and 4800 bits/s. The audio input is a stream of 16
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bit samples, at 8000 samples/second. The transmit and receive side of V.29
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modems operate independantly. V.29 is mostly used for FAX transmission, where it
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provides the standard 9600 and 7200 bits/s rates (the 4800 bits/s mode is not
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used for FAX).
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\section v29rx_page_sec_2 How does it work?
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V.29 operates at 2400 baud for all three bit rates. It uses 16-QAM modulation for
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9600bps, 8-QAM for 7200bps, and 4-PSK for 4800bps. A training sequence is specified
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at the start of transmission, which makes the design of a V.29 receiver relatively
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straightforward.
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The first stage of the training sequence consists of 128
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symbols, alternating between two constellation positions. The receiver monitors
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the signal power, to sense the possible presence of a valid carrier. When the
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alternating signal begins, the power rising above a minimum threshold (-26dBm0)
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causes the main receiver computation to begin. The initial measured power is
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used to quickly set the gain of the receiver. After this initial settling, the
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front end gain is locked, and the adaptive equalizer tracks any subsequent
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signal level variation. The signal is oversampled to 24000 samples/second (i.e.
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signal, zero, zero, signal, zero, zero, ...) and fed to a complex root raised
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cosine pulse shaping filter. This filter has been modified from the conventional
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root raised cosine filter, by shifting it up the band, to be centred at the nominal
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carrier frequency. This filter interpolates the samples, pulse shapes, and performs
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a fractional sample delay at the same time. 48 sets of filter coefficients are used to
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achieve a set of finely spaces fractional sample delays, between zero and
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one sample. By choosing every fifth sample, and the appropriate set of filter
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coefficients, the properly tuned symbol tracker can select data samples at 4800
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samples/second from points within 1.125 degrees of the centre and mid-points of
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each symbol. The output of the filter is multiplied by a complex carrier, generated
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by a DDS. The result is a baseband signal, requiring no further filtering, apart from
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an adaptive equalizer. The baseband signal is fed to a T/2 adaptive equalizer.
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A band edge component maximisation algorithm is used to tune the sampling, so the samples
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fed to the equalizer are close to the mid point and edges of each symbol. Initially
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the algorithm is very lightly damped, to ensure the symbol alignment pulls in
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quickly. Because the sampling rate will not be precisely the same as the
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transmitter's (the spec. says the symbol timing should be within 0.01%), the
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receiver constantly evaluates and corrects this sampling throughout its
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operation. During the symbol timing maintainence phase, the algorithm uses
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a heavier damping.
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The carrier is specified as 1700Hz +-1Hz at the transmitter, and 1700 +-7Hz at
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the receiver. The receive carrier would only be this inaccurate if the link
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includes FDM sections. These are being phased out, but the design must still
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allow for the worst case. Using an initial 1700Hz signal for demodulation gives
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a worst case rotation rate for the constellation of about one degree per symbol.
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Once the symbol timing synchronisation algorithm has been given time to lock to
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the symbol timing of the initial alternating pattern, the phase of the demodulated
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signal is recorded on two successive symbols - once for each of the constellation
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positions. The receiver then tracks the symbol alternations, until a large phase jump
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occurs. This signifies the start of the next phase of the training sequence. At this
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point the total phase shift between the original recorded symbol phase, and the
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symbol phase just before the phase jump occurred is used to provide a coarse
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estimation of the rotation rate of the constellation, and it current absolute
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angle of rotation. These are used to update the current carrier phase and phase
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update rate in the carrier DDS. The working data already in the pulse shaping
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filter and equalizer buffers is given a similar step rotation to pull it all
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into line. From this point on, a heavily damped integrate and dump approach,
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based on the angular difference between each received constellation position and
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its expected position, is sufficient to track the carrier, and maintain phase
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alignment. A fast rough approximator for the arc-tangent function is adequate
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for the estimation of the angular error.
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The next phase of the training sequence is a scrambled sequence of two
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particular symbols. We train the T/2 adaptive equalizer using this sequence. The
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scrambling makes the signal sufficiently diverse to ensure the equalizer
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converges to the proper generalised solution. At the end of this sequence, the
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equalizer should be sufficiently well adapted that is can correctly resolve the
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full QAM constellation. However, the equalizer continues to adapt throughout
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operation of the modem, fine tuning on the more complex data patterns of the
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full QAM constellation.
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In the last phase of the training sequence, the modem enters normal data
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operation, with a short defined period of all ones as data. As in most high
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speed modems, data in a V.29 modem passes through a scrambler, to whiten the
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spectrum of the signal. The transmitter should initialise its data scrambler,
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and pass the ones through it. At the end of the ones, real data begins to pass
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through the scrambler, and the transmit modem is in normal operation. The
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receiver tests that ones are really received, in order to verify the modem
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trained correctly. If all is well, the data following the ones is fed to the
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application, and the receive modem is up and running. Unfortunately, some
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transmit side of some real V.29 modems fail to initialise their scrambler before
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sending the ones. This means the first 23 received bits (the length of the
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scrambler register) cannot be trusted for the test. The receive modem,
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therefore, only tests that bits starting at bit 24 are really ones.
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*/
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typedef void (*qam_report_handler_t)(void *user_data, const complexf_t *constel, const complexf_t *target, int symbol);
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/*!
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V.29 modem receive side descriptor. This defines the working state for a
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single instance of a V.29 modem receiver.
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*/
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typedef struct v29_rx_state_s v29_rx_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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/*! Initialise a V.29 modem receive context.
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\brief Initialise a V.29 modem receive context.
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\param s The modem context.
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\param bit_rate The bit rate of the modem. Valid values are 4800, 7200 and 9600.
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\param put_bit The callback routine used to put the received data.
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\param user_data An opaque pointer passed to the put_bit routine.
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\return A pointer to the modem context, or NULL if there was a problem. */
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SPAN_DECLARE(v29_rx_state_t *) v29_rx_init(v29_rx_state_t *s, int bit_rate, put_bit_func_t put_bit, void *user_data);
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/*! Reinitialise an existing V.29 modem receive context.
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\brief Reinitialise an existing V.29 modem receive context.
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\param s The modem context.
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\param bit_rate The bit rate of the modem. Valid values are 4800, 7200 and 9600.
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\param old_train TRUE if a previous trained values are to be reused.
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\return 0 for OK, -1 for bad parameter */
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SPAN_DECLARE(int) v29_rx_restart(v29_rx_state_t *s, int bit_rate, int old_train);
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/*! Release a V.29 modem receive context.
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\brief Release a V.29 modem receive context.
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\param s The modem context.
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\return 0 for OK */
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SPAN_DECLARE(int) v29_rx_release(v29_rx_state_t *s);
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/*! Free a V.29 modem receive context.
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\brief Free a V.29 modem receive context.
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\param s The modem context.
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\return 0 for OK */
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SPAN_DECLARE(int) v29_rx_free(v29_rx_state_t *s);
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/*! Get the logging context associated with a V.29 modem receive context.
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\brief Get the logging context associated with a V.29 modem receive context.
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\param s The modem context.
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\return A pointer to the logging context */
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SPAN_DECLARE(logging_state_t *) v29_rx_get_logging_state(v29_rx_state_t *s);
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/*! Change the put_bit function associated with a V.29 modem receive context.
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\brief Change the put_bit function associated with a V.29 modem receive context.
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\param s The modem context.
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\param put_bit The callback routine used to handle received bits.
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\param user_data An opaque pointer. */
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SPAN_DECLARE(void) v29_rx_set_put_bit(v29_rx_state_t *s, put_bit_func_t put_bit, void *user_data);
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/*! Change the modem status report function associated with a V.29 modem receive context.
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\brief Change the modem status report function associated with a V.29 modem receive context.
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\param s The modem context.
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\param handler The callback routine used to report modem status changes.
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\param user_data An opaque pointer. */
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SPAN_DECLARE(void) v29_rx_set_modem_status_handler(v29_rx_state_t *s, modem_status_func_t handler, void *user_data);
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/*! Process a block of received V.29 modem audio samples.
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\brief Process a block of received V.29 modem audio samples.
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\param s The modem context.
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\param amp The audio sample buffer.
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\param len The number of samples in the buffer.
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\return The number of samples unprocessed. */
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SPAN_DECLARE_NONSTD(int) v29_rx(v29_rx_state_t *s, const int16_t amp[], int len);
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/*! Fake processing of a missing block of received V.29 modem audio samples.
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(e.g due to packet loss).
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\brief Fake processing of a missing block of received V.29 modem audio samples.
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\param s The modem context.
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\param len The number of samples to fake.
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\return The number of samples unprocessed. */
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SPAN_DECLARE_NONSTD(int) v29_rx_fillin(v29_rx_state_t *s, int len);
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/*! Get a snapshot of the current equalizer coefficients.
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\brief Get a snapshot of the current equalizer coefficients.
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\param s The modem context.
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\param coeffs The vector of complex coefficients.
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\return The number of coefficients in the vector. */
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#if defined(SPANDSP_USE_FIXED_POINT)
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SPAN_DECLARE(int) v29_rx_equalizer_state(v29_rx_state_t *s, complexi16_t **coeffs);
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#else
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SPAN_DECLARE(int) v29_rx_equalizer_state(v29_rx_state_t *s, complexf_t **coeffs);
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#endif
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/*! Get the current received carrier frequency.
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\param s The modem context.
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\return The frequency, in Hertz. */
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SPAN_DECLARE(float) v29_rx_carrier_frequency(v29_rx_state_t *s);
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/*! Get the current symbol timing correction since startup.
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\param s The modem context.
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\return The correction. */
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SPAN_DECLARE(float) v29_rx_symbol_timing_correction(v29_rx_state_t *s);
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/*! Get the current received signal power.
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\param s The modem context.
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\return The signal power, in dBm0. */
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SPAN_DECLARE(float) v29_rx_signal_power(v29_rx_state_t *s);
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/*! Set the power level at which the carrier detection will cut in
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\param s The modem context.
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\param cutoff The signal cutoff power, in dBm0. */
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SPAN_DECLARE(void) v29_rx_signal_cutoff(v29_rx_state_t *s, float cutoff);
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/*! Set a handler routine to process QAM status reports
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\param s The modem context.
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\param handler The handler routine.
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\param user_data An opaque pointer passed to the handler routine. */
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SPAN_DECLARE(void) v29_rx_set_qam_report_handler(v29_rx_state_t *s, qam_report_handler_t handler, 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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