2008-09-03 19:02:00 +00:00
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/*
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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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2008-10-01 03:59:45 +00:00
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* $Id: v29rx.h,v 1.64 2008/09/18 14:59:30 steveu Exp $
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2008-09-03 19:02:00 +00:00
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*/
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/*! \file */
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#if !defined(_V29RX_H_)
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#define _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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/* Target length for the equalizer is about 63 taps, to deal with the worst stuff
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in V.56bis. */
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2008-10-01 03:59:45 +00:00
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#define V29_EQUALIZER_PRE_LEN 16 /* This much before the real event */
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#define V29_EQUALIZER_POST_LEN 14 /* This much after the real event (must be even) */
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2008-09-03 19:02:00 +00:00
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#define V29_RX_FILTER_STEPS 27
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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
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{
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/*! \brief The bit rate of the modem. Valid values are 4800, 7200 and 9600. */
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int bit_rate;
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/*! \brief The callback function used to put each bit received. */
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put_bit_func_t put_bit;
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/*! \brief A user specified opaque pointer passed to the put_bit routine. */
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void *put_bit_user_data;
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/*! \brief The callback function used to report modem status changes. */
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modem_rx_status_func_t status_handler;
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/*! \brief A user specified opaque pointer passed to the status function. */
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void *status_user_data;
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/*! \brief A callback function which may be enabled to report every symbol's
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constellation position. */
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qam_report_handler_t qam_report;
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/*! \brief A user specified opaque pointer passed to the qam_report callback
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routine. */
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void *qam_user_data;
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/*! \brief The route raised cosine (RRC) pulse shaping filter buffer. */
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#if defined(SPANDSP_USE_FIXED_POINT)
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int16_t rrc_filter[V29_RX_FILTER_STEPS];
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#else
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float rrc_filter[V29_RX_FILTER_STEPS];
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#endif
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/*! \brief Current offset into the RRC pulse shaping filter buffer. */
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int rrc_filter_step;
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/*! \brief The register for the data scrambler. */
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unsigned int scramble_reg;
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/*! \brief The register for the training scrambler. */
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uint8_t training_scramble_reg;
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/*! \brief The current step in the table of CD constellation positions. */
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int training_cd;
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/*! \brief TRUE if the previous trained values are to be reused. */
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int old_train;
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/*! \brief The section of the training data we are currently in. */
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int training_stage;
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/*! \brief A count of how far through the current training step we are. */
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int training_count;
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/*! \brief A measure of how much mismatch there is between the real constellation,
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and the decoded symbol positions. */
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float training_error;
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/*! \brief The value of the last signal sample, using the a simple HPF for signal power estimation. */
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int16_t last_sample;
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/*! \brief >0 if a signal above the minimum is present. It may or may not be a V.29 signal. */
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int signal_present;
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/*! \brief Whether or not a carrier drop was detected and the signal delivery is pending. */
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int carrier_drop_pending;
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/*! \brief A count of the current consecutive samples below the carrier off threshold. */
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int low_samples;
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/*! \brief A highest magnitude sample seen. */
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int16_t high_sample;
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2008-10-01 03:56:17 +00:00
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/*! \brief The position of the current symbol in the constellation, used for
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differential decoding. */
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int constellation_state;
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2008-09-03 19:02:00 +00:00
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/*! \brief The current phase of the carrier (i.e. the DDS parameter). */
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uint32_t carrier_phase;
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/*! \brief The update rate for the phase of the carrier (i.e. the DDS increment). */
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int32_t carrier_phase_rate;
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/*! \brief The carrier update rate saved for reuse when using short training. */
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int32_t carrier_phase_rate_save;
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2008-10-01 03:54:17 +00:00
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#if defined(SPANDSP_USE_FIXED_POINT)
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/*! \brief The proportional part of the carrier tracking filter. */
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int32_t carrier_track_p;
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/*! \brief The integral part of the carrier tracking filter. */
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int32_t carrier_track_i;
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#else
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2008-09-03 19:02:00 +00:00
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/*! \brief The proportional part of the carrier tracking filter. */
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float carrier_track_p;
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/*! \brief The integral part of the carrier tracking filter. */
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float carrier_track_i;
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#endif
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2008-09-03 19:02:00 +00:00
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/*! \brief A power meter, to measure the HPF'ed signal power in the channel. */
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power_meter_t power;
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/*! \brief The power meter level at which carrier on is declared. */
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int32_t carrier_on_power;
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/*! \brief The power meter level at which carrier off is declared. */
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int32_t carrier_off_power;
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2008-10-01 03:56:17 +00:00
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/*! \brief Current read offset into the equalizer buffer. */
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2008-09-03 19:02:00 +00:00
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int eq_step;
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/*! \brief Current write offset into the equalizer buffer. */
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int eq_put_step;
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/*! \brief Symbol counter to the next equalizer update. */
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int eq_skip;
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/*! \brief The current half of the baud. */
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int baud_half;
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#if defined(SPANDSP_USE_FIXED_POINT)
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2008-10-01 03:54:17 +00:00
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/*! \brief The scaling factor accessed by the AGC algorithm. */
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int16_t agc_scaling;
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/*! \brief The previous value of agc_scaling, needed to reuse old training. */
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int16_t agc_scaling_save;
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2008-10-01 03:54:17 +00:00
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2008-09-09 17:04:42 +00:00
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/*! \brief The current delta factor for updating the equalizer coefficients. */
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int16_t eq_delta;
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/*! \brief The adaptive equalizer coefficients. */
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complexi16_t eq_coeff[V29_EQUALIZER_PRE_LEN + 1 + V29_EQUALIZER_POST_LEN];
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/*! \brief A saved set of adaptive equalizer coefficients for use after restarts. */
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complexi16_t eq_coeff_save[V29_EQUALIZER_PRE_LEN + 1 + V29_EQUALIZER_POST_LEN];
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/*! \brief The equalizer signal buffer. */
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complexi16_t eq_buf[V29_EQUALIZER_PRE_LEN + 1 + V29_EQUALIZER_POST_LEN];
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2008-09-09 17:04:42 +00:00
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2008-09-03 19:02:00 +00:00
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/*! Low band edge filter for symbol sync. */
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int32_t symbol_sync_low[2];
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/*! High band edge filter for symbol sync. */
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int32_t symbol_sync_high[2];
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/*! DC filter for symbol sync. */
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int32_t symbol_sync_dc_filter[2];
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/*! Baud phase for symbol sync. */
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int32_t baud_phase;
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#else
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2008-10-01 03:54:17 +00:00
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/*! \brief The scaling factor accessed by the AGC algorithm. */
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float agc_scaling;
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/*! \brief The previous value of agc_scaling, needed to reuse old training. */
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float agc_scaling_save;
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2008-09-09 17:04:42 +00:00
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/*! \brief The current delta factor for updating the equalizer coefficients. */
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float eq_delta;
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/*! \brief The adaptive equalizer coefficients. */
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complexf_t eq_coeff[V29_EQUALIZER_PRE_LEN + 1 + V29_EQUALIZER_POST_LEN];
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/*! \brief A saved set of adaptive equalizer coefficients for use after restarts. */
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complexf_t eq_coeff_save[V29_EQUALIZER_PRE_LEN + 1 + V29_EQUALIZER_POST_LEN];
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/*! \brief The equalizer signal buffer. */
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2008-10-01 03:59:45 +00:00
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complexf_t eq_buf[V29_EQUALIZER_PRE_LEN + 1 + V29_EQUALIZER_POST_LEN];
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2008-09-09 17:04:42 +00:00
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2008-09-03 19:02:00 +00:00
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/*! Low band edge filter for symbol sync. */
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float symbol_sync_low[2];
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/*! High band edge filter for symbol sync. */
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float symbol_sync_high[2];
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/*! DC filter for symbol sync. */
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float symbol_sync_dc_filter[2];
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/*! Baud phase for symbol sync. */
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float baud_phase;
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#endif
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/*! \brief The total symbol timing correction since the carrier came up.
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This is only for performance analysis purposes. */
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int total_baud_timing_correction;
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/*! \brief Starting phase angles for the coarse carrier aquisition step. */
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int32_t start_angles[2];
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/*! \brief History list of phase angles for the coarse carrier aquisition step. */
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int32_t angles[16];
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/*! \brief Error and flow logging control */
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logging_state_t logging;
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} 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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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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int v29_rx_restart(v29_rx_state_t *s, int bit_rate, int old_train);
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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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int v29_rx_free(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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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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void v29_rx_set_modem_status_handler(v29_rx_state_t *s, modem_rx_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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int v29_rx(v29_rx_state_t *s, const int16_t amp[], 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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2008-09-09 17:04:42 +00:00
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#if defined(SPANDSP_USE_FIXED_POINT)
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int v29_rx_equalizer_state(v29_rx_state_t *s, complexi16_t **coeffs);
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#else
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2008-09-03 19:02:00 +00:00
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int v29_rx_equalizer_state(v29_rx_state_t *s, complexf_t **coeffs);
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2008-09-09 17:04:42 +00:00
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#endif
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2008-09-03 19:02:00 +00:00
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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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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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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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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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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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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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