--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/spandsp-0.0.6pre17/src/spandsp/v17rx.h	Fri Jun 25 15:50:58 2010 +0200
@@ -0,0 +1,334 @@
+/*
+ * SpanDSP - a series of DSP components for telephony
+ *
+ * v17rx.h - ITU V.17 modem receive part
+ *
+ * Written by Steve Underwood <steveu@coppice.org>
+ *
+ * Copyright (C) 2003 Steve Underwood
+ *
+ * All rights reserved.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License version 2.1,
+ * as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this program; if not, write to the Free Software
+ * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
+ *
+ * $Id: v17rx.h,v 1.65 2009/07/09 13:52:09 steveu Exp $
+ */
+
+/*! \file */
+
+#if !defined(_SPANDSP_V17RX_H_)
+#define _SPANDSP_V17RX_H_
+
+/*! \page v17rx_page The V.17 receiver
+\section v17rx_page_sec_1 What does it do?
+The V.17 receiver implements the receive side of a V.17 modem. This can operate
+at data rates of 14400, 12000, 9600 and 7200 bits/second. The audio input is a stream
+of 16 bit samples, at 8000 samples/second. The transmit and receive side of V.17
+modems operate independantly. V.17 is mostly used for FAX transmission over PSTN
+lines, where it provides the standard 14400 bits/second rate. 
+
+\section v17rx_page_sec_2 How does it work?
+V.17 uses QAM modulation, at 2400 baud, and trellis coding. Constellations with
+16, 32, 64, and 128 points are defined. After one bit per baud is absorbed by the
+trellis coding, this gives usable bit rates of 7200, 9600, 12000, and 14400 per
+second.
+
+V.17 specifies a training sequence at the start of transmission, which makes the
+design of a V.17 receiver relatively straightforward. The first stage of the
+training sequence consists of 256
+symbols, alternating between two constellation positions. The receiver monitors
+the signal power, to sense the possible presence of a valid carrier. When the
+alternating signal begins, the power rising above a minimum threshold (-43dBm0)
+causes the main receiver computation to begin. The initial measured power is
+used to quickly set the gain of the receiver. After this initial settling, the
+front end gain is locked, and the adaptive equalizer tracks any subsequent
+signal level variation. The signal is oversampled to 24000 samples/second (i.e.
+signal, zero, zero, signal, zero, zero, ...) and fed to a complex root raised
+cosine pulse shaping filter. This filter has been modified from the conventional
+root raised cosine filter, by shifting it up the band, to be centred at the nominal
+carrier frequency. This filter interpolates the samples, pulse shapes, and performs
+a fractional sample delay at the same time. 192 sets of filter coefficients are used
+to achieve a set of finely spaces fractional sample delays, between zero and
+one sample. By choosing every fifth sample, and the appropriate set of filter
+coefficients, the properly tuned symbol tracker can select data samples at 4800
+samples/second from points within 0.28 degrees of the centre and mid-points of
+each symbol. The output of the filter is multiplied by a complex carrier, generated
+by a DDS. The result is a baseband signal, requiring no further filtering, apart from
+an adaptive equalizer. The baseband signal is fed to a T/2 adaptive equalizer.
+A band edge component maximisation algorithm is used to tune the sampling, so the samples
+fed to the equalizer are close to the mid point and edges of each symbol. Initially
+the algorithm is very lightly damped, to ensure the symbol alignment pulls in
+quickly. Because the sampling rate will not be precisely the same as the
+transmitter's (the spec. says the symbol timing should be within 0.01%), the
+receiver constantly evaluates and corrects this sampling throughout its
+operation. During the symbol timing maintainence phase, the algorithm uses
+a heavier damping.
+
+The carrier is specified as 1800Hz +- 1Hz at the transmitter, and 1800 +-7Hz at
+the receiver. The receive carrier would only be this inaccurate if the link
+includes FDM sections. These are being phased out, but the design must still
+allow for the worst case. Using an initial 1800Hz signal for demodulation gives
+a worst case rotation rate for the constellation of about one degree per symbol.
+Once the symbol timing synchronisation algorithm has been given time to lock to the
+symbol timing of the initial alternating pattern, the phase of the demodulated signal
+is recorded on two successive symbols - once for each of the constellation positions.
+The receiver then tracks the symbol alternations, until a large phase jump occurs.
+This signifies the start of the next phase of the training sequence. At this
+point the total phase shift between the original recorded symbol phase, and the
+symbol phase just before the phase jump occurred is used to provide a coarse
+estimation of the rotation rate of the constellation, and it current absolute
+angle of rotation. These are used to update the current carrier phase and phase
+update rate in the carrier DDS. The working data already in the pulse shaping
+filter and equalizer buffers is given a similar step rotation to pull it all
+into line. From this point on, a heavily damped integrate and dump approach,
+based on the angular difference between each received constellation position and
+its expected position, is sufficient to track the carrier, and maintain phase
+alignment. A fast rough approximator for the arc-tangent function is adequate
+for the estimation of the angular error. 
+
+The next phase of the training sequence is a scrambled sequence of two
+particular symbols. We train the T/2 adaptive equalizer using this sequence. The
+scrambling makes the signal sufficiently diverse to ensure the equalizer
+converges to the proper generalised solution. At the end of this sequence, the
+equalizer should be sufficiently well adapted that is can correctly resolve the
+full QAM constellation. However, the equalizer continues to adapt throughout
+operation of the modem, fine tuning on the more complex data patterns of the
+full QAM constellation. 
+
+In the last phase of the training sequence, the modem enters normal data
+operation, with a short defined period of all ones as data. As in most high
+speed modems, data in a V.17 modem passes through a scrambler, to whiten the
+spectrum of the signal. The transmitter should initialise its data scrambler,
+and pass the ones through it. At the end of the ones, real data begins to pass
+through the scrambler, and the transmit modem is in normal operation. The
+receiver tests that ones are really received, in order to verify the modem
+trained correctly. If all is well, the data following the ones is fed to the
+application, and the receive modem is up and running. Unfortunately, some
+transmit side of some real V.17 modems fail to initialise their scrambler before
+sending the ones. This means the first 23 received bits (the length of the
+scrambler register) cannot be trusted for the test. The receive modem,
+therefore, only tests that bits starting at bit 24 are really ones.
+
+The V.17 signal is trellis coded. Two bits of each symbol are convolutionally coded
+to form a 3 bit trellis code - the two original bits, plus an extra redundant bit. It
+is possible to ignore the trellis coding, and just decode the non-redundant bits.
+However, the noise performance of the receiver would suffer. Using a proper
+trellis decoder adds several dB to the noise tolerance to the receiving modem. Trellis
+coding seems quite complex at first sight, but is fairly straightforward once you
+get to grips with it.
+
+Trellis decoding tracks the data in terms of the possible states of the convolutional
+coder at the transmitter. There are 8 possible states of the V.17 coder. The first
+step in trellis decoding is to find the best candidate constellation point
+for each of these 8 states. One of thse will be our final answer. The constellation
+has been designed so groups of 8 are spread fairly evenly across it. Locating them
+is achieved is a reasonably fast manner, by looking up the answers in a set of space
+map tables. The disadvantage is the tables are potentially large enough to affect
+cache performance. The trellis decoder works over 16 successive symbols. The result
+of decoding is not known until 16 symbols after the data enters the decoder. The
+minimum total accumulated mismatch between each received point and the actual
+constellation (termed the distance) is assessed for each of the 8 states. A little
+analysis of the coder shows that each of the 8 current states could be arrived at
+from 4 different previous states, through 4 different constellation bit patterns.
+For each new state, the running total distance is arrived at by inspecting a previous
+total plus a new distance for the appropriate 4 previous states. The minimum of the 4
+values becomes the new distance for the state. Clearly, a mechanism is needed to stop
+this distance from growing indefinitely. A sliding window, and several other schemes
+are possible. However, a simple single pole IIR is very simple, and provides adequate
+results.
+
+For each new state we store the constellation bit pattern, or path, to that state, and
+the number of the previous state. We find the minimum distance amongst the 8 new
+states for each new symbol. We then trace back through the states, until we reach the
+one 16 states ago which leads to the current minimum distance. The bit pattern stored
+there is the error corrected bit pattern for that symbol.
+
+So, what does Trellis coding actually achieve? TCM is easier to understand by looking
+at the V.23bis modem spec. The V.32bis spec. is very similar to V.17, except that it
+is a full duplex modem and has non-TCM options, as well as the TCM ones in V.17.
+
+V32bis defines two options for pumping 9600 bits per second down a phone line - one
+with and one without TCM. Both run at 2400 baud. The non-TCM one uses simple 16 point
+QAM on the raw data. The other takes two out of every four raw bits, and convolutionally
+encodes them to 3. Now we have 5 bits per symbol, and we need 32 point QAM to send the
+data.
+
+The raw error rate from simple decoding of the 32 point QAM is horrible compared to
+decoding the 16 point QAM. If a point decoded from the 32 point QAM is wrong, the likely
+correct choice should be one of the adjacent ones. It is unlikely to have been one that
+is far away across the constellation, unless there was a huge noise spike, interference,
+or something equally nasty. Now, the 32 point symbols do not exist in isolation. There
+was a kind of temporal smearing in the convolutional coding. It created a well defined
+dependency between successive symbols. If we knew for sure what the last few symbols
+were, they would lead us to a limited group of possible values for the current symbol,
+constrained by the behaviour of the convolutional coder. If you look at how the symbols
+were mapped to constellation points, you will see the mapping tries to spread those
+possible symbols as far apart as possible. This will leave only one that is pretty
+close to the received point, which must be the correct choice. However, this assumes
+we know the last few symbols for sure. Since we don't, we have a bit more work to do
+to achieve reliable decoding.
+
+Instead of decoding to the nearest point on the constellation, we decode to a group of
+likely constellation points in the neighbourhood of the received point. We record the
+mismatch for each - that is the distance across the constellation between the received
+point and the group of nearby points. To avoid square roots, recording x2 + y2 can be
+good enough. Symbol by symbol, we record this information. After a few symbols we can
+stand back and look at the recorded information.
+
+For each symbol we have a set of possible symbol values and error metric pairs. The
+dependency between symbols, created by the convolutional coder, means some paths from
+symbol to symbol are possible and some are not. It we trace back through the possible
+symbol to symbol paths, and total up the error metric through those paths, we end up
+with a set of figures of merit (or more accurately figures of demerit, since
+larger == worse) for the likelihood of each path being the correct one. The path with
+the lowest total metric is the most likely, and gives us our final choice for what we
+think the current symbol really is.
+
+That was hard work. It takes considerable computation to do this selection and traceback,
+symbol by symbol. We need to get quite a lot from this. It needs to drive the error rate
+down so far that is compensates for the much higher error rate due to the larger
+constellation, and then buys us some actual benefit. Well in the example we are looking
+at - V.32bis at 9600bps - it works out the error rate from the TCM option is like using
+the non-TCM option with several dB more signal to noise ratio. That's nice. The non-TCM
+option is pretty reasonable on most phone lines, but a better error rate is always a
+good thing. However, V32bis includes a 14,400bps option. That uses 2400 baud, and 6 bit
+symbols. Convolutional encoding increases that to 7 bits per symbol, by taking 2 bits and
+encoding them to 3. This give a 128 point QAM constellation. Again, the difference between
+using this, and using just an uncoded 64 point constellation is equivalent to maybe 5dB of
+extra signal to noise ratio. However, in this case it is the difference between the modem
+working only on the most optimal lines, and being widely usable across most phone lines.
+TCM absolutely transformed the phone line modem business.
+*/
+
+/*!
+    V.17 modem receive side descriptor. This defines the working state for a
+    single instance of a V.17 modem receiver.
+*/
+typedef struct v17_rx_state_s v17_rx_state_t;
+
+#if defined(__cplusplus)
+extern "C"
+{
+#endif
+
+/*! Initialise a V.17 modem receive context.
+    \brief Initialise a V.17 modem receive context.
+    \param s The modem context.
+    \param bit_rate The bit rate of the modem. Valid values are 7200, 9600, 12000 and 14400.
+    \param put_bit The callback routine used to put the received data.
+    \param user_data An opaque pointer passed to the put_bit routine.
+    \return A pointer to the modem context, or NULL if there was a problem. */
+SPAN_DECLARE(v17_rx_state_t *) v17_rx_init(v17_rx_state_t *s, int bit_rate, put_bit_func_t put_bit, void *user_data);
+
+/*! Reinitialise an existing V.17 modem receive context.
+    \brief Reinitialise an existing V.17 modem receive context.
+    \param s The modem context.
+    \param bit_rate The bit rate of the modem. Valid values are 7200, 9600, 12000 and 14400.
+    \param short_train TRUE if a short training sequence is expected.
+    \return 0 for OK, -1 for bad parameter */
+SPAN_DECLARE(int) v17_rx_restart(v17_rx_state_t *s, int bit_rate, int short_train);
+
+/*! Release a V.17 modem receive context.
+    \brief Release a V.17 modem receive context.
+    \param s The modem context.
+    \return 0 for OK */
+SPAN_DECLARE(int) v17_rx_release(v17_rx_state_t *s);
+
+/*! Free a V.17 modem receive context.
+    \brief Free a V.17 modem receive context.
+    \param s The modem context.
+    \return 0 for OK */
+SPAN_DECLARE(int) v17_rx_free(v17_rx_state_t *s);
+
+/*! Get the logging context associated with a V.17 modem receive context.
+    \brief Get the logging context associated with a V.17 modem receive context.
+    \param s The modem context.
+    \return A pointer to the logging context */
+SPAN_DECLARE(logging_state_t *) v17_rx_get_logging_state(v17_rx_state_t *s);
+
+/*! Change the put_bit function associated with a V.17 modem receive context.
+    \brief Change the put_bit function associated with a V.17 modem receive context.
+    \param s The modem context.
+    \param put_bit The callback routine used to handle received bits.
+    \param user_data An opaque pointer. */
+SPAN_DECLARE(void) v17_rx_set_put_bit(v17_rx_state_t *s, put_bit_func_t put_bit, void *user_data);
+
+/*! Change the modem status report function associated with a V.17 modem receive context.
+    \brief Change the modem status report function associated with a V.17 modem receive context.
+    \param s The modem context.
+    \param handler The callback routine used to report modem status changes.
+    \param user_data An opaque pointer. */
+SPAN_DECLARE(void) v17_rx_set_modem_status_handler(v17_rx_state_t *s, modem_rx_status_func_t handler, void *user_data);
+
+/*! Process a block of received V.17 modem audio samples.
+    \brief Process a block of received V.17 modem audio samples.
+    \param s The modem context.
+    \param amp The audio sample buffer.
+    \param len The number of samples in the buffer.
+    \return The number of samples unprocessed.
+*/
+SPAN_DECLARE_NONSTD(int) v17_rx(v17_rx_state_t *s, const int16_t amp[], int len);
+
+/*! Fake processing of a missing block of received V.17 modem audio samples.
+    (e.g due to packet loss).
+    \brief Fake processing of a missing block of received V.17 modem audio samples.
+    \param s The modem context.
+    \param len The number of samples to fake.
+    \return The number of samples unprocessed.
+*/
+SPAN_DECLARE(int) v17_rx_fillin(v17_rx_state_t *s, int len);
+
+/*! Get a snapshot of the current equalizer coefficients.
+    \brief Get a snapshot of the current equalizer coefficients.
+    \param s The modem context.
+    \param coeffs The vector of complex coefficients.
+    \return The number of coefficients in the vector. */
+#if defined(SPANDSP_USE_FIXED_POINTx)
+SPAN_DECLARE(int) v17_rx_equalizer_state(v17_rx_state_t *s, complexi_t **coeffs);
+#else
+SPAN_DECLARE(int) v17_rx_equalizer_state(v17_rx_state_t *s, complexf_t **coeffs);
+#endif
+
+/*! Get the current received carrier frequency.
+    \param s The modem context.
+    \return The frequency, in Hertz. */
+SPAN_DECLARE(float) v17_rx_carrier_frequency(v17_rx_state_t *s);
+
+/*! Get the current symbol timing correction since startup.
+    \param s The modem context.
+    \return The correction. */
+SPAN_DECLARE(float) v17_rx_symbol_timing_correction(v17_rx_state_t *s);
+
+/*! Get a current received signal power.
+    \param s The modem context.
+    \return The signal power, in dBm0. */
+SPAN_DECLARE(float) v17_rx_signal_power(v17_rx_state_t *s);
+
+/*! Set the power level at which the carrier detection will cut in
+    \param s The modem context.
+    \param cutoff The signal cutoff power, in dBm0. */
+SPAN_DECLARE(void) v17_rx_signal_cutoff(v17_rx_state_t *s, float cutoff);
+
+/*! Set a handler routine to process QAM status reports
+    \param s The modem context.
+    \param handler The handler routine.
+    \param user_data An opaque pointer passed to the handler routine. */
+SPAN_DECLARE(void) v17_rx_set_qam_report_handler(v17_rx_state_t *s, qam_report_handler_t handler, void *user_data);
+
+#if defined(__cplusplus)
+}
+#endif
+
+#endif
+/*- End of file ------------------------------------------------------------*/