Separate 2.4MHz demodulator into its own file.
This commit is contained in:
Oliver Jowett
parent
a6542b505b
commit
f753c2d9fe
2
Makefile
2
Makefile
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@ -21,7 +21,7 @@ all: dump1090 view1090
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%.o: %.c dump1090.h
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%.o: %.c dump1090.h
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$(CC) $(CPPFLAGS) $(CFLAGS) $(EXTRACFLAGS) -c $<
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$(CC) $(CPPFLAGS) $(CFLAGS) $(EXTRACFLAGS) -c $<
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dump1090: dump1090.o anet.o interactive.o mode_ac.o mode_s.o net_io.o crc.o demod_2000.o
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dump1090: dump1090.o anet.o interactive.o mode_ac.o mode_s.o net_io.o crc.o demod_2000.o demod_2400.o
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$(CC) -g -o dump1090 $^ $(LIBS) $(LIBS_RTL) $(LDFLAGS)
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$(CC) -g -o dump1090 $^ $(LIBS) $(LIBS_RTL) $(LDFLAGS)
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view1090: view1090.o anet.o interactive.o mode_ac.o mode_s.o net_io.o crc.o
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view1090: view1090.o anet.o interactive.o mode_ac.o mode_s.o net_io.o crc.o
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532
demod_2400.c
Normal file
532
demod_2400.c
Normal file
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@ -0,0 +1,532 @@
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// dump1090, a Mode S messages decoder for RTLSDR devices.
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//
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// Copyright (C) 2014,2015 Oliver Jowett <oliver@mutability.co.uk>
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//
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// All rights reserved.
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//
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// Redistribution and use in source and binary forms, with or without
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// modification, are permitted provided that the following conditions are
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// met:
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//
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// * Redistributions of source code must retain the above copyright
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// notice, this list of conditions and the following disclaimer.
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//
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// * Redistributions in binary form must reproduce the above copyright
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// notice, this list of conditions and the following disclaimer in the
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// documentation and/or other materials provided with the distribution.
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//
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// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
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// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
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// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
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// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
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// HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
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// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
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// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
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// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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//
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#include "dump1090.h"
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// 2.4MHz sampling rate version
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//
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// When sampling at 2.4MHz we have exactly 6 samples per 5 symbols.
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// Each symbol is 500ns wide, each sample is 416.7ns wide
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//
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// We maintain a phase offset that is expressed in units of 1/5 of a sample i.e. 1/6 of a symbol, 83.333ns
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// Each symbol we process advances the phase offset by 6 i.e. 6/5 of a sample, 500ns
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//
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// The correlation functions below correlate a 1-0 pair of symbols (i.e. manchester encoded 1 bit)
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// starting at the given sample, and assuming that the symbol starts at a fixed 0-5 phase offset within
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// m[0]. They return a correlation value, generally interpreted as >0 = 1 bit, <0 = 0 bit
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// TODO check if there are better (or more balanced) correlation functions to use here
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// nb: the correlation functions sum to zero, so we do not need to adjust for the DC offset in the input signal
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// (adding any constant value to all of m[0..3] does not change the result)
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static inline int slice_phase0(uint16_t *m) {
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return 5 * m[0] - 3 * m[1] - 2 * m[2];
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}
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static inline int slice_phase1(uint16_t *m) {
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return 4 * m[0] - m[1] - 3 * m[2];
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}
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static inline int slice_phase2(uint16_t *m) {
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return 3 * m[0] + m[1] - 4 * m[2];
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}
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static inline int slice_phase3(uint16_t *m) {
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return 2 * m[0] + 3 * m[1] - 5 * m[2];
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}
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static inline int slice_phase4(uint16_t *m) {
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return m[0] + 5 * m[1] - 5 * m[2] - m[3];
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}
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static inline int correlate_phase0(uint16_t *m) {
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return slice_phase0(m) * 26;
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}
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static inline int correlate_phase1(uint16_t *m) {
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return slice_phase1(m) * 38;
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}
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static inline int correlate_phase2(uint16_t *m) {
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return slice_phase2(m) * 38;
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}
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static inline int correlate_phase3(uint16_t *m) {
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return slice_phase3(m) * 26;
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}
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static inline int correlate_phase4(uint16_t *m) {
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return slice_phase4(m) * 19;
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}
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//
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// These functions work out the correlation quality for the 10 symbols (5 bits) starting at m[0] + given phase offset.
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// This is used to find the right phase offset to use for decoding.
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//
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static inline int correlate_check_0(uint16_t *m) {
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return
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abs(correlate_phase0(&m[0])) +
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abs(correlate_phase2(&m[2])) +
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abs(correlate_phase4(&m[4])) +
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abs(correlate_phase1(&m[7])) +
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abs(correlate_phase3(&m[9]));
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}
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static inline int correlate_check_1(uint16_t *m) {
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return
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abs(correlate_phase1(&m[0])) +
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abs(correlate_phase3(&m[2])) +
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abs(correlate_phase0(&m[5])) +
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abs(correlate_phase2(&m[7])) +
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abs(correlate_phase4(&m[9]));
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}
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static inline int correlate_check_2(uint16_t *m) {
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return
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abs(correlate_phase2(&m[0])) +
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abs(correlate_phase4(&m[2])) +
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abs(correlate_phase1(&m[5])) +
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abs(correlate_phase3(&m[7])) +
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abs(correlate_phase0(&m[10]));
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}
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static inline int correlate_check_3(uint16_t *m) {
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return
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abs(correlate_phase3(&m[0])) +
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abs(correlate_phase0(&m[3])) +
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abs(correlate_phase2(&m[5])) +
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abs(correlate_phase4(&m[7])) +
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abs(correlate_phase1(&m[10]));
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}
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static inline int correlate_check_4(uint16_t *m) {
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return
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abs(correlate_phase4(&m[0])) +
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abs(correlate_phase1(&m[3])) +
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abs(correlate_phase3(&m[5])) +
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abs(correlate_phase0(&m[8])) +
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abs(correlate_phase2(&m[10]));
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}
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// Work out the best phase offset to use for the given message.
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static int best_phase(uint16_t *m) {
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int test;
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int best = -1;
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int bestval = (m[0] + m[1] + m[2] + m[3] + m[4] + m[5]); // minimum correlation quality we will accept
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// empirical testing suggests that 4..8 is the best range to test for here
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// (testing a wider range runs the danger of picking the wrong phase for
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// a message that would otherwise be successfully decoded - the correlation
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// functions can match well with a one symbol / half bit offset)
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// this is consistent with the peak detection which should produce
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// the first data symbol with phase offset 4..8
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//test = correlate_check_2(&m[0]);
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//if (test > bestval) { bestval = test; best = 2; }
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//test = correlate_check_3(&m[0]);
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//if (test > bestval) { bestval = test; best = 3; }
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test = correlate_check_4(&m[0]);
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if (test > bestval) { bestval = test; best = 4; }
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test = correlate_check_0(&m[1]);
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if (test > bestval) { bestval = test; best = 5; }
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test = correlate_check_1(&m[1]);
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if (test > bestval) { bestval = test; best = 6; }
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test = correlate_check_2(&m[1]);
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if (test > bestval) { bestval = test; best = 7; }
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test = correlate_check_3(&m[1]);
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if (test > bestval) { bestval = test; best = 8; }
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//test = correlate_check_4(&m[1]);
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//if (test > bestval) { bestval = test; best = 9; }
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return best;
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}
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//
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//=========================================================================
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//
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// Detect a Mode S messages inside the magnitude buffer pointed by 'm' and of
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// size 'mlen' bytes. Every detected Mode S message is convert it into a
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// stream of bits and passed to the function to display it.
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//
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void demodulate2400(uint16_t *m, uint32_t mlen)
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{
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struct modesMessage mm;
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unsigned char msg[MODES_LONG_MSG_BYTES], *pMsg;
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uint32_t j;
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memset(&mm, 0, sizeof(mm));
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for (j = 0; j < mlen; j++) {
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uint16_t *preamble = &m[j];
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int high, i, initial_phase, phase, errors, errors56, errorsTy;
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int msglen, scanlen;
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uint16_t *pPtr;
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uint8_t theByte, theErrs;
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uint32_t sigLevel, noiseLevel;
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uint16_t snr;
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int try_phase;
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// Look for a message starting at around sample 0 with phase offset 3..7
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// Ideal sample values for preambles with different phase
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// Xn is the first data symbol with phase offset N
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//
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// sample#: 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0
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// phase 3: 2/4\0/5\1 0 0 0 0/5\1/3 3\0 0 0 0 0 0 X4
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// phase 4: 1/5\0/4\2 0 0 0 0/4\2 2/4\0 0 0 0 0 0 0 X0
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// phase 5: 0/5\1/3 3\0 0 0 0/3 3\1/5\0 0 0 0 0 0 0 X1
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// phase 6: 0/4\2 2/4\0 0 0 0 2/4\0/5\1 0 0 0 0 0 0 X2
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// phase 7: 0/3 3\1/5\0 0 0 0 1/5\0/4\2 0 0 0 0 0 0 X3
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//
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// quick check: we must have a rising edge 0->1 and a falling edge 12->13
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if (! (preamble[0] < preamble[1] && preamble[12] > preamble[13]) )
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continue;
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if (preamble[1] > preamble[2] && // 1
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preamble[2] < preamble[3] && preamble[3] > preamble[4] && // 3
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preamble[8] < preamble[9] && preamble[9] > preamble[10] && // 9
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preamble[10] < preamble[11]) { // 11-12
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// peaks at 1,3,9,11-12: phase 3
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high = (preamble[1] + preamble[3] + preamble[9] + preamble[11] + preamble[12]) / 4;
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sigLevel = preamble[1] + preamble[3] + preamble[9];
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noiseLevel = preamble[5] + preamble[6] + preamble[7];
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} else if (preamble[1] > preamble[2] && // 1
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preamble[2] < preamble[3] && preamble[3] > preamble[4] && // 3
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preamble[8] < preamble[9] && preamble[9] > preamble[10] && // 9
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preamble[11] < preamble[12]) { // 12
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// peaks at 1,3,9,12: phase 4
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high = (preamble[1] + preamble[3] + preamble[9] + preamble[12]) / 4;
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sigLevel = preamble[1] + preamble[3] + preamble[9] + preamble[12];
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noiseLevel = preamble[5] + preamble[6] + preamble[7] + preamble[8];
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} else if (preamble[1] > preamble[2] && // 1
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preamble[2] < preamble[3] && preamble[4] > preamble[5] && // 3-4
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preamble[8] < preamble[9] && preamble[10] > preamble[11] && // 9-10
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preamble[11] < preamble[12]) { // 12
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// peaks at 1,3-4,9-10,12: phase 5
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high = (preamble[1] + preamble[3] + preamble[4] + preamble[9] + preamble[10] + preamble[12]) / 4;
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sigLevel = preamble[1] + preamble[12];
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noiseLevel = preamble[6] + preamble[7];
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} else if (preamble[1] > preamble[2] && // 1
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preamble[3] < preamble[4] && preamble[4] > preamble[5] && // 4
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preamble[9] < preamble[10] && preamble[10] > preamble[11] && // 10
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preamble[11] < preamble[12]) { // 12
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// peaks at 1,4,10,12: phase 6
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high = (preamble[1] + preamble[4] + preamble[10] + preamble[12]) / 4;
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sigLevel = preamble[1] + preamble[4] + preamble[10] + preamble[12];
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noiseLevel = preamble[5] + preamble[6] + preamble[7] + preamble[8];
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} else if (preamble[2] > preamble[3] && // 1-2
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preamble[3] < preamble[4] && preamble[4] > preamble[5] && // 4
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preamble[9] < preamble[10] && preamble[10] > preamble[11] && // 10
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preamble[11] < preamble[12]) { // 12
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// peaks at 1-2,4,10,12: phase 7
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high = (preamble[1] + preamble[2] + preamble[4] + preamble[10] + preamble[12]) / 4;
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sigLevel = preamble[4] + preamble[10] + preamble[12];
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noiseLevel = preamble[6] + preamble[7] + preamble[8];
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} else {
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// no suitable peaks
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continue;
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}
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// Check for enough signal
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if (sigLevel * 2 < 3 * noiseLevel) // about 3.5dB SNR
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continue;
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// Check that the "quiet" bits 6,7,15,16,17 are actually quiet
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if (preamble[5] >= high ||
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preamble[6] >= high ||
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preamble[7] >= high ||
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preamble[8] >= high ||
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preamble[14] >= high ||
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preamble[15] >= high ||
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preamble[16] >= high ||
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preamble[17] >= high ||
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preamble[18] >= high) {
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++Modes.stat_preamble_not_quiet;
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continue;
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}
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// Crosscorrelate against the first few bits to find a likely phase offset
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initial_phase = best_phase(&preamble[19]);
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if (initial_phase < 0) {
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++Modes.stat_preamble_no_correlation;
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continue; // nothing satisfactory
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}
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Modes.stat_valid_preamble++;
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Modes.stat_preamble_phase[initial_phase%MODES_MAX_PHASE_STATS]++;
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try_phase = initial_phase;
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retry:
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// Rather than clear the whole mm structure, just clear the parts which are required. The clear
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// is required for every possible preamble, and we don't want to be memset-ing the whole
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// modesMessage structure if we don't have to..
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mm.bFlags =
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mm.crcok =
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mm.correctedbits = 0;
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// Decode all the next 112 bits, regardless of the actual message
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// size. We'll check the actual message type later
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pMsg = &msg[0];
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pPtr = &m[j+19] + (try_phase/5);
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phase = try_phase % 5;
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theByte = 0;
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theErrs = 0; errorsTy = 0;
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errors = 0; errors56 = 0;
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msglen = scanlen = MODES_LONG_MSG_BITS;
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for (i = 0; i < scanlen; i++) {
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int test;
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switch (phase) {
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case 0:
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test = slice_phase0(pPtr);
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phase = 2;
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pPtr += 2;
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break;
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case 1:
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test = slice_phase1(pPtr);
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||||||
|
phase = 3;
|
||||||
|
pPtr += 2;
|
||||||
|
break;
|
||||||
|
|
||||||
|
case 2:
|
||||||
|
test = slice_phase2(pPtr);
|
||||||
|
phase = 4;
|
||||||
|
pPtr += 2;
|
||||||
|
break;
|
||||||
|
|
||||||
|
case 3:
|
||||||
|
test = slice_phase3(pPtr);
|
||||||
|
phase = 0;
|
||||||
|
pPtr += 3;
|
||||||
|
break;
|
||||||
|
|
||||||
|
case 4:
|
||||||
|
test = slice_phase4(pPtr);
|
||||||
|
|
||||||
|
// A phase-4 bit exactly straddles a sample boundary.
|
||||||
|
// Here's what a 1-0 bit with phase 4 looks like:
|
||||||
|
//
|
||||||
|
// |SYM 1|
|
||||||
|
// xxx| | |xxx
|
||||||
|
// |SYM 2|
|
||||||
|
//
|
||||||
|
// 012340123401234012340 <-- sample phase
|
||||||
|
// | 0 | 1 | 2 | 3 | <-- sample boundaries
|
||||||
|
//
|
||||||
|
// Samples 1 and 2 only have power from symbols 1 and 2.
|
||||||
|
// So we can use this to extract signal/noise values
|
||||||
|
// as one of the two symbols is high (signal) and the
|
||||||
|
// other is low (noise)
|
||||||
|
//
|
||||||
|
// This also gives us an equal number of signal and noise
|
||||||
|
// samples, which is convenient. Using the first half of
|
||||||
|
// a phase 0 bit, or the second half of a phase 3 bit, would
|
||||||
|
// also work, but we have no guarantees about how many signal
|
||||||
|
// or noise bits we'd see in those phases.
|
||||||
|
|
||||||
|
if (test < 0) { // 0 1
|
||||||
|
noiseLevel += pPtr[1];
|
||||||
|
sigLevel += pPtr[2];
|
||||||
|
} else { // 1 0
|
||||||
|
sigLevel += pPtr[1];
|
||||||
|
noiseLevel += pPtr[2];
|
||||||
|
}
|
||||||
|
phase = 1;
|
||||||
|
pPtr += 3;
|
||||||
|
break;
|
||||||
|
|
||||||
|
default:
|
||||||
|
test = 0;
|
||||||
|
break;
|
||||||
|
}
|
||||||
|
|
||||||
|
if (test > 0)
|
||||||
|
theByte |= 1;
|
||||||
|
/* else if (test < 0) theByte |= 0; */
|
||||||
|
else if (test == 0) {
|
||||||
|
if (i >= MODES_SHORT_MSG_BITS) { // poor correlation, and we're in the long part of a frame
|
||||||
|
errors++;
|
||||||
|
} else if (i >= 5) { // poor correlation, and we're in the short part of a frame
|
||||||
|
scanlen = MODES_LONG_MSG_BITS;
|
||||||
|
errors56 = ++errors;
|
||||||
|
} else if (i) { // poor correlation, and we're in the message type part of a frame
|
||||||
|
errorsTy = errors56 = ++errors;
|
||||||
|
theErrs |= 1;
|
||||||
|
} else { // poor correlation, and we're in the first bit of the message type part of a frame
|
||||||
|
errorsTy = errors56 = ++errors;
|
||||||
|
theErrs |= 1;
|
||||||
|
}
|
||||||
|
}
|
||||||
|
|
||||||
|
if ((i & 7) == 7)
|
||||||
|
*pMsg++ = theByte;
|
||||||
|
|
||||||
|
theByte = theByte << 1;
|
||||||
|
|
||||||
|
if (i < 7)
|
||||||
|
{theErrs = theErrs << 1;}
|
||||||
|
|
||||||
|
// If we've exceeded the permissible number of encoding errors, abandon ship now
|
||||||
|
if (errors > MODES_MSG_ENCODER_ERRS) {
|
||||||
|
if (i < MODES_SHORT_MSG_BITS) {
|
||||||
|
msglen = 0;
|
||||||
|
} else if ((errorsTy == 1) && (theErrs == 0x80)) {
|
||||||
|
// If we only saw one error in the first bit of the byte of the frame, then it's possible
|
||||||
|
// we guessed wrongly about the value of the bit. We may be able to correct it by guessing
|
||||||
|
// the other way.
|
||||||
|
//
|
||||||
|
// We guessed a '1' at bit 7, which is the DF length bit == 112 Bits.
|
||||||
|
// Inverting bit 7 will change the message type from a long to a short.
|
||||||
|
// Invert the bit, cross your fingers and carry on.
|
||||||
|
msglen = MODES_SHORT_MSG_BITS;
|
||||||
|
msg[0] ^= theErrs; errorsTy = 0;
|
||||||
|
errors = errors56; // revert to the number of errors prior to bit 56
|
||||||
|
Modes.stat_DF_Len_Corrected++;
|
||||||
|
} else if (i < MODES_LONG_MSG_BITS) {
|
||||||
|
msglen = MODES_SHORT_MSG_BITS;
|
||||||
|
errors = errors56;
|
||||||
|
} else {
|
||||||
|
msglen = MODES_LONG_MSG_BITS;
|
||||||
|
}
|
||||||
|
|
||||||
|
break;
|
||||||
|
}
|
||||||
|
}
|
||||||
|
|
||||||
|
// Ensure msglen is consistent with the DF type
|
||||||
|
if (msglen > 0) {
|
||||||
|
i = modesMessageLenByType(msg[0] >> 3);
|
||||||
|
if (msglen > i) {msglen = i;}
|
||||||
|
else if (msglen < i) {msglen = 0;}
|
||||||
|
}
|
||||||
|
|
||||||
|
//
|
||||||
|
// If we guessed at any of the bits in the DF type field, then look to see if our guess was sensible.
|
||||||
|
// Do this by looking to see if the original guess results in the DF type being one of the ICAO defined
|
||||||
|
// message types. If it isn't then toggle the guessed bit and see if this new value is ICAO defined.
|
||||||
|
// if the new value is ICAO defined, then update it in our message.
|
||||||
|
if ((msglen) && (errorsTy == 1) && (theErrs & 0x78)) {
|
||||||
|
// We guessed at one (and only one) of the message type bits. See if our guess is "likely"
|
||||||
|
// to be correct by comparing the DF against a list of known good DF's
|
||||||
|
int thisDF = ((theByte = msg[0]) >> 3) & 0x1f;
|
||||||
|
uint32_t validDFbits = 0x017F0831; // One bit per 32 possible DF's. Set bits 0,4,5,11,16.17.18.19,20,21,22,24
|
||||||
|
uint32_t thisDFbit = (1 << thisDF);
|
||||||
|
if (0 == (validDFbits & thisDFbit)) {
|
||||||
|
// The current DF is not ICAO defined, so is probably an errors.
|
||||||
|
// Toggle the bit we guessed at and see if the resultant DF is more likely
|
||||||
|
theByte ^= theErrs;
|
||||||
|
thisDF = (theByte >> 3) & 0x1f;
|
||||||
|
thisDFbit = (1 << thisDF);
|
||||||
|
// if this DF any more likely?
|
||||||
|
if (validDFbits & thisDFbit) {
|
||||||
|
// Yep, more likely, so update the main message
|
||||||
|
msg[0] = theByte;
|
||||||
|
Modes.stat_DF_Type_Corrected++;
|
||||||
|
errors--; // decrease the error count so we attempt to use the modified DF.
|
||||||
|
}
|
||||||
|
}
|
||||||
|
}
|
||||||
|
|
||||||
|
// snr = 5 * 20log10(sigLevel / noiseLevel) (in units of 0.2dB)
|
||||||
|
// = 100log10(sigLevel) - 100log10(noiseLevel)
|
||||||
|
|
||||||
|
while (sigLevel > 65535 || noiseLevel > 65535) {
|
||||||
|
sigLevel >>= 1;
|
||||||
|
noiseLevel >>= 1;
|
||||||
|
}
|
||||||
|
snr = Modes.log10lut[sigLevel] - Modes.log10lut[noiseLevel];
|
||||||
|
|
||||||
|
// When we reach this point, if error is small, and the signal strength is large enough
|
||||||
|
// we may have a Mode S message on our hands. It may still be broken and the CRC may not
|
||||||
|
// be correct, but this can be handled by the next layer.
|
||||||
|
if ( (msglen)
|
||||||
|
// && ((2 * snr) > (int) (MODES_MSG_SQUELCH_DB * 10))
|
||||||
|
&& (errors <= MODES_MSG_ENCODER_ERRS) ) {
|
||||||
|
// Set initial mm structure details
|
||||||
|
mm.timestampMsg = Modes.timestampBlk + (j*5) + try_phase;
|
||||||
|
mm.signalLevel = (snr > 255 ? 255 : (uint8_t)snr);
|
||||||
|
mm.phase_corrected = (initial_phase != try_phase);
|
||||||
|
|
||||||
|
// Decode the received message
|
||||||
|
decodeModesMessage(&mm, msg);
|
||||||
|
|
||||||
|
// Update statistics
|
||||||
|
if (Modes.stats) {
|
||||||
|
struct demod_stats *dstats = (mm.phase_corrected ? &Modes.stat_demod_phasecorrected : &Modes.stat_demod);
|
||||||
|
|
||||||
|
switch (errors) {
|
||||||
|
case 0: dstats->demodulated0++; break;
|
||||||
|
case 1: dstats->demodulated1++; break;
|
||||||
|
case 2: dstats->demodulated2++; break;
|
||||||
|
default: dstats->demodulated3++; break;
|
||||||
|
}
|
||||||
|
|
||||||
|
if (mm.crcok) {
|
||||||
|
dstats->goodcrc++;
|
||||||
|
dstats->goodcrc_byphase[try_phase%MODES_MAX_PHASE_STATS]++;
|
||||||
|
} else if (mm.correctedbits > 0) {
|
||||||
|
dstats->badcrc++;
|
||||||
|
dstats->fixed++;
|
||||||
|
if (mm.correctedbits <= MODES_MAX_BITERRORS)
|
||||||
|
dstats->bit_fix[mm.correctedbits-1] += 1;
|
||||||
|
} else {
|
||||||
|
dstats->badcrc++;
|
||||||
|
}
|
||||||
|
}
|
||||||
|
|
||||||
|
// Skip this message if we are sure it's fine
|
||||||
|
// (we actually skip to 8 bits before the end of the message,
|
||||||
|
// because we can often decode two messages that *almost* collide,
|
||||||
|
// where the preamble of the second message clobbered the last
|
||||||
|
// few bits of the first message, but the message bits didn't
|
||||||
|
// overlap)
|
||||||
|
if (mm.crcok || mm.correctedbits) {
|
||||||
|
j += (8 + msglen - 8)*12/5 - 1;
|
||||||
|
}
|
||||||
|
|
||||||
|
// Pass data to the next layer
|
||||||
|
useModesMessage(&mm);
|
||||||
|
|
||||||
|
// Only try with different phases if we mostly demodulated OK,
|
||||||
|
// but the CRC failed. This seems to catch most of the cases
|
||||||
|
// where trying different phases actually helps, and is much
|
||||||
|
// cheaper than trying it on every single candidate that passes
|
||||||
|
// peak detection
|
||||||
|
if (Modes.phase_enhance && !mm.crcok && !mm.correctedbits) {
|
||||||
|
if (try_phase == initial_phase)
|
||||||
|
++Modes.stat_out_of_phase;
|
||||||
|
try_phase++;
|
||||||
|
if (try_phase == 9)
|
||||||
|
try_phase = 4;
|
||||||
|
if (try_phase != initial_phase)
|
||||||
|
goto retry;
|
||||||
|
}
|
||||||
|
}
|
||||||
|
}
|
||||||
|
}
|
||||||
|
|
||||||
38
demod_2400.h
Normal file
38
demod_2400.h
Normal file
|
|
@ -0,0 +1,38 @@
|
||||||
|
// dump1090, a Mode S messages decoder for RTLSDR devices.
|
||||||
|
//
|
||||||
|
// Copyright (C) 2014,2015 Oliver Jowett <oliver@mutability.co.uk>
|
||||||
|
//
|
||||||
|
// All rights reserved.
|
||||||
|
//
|
||||||
|
// Redistribution and use in source and binary forms, with or without
|
||||||
|
// modification, are permitted provided that the following conditions are
|
||||||
|
// met:
|
||||||
|
//
|
||||||
|
// * Redistributions of source code must retain the above copyright
|
||||||
|
// notice, this list of conditions and the following disclaimer.
|
||||||
|
//
|
||||||
|
// * Redistributions in binary form must reproduce the above copyright
|
||||||
|
// notice, this list of conditions and the following disclaimer in the
|
||||||
|
// documentation and/or other materials provided with the distribution.
|
||||||
|
//
|
||||||
|
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
|
||||||
|
// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
|
||||||
|
// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
|
||||||
|
// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
|
||||||
|
// HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
|
||||||
|
// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
|
||||||
|
// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
|
||||||
|
// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
|
||||||
|
// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
|
||||||
|
// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
|
||||||
|
// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
|
||||||
|
//
|
||||||
|
|
||||||
|
#ifndef DUMP1090_DEMOD_2400_H
|
||||||
|
#define DUMP1090_DEMOD_2400_H
|
||||||
|
|
||||||
|
#include <stdint.h>
|
||||||
|
|
||||||
|
void demodulate2400(uint16_t *m, uint32_t mlen);
|
||||||
|
|
||||||
|
#endif
|
||||||
|
|
@ -1039,7 +1039,7 @@ int main(int argc, char **argv) {
|
||||||
clock_gettime(CLOCK_THREAD_CPUTIME_ID, &cpu_start_time);
|
clock_gettime(CLOCK_THREAD_CPUTIME_ID, &cpu_start_time);
|
||||||
|
|
||||||
if (Modes.oversample)
|
if (Modes.oversample)
|
||||||
detectModeS_oversample(Modes.magnitude, MODES_ASYNC_BUF_SAMPLES);
|
demodulate2400(Modes.magnitude, MODES_ASYNC_BUF_SAMPLES);
|
||||||
else
|
else
|
||||||
demodulate2000(Modes.magnitude, MODES_ASYNC_BUF_SAMPLES);
|
demodulate2000(Modes.magnitude, MODES_ASYNC_BUF_SAMPLES);
|
||||||
|
|
||||||
|
|
|
||||||
|
|
@ -73,6 +73,7 @@
|
||||||
#include "anet.h"
|
#include "anet.h"
|
||||||
#include "crc.h"
|
#include "crc.h"
|
||||||
#include "demod_2000.h"
|
#include "demod_2000.h"
|
||||||
|
#include "demod_2400.h"
|
||||||
|
|
||||||
// ============================= #defines ===============================
|
// ============================= #defines ===============================
|
||||||
//
|
//
|
||||||
|
|
|
||||||
499
mode_s.c
499
mode_s.c
|
|
@ -903,505 +903,6 @@ void computeMagnitudeVector(uint16_t *p) {
|
||||||
}
|
}
|
||||||
}
|
}
|
||||||
|
|
||||||
// 2.4MHz sampling rate version
|
|
||||||
//
|
|
||||||
// When sampling at 2.4MHz we have exactly 6 samples per 5 symbols.
|
|
||||||
// Each symbol is 500ns wide, each sample is 416.7ns wide
|
|
||||||
//
|
|
||||||
// We maintain a phase offset that is expressed in units of 1/5 of a sample i.e. 1/6 of a symbol, 83.333ns
|
|
||||||
// Each symbol we process advances the phase offset by 6 i.e. 6/5 of a sample, 500ns
|
|
||||||
//
|
|
||||||
// The correlation functions below correlate a 1-0 pair of symbols (i.e. manchester encoded 1 bit)
|
|
||||||
// starting at the given sample, and assuming that the symbol starts at a fixed 0-5 phase offset within
|
|
||||||
// m[0]. They return a correlation value, generally interpreted as >0 = 1 bit, <0 = 0 bit
|
|
||||||
|
|
||||||
// TODO check if there are better (or more balanced) correlation functions to use here
|
|
||||||
|
|
||||||
// nb: the correlation functions sum to zero, so we do not need to adjust for the DC offset in the input signal
|
|
||||||
// (adding any constant value to all of m[0..3] does not change the result)
|
|
||||||
|
|
||||||
static inline int slice_phase0(uint16_t *m) {
|
|
||||||
return 5 * m[0] - 3 * m[1] - 2 * m[2];
|
|
||||||
}
|
|
||||||
static inline int slice_phase1(uint16_t *m) {
|
|
||||||
return 4 * m[0] - m[1] - 3 * m[2];
|
|
||||||
}
|
|
||||||
static inline int slice_phase2(uint16_t *m) {
|
|
||||||
return 3 * m[0] + m[1] - 4 * m[2];
|
|
||||||
}
|
|
||||||
static inline int slice_phase3(uint16_t *m) {
|
|
||||||
return 2 * m[0] + 3 * m[1] - 5 * m[2];
|
|
||||||
}
|
|
||||||
static inline int slice_phase4(uint16_t *m) {
|
|
||||||
return m[0] + 5 * m[1] - 5 * m[2] - m[3];
|
|
||||||
}
|
|
||||||
|
|
||||||
static inline int correlate_phase0(uint16_t *m) {
|
|
||||||
return slice_phase0(m) * 26;
|
|
||||||
}
|
|
||||||
static inline int correlate_phase1(uint16_t *m) {
|
|
||||||
return slice_phase1(m) * 38;
|
|
||||||
}
|
|
||||||
static inline int correlate_phase2(uint16_t *m) {
|
|
||||||
return slice_phase2(m) * 38;
|
|
||||||
}
|
|
||||||
static inline int correlate_phase3(uint16_t *m) {
|
|
||||||
return slice_phase3(m) * 26;
|
|
||||||
}
|
|
||||||
static inline int correlate_phase4(uint16_t *m) {
|
|
||||||
return slice_phase4(m) * 19;
|
|
||||||
}
|
|
||||||
|
|
||||||
//
|
|
||||||
// These functions work out the correlation quality for the 10 symbols (5 bits) starting at m[0] + given phase offset.
|
|
||||||
// This is used to find the right phase offset to use for decoding.
|
|
||||||
//
|
|
||||||
|
|
||||||
static inline int correlate_check_0(uint16_t *m) {
|
|
||||||
return
|
|
||||||
abs(correlate_phase0(&m[0])) +
|
|
||||||
abs(correlate_phase2(&m[2])) +
|
|
||||||
abs(correlate_phase4(&m[4])) +
|
|
||||||
abs(correlate_phase1(&m[7])) +
|
|
||||||
abs(correlate_phase3(&m[9]));
|
|
||||||
}
|
|
||||||
|
|
||||||
static inline int correlate_check_1(uint16_t *m) {
|
|
||||||
return
|
|
||||||
abs(correlate_phase1(&m[0])) +
|
|
||||||
abs(correlate_phase3(&m[2])) +
|
|
||||||
abs(correlate_phase0(&m[5])) +
|
|
||||||
abs(correlate_phase2(&m[7])) +
|
|
||||||
abs(correlate_phase4(&m[9]));
|
|
||||||
}
|
|
||||||
|
|
||||||
static inline int correlate_check_2(uint16_t *m) {
|
|
||||||
return
|
|
||||||
abs(correlate_phase2(&m[0])) +
|
|
||||||
abs(correlate_phase4(&m[2])) +
|
|
||||||
abs(correlate_phase1(&m[5])) +
|
|
||||||
abs(correlate_phase3(&m[7])) +
|
|
||||||
abs(correlate_phase0(&m[10]));
|
|
||||||
}
|
|
||||||
|
|
||||||
static inline int correlate_check_3(uint16_t *m) {
|
|
||||||
return
|
|
||||||
abs(correlate_phase3(&m[0])) +
|
|
||||||
abs(correlate_phase0(&m[3])) +
|
|
||||||
abs(correlate_phase2(&m[5])) +
|
|
||||||
abs(correlate_phase4(&m[7])) +
|
|
||||||
abs(correlate_phase1(&m[10]));
|
|
||||||
}
|
|
||||||
|
|
||||||
static inline int correlate_check_4(uint16_t *m) {
|
|
||||||
return
|
|
||||||
abs(correlate_phase4(&m[0])) +
|
|
||||||
abs(correlate_phase1(&m[3])) +
|
|
||||||
abs(correlate_phase3(&m[5])) +
|
|
||||||
abs(correlate_phase0(&m[8])) +
|
|
||||||
abs(correlate_phase2(&m[10]));
|
|
||||||
}
|
|
||||||
|
|
||||||
// Work out the best phase offset to use for the given message.
|
|
||||||
static int best_phase(uint16_t *m) {
|
|
||||||
int test;
|
|
||||||
int best = -1;
|
|
||||||
int bestval = (m[0] + m[1] + m[2] + m[3] + m[4] + m[5]); // minimum correlation quality we will accept
|
|
||||||
|
|
||||||
// empirical testing suggests that 4..8 is the best range to test for here
|
|
||||||
// (testing a wider range runs the danger of picking the wrong phase for
|
|
||||||
// a message that would otherwise be successfully decoded - the correlation
|
|
||||||
// functions can match well with a one symbol / half bit offset)
|
|
||||||
|
|
||||||
// this is consistent with the peak detection which should produce
|
|
||||||
// the first data symbol with phase offset 4..8
|
|
||||||
|
|
||||||
//test = correlate_check_2(&m[0]);
|
|
||||||
//if (test > bestval) { bestval = test; best = 2; }
|
|
||||||
//test = correlate_check_3(&m[0]);
|
|
||||||
//if (test > bestval) { bestval = test; best = 3; }
|
|
||||||
test = correlate_check_4(&m[0]);
|
|
||||||
if (test > bestval) { bestval = test; best = 4; }
|
|
||||||
test = correlate_check_0(&m[1]);
|
|
||||||
if (test > bestval) { bestval = test; best = 5; }
|
|
||||||
test = correlate_check_1(&m[1]);
|
|
||||||
if (test > bestval) { bestval = test; best = 6; }
|
|
||||||
test = correlate_check_2(&m[1]);
|
|
||||||
if (test > bestval) { bestval = test; best = 7; }
|
|
||||||
test = correlate_check_3(&m[1]);
|
|
||||||
if (test > bestval) { bestval = test; best = 8; }
|
|
||||||
//test = correlate_check_4(&m[1]);
|
|
||||||
//if (test > bestval) { bestval = test; best = 9; }
|
|
||||||
return best;
|
|
||||||
}
|
|
||||||
|
|
||||||
//
|
|
||||||
//=========================================================================
|
|
||||||
//
|
|
||||||
// Detect a Mode S messages inside the magnitude buffer pointed by 'm' and of
|
|
||||||
// size 'mlen' bytes. Every detected Mode S message is convert it into a
|
|
||||||
// stream of bits and passed to the function to display it.
|
|
||||||
//
|
|
||||||
void detectModeS_oversample(uint16_t *m, uint32_t mlen) {
|
|
||||||
struct modesMessage mm;
|
|
||||||
unsigned char msg[MODES_LONG_MSG_BYTES], *pMsg;
|
|
||||||
uint32_t j;
|
|
||||||
|
|
||||||
memset(&mm, 0, sizeof(mm));
|
|
||||||
|
|
||||||
for (j = 0; j < mlen; j++) {
|
|
||||||
uint16_t *preamble = &m[j];
|
|
||||||
int high, i, initial_phase, phase, errors, errors56, errorsTy;
|
|
||||||
int msglen, scanlen;
|
|
||||||
uint16_t *pPtr;
|
|
||||||
uint8_t theByte, theErrs;
|
|
||||||
uint32_t sigLevel, noiseLevel;
|
|
||||||
uint16_t snr;
|
|
||||||
int try_phase;
|
|
||||||
|
|
||||||
// Look for a message starting at around sample 0 with phase offset 3..7
|
|
||||||
|
|
||||||
// Ideal sample values for preambles with different phase
|
|
||||||
// Xn is the first data symbol with phase offset N
|
|
||||||
//
|
|
||||||
// sample#: 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0
|
|
||||||
// phase 3: 2/4\0/5\1 0 0 0 0/5\1/3 3\0 0 0 0 0 0 X4
|
|
||||||
// phase 4: 1/5\0/4\2 0 0 0 0/4\2 2/4\0 0 0 0 0 0 0 X0
|
|
||||||
// phase 5: 0/5\1/3 3\0 0 0 0/3 3\1/5\0 0 0 0 0 0 0 X1
|
|
||||||
// phase 6: 0/4\2 2/4\0 0 0 0 2/4\0/5\1 0 0 0 0 0 0 X2
|
|
||||||
// phase 7: 0/3 3\1/5\0 0 0 0 1/5\0/4\2 0 0 0 0 0 0 X3
|
|
||||||
//
|
|
||||||
|
|
||||||
// quick check: we must have a rising edge 0->1 and a falling edge 12->13
|
|
||||||
if (! (preamble[0] < preamble[1] && preamble[12] > preamble[13]) )
|
|
||||||
continue;
|
|
||||||
|
|
||||||
if (preamble[1] > preamble[2] && // 1
|
|
||||||
preamble[2] < preamble[3] && preamble[3] > preamble[4] && // 3
|
|
||||||
preamble[8] < preamble[9] && preamble[9] > preamble[10] && // 9
|
|
||||||
preamble[10] < preamble[11]) { // 11-12
|
|
||||||
// peaks at 1,3,9,11-12: phase 3
|
|
||||||
high = (preamble[1] + preamble[3] + preamble[9] + preamble[11] + preamble[12]) / 4;
|
|
||||||
sigLevel = preamble[1] + preamble[3] + preamble[9];
|
|
||||||
noiseLevel = preamble[5] + preamble[6] + preamble[7];
|
|
||||||
} else if (preamble[1] > preamble[2] && // 1
|
|
||||||
preamble[2] < preamble[3] && preamble[3] > preamble[4] && // 3
|
|
||||||
preamble[8] < preamble[9] && preamble[9] > preamble[10] && // 9
|
|
||||||
preamble[11] < preamble[12]) { // 12
|
|
||||||
// peaks at 1,3,9,12: phase 4
|
|
||||||
high = (preamble[1] + preamble[3] + preamble[9] + preamble[12]) / 4;
|
|
||||||
sigLevel = preamble[1] + preamble[3] + preamble[9] + preamble[12];
|
|
||||||
noiseLevel = preamble[5] + preamble[6] + preamble[7] + preamble[8];
|
|
||||||
} else if (preamble[1] > preamble[2] && // 1
|
|
||||||
preamble[2] < preamble[3] && preamble[4] > preamble[5] && // 3-4
|
|
||||||
preamble[8] < preamble[9] && preamble[10] > preamble[11] && // 9-10
|
|
||||||
preamble[11] < preamble[12]) { // 12
|
|
||||||
// peaks at 1,3-4,9-10,12: phase 5
|
|
||||||
high = (preamble[1] + preamble[3] + preamble[4] + preamble[9] + preamble[10] + preamble[12]) / 4;
|
|
||||||
sigLevel = preamble[1] + preamble[12];
|
|
||||||
noiseLevel = preamble[6] + preamble[7];
|
|
||||||
} else if (preamble[1] > preamble[2] && // 1
|
|
||||||
preamble[3] < preamble[4] && preamble[4] > preamble[5] && // 4
|
|
||||||
preamble[9] < preamble[10] && preamble[10] > preamble[11] && // 10
|
|
||||||
preamble[11] < preamble[12]) { // 12
|
|
||||||
// peaks at 1,4,10,12: phase 6
|
|
||||||
high = (preamble[1] + preamble[4] + preamble[10] + preamble[12]) / 4;
|
|
||||||
sigLevel = preamble[1] + preamble[4] + preamble[10] + preamble[12];
|
|
||||||
noiseLevel = preamble[5] + preamble[6] + preamble[7] + preamble[8];
|
|
||||||
} else if (preamble[2] > preamble[3] && // 1-2
|
|
||||||
preamble[3] < preamble[4] && preamble[4] > preamble[5] && // 4
|
|
||||||
preamble[9] < preamble[10] && preamble[10] > preamble[11] && // 10
|
|
||||||
preamble[11] < preamble[12]) { // 12
|
|
||||||
// peaks at 1-2,4,10,12: phase 7
|
|
||||||
high = (preamble[1] + preamble[2] + preamble[4] + preamble[10] + preamble[12]) / 4;
|
|
||||||
sigLevel = preamble[4] + preamble[10] + preamble[12];
|
|
||||||
noiseLevel = preamble[6] + preamble[7] + preamble[8];
|
|
||||||
} else {
|
|
||||||
// no suitable peaks
|
|
||||||
continue;
|
|
||||||
}
|
|
||||||
|
|
||||||
// Check for enough signal
|
|
||||||
if (sigLevel * 2 < 3 * noiseLevel) // about 3.5dB SNR
|
|
||||||
continue;
|
|
||||||
|
|
||||||
// Check that the "quiet" bits 6,7,15,16,17 are actually quiet
|
|
||||||
if (preamble[5] >= high ||
|
|
||||||
preamble[6] >= high ||
|
|
||||||
preamble[7] >= high ||
|
|
||||||
preamble[8] >= high ||
|
|
||||||
preamble[14] >= high ||
|
|
||||||
preamble[15] >= high ||
|
|
||||||
preamble[16] >= high ||
|
|
||||||
preamble[17] >= high ||
|
|
||||||
preamble[18] >= high) {
|
|
||||||
++Modes.stat_preamble_not_quiet;
|
|
||||||
continue;
|
|
||||||
}
|
|
||||||
|
|
||||||
// Crosscorrelate against the first few bits to find a likely phase offset
|
|
||||||
initial_phase = best_phase(&preamble[19]);
|
|
||||||
if (initial_phase < 0) {
|
|
||||||
++Modes.stat_preamble_no_correlation;
|
|
||||||
continue; // nothing satisfactory
|
|
||||||
}
|
|
||||||
|
|
||||||
Modes.stat_valid_preamble++;
|
|
||||||
Modes.stat_preamble_phase[initial_phase%MODES_MAX_PHASE_STATS]++;
|
|
||||||
|
|
||||||
try_phase = initial_phase;
|
|
||||||
|
|
||||||
retry:
|
|
||||||
// Rather than clear the whole mm structure, just clear the parts which are required. The clear
|
|
||||||
// is required for every possible preamble, and we don't want to be memset-ing the whole
|
|
||||||
// modesMessage structure if we don't have to..
|
|
||||||
mm.bFlags =
|
|
||||||
mm.crcok =
|
|
||||||
mm.correctedbits = 0;
|
|
||||||
|
|
||||||
// Decode all the next 112 bits, regardless of the actual message
|
|
||||||
// size. We'll check the actual message type later
|
|
||||||
|
|
||||||
pMsg = &msg[0];
|
|
||||||
pPtr = &m[j+19] + (try_phase/5);
|
|
||||||
phase = try_phase % 5;
|
|
||||||
theByte = 0;
|
|
||||||
theErrs = 0; errorsTy = 0;
|
|
||||||
errors = 0; errors56 = 0;
|
|
||||||
msglen = scanlen = MODES_LONG_MSG_BITS;
|
|
||||||
for (i = 0; i < scanlen; i++) {
|
|
||||||
int test;
|
|
||||||
|
|
||||||
switch (phase) {
|
|
||||||
case 0:
|
|
||||||
test = slice_phase0(pPtr);
|
|
||||||
phase = 2;
|
|
||||||
pPtr += 2;
|
|
||||||
break;
|
|
||||||
|
|
||||||
case 1:
|
|
||||||
test = slice_phase1(pPtr);
|
|
||||||
phase = 3;
|
|
||||||
pPtr += 2;
|
|
||||||
break;
|
|
||||||
|
|
||||||
case 2:
|
|
||||||
test = slice_phase2(pPtr);
|
|
||||||
phase = 4;
|
|
||||||
pPtr += 2;
|
|
||||||
break;
|
|
||||||
|
|
||||||
case 3:
|
|
||||||
test = slice_phase3(pPtr);
|
|
||||||
phase = 0;
|
|
||||||
pPtr += 3;
|
|
||||||
break;
|
|
||||||
|
|
||||||
case 4:
|
|
||||||
test = slice_phase4(pPtr);
|
|
||||||
|
|
||||||
// A phase-4 bit exactly straddles a sample boundary.
|
|
||||||
// Here's what a 1-0 bit with phase 4 looks like:
|
|
||||||
//
|
|
||||||
// |SYM 1|
|
|
||||||
// xxx| | |xxx
|
|
||||||
// |SYM 2|
|
|
||||||
//
|
|
||||||
// 012340123401234012340 <-- sample phase
|
|
||||||
// | 0 | 1 | 2 | 3 | <-- sample boundaries
|
|
||||||
//
|
|
||||||
// Samples 1 and 2 only have power from symbols 1 and 2.
|
|
||||||
// So we can use this to extract signal/noise values
|
|
||||||
// as one of the two symbols is high (signal) and the
|
|
||||||
// other is low (noise)
|
|
||||||
//
|
|
||||||
// This also gives us an equal number of signal and noise
|
|
||||||
// samples, which is convenient. Using the first half of
|
|
||||||
// a phase 0 bit, or the second half of a phase 3 bit, would
|
|
||||||
// also work, but we have no guarantees about how many signal
|
|
||||||
// or noise bits we'd see in those phases.
|
|
||||||
|
|
||||||
if (test < 0) { // 0 1
|
|
||||||
noiseLevel += pPtr[1];
|
|
||||||
sigLevel += pPtr[2];
|
|
||||||
} else { // 1 0
|
|
||||||
sigLevel += pPtr[1];
|
|
||||||
noiseLevel += pPtr[2];
|
|
||||||
}
|
|
||||||
phase = 1;
|
|
||||||
pPtr += 3;
|
|
||||||
break;
|
|
||||||
|
|
||||||
default:
|
|
||||||
test = 0;
|
|
||||||
break;
|
|
||||||
}
|
|
||||||
|
|
||||||
if (test > 0)
|
|
||||||
theByte |= 1;
|
|
||||||
/* else if (test < 0) theByte |= 0; */
|
|
||||||
else if (test == 0) {
|
|
||||||
if (i >= MODES_SHORT_MSG_BITS) { // poor correlation, and we're in the long part of a frame
|
|
||||||
errors++;
|
|
||||||
} else if (i >= 5) { // poor correlation, and we're in the short part of a frame
|
|
||||||
scanlen = MODES_LONG_MSG_BITS;
|
|
||||||
errors56 = ++errors;
|
|
||||||
} else if (i) { // poor correlation, and we're in the message type part of a frame
|
|
||||||
errorsTy = errors56 = ++errors;
|
|
||||||
theErrs |= 1;
|
|
||||||
} else { // poor correlation, and we're in the first bit of the message type part of a frame
|
|
||||||
errorsTy = errors56 = ++errors;
|
|
||||||
theErrs |= 1;
|
|
||||||
}
|
|
||||||
}
|
|
||||||
|
|
||||||
if ((i & 7) == 7)
|
|
||||||
*pMsg++ = theByte;
|
|
||||||
|
|
||||||
theByte = theByte << 1;
|
|
||||||
|
|
||||||
if (i < 7)
|
|
||||||
{theErrs = theErrs << 1;}
|
|
||||||
|
|
||||||
// If we've exceeded the permissible number of encoding errors, abandon ship now
|
|
||||||
if (errors > MODES_MSG_ENCODER_ERRS) {
|
|
||||||
if (i < MODES_SHORT_MSG_BITS) {
|
|
||||||
msglen = 0;
|
|
||||||
} else if ((errorsTy == 1) && (theErrs == 0x80)) {
|
|
||||||
// If we only saw one error in the first bit of the byte of the frame, then it's possible
|
|
||||||
// we guessed wrongly about the value of the bit. We may be able to correct it by guessing
|
|
||||||
// the other way.
|
|
||||||
//
|
|
||||||
// We guessed a '1' at bit 7, which is the DF length bit == 112 Bits.
|
|
||||||
// Inverting bit 7 will change the message type from a long to a short.
|
|
||||||
// Invert the bit, cross your fingers and carry on.
|
|
||||||
msglen = MODES_SHORT_MSG_BITS;
|
|
||||||
msg[0] ^= theErrs; errorsTy = 0;
|
|
||||||
errors = errors56; // revert to the number of errors prior to bit 56
|
|
||||||
Modes.stat_DF_Len_Corrected++;
|
|
||||||
} else if (i < MODES_LONG_MSG_BITS) {
|
|
||||||
msglen = MODES_SHORT_MSG_BITS;
|
|
||||||
errors = errors56;
|
|
||||||
} else {
|
|
||||||
msglen = MODES_LONG_MSG_BITS;
|
|
||||||
}
|
|
||||||
|
|
||||||
break;
|
|
||||||
}
|
|
||||||
}
|
|
||||||
|
|
||||||
// Ensure msglen is consistent with the DF type
|
|
||||||
if (msglen > 0) {
|
|
||||||
i = modesMessageLenByType(msg[0] >> 3);
|
|
||||||
if (msglen > i) {msglen = i;}
|
|
||||||
else if (msglen < i) {msglen = 0;}
|
|
||||||
}
|
|
||||||
|
|
||||||
//
|
|
||||||
// If we guessed at any of the bits in the DF type field, then look to see if our guess was sensible.
|
|
||||||
// Do this by looking to see if the original guess results in the DF type being one of the ICAO defined
|
|
||||||
// message types. If it isn't then toggle the guessed bit and see if this new value is ICAO defined.
|
|
||||||
// if the new value is ICAO defined, then update it in our message.
|
|
||||||
if ((msglen) && (errorsTy == 1) && (theErrs & 0x78)) {
|
|
||||||
// We guessed at one (and only one) of the message type bits. See if our guess is "likely"
|
|
||||||
// to be correct by comparing the DF against a list of known good DF's
|
|
||||||
int thisDF = ((theByte = msg[0]) >> 3) & 0x1f;
|
|
||||||
uint32_t validDFbits = 0x017F0831; // One bit per 32 possible DF's. Set bits 0,4,5,11,16.17.18.19,20,21,22,24
|
|
||||||
uint32_t thisDFbit = (1 << thisDF);
|
|
||||||
if (0 == (validDFbits & thisDFbit)) {
|
|
||||||
// The current DF is not ICAO defined, so is probably an errors.
|
|
||||||
// Toggle the bit we guessed at and see if the resultant DF is more likely
|
|
||||||
theByte ^= theErrs;
|
|
||||||
thisDF = (theByte >> 3) & 0x1f;
|
|
||||||
thisDFbit = (1 << thisDF);
|
|
||||||
// if this DF any more likely?
|
|
||||||
if (validDFbits & thisDFbit) {
|
|
||||||
// Yep, more likely, so update the main message
|
|
||||||
msg[0] = theByte;
|
|
||||||
Modes.stat_DF_Type_Corrected++;
|
|
||||||
errors--; // decrease the error count so we attempt to use the modified DF.
|
|
||||||
}
|
|
||||||
}
|
|
||||||
}
|
|
||||||
|
|
||||||
// snr = 5 * 20log10(sigLevel / noiseLevel) (in units of 0.2dB)
|
|
||||||
// = 100log10(sigLevel) - 100log10(noiseLevel)
|
|
||||||
|
|
||||||
while (sigLevel > 65535 || noiseLevel > 65535) {
|
|
||||||
sigLevel >>= 1;
|
|
||||||
noiseLevel >>= 1;
|
|
||||||
}
|
|
||||||
snr = Modes.log10lut[sigLevel] - Modes.log10lut[noiseLevel];
|
|
||||||
|
|
||||||
// When we reach this point, if error is small, and the signal strength is large enough
|
|
||||||
// we may have a Mode S message on our hands. It may still be broken and the CRC may not
|
|
||||||
// be correct, but this can be handled by the next layer.
|
|
||||||
if ( (msglen)
|
|
||||||
// && ((2 * snr) > (int) (MODES_MSG_SQUELCH_DB * 10))
|
|
||||||
&& (errors <= MODES_MSG_ENCODER_ERRS) ) {
|
|
||||||
// Set initial mm structure details
|
|
||||||
mm.timestampMsg = Modes.timestampBlk + (j*5) + try_phase;
|
|
||||||
mm.signalLevel = (snr > 255 ? 255 : (uint8_t)snr);
|
|
||||||
mm.phase_corrected = (initial_phase != try_phase);
|
|
||||||
|
|
||||||
// Decode the received message
|
|
||||||
decodeModesMessage(&mm, msg);
|
|
||||||
|
|
||||||
// Update statistics
|
|
||||||
if (Modes.stats) {
|
|
||||||
struct demod_stats *dstats = (mm.phase_corrected ? &Modes.stat_demod_phasecorrected : &Modes.stat_demod);
|
|
||||||
|
|
||||||
switch (errors) {
|
|
||||||
case 0: dstats->demodulated0++; break;
|
|
||||||
case 1: dstats->demodulated1++; break;
|
|
||||||
case 2: dstats->demodulated2++; break;
|
|
||||||
default: dstats->demodulated3++; break;
|
|
||||||
}
|
|
||||||
|
|
||||||
if (mm.crcok) {
|
|
||||||
dstats->goodcrc++;
|
|
||||||
dstats->goodcrc_byphase[try_phase%MODES_MAX_PHASE_STATS]++;
|
|
||||||
} else if (mm.correctedbits > 0) {
|
|
||||||
dstats->badcrc++;
|
|
||||||
dstats->fixed++;
|
|
||||||
if (mm.correctedbits <= MODES_MAX_BITERRORS)
|
|
||||||
dstats->bit_fix[mm.correctedbits-1] += 1;
|
|
||||||
} else {
|
|
||||||
dstats->badcrc++;
|
|
||||||
}
|
|
||||||
}
|
|
||||||
|
|
||||||
// Skip this message if we are sure it's fine
|
|
||||||
// (we actually skip to 8 bits before the end of the message,
|
|
||||||
// because we can often decode two messages that *almost* collide,
|
|
||||||
// where the preamble of the second message clobbered the last
|
|
||||||
// few bits of the first message, but the message bits didn't
|
|
||||||
// overlap)
|
|
||||||
if (mm.crcok || mm.correctedbits) {
|
|
||||||
j += (8 + msglen - 8)*12/5 - 1;
|
|
||||||
}
|
|
||||||
|
|
||||||
// Pass data to the next layer
|
|
||||||
useModesMessage(&mm);
|
|
||||||
|
|
||||||
// Only try with different phases if we mostly demodulated OK,
|
|
||||||
// but the CRC failed. This seems to catch most of the cases
|
|
||||||
// where trying different phases actually helps, and is much
|
|
||||||
// cheaper than trying it on every single candidate that passes
|
|
||||||
// peak detection
|
|
||||||
if (Modes.phase_enhance && !mm.crcok && !mm.correctedbits) {
|
|
||||||
if (try_phase == initial_phase)
|
|
||||||
++Modes.stat_out_of_phase;
|
|
||||||
try_phase++;
|
|
||||||
if (try_phase == 9)
|
|
||||||
try_phase = 4;
|
|
||||||
if (try_phase != initial_phase)
|
|
||||||
goto retry;
|
|
||||||
}
|
|
||||||
}
|
|
||||||
}
|
|
||||||
}
|
|
||||||
|
|
||||||
//
|
//
|
||||||
//=========================================================================
|
//=========================================================================
|
||||||
//
|
//
|
||||||
|
|
|
||||||
Loading…
Reference in a new issue