AFG/dump1090
3
0
Fork 0
Code Issues Pull requests Packages Projects Releases Wiki Activity

Oversampling, round two.

Browse source 
This now seems to be at the point where it will decode more messages
than when using 2MHz with --phase-enhance.
This commit is contained in:
Oliver Jowett 2014-09-27 13:07:23 +01:00
parent 17f73cc01a
commit 69a30535d4
3 changed files with 132 additions and 77 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

15
dump1090.c
View file

@ -508,7 +508,12 @@ static void display_stats(void) {
printf("%d sample blocks dropped\n", Modes.stat_blocks_dropped);
printf("%d ModeA/C detected\n", Modes.stat_ModeAC);
printf("%d Mode-S preambles with poor correlation\n", Modes.stat_preamble_no_correlation);
printf("%d Mode-S preambles with noise in the quiet period\n", Modes.stat_preamble_not_quiet);
printf("%d valid Mode-S preambles\n", Modes.stat_valid_preamble);
for (j = 0; j < MODES_MAX_PHASE_STATS; ++j)
if (Modes.stat_preamble_phase[j] > 0)
printf(" %d with phase offset %d\n", Modes.stat_preamble_phase[j], j);
printf("%d DF-?? fields corrected for length\n", Modes.stat_DF_Len_Corrected);
printf("%d DF-?? fields corrected for type\n", Modes.stat_DF_Type_Corrected);
printf("%d demodulated with 0 errors\n", Modes.stat_demodulated0);
@ -516,6 +521,9 @@ static void display_stats(void) {
printf("%d demodulated with 2 errors\n", Modes.stat_demodulated2);
printf("%d demodulated with > 2 errors\n", Modes.stat_demodulated3);
printf("%d with good crc\n", Modes.stat_goodcrc);
for (j = 0; j < MODES_MAX_PHASE_STATS; ++j)
if (Modes.stat_goodcrc_phase[j] > 0)
printf(" %d with phase offset %d\n", Modes.stat_goodcrc_phase[j], j);
printf("%d with bad crc\n", Modes.stat_badcrc);
printf("%d errors corrected\n", Modes.stat_fixed);
@ -545,6 +553,8 @@ static void display_stats(void) {
Modes.stat_blocks_dropped = 0;
Modes.stat_ModeAC =
Modes.stat_preamble_no_correlation =
Modes.stat_preamble_not_quiet =
Modes.stat_valid_preamble =
Modes.stat_DF_Len_Corrected =
Modes.stat_DF_Type_Corrected =
@ -569,6 +579,11 @@ static void display_stats(void) {
Modes.stat_ph_bit_fix[j] = 0;
Modes.stat_bit_fix[j] = 0;
}
for (j = 0; j < MODES_MAX_PHASE_STATS; j++) {
Modes.stat_preamble_phase[j] = 0;
Modes.stat_goodcrc_phase[j] = 0;
}
}

5
dump1090.h
View file

@ -347,12 +347,17 @@ struct { // Internal state
struct stDF *pDF; // Pointer to DF list
// Statistics
#define MODES_MAX_PHASE_STATS 12
unsigned int stat_preamble_no_correlation;
unsigned int stat_preamble_not_quiet;
unsigned int stat_valid_preamble;
unsigned int stat_preamble_phase[MODES_MAX_PHASE_STATS];
unsigned int stat_demodulated0;
unsigned int stat_demodulated1;
unsigned int stat_demodulated2;
unsigned int stat_demodulated3;
unsigned int stat_goodcrc;
unsigned int stat_goodcrc_phase[MODES_MAX_PHASE_STATS];
unsigned int stat_badcrc;
unsigned int stat_fixed;

161
mode_s.c
View file

@ -1947,20 +1947,25 @@ void detectModeS(uint16_t *m, uint32_t mlen) {
// 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 correlate_phase0(uint16_t *m) {
return 5 * m[0] - 3 * m[1] - 2 * m[2];
return (5 * m[0] - 3 * m[1] - 2 * m[2]) * 10 / 19;
}
static inline int correlate_phase1(uint16_t *m) {
return 4 * m[0] - 1 * m[1] - 3 * m[2];
return (4 * m[0] - 1 * m[1] - 3 * m[2]) * 10 / 13;
}
static inline int correlate_phase2(uint16_t *m) {
return 3 * m[0] + 1 * m[1] - 4 * m[2];
return (3 * m[0] + 1 * m[1] - 4 * m[2]) * 10 / 13;
}
static inline int correlate_phase3(uint16_t *m) {
return 2 * m[0] + 3 * m[1] - 5 * m[2];
return (2 * m[0] + 3 * m[1] - 5 * m[2]) * 10 / 14;
}
static inline int correlate_phase4(uint16_t *m) {
return 1 * m[0] + 5 * m[1] - 5 * m[2] + 1 * m[3];
return (1 * m[0] + 5 * m[1] - 5 * m[2] - 1 * m[3]) * 10 / 28;
}
//
@ -2014,25 +2019,28 @@ static inline int correlate_check_4(uint16_t *m) {
}
// Work out the best phase offset to use for the given message.
// Note that the message may start at anywhere between
// m[19] offset 1 and m[20] offset 0
static int best_phase(uint16_t *m) {
int test;
int best = -1;
int bestval = 50; // minimum correlation quality we will accept
// look at the first 5 bits with each possible phase
test = correlate_check_1(&m[19]);
if (test > bestval) { bestval = test; best = 1; }
test = correlate_check_2(&m[19]);
if (test > bestval) { bestval = test; best = 2; }
test = correlate_check_3(&m[19]);
if (test > bestval) { bestval = test; best = 3; }
test = correlate_check_4(&m[19]);
if (test > bestval) { bestval = test; best = 4; }
test = correlate_check_0(&m[20]);
if (test > bestval) { best = 5; }
// empirical testing suggests that 3..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)
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; }
return best;
}
@ -2052,69 +2060,98 @@ void detectModeS_oversample(uint16_t *m, uint32_t mlen) {
for (j = 0; j < mlen; j++) {
uint16_t *preamble = &m[j];
int high, i, phase, errors, errors56, errorsTy;
int high, i, initial_phase, phase, errors, errors56, errorsTy;
int msglen, scanlen;
uint16_t *pPtr;
uint8_t theByte, theErrs;
// Rather than clear the whole mm structure, just clear the parts which are required. The clear
// is required for every bit of the input stream, and we don't want to be memset-ing the whole
// modesMessage structure two million times per second if we don't have to..
mm.bFlags =
mm.crcok =
mm.correctedbits = 0;
// Look for a message starting at around sample 0 with phase offset 3..7
// Look for a message starting at sample 1 with phase offset 0-4
// 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
//
// Check for peaks at (1,12) or (3,9)
if (preamble[1] > preamble[0] &&
preamble[1] > preamble[2] &&
preamble[12] > preamble[11] &&
preamble[12] > preamble[13]) {
high = (preamble[1] + preamble[12]) / 2;
} else if (preamble[3] > preamble[2] &&
preamble[3] > preamble[4] &&
preamble[9] > preamble[8] &&
preamble[9] > preamble[10]) {
high = (preamble[3] + preamble[9]) / 2;
// 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;
} 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;
} 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;
} 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;
} 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;
} else {
// No peaks
// no suitable peaks
continue;
}
// The samples between the two spikes must be < than the average
// of the high spikes level. We don't test bits too near to
// the high levels as signals can be out of phase so part of the
// energy can be in the near samples
// Check that the "quiet" bits 6,7,15,16,17 are actually quiet
if (preamble[5] >= high ||
preamble[6] >= high ||
if (preamble[6] >= high ||
preamble[7] >= high ||
preamble[14] >= high ||
preamble[15] >= high ||
preamble[16] >= high ||
preamble[17] >= high ||
preamble[18] >= high)
preamble[17] >= high) {
++Modes.stat_preamble_not_quiet;
continue;
}
// Crosscorrelate against the first few bits to find a likely phase offset
phase = best_phase(preamble);
if (phase < 0) {
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]++;
// 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];
if (phase == 5) {
++pPtr;
phase = 0;
}
pPtr = &m[j+19] + (initial_phase/5);
phase = initial_phase % 5;
theByte = 0;
theErrs = 0; errorsTy = 0;
errors = 0; errors56 = 0;
@ -2158,12 +2195,10 @@ void detectModeS_oversample(uint16_t *m, uint32_t mlen) {
break;
}
if (test > 10)
theByte |= 1;
else if (test >= -10) {
if (test > 0)
theByte |= 1; // best guess
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
@ -2261,7 +2296,6 @@ void detectModeS_oversample(uint16_t *m, uint32_t mlen) {
// Update statistics
if (Modes.stats) {
if (mm.crcok || mm.correctedbits) {
switch (errors) {
case 0: {Modes.stat_demodulated0++; break;}
case 1: {Modes.stat_demodulated1++; break;}
@ -2270,8 +2304,10 @@ void detectModeS_oversample(uint16_t *m, uint32_t mlen) {
}
if (mm.correctedbits == 0) {
if (mm.crcok) {Modes.stat_goodcrc++;}
else {Modes.stat_badcrc++;}
if (mm.crcok) {
Modes.stat_goodcrc++;
Modes.stat_goodcrc_phase[initial_phase%MODES_MAX_PHASE_STATS]++;
} else {Modes.stat_badcrc++;}
} else {
Modes.stat_badcrc++;
Modes.stat_fixed++;
@ -2281,7 +2317,6 @@ void detectModeS_oversample(uint16_t *m, uint32_t mlen) {
}
}
}
}
// Skip this message if we are sure it's fine
if (mm.crcok) {