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Import new 2.4MHz demodulator from experimental branch.

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This commit is contained in:
Oliver Jowett 2015-01-21 13:32:17 +00:00
parent 0493248425
commit ef098a2461
3 changed files with 202 additions and 280 deletions
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252
demod_2400.c
View file

@ -132,10 +132,6 @@ static int best_phase(uint16_t *m) {
// 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]);
@ -146,35 +142,30 @@ static int best_phase(uint16_t *m) {
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;
}
//
//=========================================================================
// Given 'mlen' magnitude samples in 'm', sampled at 2.4MHz,
// try to demodulate some Mode S messages.
//
// 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 demodulate2400(uint16_t *m, uint32_t mlen)
{
void demodulate2400(uint16_t *m, uint32_t mlen) {
struct modesMessage mm;
unsigned char msg[MODES_LONG_MSG_BYTES], *pMsg;
unsigned char msg1[MODES_LONG_MSG_BYTES], msg2[MODES_LONG_MSG_BYTES], *msg;
uint32_t j;
unsigned char *bestmsg;
int bestscore, bestphase, bestsnr;
memset(&mm, 0, sizeof(mm));
msg = msg1;
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;
int high;
uint32_t base_signal, base_noise;
int initial_phase, first_phase, last_phase, try_phase;
int msglen;
// Look for a message starting at around sample 0 with phase offset 3..7
@ -199,47 +190,47 @@ void demodulate2400(uint16_t *m, uint32_t mlen)
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];
base_signal = preamble[1] + preamble[3] + preamble[9];
base_noise = 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];
base_signal = preamble[1] + preamble[3] + preamble[9] + preamble[12];
base_noise = 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];
base_signal = preamble[1] + preamble[12];
base_noise = 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];
base_signal = preamble[1] + preamble[4] + preamble[10] + preamble[12];
base_noise = 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];
base_signal = preamble[4] + preamble[10] + preamble[12];
base_noise = preamble[6] + preamble[7] + preamble[8];
} else {
// no suitable peaks
continue;
}
// Check for enough signal
if (sigLevel * 2 < 3 * noiseLevel) // about 3.5dB SNR
if (base_signal * 2 < 3 * base_noise) // about 3.5dB SNR
continue;
// Check that the "quiet" bits 6,7,15,16,17 are actually quiet
@ -256,6 +247,10 @@ void demodulate2400(uint16_t *m, uint32_t mlen)
continue;
}
if (Modes.phase_enhance) {
first_phase = 4;
last_phase = 8; // try all phases
} else {
// Crosscorrelate against the first few bits to find a likely phase offset
initial_phase = best_phase(&preamble[19]);
if (initial_phase < 0) {
@ -263,17 +258,18 @@ void demodulate2400(uint16_t *m, uint32_t mlen)
continue; // nothing satisfactory
}
Modes.stats_current.valid_preamble++;
Modes.stats_current.preamble_phase[initial_phase%MODES_MAX_PHASE_STATS]++;
first_phase = last_phase = initial_phase; // try only the phase we think it is
}
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.correctedbits = 0;
Modes.stats_current.valid_preamble++;
bestmsg = NULL; bestscore = -1; bestphase = -1; bestsnr = -1;
for (try_phase = first_phase; try_phase <= last_phase; ++try_phase) {
int sigLevel = base_signal, noiseLevel = base_noise;
uint8_t theByte;
uint16_t *pPtr;
unsigned char *pMsg;
int phase, errors, i, snr, score;
// Decode all the next 112 bits, regardless of the actual message
// size. We'll check the actual message type later
@ -282,10 +278,9 @@ void demodulate2400(uint16_t *m, uint32_t mlen)
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++) {
errors = 0;
for (i = 0; i < MODES_LONG_MSG_BITS && errors < MODES_MSG_ENCODER_ERRS; i++) {
int test;
switch (phase) {
@ -357,166 +352,89 @@ void demodulate2400(uint16_t *m, uint32_t mlen)
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;
}
++errors;
}
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.stats_current.DF_Len_Corrected++;
} else if (i < MODES_LONG_MSG_BITS) {
msglen = MODES_SHORT_MSG_BITS;
errors = errors56;
} else {
msglen = MODES_LONG_MSG_BITS;
}
break;
}
}
// Score the mode S message and see if it's any good.
score = scoreModesMessage(msg, i);
if (score < 0)
continue; // can't decode
// 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.stats_current.DF_Type_Corrected++;
errors--; // decrease the error count so we attempt to use the modified DF.
}
}
}
// apply the SNR to the score, so less noisy decodes are better,
// all things being equal
// 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) ) {
int message_ok;
snr = Modes.log10lut[sigLevel] - Modes.log10lut[noiseLevel];
score += snr;
if (score > bestscore) {
// new high score!
bestmsg = msg;
bestscore = score;
bestphase = try_phase;
bestsnr = snr;
// swap to using the other buffer so we don't clobber our demodulated data
// (if we find a better result then we'll swap back, but that's OK because
// we no longer need this copy if we found a better one)
msg = (msg == msg1) ? msg2 : msg1;
}
}
// Do we have a candidate?
if (!bestmsg) {
Modes.stats_current.demod.badcrc++;
continue; // nope.
}
msglen = modesMessageLenByType(bestmsg[0] >> 3);
// 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);
mm.timestampMsg = Modes.timestampBlk + (j*5) + bestphase;
mm.signalLevel = (bestsnr > 255 ? 255 : (uint8_t)bestsnr);
mm.score = bestscore;
mm.bFlags = mm.correctedbits = 0;
// Decode the received message
message_ok = (decodeModesMessage(&mm, msg) >= 0);
if (decodeModesMessage(&mm, bestmsg) < 0)
continue;
// Update statistics
if (Modes.stats) {
struct demod_stats *dstats = (mm.phase_corrected ? &Modes.stats_current.demod_phasecorrected : &Modes.stats_current.demod);
switch (errors) {
case 0: dstats->demodulated0++; break;
case 1: dstats->demodulated1++; break;
case 2: dstats->demodulated2++; break;
default: dstats->demodulated3++; break;
}
if (!message_ok) {
dstats->badcrc++;
} else if (mm.correctedbits > 0) {
dstats->badcrc++;
dstats->fixed++;
if (mm.correctedbits <= MODES_MAX_BITERRORS)
dstats->bit_fix[mm.correctedbits-1] += 1;
if (mm.correctedbits == 0) {
Modes.stats_current.demod.goodcrc++;
Modes.stats_current.demod.goodcrc_byphase[bestphase%MODES_MAX_PHASE_STATS]++;
} else {
dstats->goodcrc++;
dstats->goodcrc_byphase[try_phase%MODES_MAX_PHASE_STATS]++;
Modes.stats_current.demod.badcrc++;
Modes.stats_current.demod.fixed++;
if (mm.correctedbits)
Modes.stats_current.demod.bit_fix[mm.correctedbits-1]++;
}
}
// Skip this message if we are sure it's fine
// Skip over the message:
// (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 (message_ok) {
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 && !message_ok) {
if (try_phase == initial_phase)
++Modes.stats_current.out_of_phase;
try_phase++;
if (try_phase == 9)
try_phase = 4;
if (try_phase != initial_phase)
goto retry;
}
}
}
}

1
dump1090.h
View file

@ -403,6 +403,7 @@ struct modesMessage {
uint64_t timestampMsg; // Timestamp of the message
int remote; // If set this message is from a remote station
unsigned char signalLevel; // Signal Amplitude
int score;
// DF 11, DF 17
int ca; // Responder capabilities

3
mode_s.c
View file

@ -998,6 +998,9 @@ void displayModesMessage(struct modesMessage *mm) {
printf("SNR: %d.%d dB\n", mm->signalLevel/5, 2*(mm->signalLevel%5));
if (mm->score)
printf("Score: %d\n", mm->score);
if (mm->timestampMsg)
printf("Time: %.2fus (phase: %d)\n", mm->timestampMsg / 12.0, (unsigned int) (360 * (mm->timestampMsg % 6) / 6));