forked from flightaware/dump1090
-
Notifications
You must be signed in to change notification settings - Fork 0
/
Copy pathdemod_2400.c
661 lines (554 loc) · 25 KB
/
demod_2400.c
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
// Part of dump1090, a Mode S message decoder for RTLSDR devices.
//
// demod_2400.c: 2.4MHz Mode S demodulator.
//
// Copyright (c) 2014,2015 Oliver Jowett <[email protected]>
//
// This file is free software: you may copy, redistribute and/or modify it
// under the terms of the GNU General Public License as published by the
// Free Software Foundation, either version 2 of the License, or (at your
// option) any later version.
//
// This file 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
// General Public License for more details.
//
// You should have received a copy of the GNU General Public License
// along with this program. If not, see <http://www.gnu.org/licenses/>.
#include "dump1090.h"
#include <assert.h>
#ifdef MODEAC_DEBUG
#include <gd.h>
#endif
// 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];
}
//
// Given 'mlen' magnitude samples in 'm', sampled at 2.4MHz,
// try to demodulate some Mode S messages.
//
void demodulate2400(struct mag_buf *mag)
{
static struct modesMessage zeroMessage;
struct modesMessage mm;
unsigned char msg1[MODES_LONG_MSG_BYTES], msg2[MODES_LONG_MSG_BYTES], *msg;
uint32_t j;
unsigned char *bestmsg;
int bestscore, bestphase;
uint16_t *m = mag->data;
uint32_t mlen = mag->length;
uint64_t sum_scaled_signal_power = 0;
msg = msg1;
for (j = 0; j < mlen; j++) {
uint16_t *preamble = &m[j];
int high;
uint32_t base_signal, base_noise;
int try_phase;
int msglen;
// 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;
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;
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;
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;
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;
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 (base_signal * 2 < 3 * base_noise) // 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) {
continue;
}
// try all phases
Modes.stats_current.demod_preambles++;
bestmsg = NULL; bestscore = -2; bestphase = -1;
for (try_phase = 4; try_phase <= 8; ++try_phase) {
uint16_t *pPtr;
int phase, i, score, bytelen;
// Decode all the next 112 bits, regardless of the actual message
// size. We'll check the actual message type later
pPtr = &m[j+19] + (try_phase/5);
phase = try_phase % 5;
bytelen = MODES_LONG_MSG_BYTES;
for (i = 0; i < bytelen; ++i) {
uint8_t theByte = 0;
switch (phase) {
case 0:
theByte =
(slice_phase0(pPtr) > 0 ? 0x80 : 0) |
(slice_phase2(pPtr+2) > 0 ? 0x40 : 0) |
(slice_phase4(pPtr+4) > 0 ? 0x20 : 0) |
(slice_phase1(pPtr+7) > 0 ? 0x10 : 0) |
(slice_phase3(pPtr+9) > 0 ? 0x08 : 0) |
(slice_phase0(pPtr+12) > 0 ? 0x04 : 0) |
(slice_phase2(pPtr+14) > 0 ? 0x02 : 0) |
(slice_phase4(pPtr+16) > 0 ? 0x01 : 0);
phase = 1;
pPtr += 19;
break;
case 1:
theByte =
(slice_phase1(pPtr) > 0 ? 0x80 : 0) |
(slice_phase3(pPtr+2) > 0 ? 0x40 : 0) |
(slice_phase0(pPtr+5) > 0 ? 0x20 : 0) |
(slice_phase2(pPtr+7) > 0 ? 0x10 : 0) |
(slice_phase4(pPtr+9) > 0 ? 0x08 : 0) |
(slice_phase1(pPtr+12) > 0 ? 0x04 : 0) |
(slice_phase3(pPtr+14) > 0 ? 0x02 : 0) |
(slice_phase0(pPtr+17) > 0 ? 0x01 : 0);
phase = 2;
pPtr += 19;
break;
case 2:
theByte =
(slice_phase2(pPtr) > 0 ? 0x80 : 0) |
(slice_phase4(pPtr+2) > 0 ? 0x40 : 0) |
(slice_phase1(pPtr+5) > 0 ? 0x20 : 0) |
(slice_phase3(pPtr+7) > 0 ? 0x10 : 0) |
(slice_phase0(pPtr+10) > 0 ? 0x08 : 0) |
(slice_phase2(pPtr+12) > 0 ? 0x04 : 0) |
(slice_phase4(pPtr+14) > 0 ? 0x02 : 0) |
(slice_phase1(pPtr+17) > 0 ? 0x01 : 0);
phase = 3;
pPtr += 19;
break;
case 3:
theByte =
(slice_phase3(pPtr) > 0 ? 0x80 : 0) |
(slice_phase0(pPtr+3) > 0 ? 0x40 : 0) |
(slice_phase2(pPtr+5) > 0 ? 0x20 : 0) |
(slice_phase4(pPtr+7) > 0 ? 0x10 : 0) |
(slice_phase1(pPtr+10) > 0 ? 0x08 : 0) |
(slice_phase3(pPtr+12) > 0 ? 0x04 : 0) |
(slice_phase0(pPtr+15) > 0 ? 0x02 : 0) |
(slice_phase2(pPtr+17) > 0 ? 0x01 : 0);
phase = 4;
pPtr += 19;
break;
case 4:
theByte =
(slice_phase4(pPtr) > 0 ? 0x80 : 0) |
(slice_phase1(pPtr+3) > 0 ? 0x40 : 0) |
(slice_phase3(pPtr+5) > 0 ? 0x20 : 0) |
(slice_phase0(pPtr+8) > 0 ? 0x10 : 0) |
(slice_phase2(pPtr+10) > 0 ? 0x08 : 0) |
(slice_phase4(pPtr+12) > 0 ? 0x04 : 0) |
(slice_phase1(pPtr+15) > 0 ? 0x02 : 0) |
(slice_phase3(pPtr+17) > 0 ? 0x01 : 0);
phase = 0;
pPtr += 20;
break;
}
msg[i] = theByte;
if (i == 0) {
switch (msg[0] >> 3) {
case 0: case 4: case 5: case 11:
bytelen = MODES_SHORT_MSG_BYTES; break;
case 16: case 17: case 18: case 20: case 21: case 24:
break;
default:
bytelen = 1; // unknown DF, give up immediately
break;
}
}
}
// Score the mode S message and see if it's any good.
score = scoreModesMessage(msg, i*8);
if (score > bestscore) {
// new high score!
bestmsg = msg;
bestscore = score;
bestphase = try_phase;
// 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 (bestscore < 0) {
if (bestscore == -1)
Modes.stats_current.demod_rejected_unknown_icao++;
else
Modes.stats_current.demod_rejected_bad++;
continue; // nope.
}
msglen = modesMessageLenByType(bestmsg[0] >> 3);
// Set initial mm structure details
mm = zeroMessage;
// For consistency with how the Beast / Radarcape does it,
// we report the timestamp at the end of bit 56 (even if
// the frame is a 112-bit frame)
mm.timestampMsg = mag->sampleTimestamp + j*5 + (8 + 56) * 12 + bestphase;
// compute message receive time as block-start-time + difference in the 12MHz clock
mm.sysTimestampMsg = mag->sysTimestamp; // start of block time
mm.sysTimestampMsg.tv_nsec += receiveclock_ns_elapsed(mag->sampleTimestamp, mm.timestampMsg);
normalize_timespec(&mm.sysTimestampMsg);
mm.score = bestscore;
// Decode the received message
{
int result = decodeModesMessage(&mm, bestmsg);
if (result < 0) {
if (result == -1)
Modes.stats_current.demod_rejected_unknown_icao++;
else
Modes.stats_current.demod_rejected_bad++;
continue;
} else {
Modes.stats_current.demod_accepted[mm.correctedbits]++;
}
}
// measure signal power
{
double signal_power;
uint64_t scaled_signal_power = 0;
int signal_len = msglen*12/5;
int k;
for (k = 0; k < signal_len; ++k) {
uint32_t mag = m[j+19+k];
scaled_signal_power += mag * mag;
}
signal_power = scaled_signal_power / 65535.0 / 65535.0;
mm.signalLevel = signal_power / signal_len;
Modes.stats_current.signal_power_sum += signal_power;
Modes.stats_current.signal_power_count += signal_len;
sum_scaled_signal_power += scaled_signal_power;
if (mm.signalLevel > Modes.stats_current.peak_signal_power)
Modes.stats_current.peak_signal_power = mm.signalLevel;
if (mm.signalLevel > 0.50119)
Modes.stats_current.strong_signal_count++; // signal power above -3dBFS
}
// 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)
j += msglen*12/5;
// Pass data to the next layer
useModesMessage(&mm);
}
/* update noise power */
{
double sum_signal_power = sum_scaled_signal_power / 65535.0 / 65535.0;
Modes.stats_current.noise_power_sum += (mag->mean_power * mag->length - sum_signal_power);
Modes.stats_current.noise_power_count += mag->length;
}
}
#ifdef MODEAC_DEBUG
static int yscale(unsigned signal)
{
return (int) (299 - 299.0 * signal / 65536.0);
}
static void draw_modeac(uint16_t *m, unsigned modeac, unsigned f1_clock, unsigned noise_threshold, unsigned signal_threshold, unsigned bits, unsigned noisy_bits, unsigned uncertain_bits)
{
// 25 bits at 87*60MHz
// use 1 pixel = 30MHz = 1087 pixels
gdImagePtr im = gdImageCreate(1088, 300);
int red = gdImageColorAllocate(im, 255, 0, 0);
int brightgreen = gdImageColorAllocate(im, 0, 255, 0);
int darkgreen = gdImageColorAllocate(im, 0, 180, 0);
int blue = gdImageColorAllocate(im, 0, 0, 255);
int grey = gdImageColorAllocate(im, 200, 200, 200);
int white = gdImageColorAllocate(im, 255, 255, 255);
int black = gdImageColorAllocate(im, 0, 0, 0);
gdImageFilledRectangle(im, 0, 0, 1087, 299, white);
// draw samples
for (unsigned pixel = 0; pixel < 1088; ++pixel) {
int clock_offset = (pixel - 150) * 2;
int bit = clock_offset / 87;
int sample = (f1_clock + clock_offset) / 25;
int bitoffset = clock_offset % 87;
int color;
if (sample < 0)
continue;
if (clock_offset < 0 || bit >= 20) {
color = grey;
} else if (bitoffset < 27 && (uncertain_bits & (1 << (19-bit)))) {
color = red;
} else if (bitoffset >= 27 && (noisy_bits & (1 << (19-bit)))) {
color = red;
} else if (bitoffset >= 27) {
color = grey;
} else if (bits & (1 << (19-bit))) {
color = brightgreen;
} else {
color = darkgreen;
}
gdImageLine(im, pixel, 299, pixel, yscale(m[sample]), color);
}
// draw bit boundaries
for (unsigned bit = 0; bit < 20; ++bit) {
unsigned clock = 87 * bit;
unsigned pixel0 = clock / 2 + 150;
unsigned pixel1 = (clock + 27) / 2 + 150;
gdImageLine(im, pixel0, 0, pixel0, 299, (bit == 0 || bit == 14) ? black : grey);
gdImageLine(im, pixel1, 0, pixel1, 299, (bit == 0 || bit == 14) ? black : grey);
}
// draw thresholds
gdImageLine(im, 0, yscale(noise_threshold), 1087, yscale(noise_threshold), blue);
gdImageLine(im, 0, yscale(signal_threshold), 1087, yscale(signal_threshold), blue);
// save it
static int file_counter;
char filename[PATH_MAX];
sprintf(filename, "modeac_%04X_%04d.png", modeac, ++file_counter);
fprintf(stderr, "writing %s\n", filename);
FILE *pngout = fopen(filename, "wb");
gdImagePng(im, pngout);
fclose(pngout);
gdImageDestroy(im);
}
#endif
//////////
////////// MODE A/C
//////////
// Mode A/C bits are 1.45us wide, consisting of 0.45us on and 1.0us off
// We track this in terms of a (virtual) 60MHz clock, which is the lowest common multiple
// of the bit frequency and the 2.4MHz sampling frequency
//
// 0.45us = 27 cycles }
// 1.00us = 60 cycles } one bit period = 1.45us = 87 cycles
//
// one 2.4MHz sample = 25 cycles
void demodulate2400AC(struct mag_buf *mag)
{
struct modesMessage mm;
uint16_t *m = mag->data;
uint32_t mlen = mag->length;
unsigned f1_sample;
memset(&mm, 0, sizeof(mm));
double noise_stddev = sqrt(mag->mean_power - mag->mean_level * mag->mean_level); // Var(X) = E[(X-E[X])^2] = E[X^2] - (E[X])^2
unsigned noise_level = (unsigned) ((mag->mean_power + noise_stddev) * 65535 + 0.5);
for (f1_sample = 1; f1_sample < mlen; ++f1_sample) {
// Mode A/C messages should match this bit sequence:
// bit # value
// -1 0 quiet zone
// 0 1 framing pulse (F1)
// 1 C1
// 2 A1
// 3 C2
// 4 A2
// 5 C4
// 6 A4
// 7 0 quiet zone (X1)
// 8 B1
// 9 D1
// 10 B2
// 11 D2
// 12 B4
// 13 D4
// 14 1 framing pulse (F2)
// 15 0 quiet zone (X2)
// 16 0 quiet zone (X3)
// 17 SPI
// 18 0 quiet zone (X4)
// 19 0 quiet zone (X5)
// Look for a F1 and F2 pair,
// with F1 starting at offset f1_sample.
// the first framing pulse covers 3.5 samples:
//
// |----| |----|
// | F1 |________| C1 |_
//
// | 0 | 1 | 2 | 3 | 4 |
//
// and there is some unknown phase offset of the
// leading edge e.g.:
//
// |----| |----|
// __| F1 |________| C1 |_
//
// | 0 | 1 | 2 | 3 | 4 |
//
// in theory the "on" period can straddle 3 samples
// but it's not a big deal as at most 4% of the power
// is in the third sample.
if (!(m[f1_sample-1] < m[f1_sample+0]))
continue; // not a rising edge
if (m[f1_sample+2] > m[f1_sample+0] || m[f1_sample+2] > m[f1_sample+1])
continue; // quiet part of bit wasn't sufficiently quiet
unsigned f1_level = (m[f1_sample+0] + m[f1_sample+1]) / 2;
if (noise_level * 2 > f1_level) {
// require 6dB above noise
continue;
}
// estimate initial clock phase based on the amount of power
// that ended up in the second sample
float f1a_power = (float)m[f1_sample] * m[f1_sample];
float f1b_power = (float)m[f1_sample+1] * m[f1_sample+1];
float fraction = f1b_power / (f1a_power + f1b_power);
unsigned f1_clock = (unsigned) (25 * (f1_sample + fraction * fraction) + 0.5);
// same again for F2
// F2 is 20.3us / 14 bit periods after F1
unsigned f2_clock = f1_clock + (87 * 14);
unsigned f2_sample = f2_clock / 25;
assert(f2_sample < mlen + Modes.trailing_samples);
if (!(m[f2_sample-1] < m[f2_sample+0]))
continue;
if (m[f2_sample+2] > m[f2_sample+0] || m[f2_sample+2] > m[f2_sample+1])
continue; // quiet part of bit wasn't sufficiently quiet
unsigned f2_level = (m[f2_sample+0] + m[f2_sample+1]) / 2;
if (noise_level * 2 > f2_level) {
// require 6dB above noise
continue;
}
unsigned f1f2_level = (f1_level > f2_level ? f1_level : f2_level);
float midpoint = sqrtf(noise_level * f1f2_level); // geometric mean of the two levels
unsigned signal_threshold = (unsigned) (midpoint * M_SQRT2 + 0.5); // +3dB
unsigned noise_threshold = (unsigned) (midpoint / M_SQRT2 + 0.5); // -3dB
// Looks like a real signal. Demodulate all the bits.
unsigned uncertain_bits = 0;
unsigned noisy_bits = 0;
unsigned bits = 0;
unsigned bit;
unsigned clock;
for (bit = 0, clock = f1_clock; bit < 20; ++bit, clock += 87) {
unsigned sample = clock / 25;
bits <<= 1;
noisy_bits <<= 1;
uncertain_bits <<= 1;
// check for excessive noise in the quiet period
if (m[sample+2] >= signal_threshold) {
noisy_bits |= 1;
}
// decide if this bit is on or off
if (m[sample+0] >= signal_threshold || m[sample+1] >= signal_threshold) {
bits |= 1;
} else if (m[sample+0] > noise_threshold && m[sample+1] > noise_threshold) {
/* not certain about this bit */
uncertain_bits |= 1;
} else {
/* this bit is off */
}
}
// framing bits must be on
if ((bits & 0x80020) != 0x80020) {
continue;
}
// quiet bits must be off
if ((bits & 0x0101B) != 0) {
continue;
}
if (noisy_bits || uncertain_bits) {
continue;
}
// Convert to the form that we use elsewhere:
// 00 A4 A2 A1 00 B4 B2 B1 SPI C4 C2 C1 00 D4 D2 D1
unsigned modeac =
((bits & 0x40000) ? 0x0010 : 0) | // C1
((bits & 0x20000) ? 0x1000 : 0) | // A1
((bits & 0x10000) ? 0x0020 : 0) | // C2
((bits & 0x08000) ? 0x2000 : 0) | // A2
((bits & 0x04000) ? 0x0040 : 0) | // C4
((bits & 0x02000) ? 0x4000 : 0) | // A4
((bits & 0x00800) ? 0x0100 : 0) | // B1
((bits & 0x00400) ? 0x0001 : 0) | // D1
((bits & 0x00200) ? 0x0200 : 0) | // B2
((bits & 0x00100) ? 0x0002 : 0) | // D2
((bits & 0x00080) ? 0x0400 : 0) | // B4
((bits & 0x00040) ? 0x0004 : 0) | // D4
((bits & 0x00004) ? 0x0080 : 0); // SPI
#ifdef MODEAC_DEBUG
draw_modeac(m, modeac, f1_clock, noise_threshold, signal_threshold, bits, noisy_bits, uncertain_bits);
#endif
// This message looks good, submit it
// For consistency with how the Beast / Radarcape does it,
// we report the timestamp at the second framing pulse (F2)
mm.timestampMsg = mag->sampleTimestamp + f2_clock / 5; // 60MHz -> 12MHz
// compute message receive time as block-start-time + difference in the 12MHz clock
mm.sysTimestampMsg = mag->sysTimestamp; // start of block time
mm.sysTimestampMsg.tv_nsec += receiveclock_ns_elapsed(mag->sampleTimestamp, mm.timestampMsg);
normalize_timespec(&mm.sysTimestampMsg);
decodeModeAMessage(&mm, modeac);
// Pass data to the next layer
useModesMessage(&mm);
f1_sample += (20*87 / 25);
Modes.stats_current.demod_modeac++;
}
}