// Part of readsb, a Mode-S/ADSB/TIS message decoder. // // demod_2400.c: 2.4MHz Mode S demodulator. // // Copyright (c) 2019 Michael Wolf // // This code is based on a detached fork of dump1090-fa. // // Copyright (c) 2014,2015 Oliver Jowett // // This file is free software: you can redistribute it and/or modify // it under the terms of the GNU General Public License as published by // the Free Software Foundation, either version 3 of the License, or // 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 . #include "readsb.h" #include #ifdef MODEAC_DEBUG #include #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) // Changes 2020 by wiedehopf: // 20 units per sample, 24 units per symbol that are distributed according to phase // 1 bit has 2 symbols, in a bit representing a one the first symbol is high and the second is low // The previous assumption was that symbols beyond our control are zero. // Let's make the assumption that the symbols beyond our control are a statistical mean of 0 and 1. // Such a mean is represented by 12 units per symbol. // As an example for the above let's discuss the first slice function: // Samples 0 and 1 are completely occupied by the bit we are trying to judge thus no outside symbols. // The 3rd sample is 8 units of our bit and 12 units of the following symbol. // Our bit contributes part of a low symbol represented by -8 units // but we also get 12 units of 0.5 resulting in +6 units from the following symbol. // // The above comment is how these changes started out, i'll leave them here as food for thought. // Using --ifile the coefficients from the above thought process were iteratively tweaked by hand. // Note one of the correlation functions is no longer DC balanced (but just slightly) // Further testing on your own samples using --ifile --quiet --stats is welcome // Note you might need to use --throttle unless your using wiedehopf's readsb fork, // otherwise position stats won't work as they rely on realtime differences between // reception of CPRs. // Creating a 5 minute sample with a gain of 43.9: // timeout 300 rtl_sdr -f 1090000000 -s 2400000 -g 43.9 sample.dat // Checking a set of correlation functions using the above sample: // make && ./readsb --device-type ifile --ifile sample.dat --quiet --stats static inline __attribute__((always_inline)) int slice_phase0(uint16_t *m) { return 18 * m[0] - 15 * m[1] - 3 * m[2]; } static inline __attribute__((always_inline)) int slice_phase1(uint16_t *m) { return 14 * m[0] - 5 * m[1] - 9 * m[2]; } // slightly DC unbalanced but better results static inline __attribute__((always_inline)) int slice_phase2(uint16_t *m) { return 16 * m[0] + 5 * m[1] - 20 * m[2]; } static inline __attribute__((always_inline)) int slice_phase3(uint16_t *m) { return 7 * m[0] + 11 * m[1] - 18 * m[2]; } static inline __attribute__((always_inline)) int slice_phase4(uint16_t *m) { return 4 * m[0] + 15 * m[1] - 20 * m[2] + 1 * m[3]; } static uint32_t valid_df_short_bitset; // set of acceptable DF values for short messages static uint32_t valid_df_long_bitset; // set of acceptable DF values for long messages static uint32_t generate_damage_set(uint8_t df, unsigned damage_bits) { uint32_t result = (1 << df); if (!damage_bits) return result; for (unsigned bit = 0; bit < 5; ++bit) { unsigned damaged_df = df ^ (1 << bit); result |= generate_damage_set(damaged_df, damage_bits - 1); } return result; } static void init_bitsets() { // DFs that we directly understand without correction valid_df_short_bitset = (1 << 0) | (1 << 4) | (1 << 5) | (1 << 11); valid_df_long_bitset = (1 << 16) | (1 << 17) | (1 << 18) | (1 << 20) | (1 << 21); #ifdef ENABLE_DF24 if (1) valid_df_long_bitset |= (1 << 24) | (1 << 25) | (1 << 26) | (1 << 27) | (1 << 28) | (1 << 29) | (1 << 30) | (1 << 31); #endif // if we can also repair DF damage, include those corrections if (Modes.fixDF && Modes.nfix_crc) { // only correct for possible DF17, other types are less useful usually (DF11/18 would also be possible) valid_df_long_bitset |= generate_damage_set(17, 1); } } // extract one byte from the mag buffers using slice_phase functions // advance pPtr and phase static inline __attribute__((always_inline)) uint8_t slice_byte(uint16_t **pPtr, int *phase) { 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; } return theByte; } static void score_phase(int try_phase, uint16_t *pa, unsigned char **bestmsg, int *bestscore, int *bestphase, unsigned char **msg, unsigned char *msg1, unsigned char *msg2) { Modes.stats_current.demod_preamblePhase[try_phase - 4]++; uint16_t *pPtr; int phase, score, bytelen; pPtr = pa + 19 + (try_phase / 5); phase = try_phase % 5; (*msg)[0] = slice_byte(&pPtr, &phase); // inspect DF field early, only continue processing // messages where the DF appears valid uint32_t df = ((uint8_t) (*msg)[0]) >> 3; if (valid_df_long_bitset & (1 << df)) { bytelen = MODES_LONG_MSG_BYTES; } else if (valid_df_short_bitset & (1 << df)) { bytelen = MODES_SHORT_MSG_BYTES; } else { score = -2; if (score > *bestscore) { // this is only for preamble stats *bestscore = score; } return; } for (int i = 1; i < bytelen; ++i) { (*msg)[i] = slice_byte(&pPtr, &phase); } // Score the mode S message and see if it's any good. score = scoreModesMessage(*msg, bytelen * 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; } } // // Given 'mlen' magnitude samples in 'm', sampled at 2.4MHz, // try to demodulate some Mode S messages. // void demodulate2400(struct mag_buf *mag) { unsigned char msg1[MODES_LONG_MSG_BYTES], msg2[MODES_LONG_MSG_BYTES], *msg; unsigned char *bestmsg = NULL; int bestscore; int bestphase = 0; uint16_t *m = mag->data; uint32_t mlen = mag->length; uint64_t sum_scaled_signal_power = 0; // initialize bitsets on first call if (!valid_df_short_bitset) init_bitsets(); msg = msg1; // advance ifile artificial clock even if we don't receive anything if (Modes.sdr_type == SDR_IFILE && Modes.synthetic_now) { Modes.synthetic_now = mag->sysTimestamp; } uint16_t *pa = m; uint16_t *stop = m + mlen; for (; pa < stop; pa++) { int32_t pa_mag, base_noise, ref_level; 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 // do a pre-check to reduce CPU usage // some silly unrolling that cuts CPU cycles // due to plenty room in the message buffer for decoding // we can with pa go beyond stop without a buffer overrun ... if (pa[1] > pa[7] && pa[12] > pa[14] && pa[12] > pa[15]) { goto after_pre; } pa++; if (pa[1] > pa[7] && pa[12] > pa[14] && pa[12] > pa[15]) { goto after_pre; } pa++; if (pa[1] > pa[7] && pa[12] > pa[14] && pa[12] > pa[15]) { goto after_pre; } pa++; if (pa[1] > pa[7] && pa[12] > pa[14] && pa[12] > pa[15]) { goto after_pre; } pa++; if (pa[1] > pa[7] && pa[12] > pa[14] && pa[12] > pa[15]) { goto after_pre; } pa++; if (pa[1] > pa[7] && pa[12] > pa[14] && pa[12] > pa[15]) { goto after_pre; } pa++; if (pa[1] > pa[7] && pa[12] > pa[14] && pa[12] > pa[15]) { goto after_pre; } pa++; if (pa[1] > pa[7] && pa[12] > pa[14] && pa[12] > pa[15]) { goto after_pre; } pa++; if (pa[1] > pa[7] && pa[12] > pa[14] && pa[12] > pa[15]) { goto after_pre; } pa++; if (pa[1] > pa[7] && pa[12] > pa[14] && pa[12] > pa[15]) { goto after_pre; } continue; after_pre: // ... but we must NOT decode if have ran past stop if (!(pa < stop)) continue; // 5 noise samples base_noise = pa[5] + pa[8] + pa[16] + pa[17] + pa[18]; // pa_mag is the sum of the 4 preamble high bits // minus 2 low bits between each of high bit pairs // reduce number of preamble detections if we recently dropped samples if (Modes.stats_15min.samples_dropped) ref_level = base_noise * imax(PREAMBLE_THRESHOLD_PIZERO, Modes.preambleThreshold); else ref_level = base_noise * Modes.preambleThreshold; ref_level >>= 5; // divide by 32 bestscore = -42; int32_t diff_2_3 = pa[2] - pa[3]; int32_t sum_1_4 = pa[1] + pa[4]; int32_t diff_10_11 = pa[10] - pa[11]; int32_t common3456 = sum_1_4 - diff_2_3 + pa[9] + pa[12]; // 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 pa_mag = common3456 - diff_10_11; if (pa_mag >= ref_level) { // peaks at 1,3,9,11-12: phase 3 score_phase(4, pa, &bestmsg, &bestscore, &bestphase, &msg, msg1, msg2); // peaks at 1,3,9,12: phase 4 score_phase(5, pa, &bestmsg, &bestscore, &bestphase, &msg, msg1, msg2); } // sample#: 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 // 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 pa_mag = common3456 + diff_10_11; if (pa_mag >= ref_level) { // peaks at 1,3-4,9-10,12: phase 5 score_phase(6, pa, &bestmsg, &bestscore, &bestphase, &msg, msg1, msg2); // peaks at 1,4,10,12: phase 6 score_phase(7, pa, &bestmsg, &bestscore, &bestphase, &msg, msg1, msg2); } // peaks at 1-2,4,10,12: phase 7 // sample#: 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 // phase 7: 0/3 3\1/5\0 0 0 0 1/5\0/4\2 0 0 0 0 0 0 X3 pa_mag = sum_1_4 + 2 * diff_2_3 + diff_10_11 + pa[12]; if (pa_mag >= ref_level) score_phase(8, pa, &bestmsg, &bestscore, &bestphase, &msg, msg1, msg2); // no preamble detected if (bestscore == -42) continue; // we had at least one phase greater than the preamble threshold // and used scoremodesmessage on those bytes Modes.stats_current.demod_preambles++; // 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(getbits(bestmsg, 1, 5)); struct modesMessage *mm = netGetMM(&Modes.netMessageBuffer[0]); // 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->timestamp = mag->sampleTimestamp + (pa -m) * 5 + (8 + 56) * 12 + bestphase; // compute message receive time as block-start-time + difference in the 12MHz clock mm->sysTimestamp = mag->sysTimestamp + receiveclock_ms_elapsed(mag->sampleTimestamp, mm->timestamp); // advance ifile artifical clock for every message received if (Modes.sdr_type == SDR_IFILE && Modes.synthetic_now) { Modes.synthetic_now = mm->sysTimestamp; } mm->score = bestscore; // Decode the received message { memcpy(mm->msg, bestmsg, MODES_LONG_MSG_BYTES); int result = decodeModesMessage(mm); 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]++; } } Modes.stats_current.demod_bestPhase[bestphase - 4]++; // 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 = pa[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) //pa += msglen * 12 / 5; // // let's test something, only jump part of the message and let the preamble detection handle the rest. pa += msglen * 8 / 4; // Pass data to the next layer netUseMessage(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; } netDrainMessageBuffers(); } #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) { uint16_t *m = mag->data; uint32_t mlen = mag->length; unsigned f1_sample; 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 struct modesMessage *mm = netGetMM(&Modes.netMessageBuffer[0]); // For consistency with how the Beast / Radarcape does it, // we report the timestamp at the second framing pulse (F2) mm->timestamp = mag->sampleTimestamp + f2_clock / 5; // 60MHz -> 12MHz // compute message receive time as block-start-time + difference in the 12MHz clock mm->sysTimestamp = mag->sysTimestamp + receiveclock_ms_elapsed(mag->sampleTimestamp, mm->timestamp); decodeModeAMessage(mm, modeac); // Pass data to the next layer netUseMessage(mm); f1_sample += (20 * 87 / 25); Modes.stats_current.demod_modeac++; } netDrainMessageBuffers(); }