demod_2400.c

// Part of readsb, a Mode-S/ADSB/TIS message decoder.
//
// demod_2400.c: 2.4MHz Mode S demodulator.
//
// Copyright (c) 2020 Michael Wolf <michael@mictronics.de>
//
// This code is based on a detached fork of dump1090-fa.
//
// Copyright (c) 2014,2015 Oliver Jowett <oliver@mutability.co.uk>
//
// 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 <http://www.gnu.org/licenses/>.

#include "readsb.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)

// 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];
}

// 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;
}

// score a phase using data from the magnitude buffers
// if the score is better than the existing one passed via bestscore,
// update bestmsg, bestscore and bestphase

static void score_phase(int try_phase, uint16_t *m, int j, 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, i, score, bytelen;

    pPtr = &m[j + 19] + (try_phase / 5);
    phase = try_phase % 5;

    (*msg)[0] = slice_byte(&pPtr, &phase);

    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:
            bytelen = MODES_LONG_MSG_BYTES;
            break;

        default:
            bytelen = 1; // unknown DF, give up immediately
            break;
    }

    for (i = 1; i < bytelen; ++i) {
        (*msg)[i] = slice_byte(&pPtr, &phase);
    }

    // Score the mode S message and see if it's any good.
    if (bytelen > 1) {
        score = scoreModesMessage(*msg, i * 8);
    } else {
        score = -2;
    }

    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) {
    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->validLength - mag->overlap;

    uint64_t sum_scaled_signal_power = 0;

    msg = msg1;

    // advance ifile artificial clock even if we don't receive anything
    if (Modes.sdr_type == SDR_IFILE) {
        Modes.ifile_now = mag->sysTimestamp;
    }

    for (j = 0; j < mlen; j++) {
        uint16_t *pa = &m[j];
        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
        if (!(pa[1] > pa[7] && pa[12] > pa[14] && pa[12] > pa[15])) {
            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 * max(PREAMBLE_THRESHOLD_PIZERO, Modes.preambleThreshold);
        } else {
            ref_level = base_noise * Modes.preambleThreshold;
        }

        ref_level >>= 5; // divide by 32

        bestmsg = NULL;
        bestscore = -42;
        bestphase = -1;

        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, m, j, &bestmsg, &bestscore, &bestphase, &msg, msg1, msg2);
            // peaks at 1,3,9,12: phase 4
            score_phase(5, m, j, &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, m, j, &bestmsg, &bestscore, &bestphase, &msg, msg1, msg2);
            // peaks at 1,4,10,12: phase 6
            score_phase(7, m, j, &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, m, j, &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(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 + receiveclock_ms_elapsed(mag->sampleTimestamp, mm.timestampMsg);

        // advance ifile artifical clock for every message received
        if (Modes.sdr_type == SDR_IFILE) {
            Modes.ifile_now = 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]++;
            }
        }

        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 = 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 * mlen - sum_signal_power);
        Modes.stats_current.noise_power_count += mlen;
    }
}


#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->validLength - mag->overlap;
    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 + mag->overlap);

        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 + receiveclock_ms_elapsed(mag->sampleTimestamp, mm.timestampMsg);

        decodeModeAMessage(&mm, modeac);

        // Pass data to the next layer
        useModesMessage(&mm);

        f1_sample += (20 * 87 / 25);
        Modes.stats_current.demod_modeac++;
    }
}