Tweaks to A/C demodulator.
* use a global noise level rather than one computed from a few bits * work out level vs power confusion in some thresholds * fix the power calculation for working out the phase offset from the framing bits * require fewer quiet trailing bits * relax the bit-threshold tests
This commit is contained in:
Oliver Jowett
parent
5ef2612bb4
commit
32231cf142
191
demod_2400.c
191
demod_2400.c
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@ -19,6 +19,8 @@
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#include "dump1090.h"
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#include <assert.h>
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// 2.4MHz sampling rate version
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//
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// When sampling at 2.4MHz we have exactly 6 samples per 5 symbols.
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@ -389,6 +391,9 @@ void demodulate2400AC(struct mag_buf *mag)
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memset(&mm, 0, sizeof(mm));
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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
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unsigned noise_level = (unsigned) ((mag->mean_power + noise_stddev) * 65535 + 0.5);
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for (f1_sample = 1; f1_sample < mlen; ++f1_sample) {
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// Mode A/C messages should match this bit sequence:
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@ -414,10 +419,6 @@ void demodulate2400AC(struct mag_buf *mag)
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// 17 SPI
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// 18 0 quiet zone (X4)
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// 19 0 quiet zone (X5)
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// 20 0 quiet zone (X6)
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// 21 0 quiet zone (X7)
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// 22 0 quiet zone (X8)
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// 23 0 quiet zone (X9)
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// Look for a F1 and F2 pair,
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// with F1 starting at offset f1_sample.
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@ -447,26 +448,26 @@ void demodulate2400AC(struct mag_buf *mag)
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if (m[f1_sample+2] > m[f1_sample+0] || m[f1_sample+2] > m[f1_sample+1])
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continue; // quiet part of bit wasn't sufficiently quiet
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unsigned f1_noise = (m[f1_sample-1] + m[f1_sample+2]) / 2;
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unsigned f1_signal = (m[f1_sample+0] + m[f1_sample+1]) / 2;
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unsigned f1_level = (m[f1_sample+0] + m[f1_sample+1]) / 2;
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if (f1_noise * 4 > f1_signal) {
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// require 12dB SNR
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if (noise_level * 2 > f1_level) {
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// require 6dB above noise
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continue;
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}
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// estimate initial clock phase based on the amount of power
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// that ended up in the second sample
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unsigned f1_clock = 25 * f1_sample;
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if (m[f1_sample+1] > f1_noise) {
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f1_clock += 25 * (m[f1_sample+1] - f1_noise) / (2*(f1_signal - f1_noise));
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}
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float f1a_power = (float)m[f1_sample] * m[f1_sample];
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float f1b_power = (float)m[f1_sample+1] * m[f1_sample+1];
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float fraction = f1b_power / (f1a_power + f1b_power);
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unsigned f1_clock = (unsigned) (25 * (f1_sample + fraction * fraction) + 0.5);
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// same again for F2
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// F2 is 20.3us / 14 bit periods after F1
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unsigned f2_clock = f1_clock + (87 * 14);
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unsigned f2_sample = f2_clock / 25;
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assert(f2_sample < mlen + Modes.trailing_samples);
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if (!(m[f2_sample-1] < m[f2_sample+0]))
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continue;
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@ -474,174 +475,78 @@ void demodulate2400AC(struct mag_buf *mag)
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if (m[f2_sample+2] > m[f2_sample+0] || m[f2_sample+2] > m[f2_sample+1])
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continue; // quiet part of bit wasn't sufficiently quiet
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unsigned f2_noise = (m[f2_sample-1] + m[f2_sample+2]) / 2;
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unsigned f2_signal = (m[f2_sample+0] + m[f2_sample+1]) / 2;
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unsigned f2_level = (m[f2_sample+0] + m[f2_sample+1]) / 2;
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if (f2_noise * 4 > f2_signal) {
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// require 12dB SNR
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if (noise_level * 2 > f2_level) {
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// require 6dB above noise
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continue;
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}
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unsigned f1f2_signal = (f1_signal + f2_signal) / 2;
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unsigned f1f2_level = (f1_level > f2_level ? f1_level : f2_level);
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// look at X1, X2, X3 which should be quiet
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// (sample 0 may have part of the previous bit, but
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// it always covers the quiet part of it)
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unsigned x1_clock = f1_clock + (87 * 7);
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unsigned x1_sample = x1_clock / 25;
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unsigned x1_noise = (m[x1_sample + 0] + m[x1_sample + 1] + m[x1_sample + 2]) / 3;
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if (x1_noise * 4 >= f1f2_signal)
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continue;
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unsigned x2_clock = f1_clock + (87 * 15);
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unsigned x2_sample = x2_clock / 25;
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unsigned x2_noise = (m[x2_sample + 0] + m[x2_sample + 1] + m[x2_sample + 2]) / 3;
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if (x2_noise * 4 >= f1f2_signal)
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continue;
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unsigned x3_clock = f1_clock + (87 * 16);
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unsigned x3_sample = x3_clock / 25;
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unsigned x3_noise = (m[x3_sample + 0] + m[x3_sample + 1] + m[x3_sample + 2]) / 3;
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if (x3_noise * 4 >= f1f2_signal)
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continue;
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unsigned x1x2x3_noise = (x1_noise + x2_noise + x3_noise) / 3;
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if (x1x2x3_noise * 4 >= f1f2_signal) // require 12dB separation
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continue;
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// ----- F1/F2 average signal
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// ^
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// | at least 3dB
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// v
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// ----- minimum signal level we accept as "on"
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// ^
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// | 3dB
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// v
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// ---- midpoint between F1/F2 and X1/X2/X3
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// ^
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// | 3dB
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// v
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// ----- maximum signal level we accept as "off"
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// ^
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// | at least 3dB
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// v
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// ----- X1/X2/X3 average noise
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float midpoint = sqrtf(x1x2x3_noise * f1f2_signal); // so that signal/midpoint == midpoint/noise
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unsigned quiet_threshold = (unsigned) midpoint;
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unsigned noise_threshold = (unsigned) (midpoint * 0.707107 + 0.5); // -3dB from midpoint
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unsigned signal_threshold = (unsigned) (midpoint * 1.414214 + 0.5); // +3dB from midpoint
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#if 0
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fprintf(stderr, "f1f2 %u x1x2x3 %u midpoint %.0f noise_threshold %u signal_threshold %u\n",
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f1f2_signal, x1x2x3_noise, midpoint, noise_threshold, signal_threshold);
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fprintf(stderr, "f1 %u f2 %u x1 %u x2 %u x3 %u\n",
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f1_signal, f2_signal, x1_noise, x2_noise, x3_noise);
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#endif
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// recheck F/X bits just in case
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if (f1_signal < signal_threshold)
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continue;
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if (f2_signal < signal_threshold)
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continue;
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if (x1_noise > noise_threshold)
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continue;
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if (x2_noise > noise_threshold)
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continue;
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if (x3_noise > noise_threshold)
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continue;
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float midpoint = sqrtf(noise_level * f1f2_level); // geometric mean of the two levels
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unsigned signal_threshold = (unsigned) (midpoint * M_SQRT2 + 0.5); // +3dB
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unsigned noise_threshold = (unsigned) (midpoint / M_SQRT2 + 0.5); // -3dB
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// Looks like a real signal. Demodulate all the bits.
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unsigned uncertain_bits = 0;
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unsigned noisy_bits = 0;
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unsigned bits = 0;
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unsigned bit;
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unsigned clock;
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for (bit = 0, clock = f1_clock; bit < 24; ++bit, clock += 87) {
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for (bit = 0, clock = f1_clock; bit < 20; ++bit, clock += 87) {
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unsigned sample = clock / 25;
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bits <<= 1;
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noisy_bits <<= 1;
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uncertain_bits <<= 1;
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// check for excessive noise in the quiet period
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if (m[sample+2] >= quiet_threshold) {
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//fprintf(stderr, "bit %u was not quiet (%u > %u)\n", bit, m[sample+2], quiet_threshold);
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if (m[sample+2] >= signal_threshold) {
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noisy_bits |= 1;
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continue;
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}
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// decide if this bit is on or off
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unsigned bit_signal = (m[sample+0] + m[sample+1]) / 2;
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if (bit_signal >= signal_threshold) {
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if (m[sample+0] >= signal_threshold || m[sample+1] >= signal_threshold) {
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bits |= 1;
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} else if (bit_signal > noise_threshold) {
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} else if (m[sample+0] > noise_threshold && m[sample+1] > noise_threshold) {
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/* not certain about this bit */
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//fprintf(stderr, "bit %u was uncertain (%u < %u < %u)\n", bit, noise_threshold, bit_signal, signal_threshold);
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noisy_bits |= 1;
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uncertain_bits |= 1;
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} else {
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/* this bit is off */
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}
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}
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#if 0
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fprintf(stderr, "bits: %06X noisy: %06X\n", bits, noisy_bits);
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unsigned j, sample;
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static const char *names[24] = {
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"F1", "C1", "A1", "C2",
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"A2", "C4", "A4", "X1",
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"B1", "D1", "B2", "D2",
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"B4", "D4", "F2", "X2",
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"X3", "SPI", "X4", "X5",
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"X6", "X7", "X8", "X9"
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};
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fprintf(stderr, "-1 ... %6u\n", m[f1_sample-1]);
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for (j = 0; j < 24; ++j) {
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clock = f1_clock + 87 * j;
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sample = clock / 25;
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fprintf(stderr, "%2u %-3s %6u %6u %6u %6u ", j, names[j], m[sample+0], m[sample+1], m[sample+2], m[sample+3]);
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if ((m[sample+0] + m[sample+1])/2 >= signal_threshold) {
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fprintf(stderr, "ON\n");
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} else if ((m[sample+0] + m[sample+1])/2 <= noise_threshold) {
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fprintf(stderr, "OFF\n");
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} else {
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fprintf(stderr, "UNCERTAIN\n");
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}
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}
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#endif
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if (noisy_bits) {
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/* XX debug */
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continue;
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}
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// framing bits must be on
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if ((bits & 0x800200) != 0x800200) {
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if ((bits & 0x80020) != 0x80020) {
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continue;
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}
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// quiet bits must be off
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if ((bits & 0x0101BF) != 0) {
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if ((bits & 0x0101B) != 0) {
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continue;
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}
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if (noisy_bits || uncertain_bits) {
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continue;
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}
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// Convert to the form that we use elsewhere:
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// 00 A4 A2 A1 00 B4 B2 B1 SPI C4 C2 C1 00 D4 D2 D1
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unsigned modeac =
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((bits & 0x400000) ? 0x0010 : 0) | // C1
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((bits & 0x200000) ? 0x1000 : 0) | // A1
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((bits & 0x100000) ? 0x0020 : 0) | // C2
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((bits & 0x080000) ? 0x2000 : 0) | // A2
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((bits & 0x040000) ? 0x0040 : 0) | // C4
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((bits & 0x020000) ? 0x4000 : 0) | // A4
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((bits & 0x008000) ? 0x0100 : 0) | // B1
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((bits & 0x004000) ? 0x0001 : 0) | // D1
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((bits & 0x002000) ? 0x0200 : 0) | // B2
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((bits & 0x001000) ? 0x0002 : 0) | // D2
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((bits & 0x000800) ? 0x0400 : 0) | // B4
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((bits & 0x000400) ? 0x0004 : 0) | // D4
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((bits & 0x000040) ? 0x0080 : 0); // SPI
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((bits & 0x40000) ? 0x0010 : 0) | // C1
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((bits & 0x20000) ? 0x1000 : 0) | // A1
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((bits & 0x10000) ? 0x0020 : 0) | // C2
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((bits & 0x08000) ? 0x2000 : 0) | // A2
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((bits & 0x04000) ? 0x0040 : 0) | // C4
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((bits & 0x02000) ? 0x4000 : 0) | // A4
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((bits & 0x00800) ? 0x0100 : 0) | // B1
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((bits & 0x00400) ? 0x0001 : 0) | // D1
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((bits & 0x00200) ? 0x0200 : 0) | // B2
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((bits & 0x00100) ? 0x0002 : 0) | // D2
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((bits & 0x00080) ? 0x0400 : 0) | // B4
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((bits & 0x00040) ? 0x0004 : 0) | // D4
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((bits & 0x00004) ? 0x0080 : 0); // SPI
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// This message looks good, submit it
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@ -656,7 +561,7 @@ void demodulate2400AC(struct mag_buf *mag)
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// Pass data to the next layer
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useModesMessage(&mm);
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f1_sample += (24*87 / 25);
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f1_sample += (20*87 / 25);
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Modes.stats_current.demod_modeac++;
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}
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}
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