Improved decode by extending search in time and better windowing
This commit is contained in:
Karlis Goba
parent
70837f4cbc
commit
0c2e35b998
1 changed files with 143 additions and 79 deletions
166
decode_ft8.cpp
166
decode_ft8.cpp
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@ -10,6 +10,12 @@
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#include "common/wave.h"
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#include "fft/kiss_fftr.h"
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const int kMax_candidates = 100;
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const int kLDPC_iterations = 20;
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const int kMax_decoded_messages = 50;
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const int kMax_message_length = 20;
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void usage() {
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printf("Decode a 15-second WAV file.\n");
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}
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@ -21,10 +27,32 @@ float hann_i(int i, int N) {
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}
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float hamming_i(int i, int N) {
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const float a0 = (float)25 / 46;
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const float a1 = 1 - a0;
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float x1 = cosf(2 * (float)M_PI * i / (N - 1));
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return a0 - a1*x1;
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}
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float blackman_i(int i, int N) {
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const float alpha = 0.16f; // or 2860/18608
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const float a0 = (1 - alpha) / 2;
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const float a1 = 1.0f / 2;
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const float a2 = alpha / 2;
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float x1 = cosf(2 * (float)M_PI * i / (N - 1));
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float x2 = cosf(4 * (float)M_PI * i / (N - 1));
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return a0 - a1*x1 + a2*x2;
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}
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struct Candidate {
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int16_t score;
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uint16_t time_offset;
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uint16_t freq_offset;
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int16_t time_offset;
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int16_t freq_offset;
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uint8_t time_sub;
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uint8_t freq_sub;
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};
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@ -79,21 +107,26 @@ int find_sync(const uint8_t *power, int num_blocks, int num_bins, const uint8_t
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int heap_size = 0;
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for (int alt = 0; alt < 4; ++alt) {
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for (int time_offset = 0; time_offset < num_blocks - FT8_NN; ++time_offset) {
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for (int time_offset = -7; time_offset < num_blocks - FT8_NN + 7; ++time_offset) {
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for (int freq_offset = 0; freq_offset < num_bins - 8; ++freq_offset) {
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int score = 0;
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// Compute score over Costas symbols (0-7, 36-43, 72-79)
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int num_scores = 0;
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for (int m = 0; m <= 72; m += 36) {
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for (int k = 0; k < 7; ++k) {
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if (time_offset + k + m < 0) continue;
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if (time_offset + k + m >= num_blocks) break;
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int offset = ((time_offset + k + m) * 4 + alt) * num_bins + freq_offset;
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score += 8 * (int)power[offset + sync_map[k]] -
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power[offset + 0] - power[offset + 1] -
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power[offset + 2] - power[offset + 3] -
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power[offset + 4] - power[offset + 5] -
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power[offset + 6] - power[offset + 7];
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++num_scores;
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}
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}
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score /= num_scores;
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// If the heap is full AND the current candidate is better than
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// the worst in the heap, we remove the worst and make space
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@ -130,11 +163,8 @@ void extract_power(const float * signal, int num_blocks, int num_bins, uint8_t *
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float window[nfft];
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for (int i = 0; i < nfft; ++i) {
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window[i] = hann_i(i, nfft);
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window[i] = blackman_i(i, nfft);
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}
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// for (int i = 0; i < nfft; ++i) {
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// window[i] = (i < block_size) ? 2 * hann_i(i, block_size) : 0.0f;
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// }
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size_t fft_work_size;
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kiss_fftr_alloc(nfft, 0, 0, &fft_work_size);
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@ -187,18 +217,17 @@ void extract_power(const float * signal, int num_blocks, int num_bins, uint8_t *
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}
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printf("Max magnitude: %.1f dB\n", max_mag);
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free(fft_work);
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}
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uint8_t max2(uint8_t a, uint8_t b) {
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float max2(float a, float b) {
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return (a >= b) ? a : b;
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}
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uint8_t max4(uint8_t a, uint8_t b, uint8_t cand, uint8_t d) {
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return max2(max2(a, b), max2(cand, d));
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float max4(float a, float b, float c, float d) {
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return max2(max2(a, b), max2(c, d));
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}
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@ -207,45 +236,61 @@ uint8_t max4(uint8_t a, uint8_t b, uint8_t cand, uint8_t d) {
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void extract_likelihood(const uint8_t *power, int num_bins, const Candidate & cand, const uint8_t *code_map, float *log174) {
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int offset = (cand.time_offset * 4 + cand.time_sub * 2 + cand.freq_sub) * num_bins + cand.freq_offset;
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int k = 0;
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// Go over FSK tones and skip Costas sync symbols
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for (int i = 7; i < FT8_NN - 7; ++i) {
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if (i == 36) i += 7;
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const int n_syms = 1;
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const int n_bits = 3 * n_syms;
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const int n_tones = (1 << n_bits);
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for (int k = 0; k < FT8_ND; k += n_syms) {
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int sym_idx = (k < FT8_ND / 2) ? (k + 7) : (k + 14);
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// Pointer to 8 bins of the current symbol
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const uint8_t * ps = power + (offset + i * 4 * num_bins);
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uint8_t s2[8];
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const uint8_t *ps = power + (offset + sym_idx * 4 * num_bins);
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float s2[n_tones];
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for (int i = 0; i < 8; ++i) {
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s2[i] = ps[code_map[i]];
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for (int j = 0; j < n_tones; ++j) {
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int j1 = j & 0x07;
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s2[j] = (float)ps[code_map[j1]];
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//int j2 = (j >> 3) & 0x07;
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//s2[j] = (float)ps[code_map[j2]];
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//s2[j] += (float)ps[code_map[j1] + 4 * num_bins];
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}
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// Extract bit significance (and convert them to float)
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// 8 FSK tones = 3 bits
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log174[k + 0] = (int)max4(s2[4], s2[5], s2[6], s2[7]) - (int)max4(s2[0], s2[1], s2[2], s2[3]);
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log174[k + 1] = (int)max4(s2[2], s2[3], s2[6], s2[7]) - (int)max4(s2[0], s2[1], s2[4], s2[5]);
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log174[k + 2] = (int)max4(s2[1], s2[3], s2[5], s2[7]) - (int)max4(s2[0], s2[2], s2[4], s2[6]);
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// printf("%d %d %d %d %d %d %d %d : %.0f %.0f %.0f\n",
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// ps[0], ps[1], ps[2], ps[3], ps[4], ps[5], ps[6], ps[7],
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// log174[k + 0], log174[k + 1], log174[k + 2]);
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int bit_idx = 3 * k;
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for (int i = 0; i < n_bits; ++i) {
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uint16_t mask = (n_tones >> (i + 1));
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k += 3;
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float max_zero = -1000, max_one = -1000;
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for (int n = 0; n < n_tones; ++n) {
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if (n & mask) {
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max_one = max2(max_one, s2[n]);
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}
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else {
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max_zero = max2(max_zero, s2[n]);
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}
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}
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if (bit_idx + i >= 174) break;
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log174[bit_idx + i] = max_one - max_zero;
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}
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// log174[bit_idx + 0] = max4(s2[4], s2[5], s2[6], s2[7]) - max4(s2[0], s2[1], s2[2], s2[3]);
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// log174[bit_idx + 1] = max4(s2[2], s2[3], s2[6], s2[7]) - max4(s2[0], s2[1], s2[4], s2[5]);
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// log174[bit_idx + 2] = max4(s2[1], s2[3], s2[5], s2[7]) - max4(s2[0], s2[2], s2[4], s2[6]);
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}
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// Compute the variance of log174
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float sum = 0;
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float sum2 = 0;
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float inv_n = 1.0f / (3 * FT8_ND);
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for (int i = 0; i < 3 * FT8_ND; ++i) {
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float inv_n = 1.0f / FT8_N;
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for (int i = 0; i < FT8_N; ++i) {
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sum += log174[i];
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sum2 += log174[i] * log174[i];
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}
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float variance = (sum2 - sum * sum * inv_n) * inv_n;
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// Normalize log174 such that sigma = 2.83 (Why? It's in WSJT-X)
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float norm_factor = 2.83f / sqrtf(variance);
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for (int i = 0; i < 3 * FT8_ND; ++i) {
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float norm_factor = 3.83f / sqrtf(variance);
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for (int i = 0; i < FT8_N; ++i) {
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log174[i] *= norm_factor;
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//printf("%.1f ", log174[i]);
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}
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@ -300,61 +345,65 @@ int main(int argc, char ** argv) {
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return -1;
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}
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const float fsk_dev = 6.25f;
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const float fsk_dev = 6.25f; // tone deviation in Hz and symbol rate
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// Compute DSP parameters that depend on the sample rate
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const int num_bins = (int)(sample_rate / (2 * fsk_dev));
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const int block_size = 2 * num_bins;
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const int num_blocks = (num_samples - (block_size/2) - block_size) / block_size;
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uint8_t power[num_blocks * 4 * num_bins]; // [num_blocks][4][num_bins] ~ 200 KB
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printf("%d blocks, %d bins\n", num_blocks, num_bins);
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// Compute FFT over the whole signal and store it
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uint8_t power[num_blocks * 4 * num_bins];
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extract_power(signal, num_blocks, num_bins, power);
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int num_candidates = 100;
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Candidate heap[num_candidates];
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Candidate heap[kMax_candidates];
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char decoded[kMax_decoded_messages][kMax_message_length];
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int num_decoded = 0;
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find_sync(power, num_blocks, num_bins, kCostas_map, num_candidates, heap);
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int num_candidates = find_sync(power, num_blocks, num_bins, kCostas_map, kMax_candidates, heap);
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for (int idx = 0; idx < num_candidates; ++idx) {
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Candidate &cand = heap[idx];
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float log174[3 * FT8_ND];
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float log174[FT8_N];
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extract_likelihood(power, num_bins, cand, kGray_map, log174);
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const int num_iters = 25;
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uint8_t plain[3 * FT8_ND];
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// bp_decode() produces better decodes, uses way less memory
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uint8_t plain[FT8_N];
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int n_errors = 0;
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bp_decode(log174, kLDPC_iterations, plain, &n_errors);
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//ldpc_decode(log174, num_iters, plain, &n_errors);
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if (n_errors > 0) {
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//printf("ldpc_decode() = %d\n", n_errors);
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continue;
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}
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float freq_hz = (cand.freq_offset + cand.freq_sub / 2.0f) * fsk_dev;
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float time_sec = (cand.time_offset + cand.time_sub / 2.0f) / fsk_dev;
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printf("%03d: score = %d freq = %.1f time = %.2f\n", idx,
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cand.score, freq_hz, time_sec);
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bp_decode(log174, num_iters, plain, &n_errors);
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//ldpc_decode(log174, num_iters, plain, &n_errors);
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printf("ldpc_decode() = %d\n", n_errors);
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if (n_errors == 0) {
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// printf("%03d: score = %d freq = %.1f time = %.2f\n", idx,
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// cand.score, freq_hz, time_sec);
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//print_tones(kGray_map, log174);
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// Extract payload + CRC
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uint8_t a91[12];
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uint8_t mask = 0x80;
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uint8_t position = 0;
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int byte_idx = 0;
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for (int i = 0; i < 12; ++i) {
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a91[i] = 0;
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}
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for (int i = 0; i < FT8_K; ++i) {
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if (plain[i]) {
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a91[position] |= mask;
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a91[byte_idx] |= mask;
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}
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mask >>= 1;
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if (!mask) {
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mask = 0x80;
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++position;
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++byte_idx;
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}
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}
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@ -365,13 +414,28 @@ int main(int argc, char ** argv) {
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// }
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// printf("\n");
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char message[20];
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char message[kMax_message_length];
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unpack77(a91, message);
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// Check for duplicate messages
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bool found = false;
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for (int i = 0; i < num_decoded; ++i) {
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if (0 == strcmp(decoded[i], message)) {
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found = true;
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break;
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}
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}
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if (!found && num_decoded < kMax_decoded_messages) {
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strcpy(decoded[num_decoded], message);
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++num_decoded;
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// Fake WSJT-X-like output for now
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printf("000000 0 %4.1f %4d ~ %s\n", time_sec, (int)(freq_hz + 0.5f), message);
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int snr = 0; // TODO: compute SNR
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printf("000000 %3d %4.1f %4d ~ %s\n", snr, time_sec, (int)(freq_hz + 0.5f), message);
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}
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}
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printf("Decoded %d messages\n", num_decoded);
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return 0;
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}
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