git.sesse.net Git - c64tapwav/blob - decode.cpp

   1 #include <stdio.h>
   2 #include <string.h>
   3 #include <math.h>
   4 #include <assert.h>
   5 #include <limits.h>
   6 #include <vector>
   7 #include <algorithm>
   8
   9 #include "audioreader.h"
  10 #include "interpolate.h"
  11 #include "tap.h"
  12
  13 #define BUFSIZE 4096
  14 #define HYSTERESIS_LIMIT (3000/32768.0)
  15 #define C64_FREQUENCY 985248
  16
  17 #define SYNC_PULSE_START 1000
  18 #define SYNC_PULSE_END 20000
  19 #define SYNC_PULSE_LENGTH 378.0
  20 #define SYNC_TEST_TOLERANCE 1.10
  21
  22 // between [x,x+1]
  23 double find_zerocrossing(const std::vector<float> &pcm, int x)
  24 {
  25         if (pcm[x] == 0) {
  26                 return x;
  27         }
  28         if (pcm[x + 1] == 0) {
  29                 return x + 1;
  30         }
  31
  32         assert(pcm[x + 1] < 0);
  33         assert(pcm[x] > 0);
  34
  35         double upper = x;
  36         double lower = x + 1;
  37         while (lower - upper > 1e-3) {
  38                 double mid = 0.5f * (upper + lower);
  39                 if (lanczos_interpolate(pcm, mid) > 0) {
  40                         upper = mid;
  41                 } else {
  42                         lower = mid;
  43                 }
  44         }
  45
  46         return 0.5f * (upper + lower);
  47 }
  48
  49 struct pulse {
  50         double time;  // in seconds from start
  51         double len;   // in seconds
  52 };
  53
  54 // Calibrate on the first ~25k pulses (skip a few, just to be sure).
  55 double calibrate(const std::vector<pulse> &pulses) {
  56         if (pulses.size() < SYNC_PULSE_END) {
  57                 fprintf(stderr, "Too few pulses, not calibrating!\n");
  58                 return 1.0;
  59         }
  60
  61         int sync_pulse_end = -1;
  62         double sync_pulse_stddev = -1.0;
  63
  64         // Compute the standard deviation (to check for uneven speeds).
  65         // If it suddenly skyrockets, we assume that sync ended earlier
  66         // than we thought (it should be 25000 cycles), and that we should
  67         // calibrate on fewer cycles.
  68         for (int try_end : { 2000, 4000, 5000, 7500, 10000, 15000, SYNC_PULSE_END }) {
  69                 double sum2 = 0.0;
  70                 for (int i = SYNC_PULSE_START; i < try_end; ++i) {
  71                         double cycles = pulses[i].len * C64_FREQUENCY;
  72                         sum2 += (cycles - SYNC_PULSE_LENGTH) * (cycles - SYNC_PULSE_LENGTH);
  73                 }
  74                 double stddev = sqrt(sum2 / (try_end - SYNC_PULSE_START - 1));
  75                 if (sync_pulse_end != -1 && stddev > 5.0 && stddev / sync_pulse_stddev > 1.3) {
  76                         fprintf(stderr, "Stopping at %d sync pulses because standard deviation would be too big (%.2f cycles); shorter-than-usual trailer?\n",
  77                                 sync_pulse_end, stddev);
  78                         break;
  79                 }
  80                 sync_pulse_end = try_end;
  81                 sync_pulse_stddev = stddev;
  82         }
  83         fprintf(stderr, "Sync pulse length standard deviation: %.2f cycles\n",
  84                 sync_pulse_stddev);
  85
  86         double sum = 0.0;
  87         for (int i = SYNC_PULSE_START; i < sync_pulse_end; ++i) {
  88                 sum += pulses[i].len;
  89         }
  90         double mean_length = C64_FREQUENCY * sum / (sync_pulse_end - SYNC_PULSE_START);
  91         double calibration_factor = SYNC_PULSE_LENGTH / mean_length;
  92         fprintf(stderr, "Calibrated sync pulse length: %.2f -> %.2f (change %+.2f%%)\n",
  93                 mean_length, SYNC_PULSE_LENGTH, 100.0 * (calibration_factor - 1.0));
  94
  95         // Check for pulses outside +/- 10% (sign of misdetection).
  96         for (int i = SYNC_PULSE_START; i < sync_pulse_end; ++i) {
  97                 double cycles = pulses[i].len * calibration_factor * C64_FREQUENCY;
  98                 if (cycles < SYNC_PULSE_LENGTH / SYNC_TEST_TOLERANCE || cycles > SYNC_PULSE_LENGTH * SYNC_TEST_TOLERANCE) {
  99                         fprintf(stderr, "Sync cycle with downflank at %.6f was detected at %.0f cycles; misdetect?\n",
 100                                 pulses[i].time, cycles);
 101                 }
 102         }
 103
 104         return calibration_factor;
 105 }
 106
 107 void output_tap(const std::vector<pulse>& pulses, double calibration_factor)
 108 {
 109         std::vector<char> tap_data;
 110         for (unsigned i = 0; i < pulses.size(); ++i) {
 111                 double cycles = pulses[i].len * calibration_factor * C64_FREQUENCY;
 112                 int len = lrintf(cycles / TAP_RESOLUTION);
 113                 if (i > SYNC_PULSE_END && (cycles < 100 || cycles > 800)) {
 114                         fprintf(stderr, "Cycle with downflank at %.6f was detected at %.0f cycles; misdetect?\n",
 115                                         pulses[i].time, cycles);
 116                 }
 117                 if (len <= 255) {
 118                         tap_data.push_back(len);
 119                 } else {
 120                         int overflow_len = lrintf(cycles);
 121                         tap_data.push_back(0);
 122                         tap_data.push_back(overflow_len & 0xff);
 123                         tap_data.push_back((overflow_len >> 8) & 0xff);
 124                         tap_data.push_back(overflow_len >> 16);
 125                 }
 126         }
 127
 128         tap_header hdr;
 129         memcpy(hdr.identifier, "C64-TAPE-RAW", 12);
 130         hdr.version = 1;
 131         hdr.reserved[0] = hdr.reserved[1] = hdr.reserved[2] = 0;
 132         hdr.data_len = tap_data.size();
 133
 134         fwrite(&hdr, sizeof(hdr), 1, stdout);
 135         fwrite(tap_data.data(), tap_data.size(), 1, stdout);
 136 }
 137
 138 int main(int argc, char **argv)
 139 {
 140         make_lanczos_weight_table();
 141         std::vector<float> pcm;
 142         int sample_rate;
 143         if (!read_audio_file(argv[1], &pcm, &sample_rate)) {
 144                 exit(1);
 145         }
 146
 147 #if 0
 148         for (int i = 0; i < LEN; ++i) {
 149                 in[i] += rand() % 10000;
 150         }
 151 #endif
 152
 153 #if 0
 154         for (int i = 0; i < LEN; ++i) {
 155                 printf("%d\n", in[i]);
 156         }
 157 #endif
 158
 159         std::vector<pulse> pulses;  // in seconds
 160
 161         // Find the flanks.
 162         int last_bit = -1;
 163         double last_downflank = -1;
 164         for (unsigned i = 0; i < pcm.size(); ++i) {
 165                 int bit = (pcm[i] > 0) ? 1 : 0;
 166                 if (bit == 0 && last_bit == 1) {
 167                         // Check if we ever go up above HYSTERESIS_LIMIT before we dip down again.
 168                         bool true_pulse = false;
 169                         unsigned j;
 170                         int min_level_after = 32767;
 171                         for (j = i; j < pcm.size(); ++j) {
 172                                 min_level_after = std::min<int>(min_level_after, pcm[j]);
 173                                 if (pcm[j] > 0) break;
 174                                 if (pcm[j] < -HYSTERESIS_LIMIT) {
 175                                         true_pulse = true;
 176                                         break;
 177                                 }
 178                         }
 179
 180                         if (!true_pulse) {
 181 #if 0
 182                                 fprintf(stderr, "Ignored down-flank at %.6f seconds due to hysteresis (%d < %d).\n",
 183                                         double(i) / sample_rate, -min_level_after, HYSTERESIS_LIMIT);
 184 #endif
 185                                 i = j;
 186                                 continue;
 187                         }
 188
 189                         // down-flank!
 190                         double t = find_zerocrossing(pcm, i - 1) * (1.0 / sample_rate);
 191                         if (last_downflank > 0) {
 192                                 pulse p;
 193                                 p.time = t;
 194                                 p.len = t - last_downflank;
 195                                 pulses.push_back(p);
 196                         }
 197                         last_downflank = t;
 198                 }
 199                 last_bit = bit;
 200         }
 201
 202         double calibration_factor = calibrate(pulses);
 203
 204         FILE *fp = fopen("cycles.plot", "w");
 205         std::vector<char> tap_data;
 206         for (unsigned i = 0; i < pulses.size(); ++i) {
 207                 double cycles = pulses[i].len * calibration_factor * C64_FREQUENCY;
 208                 fprintf(fp, "%f %f\n", pulses[i].time, cycles);
 209         }
 210         fclose(fp);
 211
 212         output_tap(pulses, calibration_factor);
 213 }