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

   1 #include <stdio.h>
   2 #include <string.h>
   3 #include <math.h>
   4 #include <unistd.h>
   5 #include <assert.h>
   6 #include <limits.h>
   7 #include <vector>
   8 #include <algorithm>
   9
  10 #include "interpolate.h"
  11
  12 #define BUFSIZE 4096
  13 #define HYSTERESIS_LIMIT 3000
  14 #define SAMPLE_RATE 44100
  15 #define C64_FREQUENCY 985248
  16 #define TAP_RESOLUTION 8
  17
  18 #define SYNC_PULSE_START 1000
  19 #define SYNC_PULSE_END 15000
  20 #define SYNC_PULSE_LENGTH 378.0
  21 #define SYNC_TEST_TOLERANCE 1.10
  22
  23 struct tap_header {
  24         char identifier[12];
  25         char version;
  26         char reserved[3];
  27         unsigned int data_len;
  28 };
  29
  30 // between [x,x+1]
  31 double find_zerocrossing(const std::vector<short> &pcm, int x)
  32 {
  33         if (pcm[x] == 0) {
  34                 return x;
  35         }
  36         if (pcm[x + 1] == 0) {
  37                 return x + 1;
  38         }
  39
  40         assert(pcm[x + 1] < 0);
  41         assert(pcm[x] > 0);
  42
  43         double upper = x;
  44         double lower = x + 1;
  45         while (lower - upper > 1e-3) {
  46                 double mid = 0.5f * (upper + lower);
  47                 if (lanczos_interpolate(pcm, mid) > 0) {
  48                         upper = mid;
  49                 } else {
  50                         lower = mid;
  51                 }
  52         }
  53
  54         return 0.5f * (upper + lower);
  55 }
  56
  57 struct pulse {
  58         double time;  // in seconds from start
  59         double len;   // in seconds
  60 };
  61
  62 int main(int argc, char **argv)
  63 {
  64         make_lanczos_weight_table();
  65         std::vector<short> pcm;
  66
  67         while (!feof(stdin)) {
  68                 short buf[BUFSIZE];
  69                 ssize_t ret = fread(buf, 2, BUFSIZE, stdin);
  70                 if (ret >= 0) {
  71                         pcm.insert(pcm.end(), buf, buf + ret);
  72                 }
  73         }
  74
  75 #if 0
  76         for (int i = 0; i < LEN; ++i) {
  77                 in[i] += rand() % 10000;
  78         }
  79 #endif
  80
  81 #if 0
  82         for (int i = 0; i < LEN; ++i) {
  83                 printf("%d\n", in[i]);
  84         }
  85 #endif
  86
  87         std::vector<pulse> pulses;  // in seconds
  88
  89         // Find the flanks.
  90         int last_bit = -1;
  91         double last_downflank = -1;
  92         for (unsigned i = 0; i < pcm.size(); ++i) {
  93                 int bit = (pcm[i] > 0) ? 1 : 0;
  94                 if (bit == 0 && last_bit == 1) {
  95                         // Check if we ever go up above HYSTERESIS_LIMIT before we dip down again.
  96                         bool true_pulse = false;
  97                         unsigned j;
  98                         int min_level_after = 32767;
  99                         for (j = i; j < pcm.size(); ++j) {
 100                                 min_level_after = std::min<int>(min_level_after, pcm[j]);
 101                                 if (pcm[j] > 0) break;
 102                                 if (pcm[j] < -HYSTERESIS_LIMIT) {
 103                                         true_pulse = true;
 104                                         break;
 105                                 }
 106                         }
 107
 108                         if (!true_pulse) {
 109 #if 0
 110                                 fprintf(stderr, "Ignored down-flank at %.6f seconds due to hysteresis (%d < %d).\n",
 111                                         double(i) / SAMPLE_RATE, -min_level_after, HYSTERESIS_LIMIT);
 112 #endif
 113                                 i = j;
 114                                 continue;
 115                         }
 116
 117                         // down-flank!
 118                         double t = find_zerocrossing(pcm, i - 1) * (1.0 / SAMPLE_RATE);
 119                         if (last_downflank > 0) {
 120                                 pulse p;
 121                                 p.time = t;
 122                                 p.len = t - last_downflank;
 123                                 pulses.push_back(p);
 124                         }
 125                         last_downflank = t;
 126                 }
 127                 last_bit = bit;
 128         }
 129
 130         // Calibrate on the first ~25k pulses (skip a few, just to be sure).
 131         double calibration_factor = 1.0f;
 132         if (pulses.size() < SYNC_PULSE_END) {
 133                 fprintf(stderr, "Too few pulses, not calibrating!\n");
 134         } else {
 135                 double sum = 0.0;
 136                 for (int i = SYNC_PULSE_START; i < SYNC_PULSE_END; ++i) {
 137                         sum += pulses[i].len;
 138                 }
 139                 double mean_length = C64_FREQUENCY * sum / (SYNC_PULSE_END - SYNC_PULSE_START);
 140                 calibration_factor = SYNC_PULSE_LENGTH / mean_length;
 141                 fprintf(stderr, "Calibrated sync pulse length: %.2f -> %.2f (change %+.2f%%)\n",
 142                         mean_length, SYNC_PULSE_LENGTH, 100.0 * (calibration_factor - 1.0));
 143
 144                 // Check for pulses outside +/- 10% (sign of misdetection).
 145                 for (int i = SYNC_PULSE_START; i < SYNC_PULSE_END; ++i) {
 146                         double cycles = pulses[i].len * calibration_factor * C64_FREQUENCY;
 147                         if (cycles < SYNC_PULSE_LENGTH / SYNC_TEST_TOLERANCE || cycles > SYNC_PULSE_LENGTH * SYNC_TEST_TOLERANCE) {
 148                                 fprintf(stderr, "Sync cycle with downflank at %.6f was detected at %.0f cycles; misdetect?\n",
 149                                         pulses[i].time, cycles);
 150                         }
 151                 }
 152
 153                 // Compute the standard deviation (to check for uneven speeds).
 154                 double sum2 = 0.0;
 155                 for (int i = SYNC_PULSE_START; i < SYNC_PULSE_END; ++i) {
 156                         double cycles = pulses[i].len * calibration_factor * C64_FREQUENCY;
 157                         sum2 += (cycles - SYNC_PULSE_LENGTH) * (cycles - SYNC_PULSE_LENGTH);
 158                 }
 159                 double stddev = sqrt(sum2 / (SYNC_PULSE_END - SYNC_PULSE_START - 1));
 160                 fprintf(stderr, "Sync pulse length standard deviation: %.2f cycles\n",
 161                         stddev);
 162         }
 163
 164         FILE *fp = fopen("cycles.plot", "w");
 165         std::vector<char> tap_data;
 166         for (unsigned i = 0; i < pulses.size(); ++i) {
 167                 double cycles = pulses[i].len * calibration_factor * C64_FREQUENCY;
 168                 fprintf(fp, "%f %f\n", pulses[i].time, cycles);
 169                 int len = lrintf(cycles / TAP_RESOLUTION);
 170                 if (i > SYNC_PULSE_END && (cycles < 100 || cycles > 800)) {
 171                         fprintf(stderr, "Cycle with downflank at %.6f was detected at %.0f cycles; misdetect?\n",
 172                                         pulses[i].time, cycles);
 173                 }
 174                 if (len <= 255) {
 175                         tap_data.push_back(len);
 176                 } else {
 177                         int overflow_len = lrintf(cycles);
 178                         tap_data.push_back(0);
 179                         tap_data.push_back(overflow_len & 0xff);
 180                         tap_data.push_back((overflow_len >> 8) & 0xff);
 181                         tap_data.push_back(overflow_len >> 16);
 182                 }
 183         }
 184         fclose(fp);
 185
 186         tap_header hdr;
 187         memcpy(hdr.identifier, "C64-TAPE-RAW", 12);
 188         hdr.version = 1;
 189         hdr.reserved[0] = hdr.reserved[1] = hdr.reserved[2] = 0;
 190         hdr.data_len = tap_data.size();
 191
 192         fwrite(&hdr, sizeof(hdr), 1, stdout);
 193         fwrite(tap_data.data(), tap_data.size(), 1, stdout);
 194
 195         // Output a debug raw file with pulse detection points.
 196         fp = fopen("debug.raw", "wb");
 197         short one = 32767;
 198         short zero = 0;
 199         unsigned pulsenum = 0;
 200         for (unsigned i = 0; i < pcm.size(); ++i) {
 201                 unsigned next_pulse = (pulsenum >= pulses.size()) ? INT_MAX : int(pulses[pulsenum].time * SAMPLE_RATE);
 202                 if (i >= next_pulse) {
 203                         fwrite(&one, sizeof(one), 1, fp);
 204                         ++pulsenum;
 205                 } else {
 206                         fwrite(&zero, sizeof(zero), 1, fp);
 207                 }
 208         }
 209         fclose(fp);
 210 }