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 (upper - lower > 1e-6) {
  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         std::vector<short> pcm;
  65
  66         while (!feof(stdin)) {
  67                 short buf[BUFSIZE];
  68                 ssize_t ret = fread(buf, 2, BUFSIZE, stdin);
  69                 if (ret >= 0) {
  70                         pcm.insert(pcm.end(), buf, buf + ret);
  71                 }
  72         }
  73
  74 #if 0
  75         for (int i = 0; i < LEN; ++i) {
  76                 in[i] += rand() % 10000;
  77         }
  78 #endif
  79
  80 #if 0
  81         for (int i = 0; i < LEN; ++i) {
  82                 printf("%d\n", in[i]);
  83         }
  84 #endif
  85
  86         std::vector<pulse> pulses;  // in seconds
  87
  88         // Find the flanks.
  89         int last_bit = -1;
  90         double last_downflank = -1;
  91         for (unsigned i = 0; i < pcm.size(); ++i) {
  92                 int bit = (pcm[i] > 0) ? 1 : 0;
  93                 if (bit == 0 && last_bit == 1) {
  94                         // Check if we ever go up above HYSTERESIS_LIMIT before we dip down again.
  95                         bool true_pulse = false;
  96                         unsigned j;
  97                         int min_level_after = 32767;
  98                         for (j = i; j < pcm.size(); ++j) {
  99                                 min_level_after = std::min<int>(min_level_after, pcm[j]);
 100                                 if (pcm[j] > 0) break;
 101                                 if (pcm[j] < -HYSTERESIS_LIMIT) {
 102                                         true_pulse = true;
 103                                         break;
 104                                 }
 105                         }
 106
 107                         if (!true_pulse) {
 108 #if 0
 109                                 fprintf(stderr, "Ignored down-flank at %.6f seconds due to hysteresis (%d < %d).\n",
 110                                         double(i) / SAMPLE_RATE, -min_level_after, HYSTERESIS_LIMIT);
 111 #endif
 112                                 i = j;
 113                                 continue;
 114                         }
 115
 116                         // down-flank!
 117                         double t = find_zerocrossing(pcm, i - 1) * (1.0 / SAMPLE_RATE);
 118                         if (last_downflank > 0) {
 119                                 pulse p;
 120                                 p.time = t;
 121                                 p.len = t - last_downflank;
 122                                 pulses.push_back(p);
 123                         }
 124                         last_downflank = t;
 125                 }
 126                 last_bit = bit;
 127         }
 128
 129         // Calibrate on the first ~25k pulses (skip a few, just to be sure).
 130         double calibration_factor = 1.0f;
 131         if (pulses.size() < SYNC_PULSE_END) {
 132                 fprintf(stderr, "Too few pulses, not calibrating!\n");
 133         } else {
 134                 double sum = 0.0;
 135                 for (int i = SYNC_PULSE_START; i < SYNC_PULSE_END; ++i) {
 136                         sum += pulses[i].len;
 137                 }
 138                 double mean_length = C64_FREQUENCY * sum / (SYNC_PULSE_END - SYNC_PULSE_START);
 139                 calibration_factor = SYNC_PULSE_LENGTH / mean_length;
 140                 fprintf(stderr, "Calibrated sync pulse length: %.2f -> %.2f (change %+.2f%%)\n",
 141                         mean_length, SYNC_PULSE_LENGTH, 100.0 * (calibration_factor - 1.0));
 142
 143                 // Check for pulses outside +/- 10% (sign of misdetection).
 144                 for (int i = SYNC_PULSE_START; i < SYNC_PULSE_END; ++i) {
 145                         double cycles = pulses[i].len * calibration_factor * C64_FREQUENCY;
 146                         if (cycles < SYNC_PULSE_LENGTH / SYNC_TEST_TOLERANCE || cycles > SYNC_PULSE_LENGTH * SYNC_TEST_TOLERANCE) {
 147                                 fprintf(stderr, "Sync cycle with downflank at %.6f was detected at %.0f cycles; misdetect?\n",
 148                                         pulses[i].time, cycles);
 149                         }
 150                 }
 151
 152                 // Compute the standard deviation (to check for uneven speeds).
 153                 double sum2 = 0.0;
 154                 for (int i = SYNC_PULSE_START; i < SYNC_PULSE_END; ++i) {
 155                         double cycles = pulses[i].len * calibration_factor * C64_FREQUENCY;
 156                         sum2 += (cycles - SYNC_PULSE_LENGTH) * (cycles - SYNC_PULSE_LENGTH);
 157                 }
 158                 double stddev = sqrt(sum2 / (SYNC_PULSE_END - SYNC_PULSE_START - 1));
 159                 fprintf(stderr, "Sync pulse length standard deviation: %.2f cycles\n",
 160                         stddev);
 161         }
 162
 163         FILE *fp = fopen("cycles.plot", "w");
 164         std::vector<char> tap_data;
 165         for (unsigned i = 0; i < pulses.size(); ++i) {
 166                 double cycles = pulses[i].len * calibration_factor * C64_FREQUENCY;
 167                 fprintf(fp, "%f %f\n", pulses[i].time, cycles);
 168                 int len = lrintf(cycles / TAP_RESOLUTION);
 169                 if (i > SYNC_PULSE_END && (cycles < 100 || cycles > 800)) {
 170                         fprintf(stderr, "Cycle with downflank at %.6f was detected at %.0f cycles; misdetect?\n",
 171                                         pulses[i].time, cycles);
 172                 }
 173                 if (len <= 255) {
 174                         tap_data.push_back(len);
 175                 } else {
 176                         int overflow_len = lrintf(cycles);
 177                         tap_data.push_back(0);
 178                         tap_data.push_back(overflow_len & 0xff);
 179                         tap_data.push_back((overflow_len >> 8) & 0xff);
 180                         tap_data.push_back(overflow_len >> 16);
 181                 }
 182         }
 183         fclose(fp);
 184
 185         tap_header hdr;
 186         memcpy(hdr.identifier, "C64-TAPE-RAW", 12);
 187         hdr.version = 1;
 188         hdr.reserved[0] = hdr.reserved[1] = hdr.reserved[2] = 0;
 189         hdr.data_len = tap_data.size();
 190
 191         fwrite(&hdr, sizeof(hdr), 1, stdout);
 192         fwrite(tap_data.data(), tap_data.size(), 1, stdout);
 193
 194         // Output a debug raw file with pulse detection points.
 195         fp = fopen("debug.raw", "wb");
 196         short one = 32767;
 197         short zero = 0;
 198         unsigned pulsenum = 0;
 199         for (unsigned i = 0; i < pcm.size(); ++i) {
 200                 unsigned next_pulse = (pulsenum >= pulses.size()) ? INT_MAX : int(pulses[pulsenum].time * SAMPLE_RATE);
 201                 if (i >= next_pulse) {
 202                         fwrite(&one, sizeof(one), 1, fp);
 203                         ++pulsenum;
 204                 } else {
 205                         fwrite(&zero, sizeof(zero), 1, fp);
 206                 }
 207         }
 208         fclose(fp);
 209 }